Light and Life in the Universe. Selected Lectures in Physics, Biology and the Origin of Life

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LIGHT AND LIFE IN THE UNIVERSE

Selected Lectures in Physics, Biology and the Origin of Life

EDITED BY

S. T. BUTLER M.SC., PH.D., D.SC.

Professor of Theoretical Physics

AND

H. MESSEL B.A., B.SC, PH.D.

Head of the School of Physics

UNIVERSITY OF SYDNEY

PERGAMON PRESS

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A course of lectures contributed to the Nuclear Research Foundation Summer Science School for Fourth-year High School students at the University of Sydney, January 6-17,

1964

For Copyright reasons this Edition is not for sale

in Australasia

Copyright © 1965 Pergamon Press Ltd.

Library of Congress Catalog No. 65-18522

First published in THE COMMONWEALTH AND INTERNATIONAL LIBRARY

1965

Printed in Great Britain by Taylor Garnett Evans & Co. Lid

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THE SPONSORS

The Nuclear Research Foundation within the University of Sydney

gratefully acknowledges the generous financial assistance given by the

following group of individual philanthropists and companies, without

whose help the 1964 Summer Science School for Fourth-year High

School students and the production of this book would not have been

possible.

Ampol Petroleum Limited

Australian Factors Group

Ducon Industries Limited

H. G. Palmer, Esq.

INTRODUCTION

The lectures of the 1964 Nuclear Research Foundation Sum-mer Science School for High School Students, contained in the chapters of this book, will be concerned primarily with various aspects of life—life here on earth and life in the universe generally.

The main lectures of this Summer School, by Professors J. D. Watson and M. Yeas, are concerned basically with the following topics:

• The "units" of living matter.

• How life may have started on earth.

• The possibility of life on other planets.

It may seem to you that these topics can have very little to do with ordinary physics and chemistry. Yet it is the purpose of the first six chapters of this book to develop for you some of the basic physics and chemistry which is necessary for appreciation of the later biology lectures. You will find in these later lectures how certain large molecules are characteristic of life forms—mole-cules which themselves have amazing properties and which are able to reproduce replicas of themselves. The question of how life may have originated on earth is thus intimately connected with how these large molecules may have been formed to start with. Similarly, the question of life elsewhere in the universe is vitally concerned with a number of other planets which may exist and which may have properties that can support life.

But do you really know what a molecule is? Do you know how electro-magnetic radiation can be responsible for the building up of large molecules? Do you know what electro-magnetic radia-tion is? Do you know the theories of what the earth was like thousands of millions of years ago when life originated on it? Are

you aware of the scientific thoughts regarding the origin of our solar system, and which are essential to any estimates of the possible number of other planets with life?

These are all topics in physics and chemistry which go hand in hand with our Summer School's main lectures on the subject of life. In order that you be able to appreciate fully these later lectures we devote much of the book's first six chapters to dis-cussing the above-mentioned subjects.

Because all lectures have been specifically prepared, written and edited for fourth-year High School students, we feel that they will be of interest to the widest section of the public. We feel that the material as presented will be appreciated not only by the increas-ingly science-conscious layman in this scientific age but also, in fields other than his own, by the specialised scientist.

The 1964 Nuclear Research Foundation Summer Science School and, indeed, this book are intended to stimulate and develop science-consciousness in Australia generally, and in particular in the 150 outstanding fourth-year High School students of 1963 who won scholarships to attend the School. The Foundation wishes to applaud and reward their ability and diligence.

Finally, we accept complete responsibility for the contents of this book and apologise for any errors which may have crept into the texts.

Sydney, January, 1964. S. T. BUTLER and H. MESSEL

CONTRIBUTORS OF LECTURES

R. N. BRACEWELL

Professor of Electrical Engineering, Stanford University, Stanford, California,

S. T. BUTLER

Professor of Theoretical Physics, University of Sydney.

H. MESSEL

Professor of Physics and Head of the School of Physics, University of Sydney.

J. D. WATSON

Professor of Biology, Harvard University, Cambridge, Massachusetts.

M. YCAS Associate Professor of Microbiology,

State University of New York, New York.

CHAPTER 1

Atoms, Molecules and Nuclei

All matter, whether in life forms or in inanimate objects, is made up of elementary particles called atoms. In many substances, atoms are joined together into larger units called molecules. In this chapter we describe, briefly, some of the properties of atoms and how they join up to form molecules, for it is this very process that has built up characteristic molecules of living beings. At the same time we shall outline some of the properties of the central core of each atom—the by now famous nucleus—because, as you will see in Chapter 4, a certain amount of nuclear physics enters into any discussion as to the origin of the sun and its solar system, (a) Matter consisting of atoms or molecules.

As you know, most substances can exist in any one of three states—the solid state, liquid state or gaseous state.

In the solid state the atoms or molecules are closely bound together by forces between them, and are arranged in a regular geometrical pattern called a "solid lattice." In this case the atoms or molecules possess rather little freedom of motion ; their main motion is a small to-and-fro movement around their proper positions in the lattice. The distance between particles in such a lattice is about the same as the size of the particles themselves.

In the liquid state the atoms or molecules are also fairly closely bound together by forces between them, but they are no longer arranged in a regular pattern, and it is possible for individual atoms and molecules to wander all over the liquid. The atoms or molecules slide over one another with great freedom, allowing the liquid to assume any shape. But since the elementary particles are still quite tightly packed and held together by strong forces, the liquid resists any attempt to change its volume.

In the gaseous state the molecules are on the average widely separ-ated from each other and exert but little attraction upon each other.

9

10 LIGHT AND LIFE IN THE UNIVERSE

The molecules of a gas move freely from place to place, the spaces between them being much larger than their size.

It is on the basis of this picture of matter that we are able to understand such properties as the pressure in gases and liquids, heat content and temperature of bodies of matter, and sound pro-pagation through a medium. Indeed all properties of matter are determined by the charged and uncharged atoms and molecules of which everything consists. As we shall see later the properties of electricity and magnetism are a direct consequence of the structure within an atom. To understand light, electricity and magnetism we must make use of the fact that each atom consists of a central positively charged core called the nucleus, around which revolve negatively charged particles called electrons. It will be our purpose in the present work to discuss in some detail the relationship of all these phenomena based upon the above picture of atoms.

For the remainder of this chapter we shall present the general picture, as it is now known, of atoms and molecules.

(b) Fundamental Particles

Atoms and molecules themselves are made up from three funda-mental building blocks—neutrons, protons and electrons. Their properties are as follows:

Electron im a S S = 9*Π x 10"28 g (approx. 2 X 103 0 lb> (charge (negative) = 4-8 x 10~10 esu

Pr ton i m a S S = 1#672 X 10~24 g

{charge (positive) = 4-8 x 10-10 esu XT , (mass = 1-675 x 10"24 g Neutron { . 4 . , ,

(zero electrical charge. If we designate the mass of the electron by me, that of the proton

by mv and of the neutron by mn it has been accurately measured that mv = 1836 me

mn = 1846 me.

Thus protons and neutrons have very nearly equal masses and each is very much more massive than an electron.

The proton has a positive charge, usually designated by +e, which is exactly equal in magnitude to the negative charge of the

ATOMS, MOLECULES AND NUCLEI 11

electron — e. The neutron, however, has no charge whatsoever and this made it much more difficult to discover. All three of these fundamental particles are extremely small in size being 10'13 cm or less in diameter.

Setting aside the question of the way in which these particles were discovered and their properties determined, let us accept the fact that they exist and see how they are built up into atoms and molecules and thus into matter as we know it.

(c) Atoms Matter which is made up purely of atoms of one kind is called

an element. There are 92 different types of atoms which occur naturally, and there are thus 92 natural elements (although other unstable elements can now be created artificially). Each atom con-sists of a very dense and small central core known as the nucleus, around which electrons move in satellite or planetary orbits. The nucleus itself contains no electrons but consists of protons and neu-trons. The number of protons in a nucleus is usually designated by Z, the so-called atomic number, and the number of neutrons in a nucleus is designated by N.

The total number of protons and neutrons in a nucleus is known as the mass number A — N -\- Z. Atoms are electrically neutral, and this means that there is a number Z of planetary electrons moving around a nucleus. These electrons which move in planetary orbits around nuclei do so because of their electrostatic attraction to the protons in the nucleus. If an atom or molecule loses an electron—we say it is ionized—it becomes a singly charged positive ion. If it loses two electrons, it becomes a doubly charged posi-tive ion, and so forth. On the other hand, if a neutral atom or molecule gains an electron, it becomes a singly charged negative ion.

If a nucleus has atomic number Z it contains Z protons each of charge +e. An electron of charge — e is therefore attracted towards such a nucleus; if the electron is a distance r away, the magnitude of the Coulomb attraction is

Ze2

Attractive force on electron = — . r2

It is to be noted that this is an inverse square force, exactly analogous to the force of gravitation. (Indeed there is also a gravita-


tional attraction between an electron and a nucleus, although because of the small masses involved it is negligible compared to the electro-static attraction). An electron which is attracted to a nucleus by this electrostatic force can therefore be in satellite-like orbits around the nucleus, these orbits being circular or elliptical exactly as the corresponding planetary orbits of the gravitational problem.

The simplest atom, that of ordinary hydrogen, has a nucleus consisting of one proton only, and one electron revolving in a satellite orbit around it. The radius of this orbit of the hydrogen atom is approximately i x 10~8 cm. It is thus seen that the hydrogen atom contains a very low density of particles. In other words, it is mostly empty space. The proton nucleus has a radius less than 10~13 cm, and then there is nothing right out to i x 10-8

cm, where the planetary electron is orbiting. The radius of the electronic orbit is larger than the radius of the proton by a factor of some hundreds of thousands.

As we go to heavier nuclei, so the number of protons and neutrons in the nucleus increases and thus also does the number of orbiting electrons. In helium there are 2 protons and 2 neutrons in the nucleus, and 2 electrons both revolving in an orbit similar to that of hydrogen. Thereafter, as the number of protons and neutrons and thus orbiting electrons increases, the additional electrons go into orbits which have decreasing radii. However, even the uranium atom with its nucleus consisting of 92 electrons and 146 neutrons has a nuclear radius of only about 10~12 cm whereas its 92 electrons revolve in orbits which range from about 10-10 cm out to about 10 ~8 cm.

Thus, even the uranium atom is much more open relatively speaking and contains more " empty space " than our solar system. In this the radius of the sun is about £ million miles, and the orbiting planets range from 35 million miles out to several thousand million miles.

We thus have our picture of the basic atoms which form natural elements. They are characterized by two numbers Z and A = TV + Z. Indeed it is now customary to state these numbers together with the chemical symbol for a particular element. For example, normal oxygen ha* a nucleus containing 8 protons and 8 neutrons and therefore has 8 satellite electrons. This is illustrated in Figure 11. The chemical symbol for oxygen is O and we can specify the number A = N + Z = 16 and Z = 8 by writing this symbol as 0\6.


FIGURE IT Schematic illustration of the O16 atom consisting of a nucleus with 8 protons and

8 neutrons plus 8 electrons in " satellite " orbits.

Similarly, as another example, natural calcium has 20 neutrons, 20 protons and 20 electrons and therefore may be written Ca^.

At the end of this chapter there is a Table giving the numbers of the fundamental particles for most of the naturally occurring 92 elements of the periodic table.

(d) Approximate Integral Values of Atomic Weights One very simple rule that is immediately evident concerns the

masses of the basic atoms of various elements. Since the mass of an electron is so very much less than that of a neutron or proton, and since also the proton and neutron have approximately equal mass, it should be the case that the total mass of an atom with mass number A is close to being an integral factor A times greater than the mass of the hydrogen atom. That this is true can be seen from Table 1 in which the atomic masses of a number of different elements are given.


TABLE 1

Atom Atomic Mass

Hi . . . . 100000

Lit ci2

0? Na*] C-/28

Cn40

FP56 te26

c«g Th™

u™

5-96839

11-90692

15-87081

22-81138

27-75986

39-6527

55-5009

62-441

230-245

236-209

In this table the mass of the hydrogen atoms has been taken as the basic unit (1-00000) and the masses of the other atoms given in terms of the number of hydrogen masses which they contain. It is to be seen that these masses are very nearly equal to the number A of neutrons and protons within the nuclei concerned. For example, carbon has a total of 12 particles in its nucleus and its total atomic mass is 11-90692 times that of the hydrogen atom.

As a matter of fact it is customary when speaking of atomic and nuclear masses in nuclear physics not to refer everything to the mass of the hydrogen atom but rather to the mass of the oxygen atom. This arose more by tradition than anything else, as oxygen had been used in chemistry as a standard to which to refer other chemical masses long before nuclear physics existed. In these units the Οψ atom is said to have a mass of 1600000 units and all other atomic masses are given on this scale.*

In Table 2 we give the masses of the neutron, proton and electron and also the atoms of Table 1 in terms of these new units in which 0\e has the mass 16-00000.

♦More recently it has been decided to take carbon as the standard. In these units C!

62 is said to have a mass of 12-00000 units and all other

atomic masses are then referred to this scale.


TABLE 2

Λ7/

Neutron . Proton Electron . H\ . . . Lit ■ ■ ■

c? . . . 0's

6 (standard) Na™ . . .

sin ■ ■ ■ Cn40

FP56

te26 Cu\l 77,232 1 " 9 0 TT238

u92

Atomic Mass

100898 1-00760 000055 1-00814 6-01697 12-00384 1600000 22-99706 27-98583 39-9755 55-9527 62-949 232119 238-132

The numbers in the atomic mass column of Table 2 which are the "atomic weights" are, of course, not the masses of the atoms in grams, but give merely the relative masses of the atoms and particles listed.* Thus the ratio of the mass of the hydrogen atom to the mass

r , . 100760 Λ t of the oxygen atom is ————- . Rather than always be con-Jh 1600000 verting atomic masses to grams however, it is appropriate to define a new unit of mass called an atomic mass unit (amu) such that the mass of the Οψ atom is exactly 16-00000 amu. The numbers in the atomic mass column of Table 2 are then the actual masses of the atoms and particles measured in amu. The conversion to grams is achieved by simply knowing the actual mass of 0\6

in grams. We have 0\e = 26-5568 x 10~24 g

26-5568 x 10-24

Hence 1 amu = — g 16

= 1-6598 x 10-24 g *The term atomic weight is thus a misnomer for it is not a weight at all. Since

it merely gives the relative masses of atoms it is a dimensionless quantity.


From the picture which we have given of an atom we should be able to check that the atomic masses given in Table 2 are actually equal to the individual masses of the fundamental particles involved in their structure. Let us consider the example of calcium 40. The experimentally measured mass as given in Table 2 is 39-9755 amu. On the other hand, this atom is made up of 20 neutrons, 20 protons and 20 electrons. The combined masses of these particles are as follows:

20 neutrons: mass = 20 x 1-00898 = 20-1796 20 protons: mass = 20 x 1-00760 = 20-1520 20 electrons: mass = 20 x 0-00055 = 0-0110

Total = 40-3426

The total mass of 40-3426 amu is certainly very close to, but slightly greater than, the measured mass of 39-9755 amu. For all atoms the same is true, that the experimentally observed mass is slightly less than the combined total of the constituent particles. Why then do we assert that the calcium 40 atom, for example, consists of 20 neutrons, 20 protons and 20 electrons, when the masses do not quite balance? This is a very interesting point which we can understand of the basis of later chapters. It certainly does not mean that our picture of the atom is incorrect. What it does mean is that when neutrons and protons are packed tightly together in a nucleus they apparently lose a little mass; we will see how this arises in Chapter 5 when we briefly discuss the theory of relativity.

(e) Molecules Although atoms are electrically neutral, any two atoms will feel

a force between them if they are sufficiently close together that their orbiting electrons tend to overlap. This force can be attractive if the two atomic cores (nuclei) are not too close together, although it becomes repulsive for very close distances of approach. Sometimes the attractive force is sufficiently strong to bond the two atoms together into what is called a molecule.

This bonding can occur in a variety of ways. Consider, for example, the case of lithium fluoride. Lithium has an atomic number of 3, and fluorine an atomic number of 9. Thus lithium has 3 atomic electrons and fluorine has 9. However, when a lithium and a fluorine atom come close enough together for the outer electron orbits just to touch each other, then it is energetically


FIGURE 1-2 Illustrating the ionic bonding of the lithium fluoride molecule.

Such a bond is called an ionic bond and is responsible also for the production of such substances as those of sodium fluoride and cesium chloride.

There are many other different ways in which atoms can combine to form molecules. The carbon atom, for example, with its 6 electrons likes to combine with 4 other identical atoms of a different type. For example, methane (C7/4) has a molecule with a carbon atom sitting at the centre of a tetrahedron and hydrogen atoms at each of the four vertices. In this way the carbon is equidistant from the hydrogen atoms. It tends to share one of its electrons with each of the hydrogen atoms producing a bonding called a covalent bond. This C//4 molecule is illustrated in Figure 1-3.

FIGURE 1-3 methane molecule (CH^) has an atom of carbon at the centre of 4 hydrogen atoms

symmetrically disposed around it on the corners of a tetrahedron.

favourable for one electron of the lithium atom to transfer into an orbit around the fluorine. Thus the lithium atom is left with a positive charge +e (ionised), and the fluorine atom has acquired an additional electron and therefore its charge is — e. The two atoms now attract each other by the electrostatic attraction and remain bonded together. This is illustrated in Figure 1-2.


There are many different combinations of atoms which can thus bond together into molecules; the principle, however, is always similar to the examples discussed above.

Such bonding of atoms into molecules is called chemical bonding. It involves only the outer electrons of each atom and in no way are the nuclei themselves disturbed or affected. The subject of chemistry really consists of a complete study of all the innumerable number of molecules that can be made up from basic atoms by means of such chemical (electronic) bonding and of the chemical reactions by which such molecules can be converted into others. Organic chemistry, in particular, involves the study of quite complex molecules sometimes involving hundreds and even hundreds of thousands of atoms—as we shall see in the study of protein molecules. No matter how complicated the molecule, however, the nuclei at the centres of all the atoms are unaffected by the (to them) slight rearrangement of their outer electrons which may have occurred in the formation of the molecule.

Tn fact, as far as normal chemistry is concerned, the detailed structure of the interior nuclear core of atoms is irrelevant; all that is of importance is its mass and its overall electric charge. It is in the field of nuclear physics that we become involved in the detailed structure within an individual nucleus.

(f) Gases, Liquids and Solids It is of considerable interest to ask why it is that a large assembly

of atoms of a given element can sometimes exist in a gaseous state, at lower temperatures in a liquid state and at even lower temperatures in a solid state.

This is due to the fact that even between two atoms of the same element there will exist an attractive force when they are sufficiently close together. The situation is illustrated in Figure 1-4.

In Figure 1·4(α) the two atoms are relatively far apart and exert very little force on each other. In Figure 1 -4(b) they are fairly close together compared to their size, and in this situation there will be an attractive force tending to pull them closer together. In Figure 1 -4(c), however, when the electronic orbits are close to overlapping, there will be a repulsive force tending to push them apart again.

These forces are purely electrostatic in nature. The total force between the two atoms of Figure 1-4 can be determined as the resultant


FIGURE 1·4(α)

ATTRACTIVE FORCE

FIGURE 14(b)

ϊΐ@ί*<^-

STRONG REPULSIVE FORCE

FIGURE 14(C)

Schematic illustration of how the force between atoms varies with their separation.

of several forces. Firstly, there are repulsive forces due to the like positive charges on the two nuclei, and also due to the electrons ot one atom repelling the like negative charges on the electrons of the other atom. Opposed to these repulsive forces there are attractive forces in which the positive nucleus of one atom is attracted by the


negative electrons of the other atom. Whether the sum total of all these forces is repulsive or attractive depends on which of the forces in the above two categories is the stronger.

The resultant force between two atoms can be calculated (although it is too complicated to do so here) and the result is always similar. It is found that at large distances the force is zero, but that as the atoms approach each other an attractive force develops; this gets stronger and stronger for closer distances of approach until the orbits of the satellite electrons of each atom start "bumping into each other". There then develops a strong repulsive force.

The overall behaviour of the force between two atoms as a function of their distance apart is shown qualitatively in Figure 1-5. It is to be realized, of course, that the attractive force that develops between two atoms is, on our standards, very weak, and that the maximum such force between two atoms is never much greater than 0-0032 poundals or about 49 dynes.

FIGURE 1-5

Typical behaviour of an interatomic force.


In a gas at high temperatures the average energy of the atoms is sufficiently great that the attractive part of the force is almost irrelevant and the atoms just bump into each other, feeling only the strong repulsive force at close separations. As the temperature of the gas is decreased, however, the average kinetic energy of the atom decreases, and so the attractive force can become more effective. At some temperatures, in fact, it becomes so effective that it starts to hold together large groups of atoms whose heat motion is now not suffi-cient to shake them loose. When this occurs the gas has condensed into a liquid. At still lower temperatures again, when the attractive force is very predominant, the atoms can become welded together in a regular lattice and thus are formed into a solid.

The same type of force also exists between molecules, and so a molecular gas can also condense into first a liquid and then a solid with decreasing temperature. In fact, it is quite often the case that the attractive force between molecules is greater than between the individual atoms, so that a group of molecules can condense at higher temperatures than a gas consisting of any one of the con-stituent atoms. Thus, for example, pure hydrogen and pure chlorine each exist as a gas at nearly normal temperatures. Yet an assembly of hydrogen and chlorine atoms bond together into molecules (HCl) forming a liquid at these temperatures—hydrochloric acid.

Similarly, hydrogen and oxygen each form a gas at normal temperatures. Yet an assembly of molecules made up of two hydrogen atoms and one oxygen atom (H20) forms a liquid at everyday temperatures — ordinary water.

(g) Nuclear Forces As we have stated, the interior nucleus of an atom consists of a

closely-packed assembly of neutrons and protons. Now we know that like-charged particles repel one another with a force inversely proportional to the square of their distance apart. At first sight therefore, we might imagine that a nucleus consisting of many close-packed protons repelling each other would not be stable.

However, the forces which bond the particles of a nucleus together are not electrostatic in nature at all. These forces are known as nuclear forces and their study is one of the central problems in nuclear physics today.


FIGURE 1-6

Schematic illustration of the distribution of neutrons and protons within a nucleus.

In general, an average nucleus contains quite a large number of neutrons and protons bound together to form something like a drop of " nuclear matter " (Figure 1-6). The neutron and proton in the interior of this nucleus, or drop, are subjected to an attractive force by all the surrounding neutrons and protons; those particles in the surface, however, have an attractive force directed inwards giving a surface tension effect. It is the interior portion of a nucleus which may be considered to be made up of typical nuclear matter just as the interior of a water droplet is a small volume of typical water. In this analogy different nuclei are just "drops" of nuclear matter of different sizes.

The individual nuclear force between neutrons and protons, a pair of neutrons, and a pair of protons has been studied in great detail in nuclear physics. In the main, this force depends on the distance apart of the two particles in the manner indicated in Figure 1 -7. For very close distances of approach the force is strongly repul-


3 42000

O +1500

z

z o iu -J u D Z Z IAJ 111

i U o

REPULSIVE

1000

500

0

+500

1000

SEPARATION IN lö ' cM 3 4

ATTRACTIVE

FIGURE 1-7 77je nuclear force as a function of separation of the nuclear particles.

sive and the two particles act like hard little billiard balls. Outside this repulsive core there is a region of attraction which tails off to zero very rapidly as the separation between the particles increases. The average range of this force is about 1*3 x 10-13 cm, and, as one would expect, this is also about the average separation of neutrons and protons in nuclear matter.

The tremendous strength of this force must be emphasized. The maximum attractive force, for example, is greater than 1,000 kg weight or about 1 ton weight. Thus two tiny nuclear particles are pulled together by this huge force until they get too close when they repel each other. Each particle in a nucleus is therefore feeling an attractive force of at least a high fraction of a ton weight from each of the nearest neighbour surrounding particles.

The average distance between neutrons and protons within a nucleus is about 1-3 x 10~13 cm. The radius R of a nucleus is


therefore dependent upon the total number of neutrons and protons A, by the approximate formula

R ~ 1-3 A1** x 10-13 cm. . . . (1-1) Thus the largest radius of any naturally occurring nucleus (uranium) is approximately:

1-3 x (238)* x 10-13 cm - 10"12 cm. Of course, within a nucleus, the ordinary electrostatic force of

repulsion between protons is present but it is not sufficient to force apart the particles of naturally occurring nuclei. It does make its presence felt, however, and if it were not for this Coulomb repulsion between protons, the universe would be a very different place. There would exist not merely 92 elements but an unlimited number of heavier and heavier elements.

The reason is that in the uranium nucleus with its 92 protons and 146 neutrons the electrostatic repulsive forces between protons are almost big enough to blow the nucleus apart. A nucleus with even one more proton is not stable and when formed will decay back again by ejection of particles into a stable nucleus. Although such unstable elements—called transuranic elements—have been produced artificially by men, they are unstable in the manner des-cribed and only last for a relatively short time before decaying back into the stable element. It is for this reason that no element heavier than uranium occurs naturally in any quantity on earth.

(h) Isotopes

As we have seen, all the chemical properties of a given element are determined simply by the satellite electrons of each atom. The number of these in turn is purely dependent upon the number of protons within the central nucleus, and each element that occurs in nature is characterized by this atomic number Z. However, the number of neutrons within the nucleus can be varied without changing Z, and therefore without changing the chemical properties of the substances. Thus it is possible to have elements which are indistinguishable chemically, having the same number of protons and satellite electrons in their atoms, but in the nuclear core containing different numbers of neutrons.

Such elements are called isotopes of the same substance. Isotopes differ only in having a different mass number and therefore different atomic masses.


As we know, the simplest type of atom is the ordinary hydrogen atom Hi, consisting of one proton in the centre and one satellite electron. However, there is another type of hydrogen called heavy hydrogen or deuterium, H,; this is identical with the ordinary hydrogen atom, except that the nucleus consists in this case of a proton and a neutron. The presence of the neutron does not affect the electron and hence the two kinds of atoms are chemically almost identical. This is the simplest example of an isotope. Deuterium occurs naturally to a small extent instead of hydrogen in the water molecules of sea water.

In addition to this isotope of hydrogen there is another which is heavier still. In this instance the nucleus consists of one proton and two neutrons and the element is called tritium Hi or doubly heavy hydrogen.

To take another example, ordinary oxygen has 8 protons and 8 neutrons in its nucleus—0\6. There also occur in nature, however, two other types of oxygen atoms, namely ones in which the neutron numbers are 9 and 10 respectively instead of 8. These isotopes of ordinary oxygen therefore have the symbols θ\7 and 0\s respective-ly. They occur in nature in the following percentages: 99-759% ordinary θ\\ 0037% <9j7 and 0-204% of θ\*.

In the same way all elements have a number of isotopes. Quite often, however, the isotopes of a given element are not stable. After all, one cannot arbitrarily change the number of neutrons within a given nucleus and necessarily still have a stable nucleus. If neutrons are taken away from a nucleus there is a predominance of protons and the nucleus becomes unstable because of the electro-static repulsion between them. The isotope of a particular element which occurs predominantly has already been selected by nature as the most stable configuration of neutrons with the number of protons involved. Adding neutrons is usually not as bad as taking them away, but can still produce "neutron-rich nuclei" which are unstable and will "decay" back to a stable nucleus by the emission of one or more nuclear particles. Such an unstable isotope is called a radioactive isotope.

The Table at the end of the chapter gives a list of various isotopes as well as their masses. The list is not complete.


(i) Avogadro's Number

In section (d), by using the value 26-5568 x lO"24 grams for the mass of a single 0\6 atom and by arbitrarily setting the mass of the 0\6 atom at 16 amu we found the value of 1 amu = 1-6598 x 10-24 grams. From this it is evident that since each 0\6

atom has a mass of 16 amu, then 16 grams of Og6 atoms must contain

16 x 1-6598 x 1 0 - " ^ 1 X , 0 " individual atoms.

In the case of ordinary hydrogen H\ we have from Table 2 that the mass of an individual atom is 1-00815 amu; thus 1-00815 grams of H\ must contain

100815 1-00815 x 1-6598 X 1 0 - = <™7 X 1 0 "

individual atoms.

In general it is clear from the method of definition that if the atomic weight of an element is MA amu and since each atom of the element has a mass of 1-6598 x 10~24 MA grams, then MA grams of the element must contain

1-6598 χ " ' θ - MA ' ^™ X , 0 " individual atoms. This number—6-0247 x 1023—usually designated by the symbol N0 is a universal constant and is known as Avogadro's number.

The same thing is true not only for pure elements consisting of atoms, but also for molecular substances. A molecule consists of a number of atoms bound together of varying mass numbers. The total mass of the molecule is say, M atomic mass units, where M, the molecular weight, is close to the integer which is the sum of the mass numbers A of the constituent atoms. For example, in the case of water (H20) the mass M o f a water molecule will be close to the integer 1 + 1 + 16 = 18 amu.

Once again therefore, if M grams of a molecular substance are taken, the total number of molecules contained therein will again be Avogadro's number N0.

The name 1-gram atom has been given to that quantity of a pure


element which weighs MA grams, where MA is the atomic weight of the element.

Similarly, the name 1-gram molecule has been given to that quantity of a molecular substance of mass M grams, where M is the molecular weight of the substance.

Thus we have the general law that 1 gram atom or 1 gram molecule of a substance contains N0 atoms or molecules respectively, where N0 is Avogadro's number.

(j) "Lifetime" of the Neutron We have stated previously that all matter is made up of three

fundamental building blocks—neutrons, protons and electrons. This is certainly true, although we should mention at this stage that neutrons have a very remarkable property.

This is that any neutron by itself will not last for ever. It has a certain "lifetime". Any neutron left by itself, will eventually turn into a proton and send off an electron.

The average lifetime of such a free neutron is 12*8 minutes. This process is called the decay of a neutron. Initially, the neutron has zero electric charge, and so also the total charge of its decay products is zero.

In view of this fact it may seem that we should really say that all matter is made up of protons and electrons, since a neutron must somehow be an unstable combination of a proton and an electron.

It is nevertheless still a fact that we should consider matter to be made up of the three fundamental particles—neutrons, protons and electrons. For even though a free neutron will decay into a proton and an electron, this is not true of neutrons existing in the stable nuclei of natural elements. When a neutron is bound together with other neutrons and protons in a nucleus the strong nuclear forces producing this close-packed binding at the same time prevent the neutrons from decaying. If this were not the case the only element which could exist in the universe would be ordinary hydrogen. This is because any heavy nucleus would have its neutrons all converted into protons and would then blow apart because of the now too-strong electrostatic repulsion. Even two protons cannot be bound together into a nucleus by themselves, but need at least one neutron to provide an additional nuclear


binding force for the same electrostatic repulsion. Indeed, the most stable nucleus with two protons is one which also has two neutrons— normal helium.

This intriguing fact, that the neutron which is a basically unstable particle exists in stable form within nuclei, is connected with the other fact noted previously that when neutrons and protons are packed together within a nucleus, there is an apparent disappearance of a small amount of mass. We shall discuss both of these effects in more detail in Chapter 5.

TABLE 3 Composition of most of the Elements and Masses of their Isotopes.

Element Z

n 0

H 1

He 2

Li 3

Be 4

B 5

Mass number A of isotopes

1

1 2 3

3 4 6

6 7 8

7 8 9

10

9 10 11 12

Atomic mass, amu

(Oie = 1 6 )

1-008 982

1-008 142 2-014 735 3016 997

3-016 977 4-003 873 6020 474

6-017 021 7-018 223 8-025 018

7-019 150 8-007 850 9015 043

10016 711

9-016 190 10-016 114 11012 789 12-018 162

Relative abundance

per cent.

unstable

99-9851 0-0149

unstable

1-3 X 10-4

99-9999 unstable

7-52 92-47 unstable

unstable unstable

100 unstable

unstable 18-5 81-5 unstable


Composition of most of the Elements and Masses of their Isotopes—Continued.

Element Z

C 6

N 1

O 8

F 9

Ne 10

Na 11

Mg 12


10 11 12 13 14

13 14 15 16 17

15 16 17 18 19

17 18 19 20

19 20 21 22 23

23

24 25 26

Atomic mass, amu

(O16 = 16)

10-020 61 11-014 916 12-003 804 13-007 473 14-007 682

13-009 858 14-007 515 15-004 863 16-010 74 17-014 04

15-007 768 16-000 000 17-004 533 18-004 874 19-009 48

17-007 486 18-006 670 19-004 456 20-006 352

19-007 915 19-998 860 21-000 589 21-998 270 23-001 680

22-997 139

23-992 696 24-993 815 25-990 871

Relative abundance per cent.

unstable unstable 98-892

1-108 unstable

unstable 99-635 0-365

unstable unstable

unstable 99-758 00373 0-2039

unstable

unstable unstable

100 unstable

unstable 90-92 0-257 8-82

unstable

100

78-60 1011 11-29



Element Z

Al 13

Si 14

P 15

S 16

a 17

A 18

K 19

Ca 20

Sc 21


27

28 29 30

31

32 33 34 36

35 37

36 38 40

39 40 41

40 42 43 44 46 48

45

Atomic mass, amu

(O ie = 16)

26-990 140

27-985 837 28-985 719 29-983 313

30-983 622

31-982 265 32-981 961 33-978 773

34-980 175 36-977 624

35-978 93 37-974 88 39-975 10

38-975 93 39-976 58 40-974 84

39-975 42 41-972 04 42-972 37 43-969 20

47-967 63

44-970 000


100

92-27 4-68 3-05

100

951 0-74 4-2 0016

75-4 24-6

0-337 0063

99-600

9308 00119 6-91

96-97 0-64 0-145 206 00033 0-185

100



Element Z

77 22

V 23

Cr 24

Mn 25

Fe 26

Co 27

Ni 28

Cu 29


46 47 48 49 50

50 51

50 52 53 54

55

54 56 57 58

59

58 60 61 62 64

63 65

Atomic mass, amu

(<916 = 16)

46-967 0 47-964 05

49-962 15 50-959 53

49-959 99 51-956 93

54-955 64

53-956 54 55-952 86 56-953 65

58-951 82

57-953 60 59-949 48

63-947 33

62-948 62 64-947 49


7-95 7-75

73-45 5-51 5-34

0-24 99-76

4-31 83-76 9-55 2-38

100

5-84 91-68

2-17 0-31

100

67-76 26-16

1-25 3-66 1-16

69-1 30-9



Element Z

Zn 30

Ga 31

Ge 32

As 33

Se 34

Br 35

Kr 36


64 66 67 68 70

69 71

70 72 73 74 76

75

74 76 77 78 80 82

79 81

78 80 82 83 84 86

Atomic mass. amu

(O16 - 1-6)

81-938 42

83-938 49 85-936 58


48-89 27-81 4-11

18-56 0-62

60-2 39-8

20-55 27-37 7-61

36-74 7-67

100

0-87 902 7-58

23-52 49-82 919

50-52 49-48

0-354 2-27

11-56 11-55 56-90 17-37



Element Z

Rb 37

Sr 38

Y 39

Zr 40

Nb 41

Mo 42

Ru 44


85 87

84 86 87 88

89

90 91 92 94 96

93

92 94 95 96 97 98

100

96 98 99

100 101 102 104

Atomic mass, amu

(O16 = 16)

84-931 0 86-929 5

85-935 4 86-935 2 87-933 60

88-937 12

93-935 2

95-935 58 96-936 93

99-938 29


72-15 27-85

0-56 9-86 7-02

82-56

100

51-46 11-23 17-11 17-40 2-80

100

15-86 9-12

15-70 16-50 9-45

23-75 9-62

5-7 2-2

12-8 12-7 17-0 31-3 18-3



Element Z

Rh 45

Pd 46

Ag 47

O/48

in 49

Sn 50


103

102 104 105 106 108 110

107 109

106 108 110 111 112 113 114 116

113 115

112 114 115 116 117 118 119 120 122 124

Atomic mass, amu

( 0 l e = 16)

103-936 9

107-936 90 109-940 98

109-939 11

111-939 99 112-942 06 113-940 13 115-942 12

114-942 07

113-941 09 114-941 54 115-938 06 116-941 71

119-939 04 121-942 6


100

0-8 9-3

22-6 27-2 26-8 13-5

51-35 48-65

1-215 0-875

12-39 12-75 24-07 12-26 28-86

7-58

4-23 95-77

0-95 0-65 0-34

14-24 7-57

24-01 8-58

32-97 4-71 5-98



Element

Z

Sb 51

Te 52

/ 53

Xe 54

Cs 55

Ba 56


121 123

120 122 123 124 125 126 128 130

127

124 126 128 129 130 131 132 134 136

133

130 132 134 135 136 137 138

Atomic mass, amu

(O1« = 16)

125-942 7 127-947 1 129-946 7

126-946

128-945 33

131-947 3


57-25 42-75

0089 2-46 0-87 4-61 6-99

18-71 31-79 34-49

100

0096 0090 1-919

26-44 408

21-18 26-89 10-44 8-87

100

0101 0097 2-42 6-59 7-81

11-32 71-66



Element Z

La 57

Ce 58

Pr 59

Nd 60

Sm 62

Eu 63

Gd 64


138 139

136 138 140 142

141

142 143 144 145 146 148 150

144 147 148 149 150 152 154

151 153

152 154 155 156 157 158 160

Atomic mass, amu

(Oie = 16)

143-956 07

149-968 78


0089 99-911

0-193 0-250

88-48 11-07

100

27-13 12-20 23-87 8-30

17-18 5-72 5-60

3-16 15-07 11-27 13-84 7-47

26-63 22-53

47-77 52-23

0-20 2-15

14-73 20-47 15-68 24-87 21-90



Element Z

Tb 65

Dy 66

llo 67

Er 68

Tm 69

Yb 70

Lu 71


159

156 158 160 161 162 163 164

165

162 164 166 167 168 170

169

168 170 171 172 173 174 176

175 176

Atomic mass, amu

(Oie = 16)


100

0-0524 00902 2-294

18-88 25-53 24-97 28-18

100

0-136 1-56

33-41 22-94 27-07 14-88

100

0140 303

14-31 21-82 1613 31-84 12-73

97-40 2-60



Element Z

Hfll

Ta 73

if 74

Re 75

Os 76

h 77

Pt 78


174 176 177 178 179 180

181

180 182 183 184 186

185 187

184 186 187 188 189 190 192

191 193

190 192 194 195 196 198

Atomic mass, amu

(Oie = 16)

175-992 3

177-993 8

180-004 4

182-003 8 183003 21 184006 0

194024 0 195026 42


0-18 5-15

18-39 27-08 13-78 35-44

100

0135 26-4 14-4 30-6 28-4

37-07 62-93

0018 1-59 1-64

13-3 16·! 26-4 410

38-5 61-5

0012 0-78

32-8 33-7 25-4

7-23



Element Z

Au 79

Hg 80

77 81

Pb 82

Bi 83

Th 90

U 92


197

196 198 199 200 201 202 204

203 205

204 206 207 208

209

232

234 235 238

Atomic mass. amu

(O16 = 16)

203-035 0 205037 9

204-036 1 206-038 6 207-040 3 208041 4

209045 5

232-110 3

234-113 8 235-117 0 238-124 9


100

0-146 10-02 16-84 23-13 13-22 29-80 6-85

29-50 70-50

1-48 23-6 22-6 52-3

100

100

0-0058 0-715

99-28

CHAPTER 2

Electro-Magnetic Radiation —

Bohr's Theory of the Atom

Throughout this book you will find yourself continually referring to the information regarding atoms and molecules con-tained in the previous chapter. In addition, however, you will also find continual reference being made to the action of light. As you will see all life is completely dependent on light—one form of electro-magnetic radiation—and it is almost certainly true that electro-magnetic radiation in general was responsible for the origin-ating of life on our planet.

Thus, an understanding of what electro-magnetic radiation is and how it is formed must be a very desirable ingredient in any series of lectures regarding life; and, although you may not realise it at present, electro-magnetic radiation is really a part of atomic physics, for it is from within atoms that electro-magnetic radiation originates.

The picture of the atom which we have outlined in Chapter 1 as consisting of satellite electrons orbiting central nuclei was first proposed by the great nuclear physicist, Ernest Rutherford, early in this century. Rutherford was led to this picture by a series of detailed experiments which he and his collaborators performed, the details of which are beyond the subject scope of this Summer School. The main point of interest to us, however, is that, although the evidence which led Rutherford to his picture of the atom was extremely strong, this picture at first had an extremely grave difficulty associated with it which made it completely untenable without further modification. This difficulty is intimately concerned with the whole subject of light and of electro-magnetic radiation, and its solution gave rise to a detailed understanding of these phenomena.

41


The present chapter is intended to introduce you to the subject of electro-magnetic radiation, through the difficulties originally encountered by the Rutherford picture of the atom. This chapter will contain most of the limited amount of mathematics that appears in this book; we hope, however, that you read it carefully for it will stand you in good stead not only in the reading of this book but in your future studies of science.

(a) Electric and Magnetic Fields produced by accelerating charge.

Our starting point will be with one of the most fundamental laws of physics—namely Coulomb's law, mentioned in Chapter 1, which expresses the force F in dynes, between two stationary charges qx and q2 (measured in esu) distant r cm from one another in a vacuum. This law states that the force F is given by the expression.

F = ± 4 * . (2-D r2

and is known as an electrostatic force.

+ _.,,., ->·

JT

+ +

Ψ V FIGURE 21a

*-TM +

Two stationary charges qt and q2 in a vacuum. If both charges are positive or both negative then the force F is repulsive—tending to force the charges apart as shown. If the charges are unlike then the force is an attractive one tending to bring the charges closer together—in this case F is negative.

Now one may immediately ask how do the particles interact? How do they influence one another ? We say that each charge has a sphere of influence around it known as a ' field ', and that the * electric field intensity ' due to a charge qx at some point P which is r cm away is defined by

E ( i i ) = J = § (2-2)

ELECTRO - MAGNETIC RADIATION 43

That is, we define the electric or electrostatic field E (qj to be the ratio of the force F on some small positive charge q2 at P9 to the magnitude of q2. The electric field

+ p • T . _ ^ ,

Ψ FIGURE 21b

The electric fieldE(qt) at P is given by qjr2 and has the direction shown in the Figure for the positive charge qv Similarly for the positive charge q2 it would be q2/r

2,

E (qj has the direction shown in Figure 2 · lb for the positive charge qv The electric field due to the positive charge q2 is defined in an entirely analogous way and hence is given by

E(ti = -t (2-3)

Next let us consider the case of two charges moving parallel to one another, one charge qx moving with uniform velocity vx and another charge q2 moving with uniform velocity v2 (see Figure 2-4). Furthermore let us consider the charges at the moment when both vx and v2 are perpendicular to r—the distance between the charges— and assume that the velocities vx and v2 are both small compared to the velocity of l ights = 3 x 101θ cm/sec. Now—what is the force or forces acting on the charges? It has been found experimentally (and it can be shown theoretically from Einstein's theory of special relativity) that the force F can no longer be given by the simple equation (2-1) , but instead consists essentially of two terms which are given by*

, = * * + * * ™ (2-4) r2 r2 c2

An understanding of the expression for F given by equation (2-4) is of tremendous importance for it introduces the whole field of

♦The expression given for the force assumes that both velocities V! and v2 are small compared with the velocity of light c. If this is not the case, then in the present instance the force on qx would not be equal to the force on q2.


" magnetism ". We shall rewrite the expression for F in a number of different ways in order to bring out various points :

F = i i

= Λ or

F = ft

= *i where

* ( * ) Ί (fl) * ( f t )

Λ (fl)

£(<&) Mfc) *(fc)

Jp"2 (fc)

and ,

*Ί (ft)

^2 (<7l)

E (ft) + ^ 1 2? (ft)

(ft) + *i (ft)

E (ft) + ^ 2 5 (ft) c

(ft) + 2̂ (ft)

= ft/'2 - · = ft £ (ft) = ft ftA2

= ft V A r .

■^W-7· = iJr* ■ . =-q*E (ft) = ft ft/r2

= ft Vi/rV .

c rz

= *i (ft) = 2̂ (ft)

.

.

.

. . .

.

.

.

Vi V2

c2

.

.

.

Vl V2

c2

.

.

(2-5a)

(2-5b)

(2-6a)

(2-6b)

(2-7a) (2-7b) (2-7c)

(2-7d)

(2-8a) (2-8b) (2-8c)

(2-8d)

(2-9a)

(2.9b)

First it should be noted that the force F acting on each of the charges is the same in magnitude. The first term in equation (2-4) is the same as that given by Coulomb's law for stationary charges. It expresses the force experienced [Fx (q^] by the charge qx due to the ordinary electric field E (q2) of the charge q2 and equally well the force experienced [Fx (q2)] by the charge q2 due to the ordinary electric field E (qx) of the charge qv Naturally, as we have already seen in equations (2-7b), (2-8b) and (2 -9a), these forces are equal in magnitude.

Now what about the second term in the expression for the force F given by equation (2-4) ? This second component F2 of the force F is the same for both charges—see equations (2-4), (2-5b), (2-6b), (2-7d), (2-8d) and (2-9b).


Let us consider the charge ft in relation to the force component F2 (ft) on it. As we can see from equation (2-7d),

r , x ft ft . VX V2 i i Vi ^2 (ft) = —5- —7- = £ (ft)

this component of the force turns out to be proportional to the velocity vx of charge ft. The influence of charge q2 travelling with velocity v2 is contained in the term B (ft) given by equation (2-7c), that is by the term q2 v2/r

2 c and hence is seen to be due to the uniform velocity v2 of charge q2 itself. In analogy to the electric field where we have in equation (2-7b),

Fi (ft) = ?i E (q2) we define this influence due to the uniform motion of charge q2, also as a "field" ; in this case we call it a magnetic field. The magnetic field intensity B (q2) at a distance r is defined by equation (2-7c)

B (ft) = ft V**2 c> and in a similar manner the magnetic field intensity of the charge q1

is given by equation (2-8c)

B (ft) = ft νχ/Α·2 c.

FIGURE 2 1 C

For a uniformly moving charge q the electric field E at a point P is along the direction qP and the magnetic field is at right angles to E and v (at right angles to the plane of the paper). Θ is the angle between the line joining the charge to point P and the

direction of the velocity v.


In our example we took r to be perpendicular both to vx and v2. If r were to make an angle Θ with the velocity v2 then we would have

» M = ^ f (2-.0) where B is measured in units known as gauss when q2 is given in esu, r in cm and c in cm/sec. The direction of B is perpendicular to both v2 and the ordinary electric field E as shown in Figure 2-lc. Thus we see that a magnetic field, or magnetism, is simply the result of the uniform motion of charged particles.* Again let us draw the attention of the student to the expression for the force component F2

(ft) given above which tells us the force on the charge qx moving with velocity v1 perpendicular to the direction of the magnetic field B (q2), which is itself due to the uniform motion of charge q2 mov-ing with velocity v2.

We now see that a charge q moving with uniform velocity v has associated with it at a distance r, two fields, one of these is known as the electric field E, the other as the magnetic field B, given by

E = q/r2 (2-1 la) av sin Θ

B = q— (211b) r2 c

The joint electric and magnetic fields are known as an electro-magnetic field.

The difference between an electric field E and magnetic field B is that the electric field produces the same force on a charge at P irrespective of whether this charge is moving or stationary—that is, the electric field produces a force on a charge irrespective of this charge's own motion. On the other hand, a magnetic field B only produces a force on a charge at P if this charge is moving. Indeed, the whole terminology of electric fields and magnetic fields has been introduced as a convenient way to describe the general forces operating between moving charges. That component of the force on one par-ticle which is independent of the particle's motion is said to be due to the electric field of the other particle, and that part of the force which is proportional to the particle's velocity is said to be due to the magnetic field of the other particle.

* The familiar bar magnet can also be readily understood on this basis.


Our entire discussion, so far, has automatically assumed, however, that the charges involved were moving with uniform velocity. These forces become slightly modified if the charges also have accelerations. On the theory of relativity the general force between two charges which are accelerating is modified by the accelerations. In other words, the electric and magnetic fields in the vicinity of a moving charge are modified from equations (2-1 la) and (2-1 lb), if the charge also has an acceleration. Suppose, for example, a charge q not only has a velocity v, but also an acceleration a which for definiteness we can consider to be in the direction of the velocity vector v. The electric field intensity now has two components at right angles to each other, say E1 and Ev The first E1 is still given by equation (2-1 la) and is still directed outwards along the line joining the charge q to point P. Thus we have

Εχ = \ (2 12)

The other component E2 is at right angles to El and given by

a a sin Θ £,2 — -

rc (2-13)

Once again θ is the angle between the direction of motion of the charge q and the line connecting it to point P. These components

FIGURE 2-2

When a charge q is being accelerated, the two components of electric field intensity Ex and E2 are in the directions shown.


are illustrated in Figure 2-2. The meaning of the component E2

is that when the charge q is being accelerated then any other charge in its vicinity is subjected not merely to the coulomb force El9 but to another component of electric force at right angles to it.

Similarly, the magnetic field in the vicinity of the charge q now has two components B1 and B2. The first is simply as before

q v sin Θ «i = — i . . . . . (214)

re and this is in the same direction as for equation (211b). However, the second component B2 is

q a S ' n θ . . . . (2-15) z?2 = rc-

The direction of this component is at right angles to v and also at right angles to E2 as shown in Figure 2-3.

It is seen that both components E2 and B2 go to zero if the accelera-tion a is zero, and are at right angles to each other and to the direction

FIGURE 2 3

When a charge q has acceleration a in the direction of its velocity v, the two magnetic field components are both at right angles to qP and to v and a as shown. (In the

diagram Bx is directed outwards from the paper and B2 inwards.)


of the original electric field component Ελ. It is also to be seen that, when q is expressed in e s u throughout, E2 and B2 are equal to each other in magnitude.

These new components E2 and B2 are of extreme importance in nature. This importance stems from the fact that they fall off

with increasing distance r only as -, whereas the normal com-

ponents B1 and Ex fall off as —. Thus, no matter how small the

acceleration a, at large distances away from the charge q it will be the

new components E2 and B2 which predominate, even though they may

be small effects at distances close to the charge q.

(b) Velocity of Propagation of Electric and Magnetic Influence

Before we go on we should pause to enquire how the formulae of section (a) arise. In the form presented they are given naturally by the theory of relativity. As such they can only be accepted on face value by the student, although he must surely be asking why it is that the velocity of light c is so intimately involved.

Electricity and magnetism are, of course, very old scientific subjects, and until late in the last century were developed and investigated purely experimentally. The magnetic effect of an electric current was first demonstrated by Oersted (1777-1851), and, as a result of further work by Biot, Savart and Ampere, equations were set up describing the force exerted by one current carrying conductor on another. These equations were derived in an empirical manner, i.e., they were simply adjusted to fit the experimental observations. Once it is realized that an electric current is due purely to moving electric charges, the equations of section (a) are a natural consequence of the empirical laws of Biot, Savart and Ampere. Of course, the general empirical equations involve alternating currents and therefore moving charges which are undergoing accelerations ; this is how the dependence of electric and magnetic fields associated with accelerating charges can be derived from empirical laws on electro-magnetism.


Thus we may consider that the formulae of section (a) were to start with empirical formulae derived from empirical laws, once it is realized that electric currents are moving charges. A complete theoretical understanding of the forces between two moving charges, and thus of the electric and magnetic fields associated with any one moving charge, had to await the relativity theory of Einstein early in the present century, but the great theoretical physicist, Clerk Maxwell, succeeded in 1864 in obtaining at least a qualitative understanding of the physical reasons as to how these forces arose. Of course, when derived from the empirical laws of Biot, Savart and Ampere, the expressions for electric and magnetic field strengths of a moving and accelerating charge had no obvious connection with the velocity of light. It was Maxwell who first derived our formulae of section (a) from the empirical laws and observed that, whenever there was a constant in the formulae with the dimensions of a velocity, it always had the value 3 x 1010 cm/sec. This he observed was equal to the velocity of light and it became obvious to him that somehow all of electro-magnetism must be connected with light and its velocity.

He conjectured that the only way in which the velocity of light could enter would be if the electric influence of a charge spread out from that charge at the velocity of light. Once he postulated this, he immediately obtained a qualitative understanding of all

FIGURE 2-4

Charges qx and q2 moving with velocities vx and v2 respectively.


electro-magnetic phenomena. He assumed that all charges in-fluence other charges merely by the simple coulomb law except that the field of influence of any one charge is not instantaneous, but can only propagate out with a finite velocity—that of light.

Let us sec where this leads us. Suppose we first assume, contrary to Maxwell's conjecture, that any one charge can instantaneously affect every other charge. Then we can consider the case of two charges, qx and q2 each moving with certain speeds vx and v2 (see Figure 2-4). Suppose, at some instant their separation is r ; then if each one is instantaneously influencing the other, their speeds must be irrelevant and the force between them given simply by the coulomb law

F = ?if2 . . . . (216) r2

However, suppose the electric influence of each charge is travelling out at the finite velocity c. Then at the instant under consideration charge q2 is not feeling the influence of qx in its present location, but where it was a little earlier, for in the time it takes the electric influence to travel from charge qx to charge q2, qx has itself moved. Thus, at each instant, q2 feels the coulomb force of charge qx when it was at its previous position, e.g., position Qx of Figure 2-4. This is such that in the time taken for the electric influence to travel the distance rx = Q1 q2i charge qx has moved the distance Qx qx.

Thus the force on charge q2 would not be given by equation (216) but rather by

F = ^ . . . . (2-17) rx

2

and this is directed along the direction Qx q2 rather than the direction d i s -

similarly, at the instant under consideration, charge q1 is feeling the influence of q2 when it was back at Q2. Thus again, the force that qx is experiencing is not along the line joining qx q2, but rather along the direction Q2 qx. Moreover, its magnitude is given by

F = ^ , . . . . (2-18)

where r2 = Q2 qv


These equations involve the velocities of the moving charges and the velocity of light. At the same time the force on each charge can be resolved not only into a component along the line qx ^joining the two charges, but also into a component at right angles to this line.

It is similarly true that if the charges are undergoing accelerations, this will also influence the formulae since the positions Qx and Q2

will be dependent on the accelerations which the charges are under-going.

It was in this way that Maxwell realized that almost certainly the entire field of magnetism and the forces which we called magnetic forces, due to moving charges, were due simply to the fact that a charge did not instantaneously exert its coulomb force on all other charges in the neighbourhood, but that its influence propagated out from it at the speed of light.

Maxwell couldnot completely derive the equations (211a) to (215) —this had. to await the work of Einstein. However, there was such qualitative understanding to be obtained in Maxwell's ideas that he had no doubt that his conjecture concerning the finite speed of pro-pagation through vacuum of electrical influences was correct, and that this was equal to the velocity of light. Having arrived at this connection between electro-magnetic phenomena and light, it seemed to Maxwell that it was more than an accident that both light and electric influences would propagate through vacuum at exactly the same speed. He was, therefore, led into the extremely important theoretical investigation of the next section.

(c) Radiation from Accelerating Charges

We now come to a very fundamental question. We have seen that a moving charge produces in its vicinity electric and magnetic fields, and if it is accelerating, these extend to much greater distances away than if the acceleration is zero. These electric and magnetic fields really mean that there is a "field of influence" around the charge q in which it will exert forces on, and influence, other charges. Now, of course, when a force acts on another charged particle and accelerates it, this changes the energy. It is, therefore, as if the particle has extracted energy out of the electric and magnetic fields produced by the first moving charge q. It was


Maxwell, inspired by his theory connecting the speeds of propa-gation of light and electrical influences, who first enquired whether a moving charge q is somehow sending out energy in its electric and magnetic fields which can then be used to modify the passage of charged particles through these fields.

To answer this question let us again consider the situation of a charge q moving with velocity v and acceleration a in the direction of the velocity, and let us enquire whether the electric and magnetic fields at some point P are in some way carrying energy. We can ask, for example, what energy per second, if any, is passing through unit area around P at right angles to the line joining q to P. Thus we can ask for the power, if any, being radiated by the moving charge q.

We can answer this question by making use of "dimensional analysis". Any quantity which is a mass is designated by M; any-thing proportional to a time is designated by T; and anything pro-portional to a length is designated by L. You may readily check that the quantity of energy passing per second through unit area has the dimensions of MT~:t. What combination of magnetic and electric field strength E and B can give these dimensions? From equation (2-2) we see that any electric field intensity E must have dimensions given by

E = Q L·2

where Q represents the dimensions of an electric charge measured in esu. Similarly, from equation (2 10) we see that any magnetic field intensity must also have dimensions given by

B = Q L-2.

We also know that a force

F — Charge (Q) E,

and force has dimensions MLT'2. Thus we have

Force = Q2 Lr2 — MLT2,

and so

ρ-' == ML '· T-2

where Q represents electric charge measured in esu. We now


have the dimensions of E and B explicitly in terms M, L and T as follows:—

E — M*/2 L-i/2 T-I9

B — MV2 L 1 / 2 Γ 1 .

We are now in a position to enquire what combination of E and B is equal to energy passing through unit area per second. We must, however, allow that in the expression for this, the velocity of light might enter as this is a fundamental quantity governing the velocity of propagation of the electric and magnetic fields and occurs in the formulae for E and B themselves. Thus we may ask for the values of x, y, z for which we have

Ex Bv cz — MT*.

On substituting the dimensions of E, B and c we have (Mi/2 £-1/2 r-i)*(Afi/a L-i/2 T-ya^T1)*— MT'3

This requires that the following set of equations be satisfied:

i (x + v) - 1,

— i (x + y) + z - 0,

—(x + y) — z = — 3 .

These equations have as their solution £ = l , ; t - | - y = 2.

Thus the expressions

E*c (x = 2, y - 0),

EBc (x — 1, y = 1),

B°-c (x — 0, y — 2),

all have the dimension of energy per unit area per second. We can therefore have

(KE2c or

energy per unit area per second = \KB2c or

KEBc

where K in some dimensionless constant which cannot be derived by simple dimensional analysis.

If we consider a point P in the "field of influence" of a moving and accelerating charge q and at a sufficiently great distance that


only E2 and B2 are significant, then since E2 = B2 each of the possibilities above yields the same result. We have

energy per unit area per second = K ^

This energy passing through unit area per second at the point P 1

falls off only as —, that is, it obeys an "inverse square law". From r2

this we can now find the total energy through an entire sphere surrounding the charge q with radius r. The total surface area of this sphere is, of course, 4nr2, and as we let out point P move over the surface of this sphere, the only thing that changes in the above equation is the factor sin2©.

It can be shown that the average value of sin2© over the surface 2

of this sphere is —. Thus we have the Total energy per second 3

passing outwards through a sphere of radius r around the charge q is given by

q2a2

Total energy per second -= 4nr2K X X < sin2© > aT· cr2

q-a-= 4πΚ X } .

cs

As a matter of fact, the dimensionless constant K can be shown 1

to be equal to —, so that the constant 4πΚ of the above equation is An

actually just unity. The correct formula is thus qW

Total energy per second = i . . . (2-19)

The interesting thing about this formula is that it is independent of r, that is, no matter how big we consider the surrounding sphere, there will always be this amount of energy passing outwards through its surface per second. It represents the total energy being "radiated" per second by the charge. Equation (2* 19) was derived by Maxwell in his 1864 work, and since he had shown on the one


hand that accelerating charges should radiate energy into electro-magnetic fields, and on the other that this energy is carried by the fields E-2 and B2 with the velocity of light, it was natural for him to postulate that light itself was one example of this, what he called, "electro-magnetic radiation". On this basis the production of light itself is due simply to the accelerated motion of electrical charges.

In order to confirm his hypothesis Maxwell carried out an analysis as to how electro-magnetic radiation of a given frequency and wavelength could be produced. We will discuss this in the next section.

It can be shown that equation (2-19) is correct for a charge q with acceleration a irrespective of the direction in which this acceleration is occurring. If the acceleration a = 0 and only the components Ei and Βλ are present, it can be shown that the total energy passing outwards through a sphere of radius r goes to O as r goes to infinity. Thus, in this case, there is only a "local" field of influence around q and no overall loss of energy into the electric and magnetic fields. Once the charge is accelerated, however, it loses energy into the electric and magnetic fields which travel out-wards with the velocity of light. This energy is called electro-magnetic radiation.

(d) Electro-magnetic Radiation

As an example of electro-magnetic radiation, we can, following Maxwell, consider a charge q performing simple harmonic (oscillatory) motion around some central point O with frequency n. This will be continuously radiating energy, i.e., energy being carried outwards in the electric and magnetic fields since an oscillatory motion has an acceleration associated with it at all times. At some distant point P (Figure 2-3) the electric and magnetic fields will consist of E2 and B2 at right angles to the line qP. Since the acceleration is now varying and changing direction, as q passes through O and reaches a maximum at the turn around points— E2 and B2 will also be varying in simple harmonic fashion. They will be zero when q is exactly at 0 , changing sign as q passes through the mid-point. The frequency of oscillation of the fields E2 and B2 is also known—exactly the same as the frequency of oscillation of the charge q.

Here then we have precisely an analogous situation as in the case of sound waves. There some vibrating object, e.g.,.


string or diaphragm, provided a source of sound and in its vicinity there were pressure fluctuations in the atmosphere, the amplitude

of these fluctuations decreasing as -, where r is the distance from

the source. These pressure fluctuations propagated outwards from the source with the velocity vs carrying energy out with them. The

intensity of sound at any point P was proportional to — (inverse

square law), but the total energy passing outwards per second through a sphere of radius r was independent of r and represented the energy being supplied by the source per second into sound waves.

In the present case we have an oscillating charge q and at some point P electric and magnetic field strengths are fluctuating up and

down with an amplitude proportional to - and such that the inten-

sity of the radiation at point P (energy per second per unit area)

is proportional to — (inverse square law). The total energy passing r2

outwards through a sphere of radius r per second is a constant in-dependent of r and represents the energy being supplied per second by the oscillating charge to the electric and magnetic fields.

Unlike sound, this latter radiation, which is carried purely by electric and magnetic fields, does not need any medium for its pro-pagation and travels outwards even in vacuum. This electro-magnetic radiation may be considered to consist of a transverse wave motion, since the electric and magnetic fields at any point are oscillating up and down in a direction at right angles to the direction of propagation of the radiation. It is precisely this type of electric and magnetic field wave motion that makes up the entire range of electro-magnetic radiation from long radio waves to ordinary light and to X-rays.

Just as there is a characteristic distance between compressions in sound waves, so there is a characteristic distance between maxima of the electric and magnetic fields in the present case. This distance is precisely the distance that the radiation travels while the oscillating


charge q makes one complete oscillation. If we call this distance the wavelength λ, we must thus have

, 1 A = C - ,

n that is c = ηλ (2-20)

where n is the frequency of oscillation of the charge q. This is in complete analogy to the equations vs = nX in sound waves.

It is interesting to see what frequencies of oscillations of the charge q are necessary to produce certain wavelengths. If the fre-quency of oscillation of the charge is, for example, a million oscil-lations per second, we have

3 x 1010

λ = ΐΑ6 = 3 x 104 cm, 106

= 300 metres.

This is in the category of a radiowave. Again, if the frequency of oscillation is 15 million oscillations per second we have

3 x 1010

λ = κΠΠΟ' = 2 x ,03 cm' = 20 metres.

This, then, constitutes radiowaves in the "20 metre band" used, for example, by amateur radio enthusiasts. Of course, in the production of radiowaves it is not a single charge q which is made to oscillate up and down, but rather high frequency alter-nating currents are made to occur in an antenna. But since such alternating currents are produced simply by an assembly of oscillating charges (electrons), the effect is the same.

It was Hertz in 1887 who first succeeded in actually detecting electro-magnetic waves produced by an oscillatory current, thereby experi-mentally proving the correctness of Maxwell's theory. Modern wireless telegraphy, radio and television are practical developments based upon the work of Maxwell and Hertz. By the turn of the century, therefore, and particularly after the theory of relativity in 1905 whereby equations (2-1) to (2*6) could be theoretically derived as direct consequences of moving charges propagating their coulomb field of influence with finite velocity c, it was felt that a complete understanding of electro-magnetic phenomena had been achieved. It was also clear that light itself must be the result of the moving


electrons within atoms, for it is only motions on an atomic scale that can produce the short wavelength of light. Visible light was analysed to have wavelength in the region of 4 x 10-5 to 7 x 10~5

cm, which is very much shorter than the hundreds of metres encountered in radiowaves. A wavelength of 6 x 10-5 cm, for example, must be associated with a frequency of oscillation of charges n given by

c 3 x 1010

n = - = —- = 5 x 1014 oscillations per second. λ 6 x 10-5 F

How such rapid oscillations could arise was understood once the Rutherford picture of an atom was proposed. Consider the hydrogen atom, for example, with its satellite electron revolving around the central proton. Suppose its orbit is circular of radius r and the speed is v ; these quantities must be connected by the equation*

^ = ^ (2-21)

The total time for one complete revolution of the electron is ^ 27ΓΓ

v

But from equation (2*21) we have e

Vmr and hence

_ lirr Vmr _ 2π/π1/8 r3/2

e e

Thus the frequency of oscillation n is

" = 2»mZ r*» (222)

When viewed from side on, this electron would simply appear to be oscillating up and down in simply harmonic motion with the above

*On the left hand side of the equation we have the expression of Coulomb's law giving the force of attraction between two opposite charges each of magnitude

e2

e at a distance r cm apart—namely F = —r This force is counterbalanced by

the outward force given by F = m x acceleration = m x — due to the

particle's circular motion. See Chapter 3 for an alternative way of deriv-ing this equation.


frequency, and will, therefore, be emitting electro-magnetic radiation characteristic of this frequency. On substituting for r the approximate radius of an atom

r ~ £ x 10-8 cm and the known values for e and m, we find this frequency is approxi-mately given by

n ~ 8 x 1015 oscillations per second. It was thus clear that with the atoms, frequencies more than high enough to explain the production of light certainly do arise.

At the same time this immediately posed a tremendous problem for the Rutherford picture of the atom. To start with, it immediately had the consequence that all satellite electrons in atoms would be continually radiating energy and thereby gradually losing energy into electro-magnetic radiation. Exactly as artificial earth satellites lose energy due to atmospheric friction and spiral in, eventually to crash on the earth, so satellite electrons in atoms should gradually spiral in due to their loss of energy into radiation and eventually "crash" into the proton at the centre. Thus any atom would not be permanently stable, but would have a very definite lifetime.

It was clear that there was something wrong with the picture somewhere because, after all, an ordinary substance such as a piece of wood, is not radiating electro-magnetic radiation under normal circumstances.

(e) The Hydrogen Atom and Bohr Theory

Quite apart from the fact that elements are not naturally occurring in a radiating state, it was known that when they are excited so as to emit light, e.g., by heating them to very high temperatures— the light is emitted only in certain wavelengths characteristic of the element. The analysis of light radiation into its constituent wave-length is made by means of an instrument called a spectroscope or spectrograph. For the present, we need only be concerned with the fact that any element can only emit certain characteristic frequen-cies.

Take hydrogen, for example. The radiation which can be emitted from this simplest of all elements had been investigated most exten-sively even by the turn of the century. As long ago as 1885 Balmer


observed that all the different wavelengths in the visible spectrum which hydrogen could be made to emit had a simple relationship to each other. In 1889 Rydberg showed that the simple relation-ship of Balmer could be expressed in the following way :

~\ = RH \22 ~~ k*\

(k = 3, 4, 5 . . . ) . . . (2-23) where λ is the wavelength, RH is a constant known as Rydberg's constant, and k is an integer greater than 2. The empirical value of Rydberg's constant for Hydrogen is

RH = 109,680 cm"1. By substituting for k in equation (2*23) the successive values 3, 4, 5, 6 . . . . , the wavelengths in the Balmer series are obtained.

A spectrograph is an instrument which separates light of different wavelengths into its various constituents. Light is allowed to enter the instrument from the source through a very narrow slit. It then makes light of different wavelengths separate out and fall on to different positions on a photographic plate. Thus, a photograph of the slit is obtained with each wavelength constituent in a different position on the plate. The result is a series of lines each one of which is a photograph of the slit with a different wavelength. The position of each line is determined by the wavelength producing it. Such a series of lines is called the line spectrum of the element producing the light.

i

I n *> CD

«* 1

Ö ■*

CO

^ 1

K o ■<

1

I Ηβ

I I 1

Ha H.

FIGURE 2-5

The Balmer series for hydrogen HOD marks the theoretical position of the end of the series. The numbers attached to the lines give the wavelengths in Angström

units.


In Figure 2-5 we give a photograph of the line spectrum of hydrogen showing the Balmer series lines. In this Figure the wavelengths of the lines are shown in units of 10~8 cm, this unit being known as an Angström unit. The first four of these lines corresponding to k = 3, 4, 5, 6 . . . have been given the names Ha, i/ß, i/8, ΗΎ. As s obvious from the equation, the lines crowd closer and closer together as p approaches infinity and, in fact, approach a limit known as the series limit. The value of the limit of the Balmer series is

λ = —- cm. RH

All elements have the same characteristic as hydrogen, in that they produce "line spectra" when excited into radiating light, i.e., their light consists of a number of definite wavelengths. This provided an additional problem for the scientists early in the present century. In order to produce radiation of given frequencies, satellite electrons in the Rutherford atom would have to be orbiting in certain very definite orbits. On the other hand, the electro-magnetic theory of Maxwell would have the electrons continually radiating, steadily spiralling inwards, and thereby emitting radiation of continually changing frequency and wavelength.

This was, indeed, a very major dilemma. Was the Rutherford picture of the atom incorrect ? Or was the electro-magnetic theory incorrect ? Then came a theoretical analysis by the Danish physicist, Niels Bohr, in 1913, which has determined the whole course of physics during the present century. Bohr took the attitude that there was so much experimental evidence for the Rutherford atom that this must certainly be correct. The only way out of the dilemma then was to assume that somehow the ordinary equations governing the motion of charges became incorrect when we consider motions on such a small scale as is involved in an atom. After all, all the equations of mechanics, and of electricity and magnetism, were developed through experiments involving the motion of particle on a "Laboratory Scale". Was it true, he argued, that when we go down to atomic dimensions, these laws might be slightly modified ?

Following this line of thought he attempted to find some new law governing the motion of an electron in an atom which would firstly permit the atom to be stable and secondly, would yield the observed spectra of radiation from excited atoms. He succeeded quite


dramatically in what is now known as the Bohr theory of the atom. This was an empirical theory designed to give agreement with the Balmer and Rydberg results. Bohr found that he had to make two major postulates in order to obtain this agreement.

Postulate (1)

An electron revolving in a satellite orbit around a nucleus does not always radiate electro-magnetic energy according to the Maxwell equations. There are certain well-defined orbits for which the electron radiates very slowly or not at all. These orbits are defined by the condition that the angular momentum

ih or moment of momentum of the electron is equal to —'

where j is any integer 1, 2, 3 . . . and h is a constant of magnitude h = 6-625 x 10"27 erg sec.

Let us examine this postulate before proceeding to the second one. Consider, for example, a hydrogen atom in which one electron travels around a single proton. If we assume the planetary orbit to be circular of radius r and the speed of the electron to be v, we must have

£ 2 /MV2

-2 = — ■ (2-24) rz r

This merely states that the electrostatic force of attraction provides the necessary centripetal force for the circular orbit. The Bohr postulate (1) is that there must be stable or almost stable orbits for which

ih angular momentum = mvr = — . . . (2-25)

2-n If we solve these equations for r and v we find

1 f 2 ' . . . . . .

me1

and - (rJ

- 7 (Ϊ)

(2-26)

(2-27)

Thus on this postulate there is a whole host of possible circular orbits starting withy = l,y" = 2, and so on, for which the electron is assumed not to radiate. The smallest orbit will be that for j == 1;


its radius, say rl9 can be determined by substituting the values of the constants A, m and e. This yields

rx = 0-529 X 10-8 cm - 0-529 A . . . (2-28) Where A stands for Angström unit = 10-8 cm.

It will be noticed that this value is of the same order of magnitude as that obtained for the radius of an atom on the basis of the kinetic theory of gases. This value rx is called the radius of the first Bohr orbit.

The radius r} of any other orbit is given by ' , = . Λ Ί (2-29)

so that the radii of successive stable orbits increase as the square o f /

So far, the Bohr theory has just postulated that there must be certain stable non-radiating orbits, although in any other orbit the electron is considered to be able to radiate. On this picture, the lowest orbit with radius r1 must be considered completely stable, i.e., the electron can never move into smaller orbits. Thus, in ordinary hydrogen each atomic electron is in this orbit in its res-pective atom. However, if a sample of hydrogen is " excited " in some way, e.g., by heating it to a very high temperature, the atoms will collide with each other quite violently and many of the atomic electrons may be knocked out into higher orbits, corresponding to j > 1. Although the j = 1 orbit is completely stable, the higher orbits are only partially stable and the electrons will cascade back to the "ground state" (j = 1) by dropping from their excited orbits to a lower one, and then to another lower and then to a still lower one, and so on. The electron spends a relatively long time in each of the orbits, but the passage between one orbit to another is fast.

Bohr considered that when an electron drops from one orbit to another, the energy difference is radiated in the form of a flash of light (or, in general, a flash of electro-magnetic radiation). After the source of excitation of the hydrogen is removed all atomic electrons will cascade back to the ground state (j = 1) and the hydrogen will now be completely stable and no longer emitting radiation.

We now come to the second of the Bohr postulates which concerns


the frequency of the electro-magnetic radiation involved when an electron jumps from the higher Bohr orbit to another one.

Postulate (2)

In addition to the stable, orbits just discussed Bohr had to assume that when an electron cascades from one orbit down to another, the drop in energy between the two orbits is radiated as a flash of electro-magnetic radiation of frequency n given by the condition

decrease in electron energy = hn, . . (2*30) where h is the same constant introduced in postulate (1).

Let us now examine the consequences of this postulate. To do this we must first calculate the loss of energy as an electron drops from one orbit to another, i.e., the energy carried off in the pulse of electro-magnetic radiation. Suppose first that the electron was in an orbit with j = y2, and that it drops to an orbit with j = jx

O2 > ji)- Le t t n e radius of the outer orbit be r2 with electron velocity v2, and the radius of the inner orbit be rx with electron velocity.

The total energy of our system (proton plus one electron revolving around it in a circular orbit of radius r) is given by the kinetic plus potential energy* of the system. The kinetic energy of an electron in a circular orbit of radius r is from equation (2-24).

^ = T (2-31) 2 2r

The potential energy of a charge q at a point distant r from it is given by

V = q/r.

*To many students the term potential energy may be thought to refer only to gravity and to have the value mgh, for a mass /wata height h above the earth's surface. However, potential energy is a general property which applies to all bodies. If a body is under the influence of a force (gravitational, electrical, magnetic) the change in potential energy is defined as the work done to move the body from one point to another. The body is said to have zero potential energy when it is completely free of the field of force. By convention the sign of the potential energy is so chosen that it is negative for a body under the influence of an attractive force and correspondingly positive when under the influence of a repulsive force.


Thus the potential energy of an electron in the orbit of radius r is given by

eV = -e2/r, (2-32) since q = +e for the proton constituting the hydrogen nucleus and the electron has charge —e. The total energy (T.E.) is therefore

e2 e2 e2

T.E. = - - - = - i (2-33) 2r r 2r From equation (2-31) it is seen that the kinetic energy in an inner

orbit is, in fact, greater than in an outer orbit. Thus, the electron gains kinetic energy in dropping down to a lower orbit. On the other hand, from equation (2-32) we see from the minus sign that at the same time the electron loses potential energy. When r is infinite—that is, the electron is completely removed—the potential energy of the system is zero and is at its maximum value. For decreasing values of r, the potential energy increases in negative value and thus decreases. Thus the kinetic and potential energies vary in opposite directions : when the kinetic energy is at its maximum value (minimum value of r) the potential energy is at its minimum; and when the kinetic energy is at its minimum value (r = oo), the potential energy is at its maximum.

FIGURE 2-6 Electrons in two different orbits in a hydrogen atom. The total energy of the electron

e2 el is —I - in the first case and —\ — in the second. Thus the energy is more

ri rt negative in the first case, and positive energy must be added to raise the electron

out to the second orbit.


The total energy given by equation (2*33), behaves in the same manner as the potential energy. Thus, when r is infinite and the electron is completely removed—we say the atom is ionized in this instance—the total energy of the system is at its maximum value, namely zero. As the electron falls into orbits with decreasing radius the total energy decreases. For instance, in falling from an orbit with radius r2 to an orbit with radius r1 (r2 > r^ the change in total energy is given by

—e4 change in T.E. = ——

1 = 2

2/ϊ

(2-34)

and this must be the energy which the electron loses (Figure 2*6).

/ X He

j=3

r'

^~~Fi

r

m

J ) fl

ι y y

%

6

ψ&0*\ V«^«sd

Μη

FIGURE 2-7

Quantum jumps giving rise to the different spectral lines of hydrogen.


This is the amount of energy which, according to Bohr's second postulate, must be equal to hn9 where n is the frequency of the radia-tion. On substituting the values for rx and r2 we immediately obtain the following formula for the frequency :

mei · ■ ■ ■ (2-35) n = A»

e* |"J Π

1 „

Alternatively, since - = -, where c is the velocity of light, we λ c

have 1 2π2 me me* Γ 1 11

(2-36)

On inserting the values of m, e, c and h we find

^ - ^ = 109,740 cm- , chz

in close agreement, to the accuracy given, with the Rydberg constant RH of equation (2-23). It is to be noted that from the first Bohr postulate the j \ and j2 of equation (2-35) are integers with j2 > j \ . In the case that j \ = 2, equation (2-35) automatically gives the spectral lines of the Balmer series. The first line is given with j2 = 3, i.e., corresponding to an electron dropping from the Bohr orbit r3 down to r2. The second line of the Balmer series, /2 = 4 corresponds to an electron dropping from the Bohr orbit r4 directly down to the Bohr orbit r2 (missing orbit r3) and so on. This is represented diagramatically in Figure 2-7.

Of course, the Bohr equation (2-36) also gives other series of spectral wavelengths. For example, when j \ = 1, there are spectral lines due to the electron cascading down to this orbit. This series of lines is then given by the equation

h = 2,3 (2-37) and is called the Lyman series. Similarly, there is another series of lines corresponding to j \ = 3, for which

ϊ= RH & " J? h = 3, 4 (2-39)

This is called the Paschen series.


Similarly, the lines corresponding to electrons cascading down to the r4 orbit (j\ = 4) is called the Brackett series, and for j \ = 5 we have the Pfund series.

These other series of wavelengths have all been discovered and agree extremely well with the Bohr predictions. The reason why the Balmer series was the one first discovered was that this corres-ponds to wavelengths which our eyes see as visible radiation, i.e., light. The Lyman series gives wavelengths much shorter than we can see and which correspond to what we call ultra-violet radiation. Similarly, the Paschen, Brackett and Pfund, etc., series give lines with wavelengths longer than we can see; these are what we call heat rays, or infra-red radiation.

We thus see that the Bohr theory obtains spectacular agreement with experiment. Yet it depended on two postulates which, in 1913, were completely new and could not be understood in terms of previous theory. Because it worked so well for the hydrogen atom, and the same rules were also shown to give good agreement for the spectral lines of other atoms, it had to be believed. We now know that it is indeed correct that on the atomic scale of things ordinary Newtonian mechanics and electro-magnetic theory does require modification. In modern physics we know that for such dimensions we must use a detailed theory which has been developed, called quantum mechanics, and in which the original Bohr theory appears automatically as an approximation to the true situation. A discussion of quantum mechanics is beyond the scale of this book ; for us it will suffice that the original Bohr theory is essentially correct and that we can now understand it in terms of our modern mechanics.

The development of quantum mechanics during this century has provided one of the biggest advances ever made in physics, and the impetus certainly came very largely from the Bohr theory of the atom. In all fairness, however, it should be mentioned that the basis for Bohr's ideas was laid by the previous work of Planck in 1900. This work of Planck was concerned with an aspect of electro-magnetic radiation which in turn arose from some important experimental work by Wien. Wien in 1893 discovered a law concerning the distribution among the various frequencies of electro-magnetic radiation emitted by a hot object. For any object, which may be


made up of an accumulation of elements—e.g., the sun—the inten-sity of the radiation emitted as a function of the frequency is always as indicated in Figure 2-8. This figure pertains to the radia-tion emitted by the sun. The same sort of curve is also obtained for all hot objects. Wien observed that the frequency for which the maximum intensity occurred was connected to the absolute tempera-ture by means of the law

— Constant, nmax

where the constant is 9-63 x 10-12, when T is expressed in degrees absolute and nmax in vibrations/sec.

Planck observed that he could derive an intensity versus frequency curve of precisely the right form, and which would automatically yield Wien's law, provided he assumed that the radiation from a

>-

z 111 H Z

(

INFRA-RED VISIBLE

| 1

| 1 ^ 1

/ \ ' l· \ 1 / ' X1

/ ' \ 1 l ' \

- I I 1 ^

/ 1 '

/ · · J i '

y i i 3 0.5 1

FREQUENCY (in

ULTRA-VIOLET

1 1 ' " 1 1

1.5 2 2.5

units of 10 cycUsW.)

FIGURE 2-8

A graph of the intensity of radiation emitted by the sun plotted against frequency of the radiation. The maximum energy is radiated at a frequency of 61 x /01 4

cycles/sec.


hot object could be emitted only in "bursts" of energy, each burst containing the energy hn, where n is the frequency of the radiation contained in the burst.

More specifically Planck's revolutionary quantum theory per-taining to oscillators postulated :

(1) An oscillating charge, or any other physical system has a discrete set of possible energy values or levels ; energies inter-mediate between these allowed values never occur.

(2) The emission and absorption of radiation are associated with transitions or jumps between two of these levels, the energy lost or absorbed, respectively, as a quantum of radiant energy of magnitude hn, where n is the frequency of the radiation and h is Planck's universal constant equal to 6-625 x 10~27 erg seconds.

The value of h, which Planck derived in order to obtain agreement with experiment, was precisely the value which Bohr took in his theory. Thus h is known as Planck's constant. It is an extremely important constant occurring in all of quantum mechanics.

It can be seen that in his postulate (2) Bohr was directly following the lead given to him by Planck ; if radiant energy has to be emitted in bursts or quanta of energy hn, then it is clear that this is the amount of energy which an electron must radiate as it drops from one Bohr orbit to another. Bohr's first postulate, however, was a completely new and independent one which he had to make in order to obtain his stable or almost stable orbits in the first place. It was a great success of his theory, however, that he did not have to introduce any arbitrary parameters or unknown constants. He used for h the value previously suggested by Planck, and apart from this, his theory involved simply the known values of e, m and c.

Finally, it should be mentioned that a curve of intensity versus frequency as shown in Figure 2-8 pertains to the radiation from a hot object, consisting of many different elements. We have already seen that with a pure element such as hydrogen, the radiation is emitted only in certain definite frequencies. Under conditions such as on the sun where the hydrogen is ionised, the discrete energy


states of the electrons disappear and a continuum spectrum is pro-duced. Similar conditions exist more or less for complex atoms in liquids and solids where there can be " free " electrons which wander from atom to atom.

(f) Summary and Discussion

In this chapter we have seen firstly that accelerating charges should radiate energy into the electro-magnetic field, and this is the phenomenon involved in the production, for example, of radio-waves by means of alternating electric currents. The equations which we derived in this connection were based on the Maxwell theory of the electric and magnetic fields—the so-called classical theory. These equations certainly give essentially correct results as regards "long" wavelength radiation such as radio waves. In this instance, the radiation is due to oscillations of multitudes of charges—the free conduction electrons. These form oscillators of very low frequency and large amplitude on the atomic scale since they are free. Of course, the wavelength of the emitted radiation is correspondingly long. However, the classical equations have to be modified for an understanding of the production of radiation with wavelength in the infra-red visible ultra-violet regions and lower, because this radiation is produced from the motion of the electrons within individual atoms. Here the Bohr theory is appropriate.

Incidentally, the Bohr theory not only removed the major difficulty of the Rutherford model of the atom, but it also provided an immediate explanation of the previously observed rays called X-rays or Röntgen rays, which had been detected emanating from the walls of a cathode ray tube. These simply consisted of electro-magnetic radiation of extremely short wavelength and very high frequencies, which could be produced from heavy atoms when they were strongly excited. When, for example, cathode rays bombard a target, e.g., the anode, they produce a very severe excitation of the atom in the anode. In other words, some of the orbiting electrons in the atoms involved are raised to very high Bohr orbits ; when they drop back to their normal orbits the energy emitted is high, the frequency is high and thus the wavelength is short. Thus X-rays became under-stood as being simply electro-magnetic radiation of short wave-length.

It also becaine understood how X-rays could "ionize" atoms, i.e., completely remove an electron from an atom. If an atom is


bombarded with energetic X-rays the oscillating electric and magnetic fields of the rays exert powerful forces on the orbiting atomic electrons. They can certainly excite the atom, and can even be capable of completely "shaking loose" one of the electrons.

Finally, there is one important point which must be discussed. You might well ask : Why is it that we do not see a source of light "twinkling" as it emits its pulses or individual flashes of light? The answer, of course, is that the number of atoms in any source is extremely great—of the order of Avogadro's number—and the light appears to us to be continuous simply because of the very large number of atoms in any quantity of matter. It is completely impossible for our eyes to detect the individual quanta, although this has been done in certain extremely sensitive laboratory tests.

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The fact that light, or electro-magnetic radiation in general, is emitted in individual flashes does not in any way influence the fact that electro-magnetic radiation is a wave motion. Consider, for example, just one atom in which one electron has been excited into a higher orbit. When this electron drops back to the ground state, a pulse of radiation of a certain frequency is emitted. If we were to have an extremely sensitive measuring device some distance from the atom and which could detect electric and magnetic fields, we would detect oscillating fields lasting for a very short period of time as the "burst" of radiation energy passed us. At any such point


the directions of these fields would be at right angles to the line joining the measuring point to the atom, just as discussed earlier in this chapter for the classical theory. If the magnitude of one of these fields is measured as a function of time, it would appear as in Figure 2*9. There would be a period of zero field before the pulse reached the measuring point, then the field would start oscillating at the frequency involved, and then down again to zero as the pulse passed. Such a pulse of oscillating electric and magnetic field, which in itself contains wave motion, is called a wave packet.

When there is a source containing a very large number of atoms continuously being excited into radiating energy, the total effect observed at any measuring point is the combined effect of multi-millions of wave packets being emitted continuously. In this case the measuring instrument will continuously detect oscillating electric and magnetic fields containing all the frequencies which are being emitted by the atoms of the source. If we filter out all the light except for one frequency, the electric and magnetic field corresponding to this unique frequency will be continuously oscillating up and down "sinusoidally", in the same way as pressure variations corresponding to a pure note in sound (although, of course, the frequencies involved are vastly different). In the case of light such "a pure note" is called monochromatic light. As before, the distance between the two neigh-bouring "crests" of the electric and magnetic fields is the wavelength of the radiation.

Thus electro-magnetic radiation is still to be considered as a wave motion—a transverse wave motion—in which the electric and magnetic fields are oscillating up and down around a zero value in a direction at right angles to the direction of propagation of radiation. Reflection, refraction, interference and diffraction are all effects which occur for light just as they do for sound.

At the same time we should always bear in mind that electro-magnetic radiation is fundamentally made up of individual atomic bursts or quanta, sometimes called photons. In all cases in which the properties of a wave motion are unimportant, it is quite sufficient to think of a beam of light just as a beam of photons. In view of Einstein's equivalence of energy and mass—see Chapter 5— it is even possible to think of these photons as actual particles endowed with mass. Thus, in these cases it is sufficient to consider a beam of light simply as a beam of little "bullets", i.e., photons.

CHAPTER 3

The Influence of Gravitational Fields

(a) The Solar System

The things that seem normal to us and to which we are accustomed are things on earth because after all that is where we live. However, for an understanding of life processes on earth we must first delve into the sub-microscopic realm of atoms, of molecules, and of the production of electro-magnetic radiation. This has been the subject of the previous two chapters.

Similarly, in order to have any full appreciation of how the earth has achieved its conditions which support life, of how life may have originated on the earth, and of whether there can be life elsewhere, we must do something more. We must not be content simply to look at the earth itself and try to understand everything that has happened on earth in isolation from the rest of the universe. We must in fact now look outwards into the universe itself, into what one might call the ultra-macroscopic scale of things so that we can see our earth in proper perspective.

As you know, our earth is one of a number of planets, or satellites, revolving around the sun. The closest planet to the sun is Mercury, circling about 36,000,000 miles from the sun; then come Venus, Earth—about 93,000,000 miles from the sun—Mars, Jupiter, Saturn, Uranus, Neptune and finally the outermost planet, Pluto, which travels in its orbit at an average distance of 3,000,000,000 miles from the sun. Most of you would have seen a diagram such as that of the solar system in Figure 3.1. The relative sizes of the different planets are shown in Figure 3.2.

75


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Many planets have their own satellites or moons revolving around them. Our own earth has one moon, other planets have several moons and only a few have none. The sun and its planets and these planets with their moons we call our solar system.

You have only to look at the sky on a clear night, however, to realise that this solar system is not the entire universe. It is in fact merely a small speck in that part of the universe which we can see. Nearly all the stars which we see in the sky are other suns, millions upon millions of miles away. The only exception to this are the planets which, by reflecting sunlight, appear like stars to us. In Chapter 4 we will discuss the question as to whether these other suns may also have planets revolving around them. The nearest sun to us is the brighter of the two pointers of the Southern Cross, and this is so far away that light from it takes A\ years to reach us. As light travels at 186,000 miles per second, you may readily work out that the distance to this star is about 27 million million miles.

(b) Galaxies Any star which we can see with the naked eye, however, is

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THE INFLUENCE OF GRAVITATIONAL FIELDS 79

ascertained that our sun is but one of a large cluster of suns which we call a galaxy. There are in fact something like one hundred thousand million suns in our galaxy. We can also tell that these suns in our galaxy are formed into a pattern so that the overall shape is flat but with a bulging centre. It is about one hundred thousand light years across the long diameter and about ten thousand light years thick. (A light year is, of course, the distance that light travels in one year.)

With large telescopes it is possible to look much further than this. Astronomers can look beyond our galaxy and find that there is mostly empty space for millions of light years. Eventually, however, the big telescopes show up in the distance another cluster of stars something like our own galaxy; and then after another few million light years of empty space there is another cluster of stars and at greater distance again yet another and again another and so on. So the universe goes on as far as we can see—which with to-day's largest of telescopes is a few thousand million light years of several thousand million million million miles. It is an interesting thought that when we look at such a star cluster we are seeing it not as it is now, but as it was when it emitted the light several thousand million years ago. Looking a long way away is also looking back in time.

Most galaxies of the universe are spinning in space somewhat like a giant Catherine wheel; this is true of our own galaxy. Examples of galaxies are shown in Figures 3.3, 3.4 and 3.5. In Figure 3.3, for example, we see a galaxy which is relatively close to our own, called the Andromeda Nebula. The Catherine wheel effect can be clearly seen; the small bright spots dotted all over the foreground are the stars of our own galaxy through which the telescope must look to see the distant galaxies. The big glowing spots are other galaxies about the same distance away as Andromeda, although smaller than their large companions. Similarly, in Figures 3.4 and 3.5 we see other examples of distant galaxies which are orientated with respect to us at different angles.

It is because of the Catherine wheel shape of our own galaxy that we see what we call the Milky Way. When we look at the sky in such a direction that we are looking through the flat part of the galaxy, we are looking through a large number of stars of our galaxy—so many in fact that they give a milky appearance. This


Figure 3.4.—A spiral galaxy, showing the swirling "catherine-wheel" effect.


Figure 3.5.—A distant galaxy, looking like a fried egg hanging in space.


is the Milky Way, which is thickest when we are looking towards the centre of the galaxy. However, when we look in a direction which leads very quickly out of the galaxy—at right angles to the flat plane—we do not see nearly as many stars and therefore do not see a milky appearance.

Each of the galaxies in the universe contains from a hundred million to a hundred thousand million stars. It is possible that large numbers of these stars or suns have planets revolving around them. We will see in Chapter 4 that one estimate is that there are possibly 50 thousand million suns in our own galaxy alone which have planets revolving around them. What an insignificant fraction of all the life in the universe we may form ! (c) Gravitational Fields

Most bodies of the universe seem to be moving in circular, or at least elliptical paths. Our moon moves in a near-circular orbit around the earth; artificial satellites also move in near-circular paths around the earth, and the other planets move in orbits that are nearly circular around the sun. Then again galaxies of millions upon millions of suns turn in space like huge Catherine wheels. Why is this so ?

One of the most famous and far reaching laws of nature, which was first realised by Sir Isaac Newton in the 17th century, is Newton's Law of Universal Gravitation. Newton realised that the only way of understanding many observations on the behaviour of objects in our solar system was to assume that an object automatically attracts any other object; this is called the force of gravitation. We, as objects on the earth, are attracted by the earth and pulled towards its centre. This gives us our weight. If some object is freed some distance away from the earth, it is pulled towards the earth by gravitational forces. To us the object falls.

It was Newton himself who realised that earth satellites might be developed. In Figure 3.6 we show a diagram similar to one which Newton drew, in which we have an imaginary mountain reaching hundreds of miles into the upper atmosphere. Newton imagined a cannon being fired from the top of this mountain; the faster the cannon ball is fired the further it will fall away from the base of the mountain.

Eventually, if fired fast enough, it will travel so far sideways while falling that the earth's surface falls away underneath it at the


Figure 3.6.—Newton's diagram to illustrate the principle of earth satellites.

same rate. Although falling continuously, the cannon ball will never hit the earth's surface and will in fact be an earth satellite. We shall use this picture shortly to calculate the speeds with which satellites travel. Of course, in practice Newton's mountain is replaced by rockets which take satellites up to high altitudes and fire them sideways into their orbits.

It is thus clear why satellites must orbit the earth in near-circular orbits. There can never be a satellite which is standing still in space several hundred miles away from the earth because it would be drawn towards the earth by the gravitational pull and crash on the earth's surface. If an object is going to remain in space near the earth it will have to be moving around the earth all the time; although being under the action of the gravitational pull and falling all the time, its sideways movement keeps carrying it fast enough so that the earth's surface falls away underneath all the time. Our moon similarly makes a complete circuit around the earth once in a little over 27 days, and is thus a satellite of the earth.

The earth in turn is a satellite of the sun. We could never lead a nice comfortable life 93 million miles away from the sun unless we were moving around it. If we were not moving sideways around the sun we would be drawn straight in towards it and would crash through its fiery surface within a matter of a few days. We only avoid this disaster because the earth is travelling around the sun at


about 65,000 miles per hour, which is what is required for it to be a satellite of the Sun. The other planets also are moving around the sun at speeds which keep them in their orbits. The planet Mars, for example, makes one complete revolution around the sun in 687 days; the Martian year is thus almost twice as long as our year. On the other hand Venus, which is closer to the sun than we are, makes one complete revolution in about eight earth months.

If all the suns of the galaxies were still they would be drawn together by their gravitational attraction for each other and eventually collide. The continual movement of galaxies, whirling like giant Catherine wheels, prevents them from collapsing into a huge central star.

You can thus realise what a major influence this force of gravity has in our universe. We live continually under the gravitational influence of our earth which gives us our weight. In science we say we live in the earth's gravitational field. Similarly, the earth and everything on it is moving under the influence of the sun's gravitational field. Then again our sun and its planets are moving in a giant circle under the influence of the gravitational field of our galaxy—the assembly of 100,000 million suns. (d) Calculation of Satellite Speeds

The calculation of satellite speeds is of such fundamental importance in any discussion of details of the solar system and of its possible formation that we now consider this subject quantitatively.

Our starting point must be a quantitative statement of Newton's law of gravitation. This is that any two objects of masses m, and mt respectively will attract each other gravitationally by a force F given by

F = Gmxm2/r2 . . . . . . . . . . (3.1) Here r is the distance apart of the two masses and G is the universal constant of gravitation whose value is

G = 6-67 x 10-8 dyne cm2/gm3 .. . . (3.2) Thus if the masses mx and m2 are expressed in grams and their

distance apart in centimetres, equation (3.1) gives the gravitational force of attraction in dynes.

We can use equation (3.1) to express the weight of an object on the surface of the earth in terms of the mass of the earth and its radius. It can be shown that the force of attraction of the earth on any object on or above the earth's surface is the same as if all


the mass of the earth were concentrated at its centre. Thus the force of gravity which the earth, of mass mE say, exerts on an object of mass m at its surface is given by

F = W = GmEm/r02 (3.3)

where r0 = 6-38 x 108 cm (approximately 4,000 miles) is the radius of the earth.

This force is, however, just what is known as the weight of the object of mass m, which is normally written

W = mg (3.4) where g is the acceleration of gravity at the earth's surface. Thus we see that the acceleration of gravity g is related to the universal gravitational constant G, the mass of the earth mE, and the radius of the earth r0 by the equation

g = GmE/r0* (3.5) From this equation we can determine the mass of the earth.

If we substitute g = 980 cm per sec2 we obtain the value mE = 5-98 x 1027 gm. This is approximately 6 x 1021 tons.

If an object of mass m is above the earth's surface at a distance /* from its centre its weight—say Wr—is given by equation (3.1) as

Wr = GmEm/r2 . . . . . . . . . . (3.6) It is sometimes convenient to express this in terms of g, the acceleration of gravity at the earth's surface; we obtain

Wr = mg(r0/r)~ (3.7) Thus the acceleration of gravity at a distance r from the centre of the earth, say gr, is

gr = g(ro/r)- (3.8) Now let us consider the speed of a satellite in orbit around

the earth. Suppose at one instant our satellite is at point A as shown in Figure 3.7. We assume that its distance from the centre of the earth AO = r.

The satellite clearly must have some speed v which in one second would carry it along the straight line AB if it were not for the pull of the earth's gravitational speed. The distance of the line AB which it would thereby travel in one second is v.

In point of fact, however, the satellite at the same time is falling towards the earth's centre, and in one second its distance of vertical fall will be the distance BC of our diagram. If the satellite is to move on a circular orbit the arc AC must be the arc of a circle—


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that is, the satellite must not be getting any closer to the earth's centre.

When the satellite is at A we assume that its motion is at right angles to OA, and we can calculate how far it falls in one second by simply knowing the acceleration of gravity gr. We have in general that the distance fallen in time t is \grt

2. Hence we know that

BC = \gr = ig(ro/r)2

On the assumption that AC is the arc of a circle we can actually obtain a connection between AB and BC. Because ABC is a right angled triangle we have

.·. AB2 =B02 - AO2

= (BC + CO)2 - AO2

= BC2 +2.BC .CO + CO2 - AO2

= BC2 +2.BC .CO (CO = AO) = 2 . BC . CO (approximately, as BC is very

small compared to CO). From what we have said above, however, and considering a

time interval of one second, we know that AB = v, CO = r, and


BC = ig(r0/r)2. Thus we immediately have the equation v2 = gr0

2/r (3.9) You can experiment by considering any time interval /, imagining

it to be perhaps of a very small fraction of a second. You will find that the time / cancels from your equations and that you always arrive at the formula (3.9). This is then the equation governing the speed with which an earth satellite has to move in a circular orbit distant r from the centre of the earth.

Those of you who have studied circular motion will realise that this equation could have been written down very quickly. We know that the gravitational force of attraction must provide the so-called centripetal force mv2/r necessary to keep the satellite in orbit. This gives us mv2/r = mg(r0/r)2 and hence v2 — gr0

2/r. Our first calculation is instructive, however, in that it uses the

very simple idea of a satellite always falling but never getting any closer to the earth's centre. The same method could also be used to calculate equation (2.21) governing the motion of an electron in an atom. We mentioned in Chapter 2 that if you did not understand that equation you could refer to this chapter to see how to derive it. You should now be able to use exactly the same arguments as in the above to re-derive equation (2.21).

While referring back to the subject of atoms, some of you may wonder why gravitational forces were not also used in addition to the electrical forces. You simply have to substitute the appropriate values of the masses involved to see that the gravitational force between an electron and a proton for example is many billions of times smaller than the electrical coloumb forces, and therefore plays a negligible role in determining the structure of an atom. Thus gravitational forces are negligible on the atomic scale. Atoms are, however, electrically neutral so that when one electrically neutral atom is far away from another electrically neutral atom there is no electrical force left at all and the only remaining force is gravitational. These gravitational forces between individual atoms are tiny, but when there are enormous numbers of atoms present as in the large planets and suns of the universe, they build up to be of major significance. (e) The Energy of an Earth Satellite

From equation (3.9) we see that the further out a satellite is, the more slowly it need move to maintain a circular orbit. This


is simply due to the fact that the gravitational attraction by the earth diminishes with increasing distance and the less the satellite falls per second—that is, the smaller the distance BC in Figure 3.7.

You can, for example, easily calculate that at a height of 200 miles above the earth's surface (r = 4,200 miles approximately) a satellite's speed is about 17,300 miles per hour. At a height of 12,000 miles (r =- 16,000 miles) the speed of a satellite in a circular orbit is approximately 8,850 miles per hour. The moon's speed in its orbit is approximately 2,300 miles per hour.

An interesting situation applies when a satellite is about 26,000 miles from the earth's centre—that is, 22,000 miles from the surface. It then orbits the earth in 24 hours—exactly one day. If its orbit were directly above the equator and if it were moving from west to east, the satellite would always remain over the same spot, as the earth would also be making one revolution in 24 hours and also moving from west to east. Thus a spot on the earth's equator would keep pace exactly with the satellite and would always stay beneath it.

It is clear then that the further out a satellite is, the smaller is its kinetic energy, and some of you may therefore think that the total energy of a satellite is less, the further out it is. This, however, is not correct; the total energy of a satellite is less the nearer it is to the earth's surface.

To show this, let us compare two satellites each in circular orbits, one at a height r and the other at a very slightly greater height say r + h.

The kinetic energy of the higher satellite is less than the lower one by the amount

\mgr*{\/r— l/r + hh} = lmgr0*{h/r(r + h)}

Since in this example we are comparing two orbits close together for which h is very much less than r we have that the upper satellite has less kinetic energy than the lower one essentially by the amount

lmgr0*h/r* (3.10) The potential energy of the upper satellite is, however, greater

than the lower one. With h being small compared to r, we can neglect any change in the acceleration due to gravity between the two levels so that this difference in potential energy is simply

mgrh = mgra2h/r2 (3.11)


It is seen therefore that the total energy (kinetic + potential) of the upper satellite is greater than the lower one by the amount

\mgr0Vi/r* (3.12) We can now see why satellites gradually spiral in, if they have

energy being taken away from them by atmospheric friction. They gradually come in to orbits of lower total energy, which rrieans that they come in to orbits closer to the earth's surface. In these closer orbits the satellite has more kinetic energy, a fact which many people find difficult to understand. We see, however, that the satellite is indeed losing energy since the rate at which its potential energy decreases is exactly twice the rate at which the kinetic energy increases.

At this point you may wish to refer back to our discussion of the energy of an electron orbiting a proton in the hydrogen atom. Once again we have a very close analogy between an earth satellite and the hydrogen atom; you may now understand a little more clearly why the electron drops to a lower orbit when it loses energy by the emission of electro-magnetic radiation.

To end this chapter we imagine a situation which actually does not arise, but which will help us to understand the theory of the origin of our solar system in Chapter 4. Suppose the earth's atmosphere really extended more than 22,000 miles from the earth's surface, and suppose that this atmosphere rotated with the earth. What would happen to an earth satellite a little beyond the "stationary orbit" 22,000 miles up from the surface ? In this case the satellite would be making slightly less than one orbit per day; the earth would be rotating slightly faster than the satellite. Thus no longer would there be atmospheric friction but the atmosphere would tend to help the satellite along. This would add energy to the satellite and it would gradually move to orbits with greater total energy. In other words, it would gradually spiral outwards and slow down.

In the next chapter you will see how this phenomenon, applied to the sun and planets, could well have played a role in the formation of our solar system.

CHAPTER 4

The Origin of the Solar System

(a) Laplace's theory.

Chapter 3 has now given us most of the background we need to discuss possible mechanisms whereby our solar system could have been formed.

Any successful theory of the origin of the solar system must, of course, account for its striking regularities. The planets move around the sun in almost circular orbits, and all in the same direction. The region between Mars and Jupiter contains a host of small objects or fragments called the asteroids which also move around the sun in the same direction. Moreover the orbits of the planets lie fairly closely in the same plane. The plane of rotation of the earth around the sun is called the plane of the ecliptic, and the orbits of the other planets in general lie remarkably close to this plane.

The inclinations of the orbits of the various planets to the plane of the ecliptic are as follows:

Mercury, 7°; Venus, 3°; Mars, 2°; Jupiter, 1°; Saturn 2°; Uranus, 1°; Neptune, 2°; Pluto, 17°.

The first real theory of the solar system was the so-called nebula hypothesis of Laplace, proposed more than 150 years ago. This theory was generally accepted throughout the 19th century. Laplace envisaged that our solar system was once a very large gas cloud, extending well beyond the reaches of our present solar system. The gas of this low density nebula would have been initially at a very high temperature, the temperature being so high as to prevent immediate collapse due to gravitational forces pulling the mass together.

91


Moreover in this initial condition it is very unlikely that such a gas cloud would not be slowly rotating in some way. As such a gas cloud accumulates over the thousands of millions of years, some gas coming from one direction and other gas coming from another direction with different relative speeds, it would be a remarkable coincidence if the final accumulation did not have some rotation.

As this nebula gradually cooled, Laplace considered that it should begin to contract and that rings of gas would be successively shed by the parent nebula. Laplace then expected that these rings, under their own gravitational influences, would break up into separate condensations and the beginnings of the planets would thus occur. Eventually the situation as we see it to-day would be reached, in which the central part of the original nebula is now our sun, and the material left behind has formed the planets. Even the moons of the various planets could be envisaged as having formed in a similar way.

We will not develop this theory further, however, because it very quickly runs into a major difficulty. It is of considerable interest to see precisely what this difficulty is, because this points the way to what is at present considered to be the most likely theory on the origin of the solar system.

(b) Angular momentum of the solar system.

In Chapter 2 we introduced you to the idea of angular momentum. As you know, any object of mass m moving with speed v has momentum mv. If the object is moving in a circular orbit of radius r at this speed v, the quantity mvr is called the angular momentum or the moment of momentum of the moving body.

The concept of angular momentum can be used not only for a single object moving in some circular orbit, but also for a large extended object rotating about some axis. Consider, for example, a spinning top. Each tiny particle in the top is moving in a circular orbit and has a certain angular momentum; the total angular momentum of the spinning top is thus the sum of all these contributions from its individual particles.

You will all recall Newton's First Law of Motion which says that any object remains at rest or continues moving with uniform speed in a straight line unless acted upon by some external force.

THE ORIGIN OF THE SOLAR SYSTEM 93

Another way of saying this is that the momentum of a moving particle is constant unless some external force comes into play.

Now there is an extension of Newton's First Law to the case of spinning objects; this says that the angular momentum of any spinning object remains constant unless external forces act on the object which can alter the rotation. There is a simple demonstration which illustrates this. Suppose you sit in a well-oiled swivel chair and have someone set you rotating while you keep your arms extended—preferably holding a weight in each hand. When you are spinning at a reasonable rate fold your arms across your chest, as in Figure 4.1.

Your rate of spin will increase remarkably (provided there is very little friction in the chair). The reason for this is the

SLOW SPINNING

HEAVY MASSES

FAST SPINNING

HEAVY MASSES

AS THE HANDS ARE RETURNED TO THF SIDES THE SPIN INCREASES. IN BOTH CASES YOU HAVE ALMOST THE SAME AMOUNT OF ENERGY

Figure 4.1.—Spinning on a swivel chair.


conservation of angular momentum. Initially your arms and the masses you are holding had a certain amount of angular momentum; but when you fold your arms the masses are brought in to be very much closer to the axis of rotation and nearly all their angular momentum disappears. This cannot be lost and so the rate of rotation must increase so that, ignoring friction, the total angular momentum remains constant.

At this point you may be wondering how angular momentum can have any bearing on the origin of the solar system. Let us therefore return to Laplace's theory in which the solar system was considered to be once a large, slowly-rotating gas cloud. This gas cloud would have originally had a certain angular momentum, and the solar system to-day must still have this same angular momentum. Now the total amount of mass that occurs in all the planets and other objects circling the sun is only 0-1 per cent, of the total mass of the solar system. Moreover, at the time when a particular Laplacian ring of gas was shed by the parent nebula, it would have represented a fraction of the total angular momentum which would be roughly similar to the fraction of mass which it represented. Since the gas ring would be rotating at the extremities of the cloud, it would have a greater value of radius r and a greater speed v than the average within the cloud. Because of this it can actually be shown that if the mass of such a ring contributed say x% of the total mass its angular momentum would be very roughly 2-5 x% of the total angular momentum.

On these grounds we would therefore expect that the total angular momentum content contributed by the planets is really only a fraction of a per cent., say 0-25%, of the total angular momentum of the solar system. The sun now having contracted and therefore spinning much more rapidly, would be expected to contain something like 99-75% of the total angular momentum.

However, the sun is spinning on its axis once in 28 days, and its angular momentum can be calculated; similarly the angular momenta of the planets can be calculated. The surprising result is found that the sun, which has 99-9% of the mass of the solar system contributes only 2% of the total angular momentum of the solar system.

In any theory of the solar system therefore some mechanism must be found for this remarkable distribution of angular momentum


and Laplace's nebula hypothesis does not provide such a mechanism. Thus it was that, by about the turn of the century, new theories of the origin of the solar system were beginning to be explored. One of these considered that our solar system could have been formed from a collision between our sun and another star. It was considered that a close encounter between the sun and another star could have produced great eruptions from the sun from which the planets and satellites might have condensed. It was calculated, however, that for any appreciable effects to occur, the encounter had to be so close as to represent a very rare phenomenon in the galaxy. It was estimated that a star such as our sun could be expected to collide with another star in this way on the average only once in 100,000,000,000,000,000 years. Since the age of the solar system is probably only in the vicinity of a few thousand million years, the birth of our solar system by this mechanism would have been a very freak accident.

If this were the correct explanation we could expect very few other planets to exist and we could, in fact, be the only one in our galaxy. This would mean that any other life in the galaxy would be very unlikely indeed.

The encounter theory has, however, now been abandoned, not of course because we feel there should be life elsewhere in the galaxy, but because the mechanism does not seem capable of producing a solar system with the right properties; it would appear unlikely to produce planets moving in circular orbits almost in the same plane and it similarly runs into very great difficulties with regard to the distribution of angular momentum.

(c) Modern theory of solar system formation.

The clue to a mechanism whereby the solar system could have been formed with the correct properties is contained in our previous chapter. Suppose we return to our consideration of an artificial satellite out further than 22,000 miles which is being helped along by an extended earth atmosphere. We saw that this satellite would gradually spiral outwards. Let us now enquire as to what happens to its angular momentum as it works its way out.

If such a satellite has a circular orbit with radius r, its angular momentum is mvr; on substituting from equation (3.9) for the speed


v we find that the angular momentum of the satellite is given by mr0y/gr. Thus we see that as a satellite works its way out to larger orbits and the value of r increases, its angular momentum increases. Since the angular momentum of the whole system earth + satellite must be constant, this result means that the earth would be imparting some of its angular momentum to the satellite. Thus as the satellite moves out with increasing angular momentum the rotation of the earth would very slightly slow down.

In the case of the sun we can now formulate a theory for the solar system. We can again consider a starting point similar to Laplace's with a large nebulous gas slowly rotating but cooling and contracting over the millions of years, it would speed up as it got smaller in size. Eventually when the sun was considerably smaller perhaps than the dimensions of the orbit of Mercury, it would have been rotating so rapidly that some of the gaseous material around the equatorial regions would, because of centrifugal forces, tend to spread out into a disc-like shape. With further contraction of the sun, some of this material would remain spread out in a disc much as in Laplace's theory. The main difference, however, is that in this case the whole system would be much smaller than the present solar system and would all be contained well within the present orbit of Mercury. As the central mass of the sun—still 99-9% of the total mass—contracted further so that its rate of spin increased more and more, it would be rotating faster than the material it left behind in orbit around it. Any effects which would try to "help this orbiting material along" would make it gradually work its way out to more distant orbits, thereby gradually taking angular momentum from the sun.

if this coupling between the central spinning sun and the outer material continued it is quite easy to imagine that the material would gradually work its way out to the present dimensions of the solar system and acquire nearly all the angular momentum.

Indeed if we accept this explanation we can easily work back to see how fast the sun would be spinning if the planets were all made to spiral in and mix with it. This would be similar to our experiment of a person sitting in a rotating chair when he folds his arms across his chest. It turns out that, if the 0-1% of the mass of the solar system contained in the planets were to return to the central sun,


the sun's rotation would speed up from one revolution in 28 days to one revolution in a few hours.

The important point of this theory is to provide an explanation of the coupling between the sun and the outer material which would make this gradually work its way outwards. It can be shown that it is very unlikely that the sun's actual "atmosphere" is influencing the planets to anything like the required effect. However, in recent years it has been discovered that the sun does emit enormous solar flares which send streams of charged particles—protons and electrons—out into the extremities of the solar system. It is the trapping of such particles by the earth's magnetic field that has built up the Van Allen radiation belts—discovered by earth satellites. Moreover, any such motion of charged particles from the sun is accompanied by magnetic fields which do exert small forces on the earth.

Thus it is possible that magnetic fields such as these could have stretched out from the sura and exerted significant forces on the surrounding gas. Over periods of hundreds or even thousands of millions of years this could have provided the mechanism for producing the distribution of angular momentum.

How can a conjecture of this sort ever be checked ? It is, of course, impossible that we shall have an opportunity to watch the formation of any other solar system, but we may have the opportunity to observe the same kind of forces still at work. The quantity of gas now left in the solar system is of course a great deal less than that which must have been there to provide the building material of the planets. Even so there is some gas, and with space probes it should be possible to measure its state of motion and any magnetic fields it carries. It will be possible to deduce from such measurements whether the sun is at the present time still shedding angular momentum. We will not see more than the very tail end of this process, but even so it should still be possible to recognise clearly all its features.

Thus, in our present era, when more and more instrumented space vehicles are being sent into space, we may well have verification of this most plausible theory of the solar system origin. And if we do find these magnetic effects at work we have a mechanism which would operate for all stars, and we could anticipate many solar


systems within our galaxy. Indeed there is already indirect evidence that there may well be more than some 5 x 1010 planetary systems in our galaxy alone.

It is known through spectroscopic examination of the light from other stars, that stars rotate at very different speeds. It appears, however, as if the rotation speeds are not just randomly distributed among the stars but as if instead there are two definite clusters, the fast rotating and the slow rotating stars. Stars which are predominantly blue in colour, and which are therefore considered to be young stars in the process of formation, are fast rotaters; on the other hand stars which are a brighter red colour and which are considered to be older stars, tend to be slow rotaters. It may well be, therefore, that all of the slow rotaters are suns that have developed a planetary system and have had their rotations slowed down.

(d) The accumulation theory of planets.

So far we have only considered how gaseous material could work its way out and form a great gaseous disc around the sun, with most of the angular momentum being transferred to the disc. We have not, however, said anything about how the actual planets might form. The currently favoured theory in this regard is the so-called "accumulation theory", whose development was contributed to by Professor Thomas Gold, of Cornell University, and presented by him at our 1960 Summer School for High School Teachers. In this section we will present Professor Gold's arguments regarding this theory.

Let us consider the question of the physical and chemical processes that might occur in the original extended gaseous disc around the sun—the disc having become extended right out to the extremities of our present solar system. By being distributed to great distances, the outer parts would receive very little solar heat, and would therefore cool down to very low temperatures. At these temperatures of only a few tens of degrees absolute, many molecules would, of course, be formed and these in turn would make up solid objects. The solar composition, and therefore also that of this gas, is very largely hydrogen, with the light elements, carbon, nitrogen, oxygen making up a large fraction of the remainder. Materials as heavy as silicon or iron in atomic weight are only present in concentrations of the order of -1 of 1 per cent. In the cold outer


regions by far the most abundant solids that would forrri would be substances like ice, solid ammonia, methane, and other combinations of the light elements. Closer in towards the sun where the temperatures are higher only the higher melting point materials could go into a solid form and make small particles. The gaseous disc would therefore develop a distribution, graded according to vapour pressure and abundance, of little solid dust particles, and the next stage would depend critically on their surface properties. Small dust particles would be moving with the gas. When perchance neighbouring particles came to touch each other, they would very likely do this at a low relative velocity and might then easily stick together if the surfaces are slightly adhesive. This, of course, is very much like the formation of snow flakes which grow together by such chance encounters. Snowballs of the light element ices would form far out in the system and agglomerates of various chemical combinations of the heavier elements like silicon and iron that tend to make refractory materials would form in the inner part.

What will happen to these agglomerations ? How many of them will form ? And will they sweep up all the initial small grains ?

Individual agglomerations will grow until they have significantly depleted the density of grains in their vicinity. Neighbouring agglomerations will, of course, compete for material and larger and larger agglomerations will form both by picking up individual grains and by picking up other smaller agglomerates. When bodies of the order of some miles across have been formed then their gravitational attraction becomes important and they are able to grab much more material than would have come into contact with their surface otherwise. This, then, constitutes a race for the survival of the biggest, for those that are bigger can grow faster. A number of major bodies will therefore form, proceeding around the sun in orbits but perturbing each other significantly. Every now and again it will happen that two major pieces collide with one another, and get partly scattered again in this collision. New agglomeration then happens and new collisions occur for long periods of time. In all this process of agglomeration and collision, energy is dissipated for heat is produced and radiated away. It is clear, therefore, that the process cannot go on indefinitely. The source of the energy is, of course, the gravitation potential energy released by the formation of the massive bodies and therefore a


loss of energy from the entire system must correspond to a growth of those bodies.

Of course, bodies that were formed on orbits very far from one another can never collide and, therefore, there will be a process akin to one of natural selection in which those bodies that are not on possible collision orbits with others large enough to destroy them will be the eventual survivors. In other words the system will sort itself out and will grow bigger bodies, each on a lane that it dominates, and collisions will become rarer and rarer as time goes on.

In a system that has formed in this way it is clear that there would be a lot of order but not to complete perfection. All the major bodies must still be going around in the same direction for the collisions will always have been with objects going around in that same direction. The collection of the original finely divided material will always tend to make for regularity in the objects so formed; the plane of an orbit will have small inclination, the axis of spin will be normal to the plane, and the eccentricity of the orbit will be small. Then, however, when major collisions and perturbations occur, some irregularity will be produced. The spin angular momentum which is given to a body by such a collision is obviously quite arbitrary since it depends on the precise place of the impact, and it would therefore not be surprising that the Spin axes should all have been knocked about somewhat by these processes. In fact, the remaining level of regularity allows one to estimate that the planets have been put together in their final form with only a small fraction of the mass added in large lumps and most in small pieces. This does not say, of course, whether these small pieces were mostly the original dust grains or shattered pieces from any of the many preceding collision processes.

There are many points in this general scheme which need to be argued out in considerable detail. For example, why did the two major planets, Jupiter and Saturn, become so enormous? Why are there so many satellites ? Why is the moon so much less dense than the earth?

If the original material of the gaseous disc was of approximately the same composition as the sun and most of the stars, then it is clear that by now the planetary system has lost a great deal of hydrogen and has therefore concentrated the heavier elements.


Jupiter and Saturn, however, have a large proportion of hydrogen, and this could be understood if they were formed in the way we have discussed, mostly by the agglomeration of light element ices, at a time when a lot of this hydrogen was still there. Once the bodies had reached a certain size, their gravitational field and the rather low temperature would have allowed a hydrogen atmosphere to be accumulated. The inner, smaller planets could never do this however much hydrogen there was around, for their gravitational field is not strong enough and the temperature at this distance from the sun is high so that the hydrogen will readily escape. But Jupiter and Saturn could sweep up more and more hydrogen and indeed this would only increase the mass and therefore the gravitational field of the planet, and therefore accelerate the sweeping up of more. The amount swept up must therefore relate to the amount that was there and within their grasp, so to speak.

Uranus and Neptune further out would, of course, have had the same possibility, and the fact that they did not bloat themselves with hydrogen but remained bodies largely composed of elements like carbon, nitrogen, and oxygen could mean that on their orbits there was not much hydrogen available. Perhaps at this great distance hydrogen gas was too weakly bound in the overall solar gravitational field and it could escape into space either through its own thermal motion or through the sweeping and heating action of the interstellar gas through which the whole solar system must constantly be ploughing at speeds of the order of a few kilometres per second. It is not at all unreasonable that there should be ä cut off to the distance at which a hydrogen cloud could be held by the sun for long periods of time.

The existence of so many satellites whose orbits are closely related to the spin of the parent, strongly suggests that some frictional force between the planet and the satellite material was in existence at one time. It is only by the operation of a quite unreasonable amount of chance that satellites could be formed without the intervention of a resisting medium, and so for this reason alone, one would have to think that enormously more gaseous matter has been present at one time than we now have. Very likely, large amounts of hydrogen, some tens of times more than the total mass of the planets, were responsible for this.


The earth, as we have said, could not hold a hydrogen atmosphere. But on the other hand, if the whole region of the solar system in which the earth moves contained a large amount of hydrogen, then the earth's gravitational field would make a very substantial concentration. Any particular hydrogen atom might be acquired and lost again in this atmosphere, but the density would be high. Most of such a hydrogen atmosphere would rotate with the planet and it is in such circumstances that small particles could be captured into orbits encircling the planet. Such particles in turn will make agglomerations and collisions, and from there on the story of the formation of satellites around planets is much the same as it was on the larger scale for the formation of the planets around the sun. It is significant that no case occurs in which the angular momentum contained in the satellites is so great that one could not suppose it to have come from the planet. If the angular momentum of the satellites were put back into the planets, no planet would be rotating at bursting speed.

We all know that an artificial satellite revolving around the earth at a small distance, takes about \\ hours for a revolution, whereas the earth takes 24 hours to make one rotation about its axis. If the satellite is moving in the region where there are still atmospheric gases revolving with the earth, then there will be some friction and the force applied to the satellite will be in the sense of opposing its motion. The change in the orbit that results, brings the satellite closer to the earth and diminishes the length of its period. Eventually, of course, it crashes to the earth. This, however, is not the situation at all heights. As we have shown in the previous chapter, if there were still a significant drag at a distance of 22,000 miles and more, then the sense of the result would be reversed. There would then be no standing friction with any gases which, rotating with the earth, take just the same time to make a complete rotation as does the earth. For any satellites out of this distance, a frictional drag would work in the opposite sense. It would push them in the direction in which they are going and it would therefore lead to larger orbits possessing more angular momentum. For the example of the present earth, this critical level is rather high for there to be in reality any significant atmospheric effects. But the primitive earth will have revolved much faster because it had not yet distributed so much angular momentum to the moon, and


correspondingly, the critical point will have to be much lower. However, as we shall see in the next chapter, once the moon had "accumulated" itself into a single object, tidal effects could well have been sufficient for it to work its way outwards from a relatively close orbit to its present orbit 240,000 miles distant.

In the planetary system we have one case where the process of collision and accumulation has not yet been brought to its final conclusion. The belt of asteroids between Mars and Jupiter is a region in which collisions must still be happening, in which energy therefore is still being dissipated, and in which a change to a different configuration must still be taking place. For the satellite system, we have another case of a similar sort, namely, the rings of Saturn. These, it is known, must be composed of smaller pieces of matter and again processes of collision and accumulation must be occurring. Most probably these rings will one day make a number of further satellites for Saturn.

The situation to which the solar system must be tending is one in which no further collisions can occur. This, it can be shown, is one in which there are only planets going around in orbits sufficiently far from one another so that no large perturbations can occur. Each planet must have a clear lane assigned to it all around the orbit. An object like the moon cannot be going around somewhere else on the lane assigned to the earth without eventually coming into collision; but this, of course^ does not prevent the moon from going around together with the earth on a satellite orbit. Each planet may possess satellites without risking collision. Thus the demand for a collision-free system with the motions nearly confined to a common plane is already enough to specify something that looks quite a bit like the planetary and satellite system, (e) Life cycle of a sun.

So far we have considered only the formation of a solar system. We now consider very briefly the nuclear processes which occur within a sun and form the source of its energy.

When gases contract to form a star the young star consists mainly of fundamental atomic particles—that is, protons and electrons and a relatively small number of other nuclei. As the sun contracts gravitationally, its internal potential energy decreases and is converted into kinetic energy of movement of the particles. Thus the temperature of a young star is very high.


When the contracting mass of gas becomes hot enough, colliding protons can form deuterium nuclei with emission of an electron. Each deuterium nucleus consists of a positive proton and a neutral neutron and is thus positively charged. Then again deuterium nuclei themselves—whether originally existing in the sun or formed from protons—can collide and produce further nuclear reactions.

It is clear, however, that such processes can only occur in a gas which is at an extremely high temperature so that the elementary particles have very high speeds. Any two nuclei approaching one another tend to be pushed apart by the Coulomb forces between them; the closer they approach together the more strongly these forces tend to push them apart. We can understand this by a simple example. Imagine two small ping-pong balls hanging from threads, each carrying a positive electric charge. If they travel towards each other fairly slowly they will be pushed apart before they collide. The faster they travel the closer they will approach and the more strongly they will be pushed apart. Now imagine each ball to be covered with glue. If their speeds are high enough for them to collide, then having collided, they will stick together by the glue on each one. The glue would hold them together despite the charges on them which would tend to push them apart. In actual fact, of course, what happens is that when two nuclei approach each other with sufficient speed that they could actually come into contact, the nuclear forces, which are stronger than the Coulomb forces, take over and the two nuclei fuse into one mass. When two deuterium nuclei for example fuse together a new and different nucleus is formed. It is one made up of two protons and two neutrons and is the nucleus of the ordinary helium atom. In the fusion process this nucleus lasts only for a fraction of a second in a seething, unstable condition, and then throws off a neutron or a proton and becomes stable.

Such a process is illustrated schematically in Figure 4.2. When the deuterium nuclei are fused and the helium or tritium is formed, the total surface area of the fusion products is less than the surface area of the two deuterium nuclei. You could show this by getting two bubbles the same size and causing them to fuse together. Now a reduction in the surface area is accompanied by a decrease in energy. In the case of the deuterium fusion this is clearly indicated as follows: with two deuterium nuclei separated there are simply


Figure 4.2.—Fusion of deuterium.

two nuclear bonds between particles. After the reaction, however, there are three bonds within the helium or tritium nucleus, so that the attractive nuclear forces have more effect. The potential energy of the final combination is less than the initial.

The decrease in potential energy in the process means that the final products of the reaction fly apart with considerably greater kinetic energy than was available initially.

Thus, as fusion reactions occur within a young star, kinetic energy is released and the temperature of the star increases. A young star, therefore, gradually uses up its hydrogen and then its deuterium, converting it by fusion reactions into helium or tritium. The temperature of the star rises until a balance is reached—that is, until it radiates into space as much energy as is being released from nuclear reactions.


Eventually, of course, in any star the supply of protons and deuterium nuclei is gradually used up, and fusion reactions among the heavier nuclei which have been formed begin to predominate. As the heavier elements form and the average density of the star material increases the star contracts further. More violent collisions occur, more nuclear reactions take place, and more energy is released as kinetic energy and electro-magnetic radiation. Some times the amount of energy released in this process gives the nuclei so much kinetic energy that gravity can no longer hold the star together. When this stage is reached the star blows apart; such an occurrence was actually observed from earth in the year 1054, when a certain star suddenly began to glow with amazing brilliance. Night after night it grew brighter and to those who watched it it appeared to be expanding.

Eventually it started to dim; after two months it was about one-third as bright as it had been at its maximum and after about six months it had disappeared from view. A star had "died".

The description of this event has been found in Chinese writings. Large telescopes pointed to the spot where the star appeared show what remains. It is called the "Crab Nebula" and represents the remains of the star distributed in a great gaseous cloud and still expanding outwards rapidly. Such an explosion of a star—called a super nova—occurs in our galaxy about once every 50 years on the average. After such an explosion, nuclei of the various elements which were formed in the star are scattered across millions of miles of space. This may well be the fate of our own sun eventually. It has used up about one-half of its original supply of hydrogen, but this has already taken several thousand million years. Our sun is in the equilibrium process where it still has ample hydrogen left and is radiating energy into space at the same rate as it is being released within it. Thus the sun will probably last for a few thousand million years yet.

When a star explodes and hurls nuclei into space, what happens to these nuclei ? Some of them—particularly the heavy ones—may eventually provide the heavy elements for some future planet. It is quite possible that in our own earth the heavy elements were "manufactured" thousands of millions of years ago in other suns nearing the end of their evolution.

CHAPTER 5

Evolution of the Earth

So far we have discussed the formation of the solar system and the accumulation theory for planet formation. We now consider the earth itself in more detail and how it could have evolved into its present form from such beginnings as we have discussed. These considerations are of vital importance for discussion on the evolution of life on earth.

(a) Age of the earth.

Naturally one of the first questions that we should ask is How old is the earth ? so that we know the time scale involved for the development of life.

Geologists have developed a variety of ways of arriving at ages of various rock formations, but perhaps the most accurate methods for assessing the earth's age are based on physical or chemical methods. One very simple method is based on the salinity of the ocean water. Salt is continually brought into oceans by the rivers which contain a small amount of salt dissolved by rain waters coming into them. Water evaporates from the ocean surface leaving the salt behind, and goes through successive cycles of condensation and evaporation while more and more salt is accumulated in the ocean. If we divide the total amount of salt in the ocean water by the annual input of salt by rivers we find that it must have taken several thousand million years to build up the present salinity.

This method, first suggested by the astronomer Halley more than three centuries ago and improved by using modern geological information, can give only the order of magnitude of the ocean's age because of great uncertainties in the rate of erosion during the past geological eras.

Another rather interesting method is that one can determine something about the age of the earth-moon system. As you are probably aware, the tides of the oceans are caused by the gravitational

107


effects of the moon, and to a lesser extent the sun. We can understand tide production in terms of our discussion in Chapter 3.

Let us first consider the earth moving in orbit around the sun. The centre of gravity of the earth—the earth's centre—is moving at just the right speed to maintain its near-circular orbit. Consider a point, however, on the earth's surface farthest away from the sun— such as the point P in Figure 5.1.

Figure 5.1.

If there were a small object at P moving in orbit around the sun, its speed would be slightly slower—since it is farther away— than the speed of the earth's centre in its orbit. Thus the point P is being carried by the earth at slightly too fast a speed. In the

EVOLUTION OF THE EARTH 109

effort to slow down, material in the vicinity of P has a tendency to move outwards; there is, therefore, a slight outward pull on material on the "dark" side of the earth.

In turn, material in the vicinity of point Q is not travelling fast enough to be in a balanced orbit. If the earth's surface were simply one large ocean, this ocean would be slightly egg-shaped with bulges away from the sun and towards the sun. The solid earth underneath could well be rotating but the bulges would always remain towards and away from the sun.

In actual fact, the oceans of the earth tend to do just this and as the earth spins on its axis, continental coast lines can run into a bulge (high tide) or out of a bulge (low tide).

In turn the earth-moon system is really one in which the earth and the moon are each moving around their common centre of gravity. The same sort of considerations apply in this case too, and in fact the moon produces larger tides on the earth than the sun does. Now tidal effects on the earth clearly have energy associated with them and this energy must be coming from somewhere. It is in fact very slowly being taken from the energy of the earth's spin. If the earth were not spinning we would have a static situation with no varying tides and no tidal energy being dissipated.

Thus the rotation of the earth is continually slowing down and it can be estimated that each day is longer than the previous one by two thousand-millionths of a second; in spite of the small rate of increase the cumulative effect becomes appreciable over long periods of time and during one century the total discrepancy in clock readings, becomes 14 seconds. This figure has even been checked by observing slight discrepancies in astronomical data, between present observations and those carried out in the middle of the last century. The earth's spin is therefore going to cease altogether in something less than a thousand million years' time; at such a stage the earth will presumably have captured rotation, that is, with one face always being directed towards the sun.

Another aspect of this effect depends once again on angular momentum. There is a certain angular momentum associated with the earth's spin; where is this going? The answer is that if we consider the earth-moon system and the lunar tides, angular momentum of the earth is being gradually transformed into additional

no LIGHT AND LIFE IN THE UNIVERSE

angular momentum of the moon in its orbit around the earth. As you know, this must mean that the moon is gradually moving out-wards into mored ist ant orbits. Thus the tides due to the moon form a coupling between the earth and moon of just the kind discussed in the last chapter; we thus see how this effect can occur without needing an extended earth atmosphere. In turn the smaller tidal effects due to the sun are causing the earth very gradually to move to more distant orbits although this is an extremely small effect, and cannot be invoked to account for the original spreading out of the solar system.

In the case of the moon, however, the effect is quite significant; each time we see a new moon it is 10 cm farther away than was the previous new moon. If we work backwards through time it can be calculated that the moon would have been very close to the earth five thousand million years ago, or 5 x 109 years ago. If the moon-formation theory which we discussed in the last chapter is correct, then our estimate of 5 x 109 years is a reasonable one for the age of the earth-moon system. It must then also refer to a very young stage in the earth's history; the figure 5 X 109 years could therefore reasonably be given as the age of the earth.

Arguments such as this are, however, somewhat circuitous and are dependent on the validity of other theories. For example, the amount of energy dissipated by tidal effects is dependent on the nature of the earth's surface and the amount of movable matter on it. This may well have changed drastically over the billions of years, so once again the above estimate is subject to possible errors. The most accurate methods of age determination are based on the radioactivity of certain isotopes, and this is such an important field that we shall treat it separately.

(b) Radioactivity.

As we pointed out in Chapter 1, a radioactive substance is simply one whose nuclei are unstable and can automatically decay into something else.

It is consequence of the theory of relativity, which does not form a subject in this Summer School, that energy and mass are equivalent. The connection is that

E = mc2 (5.1) This means that a certain mass m grams is equivalent to an energy


of mc2 ergs, where c represents the speed of light in cm per second. When a nucleus is formed, its total energy is negative, meaning that work has to be done to pull all the separate neutrons and protons apart. Thus the mass of a nucleus is slightly less than the sum of the masses of the separate neutrons and protons. The mass of a nucleus is usually designated by the symbol M and the difference between its actual mass and the mass of the separate constituents is designated l\M. Thus the quantity l\Mc2 represents the so-called binding energy of the nucleus; it is the amount of work that would have to be done to separate the individual neutrons and protons.

If the initial mass M of a nucleus is greater than M+ + w0, where M+ is the mass of the nucleus which results when a neutron is converted to a proton, then such a nucleus will be unstable. The substance itself will continually be emitting electrons as its nuclei are steadily converting themselves into different nuclei by electron emission. Such a substance is said to be unstable against 0-emission since electrons emitted from nuclei traditionally are called ß rays.

Towards the end of the periodic table there are also many substances which are unstable against emission of a helium nucleus— or so-called a particle. The condition for this to occur is that the initial atomic mass M must exceed the sum of the masses M+ + Ma, where M+ is the atomic mass of the nucleus resulting after the α-particle has been emitted, and Ma is the mass of the a-particle.

When any such decay (including ß decay) occurs, it is not necessary that the nucleus left behind after the decay is in its lowest possible energy state—or ground state. In α-particle emission, for example, if there is an excited state of the final nucleus with mass M+, for which M > M+ + Λ/α, it is quite possible that in some cases the excited state of the final nucleus will be formed. This will then itself decay to the ground state by emission of a very energetic packet of electro-magnetic radiation—called a y-ray. This process is very similar to the situation in atoms, although the energies involved are about a million times greater.

In Figure 5.2 we show the natural radioactive series, whereby a U238 nucleus can cascade right down to become a lead nucleus by a series of a- and ß-emissions. At each stage there is a likelihood of a y-ray also being produced. Hence the radiation being emitted in this combination of decays consists of α-particles, j8-particles (electrons) and y-rays. These form the original α, β and y-radiation

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first discovered towards the end of the last century by Madame Curie and others.

It should be said, however, that the fact that such radioactive decays can occur does not necessarily mean that they will occur rapidly. Even though it may be energetically possible for the decay to occur, a nucleus may still last a long time before the decay actually happens. In this connection the term "half life" has been introduced. The half life of a particular element or isotope is the time which it takes for half of the original number of nuclei present to decay. For example, the half life of Ra22ß is 1620 years; this means that of every, say, 10 grams of radium which exists today, only 5 grams will remain in 1620 years' time, and half of this—2\ grams—will remain after another 1620 years.

Apart from substances which occur naturally, it is now possible to produce many types of radioactive isotopes—see Table 3 in Chapter 1—by means of nuclear reactions, in the laboratory. One extremely important method of artificially producing radioactive isotopes is by the use of neutrons themselves as the bombarding particle. In nuclear reactors, for example, which have been developed to produce nuclear power, large numbers of neutrons are produced which can be used in turn to manufacture useful radioactive isotopes.

One such example is the radioactive isotope of Co^. The normal cobalt nucleus has mass number 59 and atomic number 27— C027. However, when cobalt is subjected to intense neutron bombardment within a nuclear reactor, a significant fraction of the nuclei absorb neutrons to become nuclei of Cof?. Initially an excited state of the new nucleus is formed, which then decays to the ground state by y-ray emission. However, even the ground state of Co™ is unstable and permits a neutron to change into a proton with the emission of a ß-electron. This produces nickel—Ni^?—in an excited state which, in turn, drops to the ground state by emission of an intense very energetic y-ray.

It is this intense y-radiation from radioactive cobalt which is now widely used in the so-called cobalt bomb for the treatment of cancer. The radioactive isotope C02? has a half life of about 5 years so that once a quantity of it is produced it lasts for a reasonable length of time.

Many substances have nuclei which absorb neutrons in this manner. The resulting radioactive isotopes, however, have half-lives


which differ in different cases. For example, silver—Ag1^—can be turned into the radioactive isotope Ag1^ by neutron bombardment, and this decays by emission of a ß-electron and then y-radiation in the same way as with cobalt. The radioactive silver isotope, however, has a half-life of only a few minutes and hence does not last sufficiently long to be useful in the same manner as the cobalt isotope.

A very large number of artificially-produced radioactive isotopes are now being used in biology and other fields. (c) Radioactive dating.

Since many radioactive materials occur in the earth the known life-times of radioactive nuclei can be used to give evidence on the ages of rocks. One well-known method uses the uranium series of Figure 5.2. Uranium ores, for example, can be found in igneous rocks—that is, in rocks which have welled up on to or near the earth's surface, in a molten state because of volcanic activity. It is, however, impossible to find any uranium ore which is pure because some of the uranium will have decayed and there will be associated with it elements right down the decay series to Pb206.

By a careful selection of ores, however, samples can be found which give clear indications that originally no lead, for example, would have been present. Analysis of such samples gives indications of the chemical processes which must have gone on in the original formation of the particular uranium ore in. question and in many cases it can be said with fair certainty that no lead could have been present when these chemical processes went on. Thus a careful analysis of the percentage of lead now present in the samples yields a clear cut indication of how long the uranium must have had to produce this lead. The older the rock, the larger is the amount of deposited lead as compared with the amount of radioactive element still present.

Even this method has a flaw, however, since in the series of elements leading from uranium to lead there is one member, radon, which is a gas which could have partially escaped by diffusion through the rock, thus lowering the estimate of the age of the rock. Other radioactive dating methods are also available and one relatively reliable method is based on the radioactivity of rubidon, with a half life of 6 x 1010 years, which emits an electron and decays into stable strontium. Another radioactive dating method involves the potassium isotope K*% which can change into Ca^J with a half life of 1-2 x 109 years.


The results from all these methods give slightly varying age estimates as, of course, is to be expected because of the type of uncertainties mentioned. The variations are, however, not large, and the best present estimate of the age of the earth's crust obtained by radioactive dating is 4-5 x 109 years.

It is interesting to see how close this is to the previous estimate of the age of the earth-moon system.

(d) Evolution of the earth.

Naturally, it is impossible to formulate with any degree of certainty the details of how the earth has evolved over the billions of years. We can discuss, however, whether the huge amount of information which has been collected concerning the earth is possibly in agreement with the theory of the earth's origin which we have outlined, and once again we follow the reasoning outlined by Professor Gold.

The construction of the earth, as it is known from seismic evidence, is, crudely speaking, in three layers—see Figure 5.3. On the top there is a crust of very variable thickness some fifty to a hundred kilometres thick where the continents are, and possibly only ten kilometres or less under the oceans. This crust is composed of a material of considerably lower specific gravity than all that is below. The mean specific gravity of the crust is about 2.8 and the specific gravity of the material just below is probably in the neighbourhood of 3.3. The mean specific gravity of the entire mantle is about 4.5. Then, below all this, the innermost layer (approximately half of the earth's radius and therefore one-eighth of the volume) appears to be a liquid core with a specific gravity a little in excess of 10. The interpretation which is usually given is that the relatively thin crust is composed of a particularly light variety of silicate rocks, while the mantle below is made of the heaviest silicates containing iron and magnesium, possibly also iron sulphate as well as perhaps some metallic nickel-iron. The liquid core is thought to be made of iron and nickel, just because these are the abundant elements that would be expected to liquify sooner than the dense silicates and they would possess approximately the correct density. This picture of the structure of the earth is schematically illustrated in Figure 5.3.

How could a body like this have been put together ? By the


Figure 5.3.—Schematic illustration of the interior of the earth.

accumulation of big and small pieces in the manner we have discussed ?

In many scientific discussions and theories in the past it was assumed that the earth was formed in a completely molten state. This is contrary to the presently accepted theory in which the earth was formed by an accumulation of "cold" chunks of matter. There never has existed a theory, argued out in physical detail, for a way of condensing as small a body as the earth in a hot condition, as was mostly implied in such discussions. The very patchy distribution


of the lighter continental material argues heavily against there ever having been complete liquification, for in that case one could not understand why the materials of different density did not distribute themselves in the hydrostatic equilibrium of a set of spherical shells. The continental masses clearly need substantial forces of rigidity to hold the continents together and prevent them from flowing out over the surface into a uniform layer.

Much of the tendency to regard the earth as having been originally liquid has no doubt come from the geological evidence which shows that a very major fraction of, if not all, the material that is now on the earth's surface has once been liquid. But there are other ways, as we shall see, in which such a state of affairs could have come about.

If we think of an earth growing by an accumulation process of small particles interspersed with a few big ones, it is clear that the interior will not be entirely homogeneous, for the different regions will have suffered different treatment as a consequence of the major impacts. At any stage during the growth, the surface will have been pitted with a large number of impact craters in a way perhaps not very different from the present surface of the moon. As more and more material was piled on, some heating will have occurred in the interior for three different reasons. Firstly, the impacts result in pressure waves which transport energy and some of them get attenuated deep down in depths, from where heat conduction to the surface is an exceedingly slow process. Secondly, as more material was piled on the top, there will have been a gradual increase of pressure inside and an amount of heat will have been released by the compression of the material in a way that depends in detail upon the relation of pressure, volume, and internal energy of the substance at these high pressures. Our knowledge of these quantities, for very high pressures, is inadequate and therefore the amount of heat released by this process cannot be estimated very accurately. Thirdly, there will have been the amount of heat released by the radioactive minerals which has often been discussed, but with still rather indefinite conclusions.

Although different authors have made models of the earth's interior with certain distribution of radioactive minerals and then calculated in some detail the temperatures that would result from these models, it must be admitted that the basic information still


allows a wide range of possibilities. These sources of heat could easily amount to what is necessary to heat the interior of the earth to temperatures like 4,000°/5000° Centigrade. Probably we cannot do much better at the present time than to estimate the range of temperatures at which a solid mantle and a liquid core could co-exist. The melting points of most substances increases with pressure and it has been estimated that iron would be a liquid at the interior of the earth at a temperature in excess of about 4,500° C. On the other hand, in order to retain the solid phase of the other materials, the heavy silicates, in the inner part of the mantle, the temperature would probably have to be lower than about 7,000 degrees. As we go upward from the core, the melting points of the substances decreases as the pressure decreases but equally the amount of heat supplied will diminish somewhat because the contribution from the compressional effect becomes less important; and as we approach the surface, the amount of heat lost to the exterior will increase.

The randomly mixed up collection of materials that may make up the earth have the property that over quite a wide range of temperatures a certain fraction, but not all, would be in a liquid state. This range of temperatures is wide enough for it to be quite plausible that the whole solid mantle could be in this condition. Near the top we know after all, that the lowest melting point substance, namely certain light silicates, can exist as lava. In the deep interior we believe that liquid iron can occur. Yet the seismic evidence is perfectly definite in showing that the entire mantle is capable of transmitting transverse seismic waves and therefore is in the main solid. This, however, does not prove that it is not pervaded by veins of liquid of the light or the heavy sort, whose seismic effects would be small.

If we imagined a mixed up—heterogeneous—collection of substances to be heated up gradually until a certain fraction of the material was liquid, then at a certain stage enough communication between the liquid parts wijl occur for a migration to be possible. In actual fact, the big earthquakes connected with the major impacts will probably have helped greatly to allow a small fraction of the liquid to establish communicating channels. Perhaps we may anticipate here a point that will come up again later: The structure of the meteorites proves that they have once been parts of some


major bodies and they indeed show a structure for the most part, which must have resulted from the combination of metallic and silicate liquid veins pervading a solid agglomerate. Perhaps we are seeing there, representative cases of the kind of structure that is set up in the interior of a planet.

Once the growing earth had heated to internal temperatures at which some communicating liquid channels existed, gravitational sorting of materials will have taken place. The much heavier liquid iron will have drained towards what is the centre, distending the metal veins lower down and allowing the ones higher up to contract. The mean density of the material will therefore increase systematically towards the centre. Eventually the inward draining metal will form a dense liquid core, such as the earth now has. The liquid fraction which is lighter than the main mass of the mantle similarly will have a tendency to rise upwards. One might think that it would then be expected to come up to the surface and to make a layer of low melting point, light material there. There is, however, an added complication which it is well worth discussing.

The deep interior of the earth, below a depth of a few hundred kilometres, cannot lose much heat to the outside because of the great thermal insulation provided by the overlying material. The outermost layer is, however, kept quite cold by radiation into space; so cold that no structure of pores or veins of liquid constituents could exist there in the rock. The porous structure and the percolation of liquids can therefore not apply to the topmost layer, and all communication is blocked through 'freezing'. The lighter liquid constituents down below will be held under by being unable to penetrate the cold layer above and while this situation may be in hydrostatic equilibrium, it is certainly not stable. If any sufficiently large crack develops in the cold upper layer, then the accumulated liquid will make its way up through this crack. Now it is a very important feature of this process that a small crack will not suffice. Through a small crack, there could only be a slow rate of flow and the upwelling material would soon freeze in contact with the cold solid. If however there is a crack which is large enough, the upward draining can continue, for then the upwelling material will bring enough heat up with it to prevent freezing. This of course, is just the same reason as the one that causes a small water pipe to freeze


up in the winter out in the open while a very large one carrying a lot of water may be quite safe.

This thermal blockage therefore assures that the light material cannot ooze out uniformly all over the earth as would have been otherwise the natural consequence of the hydrostatic situation. Instead it is only through some very large cracks that the light material can flow, and then once having come up through a certain crack, it will tend to keep that region hot and therefore keep the flow going there. A few major lines of upwelling must therefore be expected and this would seem to account for the existence of the continental masses distributed along a bold pattern over the globe. The light continental rock that has come up in this way can, of course, not pile up indefinitely above the cracks out of which it has come. Its mechanical strength is inadequate to support more than a certain height and it must therefore flow out laterally once this height has been reached. A crack would therefore provide a line from which continental growth would take place.

One can see no way in which the lighter continental materials could have been put into their present places except Jby a gradual differentiation process from the interior. It is therefore not so much a question whether continental growth has occurred but only whether it occurred as a process during the period covered by the geological record. This record is, of course, enormously complicated by the large extent to which erosion has occurred, so that most of the material can no longer be seen in its original form but only as combinations of sedimentary, metamorphosed and intrusive rocks. It is of great interest in this connection that all volcanic activity is distributed in a few major lines around the globe and that those same lines are associated with mountain building activity. It is presumably along these major crack lines that enough heat is being transported up to prevent the freezing of the lavas; volcanoes and mountain building must therefore occur together. Deep down below along these lines the lighter materials are able to drain to the top and the mean density will therefore increase in the course of time. Perhaps this is the explanation of the deep ocean trenches that tend to occur close to these lines. The augmentation of the density below will, of course, lower the equilibrium position of the ocean floor. As light material pours out, the vicinity of the outpouring tends to sink.

CHAPTER 6

The Primordial Atmosphere and

the Origin of Life

(a) The primordial atmosphere.

As you will see in later chapters of this book, the question of the origin of life is intimately associated with the constitution which the earth's atmosphere must have had in the early stages of the earth's evolution.

Perhaps the greatest exponent of this subject is the American chemist, Professor Harold Urey, and you are recommended to read his book, The Planets, Their Origin and Development. Although we cannot be certain of the detailed composition of the atmosphere thousands of millions of years ago, one thing is certain—the primordial atmosphere was undoubtedly vastly different from our present-day atmosphere.

As you know, oxygen forms a crucial component of the present terrestrial atmosphere. However, an overwhelming percentage of the free oxygen in the present atmosphere has been formed by the process of photosynthesis whereby plant life converts C0 2 into oxygen, and retains the carbon necessary for the construction of its own molecules. Originally there would have been extremely little oxygen in the atmosphere.

Professor Urey's analysis of the types of chemical reactions going on within the young earth indicates that the original atmosphere must have consisted largely of helium and hydrogen with smaller but very significant amounts of water vapour, ammonia and methane. This is indicated in Figure 6.1.

The quantitative details shown in this Figure are almost certainly not correct in detail—there are too many unknowns involved. However, the qualitative details are likely to be correct, and here

121


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the significant factor is that in water vapour (H20), ammonia (NH3) and methane (CH4), we have the basic elements associated with the molecules of living organisms.

THE PRIMORDIAL ATMOSPHERE 123

As you will see in later chapters the most important substances forming living organisms are proteins, the molecules of which are formed by very long chains of comparatively simple chemical compounds called amino acids. Apart from the role of phosphorus atoms in these large molecules—phosphorus being readily plentiful in the earth's material—it is the joining together of various amino acid molecules which make up the structure of the large molecules which characterise life.

It is now known that the action of the sun's radiation on such a primordial atmosphere could have produced molecules of amino acids. It is this process which is considered to have formed the start of the evolution of life on earth. It is not our purpose in these chapters to take over the role of the biologists and to discuss possible life-forming processes in detail. This will be done in later chapters. It is our purpose, however, to consider the physical processes involved.

It has been proved experimentally that irradiation of a mixture of methane, ammonia and water vapour by ultra-violet light can produce amino acids. To be told that this can happen is one thing but you may well wonder how such a process can occur. We will, therefore, consider a very simple process that is going on continuously to-day which shows how the action of the sun's radiation can be responsible for building up more complex molecules from simple ones.

(b) The production of ozone in the earth's atmosphere.

As you know most of the oxygen in the earth's atmosphere at sea level exists as diatomic molecules—that is, in the standard conditions pertaining on the earth's surface all oxygen gas tends to exist in this form.

As altitude increases, however, we find that gradually more and more ozone occurs; ozone is a form of oxygen gas consisting of 0 3 molecules. The percentage of ozone increases to a maximum at around the 40 kilometres level and then gradually drops again. The reason for this ozone concentration is simply the fact that the sun's irradiation contains ultra-violet light, and if this could be shielded out, the amount of ozone in the earth's atmosphere would drop to negligible proportions.

Let us understand this process in a little more detail. To do so, recall our discussion in Chapter 2 on the Bohr atom and Bohr's


postulates. There we saw that when electro-magnetic radiation falls on atoms, for example, certain wave-lengths can be absorbed. These correspond to the wave-lengths associated with transitions between electronic orbits in the atom. If an atom when excited is capable of emitting radiation at certain definite frequencies it will also absorb these frequencies out of a continuous spectrum of radiation. We also saw in Chapter 2 that an atom can even be ionised by radiation, if it absorbs a photon of high enough frequency to eject the electron out of the atom entirely.

The same is true of molecules. A gas of molecules, when excited, can emit radiation in what is called a molecular spectrum; the wave-lengths of this spectrum are associated with quite complicated electronic movements that can occur within the molecule, and even of vibrations of the molecule. In turn these same wave-lengths will be absorbed by the molecule when it is bombarded with a spectrum of radiation. Certain frequencies may even be able to dissociate a molecule, that is, split it up into smaller atomic units.

The ordinary diatomic oxygen within the atmosphere can be dissociated in this way by ultra-violet wave-lengths—photons of the correct frequency—in the solar radiation. Thus the following process can occur:

02 + photon-* O + O (6.1)

So far you would say that this is not a build up of heavier molecules at all, but simply a breaking down of molecules. This is true, but equation (6.1) simply represents the first step in a chain of events. Once there are single oxygen atoms in the atmosphere there arises the possibility of other processes. One of these is the fact that an individual oxygen atom may collide with an 0 2 molecule. Can this form 0 3 ?

If you remember that we must always have conservation of energy and momentum, and if you wrote down the appropriate equations very carefully, you would find that this is impossible as it stands. However, occasionally we will have a "three body" collision in which an O atom, an 0 2 molecule, and some other molecule whose identity is irrelevant—call it M—all collide together. In this case it is quite possible for the following reaction to occur:

02 + O + M ~+ 0 3 + M . . . . . . (6.2) In this equation the only function of the M molecule is to conserve


energy and momentum; its kinetic energy and momentum after the reaction will be different from what it was before the collision in just such a way that the total energy and momentum of all the particles involved is conserved. Thus the M molecule acts, as it were, as a catalyst in making the reaction possible.

In turn the 0 3 molecule thus formed does not last forever but itself can be broken up by the absorption of ultra-violet radiation. The following process, for example, occurs:

0 3 + photon -> 02 + O (6.3) Moreover, individual O atoms need not necessarily go into the formation of 0 3 but can recombine to form 0 2 as follows:

0 + 0 + M->02 + M (6.4) Equations (6.1) to (6.4) represent a cycle of events which are

going on in the atmosphere continuously. Thus, although no individual 0 3 molecule lasts indefinitely, there will always be a certain concentration of ozone determined by the equilibrium balance between these four reactions. The maximum concentration of ozone occurs at the 40 kilometres level because it is at this height that the solar radiation meets atmosphere of sufficient density that the probability of reaction (6.1) occurring becomes quite high. This initiates the whole cycle of events. It is interesting that the 0 3 absorbs wave-lengths mainly between 2,000 to 3,000 Angstrom units. The equilibrium concentration of ozone which occurs around the 40 kilometres height is actually sufficient to prevent any radiation at wave-lengths less than 3,000 Angström units from penetrating to regions near the earth's surface. This is why the solar radiation reaching the earth's surface has a sharp cut-off, with no wave-lengths shorter than this. It is for the same reason that very little ozone is formed near the earth's surface—because there is no ultra-violet of the right frequencies left to take part in the ozone cycle.

This formation of ozone is a very interesting phenomenon in itself, and it plays an important role in determining properties of the earth's atmosphere—including the winds of the upper atmosphere. This subject does not, of course, concern us here but the ozone cycle does illustrate the important point that solar radiation can be responsible—combined with ordinary collision phenomena between molecules—for the formation of more complex molecules from simpler ones.


It is in a similar manner that a primordial atmosphere of HaO, NH3 and CH4 can have formed certain amounts of more complex

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CHN molecules. In this case, we are dependent on a much more complicated chain of events, because amino acids have relatively long chain molecules. One of the simplest amino acids is lysine, whose molecule is illustrated schematically in Figure 6.2. As you will learn in later chapters, other amino acid molecules which form basic building blocks in the molecules of living organisms are considerably more complex. But the fact remains, as has been experimentally verified, that they can be formed by ultra-violet radiation from an ammonia, methane and water vapour mixture.

These amino acids, if they originated in the outer reaches of the primordial atmosphere, presumably descended to the ground level, since they represented quite heavy molecules and, being dissolved in the ocean waters, formed an organic "brew"—perhaps the first stage in the development of life. On the other hand, we pointed out that the primordial atmosphere must have contained very little oxygen, since photosynthesis was non-existent. This being the case, ultra-violet radiation may have been able to reach ground level and the formation of the more complex molecules could have taken place


there and in the oceans. This is an exceedingly interesting field of study and many questions remain to be answered.

Of course, it is a very large step from the water solution of amino acids to the simplest living organism, and we still do not know how to close that gap. You will have a better idea of present biological knowledge in this direction after you have read the chapters by Dr. Yeas and Dr. Watson. As regards closing the gap between amino acids and living organisms, we also give you the thoughts expressed by Professor George Gamow at our Summer School here in 1960:

"One can visualise that during the several thousand million years which were available, amino acid molecules, uniting into long chains, went through a complicated evolutionary development. One can speculate that, at these early stages of development, Darwin's principle of the struggle for existence was already operating in full force, and that the most efficient chemical reactions between primordial protein molecules were getting an upper hand over the less successful ones. Organism structures were becoming more and more complicated and more and more adapted to their surroundings. And so here we are! " You will be able to compare these with the views expressed in the chapters which follow. The lectures of Professor Yeas will also develop in considerable detail many of the points we have touched only briefly in this and the previous chapters.

ARE WE ALONE?

For some time men have speculated on the possibility that there is intelligent life elsewhere in the universe. Most of these speculations have referred to the moon and to the other bodies of our solar system, especially Mars and Venus, but there is nowadays no expectation among astronomers that intelligent life exists on Mars or Venus, nor any life whatever on the moon. In this chapter we are going to consider intelligent life, and so we shall be concerned with regions of space far beyond the confines of our solar system, and shall be dealing instead with the planetary systems of stars other than our sun.

Do we really expect to find intelligent life out there? Well, man has often assumed that he occupies a special position in the natural scheme of things and often with humbling results. For example, it was apparently once thought that the Mediterranean was the centre of the world. Later, when the earth's ball-like character was appreciated, it was thought that the earth was at the centre and that the sun and planets moved about the earth. Then it was realized that the earth was a rather minor blob of stuff moving around a mighty central sun, and in due course we have learnt that our sun is in no way privileged, but is rather a common type of star moving with a billion others around the centre of our galaxy. Until quite recently it seemed that our galaxy was larger than other galaxies, but the earlier experiences were enough to give cause for suspicion, and it was a relief when, more recently still, further astronomical studies made it possible to demote our galaxy to a more mediocre status.

So we see that it has been a good guiding principle to assume that we are not specially privileged, or unique, and on this reason-ing alone we would be more prepared to accept the existence of other intelligent life in the universe than to deny it.

However, the cogency of this reasoning has been offset in modern times by opinion regarding the origin of the planetary system. Every-

131


one has heard the theory that the earth and other planets were formed as a result of a collision, or near collision, between the sun and another star, and other catastrophic origins have been discussed, which have permeated our intellectual environment for over a century. One can estimate how often such collisions could occur and the conclusion is that they are extraordinarily rare. Hence, if the earth really did have its origin in this way, we might really be here on our own.

For various reasons, it is now no longer generally believed that the earth originated in this way. For one thing, modern calculations have shown that the fragments drawn out of the sun by a grazing star would fall back into the sun. Another difficulty is to explain how 98 per cent of the angular momentum of the solar system is contained in the planets, and why only two per cent resides in the sun itself. For these reasons, other types of explanation have been studied, and it is now widely considered that both the sun and planets condensed simultaneously out of the one original gas cloud. In other words, instead of a catastrophic mechanism, we now enter-tain one whereby the earth and planets are here as a normal by-product of the process of star formation. We think all stars were formed in this way; consequently, planets should be a more or less general accompaniment of stars.

We do know that many stars are double, or have even more components, and this is explained by the accidental details of how the original cloud condensed. We also think that sometimes stars were formed that were not much more massive than Jupiter, our largest planet, and that there is no basic difference in origin between planets and stars.

The very massive stars, those that condensed from bigger gas clouds, have been found to be rotating about 50 times faster than would have been expected by comparison with the sun.

Possibly, the strong gravitational attraction and physical extent of the central condensation allowed it to engulf the forming planets, and to keep the whole angular momentum of the cloud. We there-fore think that the rapidly rotating stars have no planets and that the rest do.

We can now make an interesting application of the principle that we are only average. Of the other intelligent communities that

LIFE IN THE GALAXY 133

have evolved on other planets in the galaxy, some will be more advanced technologically than we are, and some will be less advanced. But those that are more advanced may be very much more advanced indeed. We have only to look at the rate of tech-nological development to see that it is accelerating, that the advances of the last century outdistance those of previous millennia and that the advances of the last two decades surpass those of the preceding century. I am referring here to advances in our understanding and control of our physical environment, and not to moral or political advances. It is impossible to predict what will have been done a century hence. The weather may be under control, the night sky may be illuminated, there may be international television with simul-taneous machine translation, disease may have been eliminated, and so on.

Let us now go on to consider what we could do to make contact with these communities. If we embark on exploration of the neigh-bouring planets, will we find them? The answer to this is given by the experience of Columbus. He did not find in the Americas a civilization that was technologically more advanced than that of Europe. Had there been one there, it would have discovered Europe. In general then, when we go out exploring, we discover inferior things—for example, we expect at the most to discover lowly forms of vegetation on Mars. The startling conclusion to this reasoning is that the more advanced communities, whose existence we have surmised, ought to be here discovering us. Have they discovered us, were they here long ago, and if so, might they have left some sign of their visit? These questions will now be followed up.

THE GALACTIC CLUB

As there are about one billion stars in our galaxy, the number of planets would be about 10 billion, if astronomers are right in thinking that stars like the sun normally possess planets. Now not all of these would be habitable, some would be too hot and some too cold, deoending on their distance from their central star; so that on the whole we need only pay attention to planets situated as our earth is with respect to the sun. Let's describe such a situation as being within the habitable zone.


This is not to imply that no life would be found outside the habitable zone. There may very well be living things existing under most arduous physical conditions, and it has often been con-jectured that some exotic forms of life might depend on the chemistry of the silicon atom, instead of the carbon atom on which all terrestrial life depends. But carbon is a plentiful atom all through the visible universe and we may confidently expect the bulk of living things to have made use of the rich chemistry of molecules containing carbon. However, if there are any silicon communities it would certainly be fascinating. Instead of breathing out carbon dioxide as we do they would breathe out silicon dioxide, which is sand.

After elimination of frozen planets, and planets sterilized by heat, we estimate that there are about 1010 (or ten thousand million) likely planets in the galaxy. We also leave out the planets of double stars because we do not think that such planets would remain in stable circular orbits, and at a steady enough temperature, for the millions of years needed for organisms to evolve.

Of the 1010 likely planets, we frankly do not know how many of them support intelligent life. Therefore, we explore all possibilities, beginning with the possibility that intelligent life is abundant and in fact occurs on practically every likely planet. In this case, the average distance from one intelligent community to the next is 10 light years. For comparison, the nearest star, of any kind, is about one light year away.

Ten light years is a very large distance. A radio signal would take 10 years to cover the distance, and as far as is known at present, signals of every kind would take at least as long. Conse-quently, communicating with someone 10 light years away would not be like a telephone conversation, with its rapidfire question and answer. It would not be a conversation at all, but rather a two-way flow of information. It should better be regarded as contact between communities rather than between individuals, because human lives are not long enough for one individual to interact.

Before going into this, however, are we sure that we can send a radio signal as far as 10 light years? A definite answer can be given to this question. We are sure that space itself presents no im-pediment to the passage of radio waves, because we have already


received radio waves from much greater distances—not signals of intelligent origin, but naturally occurring emissions from radio stars. From the facts available it is clear that we can communicate over 10 light years by radio today if we use very powerful radio trans-mitters, very sensitive receivers and very large aerials such as the familiar large radio telescopes.

How do we first make contact? Well, first of all we must realize that it is pointless trying to contact communities less advanced technologically than ourselves. For instance, if they haven't got command of radio, the radio method would fail. But we have only had radio ourselves for a few decades and so any community that is lagging us, is unlikely to be of interest to us just now. On the other hand, the communities that are ahead of us are likely to be very much ahead of us in view of the accelerating rate at which technology develops. Furthermore, we should not expect that, when we do make contact outside, it will be the first occasion on which it has ever occurred. It will have occurred many times before, so that even as I write a chain of communication may exist between communities in the galaxy, who have passed the stage of development where we are. Furthermore, they are experienced at locating emerging communities such as ours and bringing them into the circle.

Consequently, we shall not be contacting them. They will be contacting us, but we must be on the alert to receive their signals. This is the reasoning behind project Ozma, an American enterprise aimed at seeing whether any attempt was being made from Epsilon Eridani or Tau Ceti, two nearby stars, to contact us by powerful radio transmissions. The 85-foot radio telescope at Green Bank, West Virginia, was pointed at these stars for a month but if there are any people up there radioing to us at this time, they were not received in this first attempt.

Thus a very intriguing guessing game is being played in which we depend on the superior powers of the other player (in this case on his superior radio transmitter power), and try to guess what he would presume us to do, when we arrive at the stage of surmising that he is there. We have not yet made this contact, but I believe we are on the eve of plugging in on the galaxy-wide communication network.


Whether project Ozma was the right move, and whether there are other things we should do, will now be considered.

THE LIFE PHENOMENON

We have seen that conditions suitable for intelligent life are believed to be widespread in our galaxy, and the main gap in our knowledge is that we don't know whether life has evolved on the planets where the conditions are favourable. But there is no reason to think it couldn't happen elsewhere if it has happened here. Of these other communities, some would be more advanced technologi-cally than ourselves, some less so, and just' as we expect some day to make contact with other communities, so others will already have done so and created a network of communication that is in existence now.

If they are so advanced, will they be interested in us? Well, some of the intelligent communities may have developed a Yoga-like philosophy and be spending their time merely in meditation. Others may have solved all political problems and just be watching tele-vision. We had better limit ourselves here to communities that have not lost their scientific curiosity about the universe they inhabit.

Now it has been suggested that we should be very careful about making contact with other civilizations because they may want gold or some other valuable mineral that is found here, or they may just want us for beef cattle. But I do not think that this is a serious risk because of the enormous cost of transporting material objects over inter-stellar distances. It is undoubtedly cheaper to synthesize steak from its elements or to be a vegetarian than to import meat from another star. Very large rockets are needed to lift even small things out of the earth's gravitational well.

The most interesting item to be transferred from star to star is information, and this can be done by radio. I think that the infor-mation we could furnish would be valued by some other scientific community. After all, we send expeditions to inhospitable places to explore them, and are contemplating expeditions to the moon. But on our expeditions, we have to extract the information we want by laborious observation. How much richer the return would be to outsiders investigating our earth to receive our full report on


the nature of our planet. How much more worthwhile antarctic expeditions would be to us if the penguins had kept weather records.

What would these other people be like? For the purposes of the present thread of reasoning it is not necessary to know. We can be content to discuss communities that are more advanced in control of their physical environment than we are, and we can say sufficiently precisely what we mean by this, namely that they should already have gained the ability to launch rockets from their planets, send out radio waves, explode atom bombs and to understand all the other things that man on earth has achieved.

Still it is nice to speculate on their physical appearance. They might be as different from us as dinosaurs or dolphins, or, to be more extreme, they may be like ants or mosquitoes, or even bacteria.

Now it has sometimes been argued that man as a tool-using animal was greatly favoured by possessing hands and prehensile fingers, and that dogs, for example, were handicapped by inability to pick up a stick and to use it as a weapon or implement. But I think that the brain is more important and that other intelligent beings not only need not have hands but may be very strange indeed. It is hard, of course, to imagine living communities that are quite unlike anything we have seen. But for all we know the inhabitants of some other planet may be spherical — just round balls—and may have adopted that shape because of peculiarities of their physical environment. Instead of handling things as we do, they might have to ingurgitate them and manipulate them as we can manipulate things with our tongues. Perhaps their tongues would be luminescent and there would be an eye in the roof of their mouths, or a microscope. Such speculations may seem most implausible, but we may be sure that the facts would be at least as strange.

The key thing as regards future contact with advanced com-munities is that they should have gained control over their environ-ment by understanding it, just as we are doing on earth. These are the only communities at stellar distances that we may hope to contact, and in due course they may inform us about their physical shape, and vice versa. This will not be as important a part of the information exchange, however, as the flow of more funda-


mental knowledge about the universe. It is on our understanding of nature that our control of it depends; for example, our ability to look inside ourselves with X-rays and take the proper steps to keep ourselves healthy, depends on our knowledge about electron beams and the structure of atoms. So it is apparent that the acquisi-tion of new information from a more advanced planet would be a tremendous experience in the culture of the human race.

Just what our destiny is as a biological phenomenon of the physi-cal universe, no one knows, but it may very well be that we are to play a role in a grander, galaxy-wide, production than we have envis-aged. Life, as an undeniably present phenomenon of the galaxy, may have more than an ephemeral significance and intelligent conscious-ness may ultimately exercise control over the evolution of the galaxy. It is clear that humans have changed the face of the earth, as may be seen by flying over it, and we may soon be extending this influence to the moon, Mars and Venus.

The extremes to which some scientists think the life phenomenon may go in altering its surroundings are exemplified by Dyson's generalized Malthusian hypothesis. He says that we are converting the inorganic matter of the earth into living protoplasm and will not stop until we have utilized all the earth and all the available power that comes from the sun.

MESSENGER PROBES IN SPACE

Although there is reason to think that we are not alone in the galaxy, and that there are other communities more advanced than ourselves in their control over their environment, we do not know how far it is to the nearest one. If such life is abundant, the nearest could be at a distance of about 10 light years. In this case, radio signals offer a good means of making contact, and attempts to receive such signals from Epsilon Eridani and Tau Ceti have already been made.

But if life is less abundant, and the nearest, more-advanced community is, say, 100 light years away, then things are different. Making contact will be more difficult, although once contact is made radio communication will be feasible. The difficulty lies in knowing where to beam the radio signals. There are about a


thousand equally likely stars within the spherical volume with 100 light years radius, and so we have to face a difficult choice. Further-more, the other party has to choose between the thousand equally likely candidates centred on itself. The probability that we are listening in their direction at a time when their signals are arriving in our direction clearly works against success. Consequently, it seems to me that some other technique might be more effective, especially as, if such a signal were received, the answer would arrive with a 200-year round-trip delay, at least, which seems a very pre-carious way of initiating relations.

To obtain a clue as to conceivable moves that might be made, we might consider what we ourselves are now planning. As is known, space probes have already been despatched towards Venus and others to Mars will shortly depart. Plans for flights to the orbit of Jupiter and beyond are being worked on, and several types of low-thrust engine suitable for sustained journeys of many years' duration are being tested. It does not seem unlikely that by the end of the century a space probe may have left our solar system for the nearest star. Such a flight would take a very long time, and the full information resulting from it would not become available in the lifetime of the launchers of the probe. Although there are prac-tically no human enterprises that are planned ahead so far, we shall have, of necessity, to contemplate such long-lived projects in connection with stellar exploration. And I think that such an imaginative project would have a good chance of being supported with the necessary funds.

Various items of interest about conditions in interstellar space would be signalled back by the probe while it was in transit, but I wish here to draw attention to one good and very simple experi-ment that could be performed by the probe on arrival at its destina-tion. Suppose that a reserve rocket was used to kick it into orbit about the distant star. It could then attempt to detect the presence of technological life, by listening for radio stations. To do this, it would not need to know where the planets were, but might hope in the course of a few years to pass close enough to one to pick up any radio waves emanating from it. It is very hard to think of a simpler and more positive experiment establishing the existence of intelligent life. The question of furnishing the complicated equip-


ment for locating a planet and the massive rocketry needed to approach and land on it is completely by-passed. What if the probe heard evidence of radio communication? It could signal back to earth. But it could do another very simple and very exciting thing. It could make its own existence known by playing back some of the transmissions it intercepted, on the same wavelength. A moment's thought will show that the recipient of the original trans-mission would be aware of something resembling an echo.

Now the point of this digression about our own plans was to help in suggesting clues as to possible moves that might be made to attract our attention to the presence of a galactic chain of com-munities, the nearest of which was so distant that direct first con-tact by radio seemed hopeless. My proposal is, then, that messenger probes might be launched to the thousand surrounding stars. One might be in our solar system now, and if this is the case then we should be very careful not to overlook unexplained radio signals that may be received. There is a great danger of such an oversight, because radio operators and other users of the radio spectrum are listening for some particular programme and deliberately reject the unexpected. It would be a tremendous experience to be the recipient of the first message from outside.

In view of the great inconvenience occasioned by 200-year round-trip delays in direct communication over a distance of 100 light years, there would be a great advantage in loading the messenger probe with lots of information to convey to any listener with which it established contact. In view of the present and projected develop-ments of automatic computers and miniaturisation techniques I do not think it is exaggerated to expect that a tremendous fund of information could be stored in a computer the size of a man's head. Indeed, it may not be unreasonable to assume that suitable prepro-gramming could produce, to us, the appearances of dealing with an intelligent being. If we contemplate the resources of biological engineering, which we have not begun to tap yet, it is conceivable that some remote community could breed a sub-race of space messengers, brains without bodies or limbs, storing the traditions of their society, mostly to be expended fruitlessly but some destined to be the instruments of the spread of intragalactic culture. Such a procedure would be unacceptable with us; we would prefer to


fabricate such a brain from inanimate material by the micromini-aturization techniques of molecular electronics. My main point is that a probe encountered at stellar distances from its place of origin may be expected to be packed with information and to be capable of reacting intelligently to interrogation.

Now it may be that even 100 light years is an under-estimate of the distance to the nearest more advanced community, and in that case I think the contacts will be made between probes—a computer from one planet engaging with a computer from another. This is a rather humbling thought for us humans. I can see these probes being launched in all directions from the parent planets, making occasional contacts, reporting back home, until ultimately the home planets are in direct communication. The attenuation of signal strength over such great distances presents no insuperable difficulty, once contact is established, since the relay principle, already used in trans-oceanic cables and telephone lines, is available also in space.

Now if our nearest neighbour is as far away as 1000 light years the situation begins to change. This would mean that there were only about 2000 advanced communities in the whole galaxy, or only one in ten million of the likely planets; and this has a surprising implication. We don't know how long it would take for life to evolve to the technological level on another planet, but we do know that in our case it took about one thousand million years. Suppose this is so in general. Then most of the ten thousand million planets resembling the earth will be supporting nothing more than primitive microbes, with millions of years of evolution still ahead of them. When they ultimately achieve our level of technology, it will flourish for a time, let us for the sake of argument say 500 years, and under these conditions about one in 10 million of the likely planets would be in flower at any given moment. There would be only about 2000 in all, spread throughout the galaxy, and from this we can verify by calculation that the distance to the nearest more advanced community would be around 1000 light years. If it is true, therefore, that technological communities are so rare that it is as much as 1000 light years to the nearest advanced neighbour, then such rarity may be ascribable to the lack of longevity of technological com-munities. It would imply a lifetime of 500 years beyond the stage we have reached. Of course you will have noticed that this would not


give us time for even one round-trip exchange, since our community would be extinct before the first answering message could arrive, and so we would never join in an intragalactic club.

It is a solemn thought that after the expenditure of so many years to evolve into a state of consciousness of the surrounding universe, and to gain partial control of its forces, technological communities may be going off pop in different parts of the galaxy, without ever knowing their neighbours, at the rate of one or two a year.

Many natural reasons leading to the extinction of living com-munities can be mentioned; most, however, involve the long time scales of geological change. For example, the climate must ulti-mately deteriorate to a point where it would be unlivable for our present society. But a short lifetime is also conceivable if the development of technology contains within itself the seeds of destruction. Unfortunately, it is only too apparent at the present time what this seed might be.

It seems at present to be a matter of chance whether we succeed in stabilizing the political situation, but apparently the achieving of this stability for a long enough period is a prerequisite to forma-tion of, and membership in, a galactic chain of communities.

CHAPTER 1

Introducing Proteins

Biology, the science of life, has been studied since the time of Aristotle, but until recently it has occupied a rather peculiar position in the eyes of those trained in chemistry and physics. The reason for this is not difficult to see. What, essentially, is the central problem of biology? To illustrate, here (Figure 1) is a biological object, which Rappens in this case to be a female of our own species. This biological object is composed of organic molecules, but at the same time shows an astonishing and interesting complexity of form and behaviour. Our problem, basically, is how a collection of organic molecules which obey the laws of physics and chemistry can do so. For a long time this problem appeared to be so intractable that physicists and chemists relegated it to the limbo of so-called "inexact" sciences, sometimes implying that it pertained more to metaphysics than to science.

Within the last hundred years, however, our knowledge has greatly increased, and at the present time advances in biology are being made so rapidly that many scientists believe we are well on the way toward understanding the functioning of biological objects. I am going to try to tell you how this problem appears to us at the present time, and what seem to be the prospects for the future.

While it is true that living organisms are composed of matter which we believe obeys all the laws of physics and chemistry, all of us have an intuitive idea that there is something peculiar and different between matter which is alive and that which is dead. Let us start by asking what is it that gives rise to this feeling.

145


Figure 1.—A biological object composed mainly of water and organic molecules.

INTRODUCING PROTEINS 147

Figure 2.—Bacteria (Bacillus cereus) growing in a nutrient medium.

I present to you a number of illustrations. In Figure 2 you see a picture of some bacteria growing in a nutritive medium. To the biologist this is a familiar sight, but what is not obvious from the picture is the way these small organisms are behaving. Penicillin has been added to their environment, and to them, of course, this is a violent poison. Some bacteria are very stupid in a situation of this kind; they simply absorb the poison and die. These bacteria, however, are behaving differently. As soon as they are aware that penicillin is present, they begin to produce a substance called penicillinase which rapidly destroys penicillin. They only do this if penicillin is present: when it is absent, they do not squander their energy to produce this counter-toxin. This behaviour strikes us as purposeful and, in a rudimentary sort of way, intelligent.

In the next picture {Figure 3), you see a moth resting on the trunk of a tree. The interesting thing about its behaviour is that it has taken up a position so that the markings on its wings blend


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in with the markings on the tree trunk, making it very hard to see. The purpose of this is to make it more difficult for its enemies, the birds, to detect. Its behaviour again shows a certain purpose, which is to avoid capture.

Next {Figure 4) you see a very interesting animal, a species of ant which lives in Brazil. These ants cut leaves into a definite form, transport them to an underground chamber, and inoculate


Figure 4.—Ants who practise agriculture. In A, a worker picks up its faeces containing fungus spores; and in B, it is placing them on a cut leaf. Below is a diagram of the underground chambers in which the cut leaves are kept. After the fungus has grown on the leaf, it is used as food. The leaf remains are then removed and replaced by fresh leaves. (After Berlese.)

them with the spores of a certain fungus or mushroom. The fungus grows on the leaf and after a while is eaten by the ants. These animals are practising a form of activity which we call agriculture and which we regard as purposeful.

Next (Figure 5) you see a work of civil engineering. A group of beavers has constructed a dam in the American Northwest. This dam has created a small lake. From the shores of this lake, the beavers will dig a system of canals through the neighbouring growths of trees. The trees will be felled and floated through the canals to the houses the beavers have built in the lake. The bark will serve as food while the timber will be used for further building and repair. Again, we say that the beavers are working for a purpose which we can easily understand.

In Figure 6 you see another animal, Homo sapiens, moving rock and earth, also to construct a dam. The purpose here is to back up the Nile at Aswan. This will irrigate a tract of territory, causing a large variety of plants to grow which will be used by these animals for food or exchanged with other animals of the same species for desirable objects.


Figure 5.—A dam constructed by beavers in Northwestern North America. (Courtesy of Professor R. T. King.)

In all these examples, the living objects, from bacteria to man, give some indication of purposive behaviour and intelligence. This behaviour, for the most part, is what makes us feel that there is a profound difference between the living and the dead.

For a start let us accept that this feeling characterizes some-thing real, and let us therefore define a living thing as a material object which behaves in a purposive and intelligent manner. We know by introspection that we are included in this category. Some living things, for example, jellyfish, turnips, and bacteria, show


Figure 6.—Building a dam at Aswan on the Nile. (Wide World Photos, Inc.).

purpose or intelligence to a lesser degree. To prevent argument in doubtful cases, let us expand the definition by adding: and such other objects as are obviously related to the above through organic evolution.

Those of you who have some acquaintance with science will notice that this is a rather unusual definition. It is perfectly true that it includes all living things, as we ordinarily understand the term, and excludes the inanimate or mineral world. The objection to such a definition might be that purpose and intelligence have no place in science; they belong rather to metaphysics or theology. To this, I would give two answers. In the first place, science must take cognizance of everything that exists. If purpose and intelli-


gence exist, there is no use shutting our eyes and pretending that they don't. Secondly, removing purpose and intelligence from the realm of scientific inquiry is quite out of date. A large part of modern technology is devoted to the design of machines which show purpose and intelligence. Consider, for example, one such device. An anti-aircraft missile, once built and installed, will search out its target, compute its trajectory, approach its target no matter what evasive action the target might take, and finally destroy it. Clearly, it is as purposeful and intelligent, in its own way, as the beavers or men who build dams. A self-guiding missile is not ordinarily considered to be alive, but rather an example of "pure mechanism". If so, "pure mechanism" need not be stupid. On the other hand, intelligence cannot exist without some mechanism which manifests it. Intelligent or purposeful mech-anisms are merely a subclass of mechanisms in general. Since science investigates all mechanisms, it also must investigate purpose and intelligence.

We might well ask, however, whether such a definition of life is useful. Would it perhaps be better to define a living organism in purely chemical terms? My reason for not doing so is that a chemical definition tends to distract us from the essential point, the organism. An organism is primarily characterized by the way it behaves, not by the atoms of which it is composed. The chemistry of an organism is an important detail, but not the organism itself. To return to our mechanical example, it is certainly very important to know the chemical properties of the propellant of a missile, but it is even more important to keep in mind that this is merely a step toward understanding the missile's function.

Granting this, our example, the missile, nevertheless shows that to understand its function fully, it is necessary to understand its construction. Since the organism is a chemical system, let us therefore start our investigations in biology with an inquiry into the chemistry of living matter. Much has already been achieved by this approach.

It is not difficult to determine from what kinds of atoms living matter is made. By drying it, we find that between 70 to 80 per cent of it is water. The rest is carbon, hydrogen, oxygen


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and nitrogen, with smaller amounts of phosphorus and sulphur, and a few metallic elements.

These facts have been known for a long time, but by them-selves they tell us very little. Those of you who have studied chemistry will understand why this is so. Atoms, in general, do not exist free, but are linked to each other to form molecules of what we call chemical compounds. As you well know, it is possible to use the same pile of bricks to build very different kinds of houses. In the same way, identical atoms can be linked


together to form very different compounds. Take, for example, a compound whose molecule contains four atoms of carbon, ten of hydrogen and one of oxygen. These atoms can be arranged to form several kinds of molecules, two of which are shown in Figure 7. One is ethyl ether, well known for its use in anesthesia; the other is butyl alcohol. You can see from the legend to the figure that these two substances are very different from each other, the difference arising from the way the atoms are arranged.

What we want to know, therefore, is what types of compounds, rather than what elements, are present in living matter. For a start, let us do the following experiment.

Take any kind of living material, say the liver, and grind it into a homogeneous paste. Now take a piece of sausage casing in the form of a tube. While you cannot see them, this cellulose casing is permeated by extremely small holes of molecular dimen-sions. The holes are so small that larger organic molecules are unable to pass through the casing; yet they are sufficiently large so that the smaller molecules, for example sugar, can pass through quite readily. You can see this from the demonstration that has been prepared for you {Figure 8). This particular casing has been filled with a red dye which consists of molecules of rather small size. When we suspend it in a beaker of water the dye passes through this material and diffuses into the surrounding water. In the other exhibit, we have placed some hemoglobin, the red oxygen-carrying substance of the blood. The molecules of this substance are very large and in this case the red colour does not pass through the walls of the tubing.

If we take our liver-mash and place it in a similar tubing, we can observe, using somewhat more refined methods, that some of the material will pass through to the outside water. This consists of various salts and small organic molecules of various kinds. However, the bulk of the material is unable to pass, indicating clearly that whatever it is, it consists of particles or molecules of very large size.

What is this material of large size? To answer this question, we need some method to separate it into its various components

INTRCDUCING PROTEINS 155

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and determine their properties. A variety of methods to do so exist, but for the moment, let us confine ourselves to a method which is in principle very simple. Suppose I have a mixture of a very fine powder and some grains of sand. When I put it into water, you may observe something that is very familiar. The fine white powder remains suspended for a considerable length of time while the large grains of sand immediately sink to the bottom. Here we are using a method of separating things according to their particle weight. The large particles sink rapidly because their surface area, and thus the resistance of the medium to their motion, is small in relation to their mass. For small particles, the reverse is the case. In theory it should be possible to separate molecules according to their sizes in the same way. In fact, molecules will not fall downwards in a resistant medium because the force of gravity on them is too small to counter-balance the random impulses, coming from all directions, of the molecules of the surrounding medium. However, it is quite possible to produce artificial gravitational fields which are enormous compared to the one we are accustomed to. This is done by putting the material into a centrifuge and spinning it very rapidly. With modern machines, it is not at all difficult to produce gravitational fields one hundred thousand or more times that found on the surface of the earth. If we put a solution of our large molecules into such a centrifuge this gravitational force is already large enough to cause our larger molecules to move to the bottom of the tube. Assuming that the molecules have the same shape, it is possible to obtain an estimate of their size and in fact, to separate one size from another. We find that many different sizes of molecules are present in our liver-mash. In general, these larger molecules have a weight somewhere of the order of a hundred thousand times or more of the weight of the hydrogen atom.

Having separated our very large molecules, we can now ask what they are made of. The standard method of answering this question is to take some of this material and degrade it into smaller molecules, say by boiling it in dilute mineral acid. The degradation products, which are smaller molecules, can be separated and identified. Great advances made recently in chemistry and biology have been made possible, in part, by the development


Figure 9.—Paper chromatography. A mixture of substances is placed on the paper at A, and one end of the paper is immersed into a suitable solvent. The solvent rises up the paper and carries the different substances up at

different rates. Each substance moves as a separate spot.

of simple and rapid methods of separation. In Figure 9 you have an example of a modern technique called paper chromatography. A mixture of various compounds (here chosen to be coloured) has been applied to one end of a paper strip. One end is then immersed into a suitable solvent, generally a mixture of water and some alcohols or acids. The solvent then percolates by capillarity


through the paper and carries the various substances along with it. However, owing to differences in solubility and to the firmness with which different substances are absorbed to the paper, they move at different rates, forming the various spots you see on this paper. In this manner, or by using other methods, it is possible to separate substances which are chemically very similar to each other.

If now we degrade our large molecules with dilute acids or alkalis, we find that for the most part they can be divided into two different types, depending on the kind of smaller molecules they yield. Most of the large molecules are proteins, which when degraded give a mixture of 20 different compounds, all of which are known to the chemist as amino acids. A smaller number, perhaps 5%, of the large molecules, called nucleic acids, give completely different products. Suitably analyzed, these products are compounds called nucleotides, each of which in turn can be shown to consist of an organic base linked to a small sugar molecule which in turn is linked to one molecule of phosphoric acid. There are two types of nucleic acids, one composed of four such nucleotides and another of four slightly different nucleotides.

I have given as an example the large molecules derived from liver. If we do the same sort of analysis on other material, say bacteria, viruses, cabbages, jellyfish, kangaroos, or man, we always obtain the same result. The large molecules of all these organisms are always proteins which are composed of the same twenty amino acids and nucleic acids composed of the same four nucleotides. Less than 30 of the same small molecules are used to build up the large molecules of any type of organism that occurs on this earth.

Our preliminary analysis thus leads us to an unexpected and rather astonishing conclusion. The number of compounds which are the building blocks from which living matter is compounded is not, as one might have expected, large, but is quite limited. How then does the great variety of living things, and their varied behaviour, arise from so small a number of fundamental building blocks? The answer is that these building blocks, like the atoms,


are arranged into "supermolecules" and a vast number of different arrangements are possible.

These "supermolecules" are of course the large molecules we called proteins and nucleic acids which did not pass through the sausage casing. Temporarily, let us ignore the important nucleic acids, which will be discussed by Dr. Watson, and turn our attention to the proteins. How exactly are the building blocks arranged to produce proteins?

To a chemist, proteins are examples of a kind of molecule called a polymer, although a polymer of a rather peculiar kind. Within the last thirty years, we have learned a great deal about the chemistry and physics of polymers, since synthetic polymers are now produced industrially on a very large scale. Examples of polymers are synthetic rubbers, various plastics and nylon.

To produce a polymer, a chemist starts with a small organic molecule which has two reactive ends, shown in Figure 10 as A and B. These are of such a nature that A and B are able to

A

^ · \ Α ^ \ # - B

A

• · / \ / \

••••A B A B

B /

A - ·

1 β

• / \

A B-

• /

/ A

V B

• / \

-A B

B

• / \

-A B·· · ·

Figure 10.—Monomers, or molecules with two groups which can join up with each other. When they have done so, they have polymerized to form a long chain, or polymer. Plastics, synthetic fibres and proteins and nucleic acids are molecules of this general type, as are such substances as rubber,

cellulose and starch.

form a stable bond between them. Because A and B are sufficiently far apart, they cannot react when they are on the same molecule. However, the chemist can so arrange conditions that A and B on different molecules can react and hook on to


each other. The single molecule is called a monomer, two linked together a dimer, when many are linked together it is a polymer. The process of linking is called polymerization. A polymer, there-fore, is a large molecule made up of similar or identical units arranged in a chain.

When chemists make a synthetic polymer, they generally use a single kind of monomer unit. As a result, their products have a certain monotonous quality, the only possible difference between one large molecule and another, given a monomer unit, being the length of the chain. If he uses more than one type of monomer unit, he can add some variety, but in this case his polymer units are a random sequence of the monomers he uses. Since the organic chemist is mainly, so far, concerned with polymers which have a structural function, he is not unduly concerned with this monotony. Nylon serves its purposes very well.

Proteins, as I said, are also polymers, but they differ in one important respect from the polymers of the organic chemist. Instead of being built from one kind of monomer unit, they are built from 20 different kinds. These units are the organic molecules called amino acids. Each amino acid molecule can be regarded as having three parts. One part is an organic (so-called carboxylic) acid, another is a basic amino group. These two groups occur in every amino acid. In addition, there is a third group, which differs from one amino acid to another and this group may be, to use the terminology of organic chemistry, aliphatic, alcoholic, carboxylic, aromatic, phenolic, heterocyclic or basic. The 20 amino acids are illustrated in Figure 11.

The two groups in amino acids which are active in poly-merization are the acid and basic groups present in every amino acid. The carboxylic acid group can react with the amino group of another amino acid as shown in Figure 12, splitting off water, to link the two molecules together. This linkage is, technically, an amide bond, but is more popularly referred to as a peptide linkage. In this way, long chain polymers of amino acids, called proteins, can be produced. (For Figure 12, see p. 162.)

All this, so far, is not difficult to understand and to produce in the laboratory. But now we come to the peculiarly biological part of the problem. As I said above, if a chemist uses more


H C — H

/ \ NH2 COOH

GLYCINE N

HCH H 1

HCH HCH

I 1 C- -H

NH2 COOH ISOLEUCINE

SH T

HCH

1 C - H

NH 2 COOH

CYSTEINE

0 j

HCH 1

/ C \ H NH 2 COOH

PHENYLALANINE NH2

NCH 1 HCH

HCH I

HCH ^ C - H

N H 2 ^ ^ C O O H LYSINE

H HCH

1

1 C—H / \

NH 2 COOH

ALANINE H

s — S H

1 H

HCH 1

HCH

C - H NH2 ^ C O O H

METHIONINE

H H HC CH

1 1 HCH C - H

\ / \ NH COOH

PROLINE OH

0 | HCH |

/ C \ H

NH2 COOH TYROSINE

1 HCH HCH

| HCH ^ - C - H

NH2 NCOOH

ARGININE

H H HCH HCH

" ^ C ^ H 1

/ c \ H

NH2 COOH

VALINE

OH 1

HCH 1 1

^ C ^ H NH2 ^ C O O H

SERINE

COOH |

HCH 1

/ C \ H

NH2 COOH ASPARTIC ACID

N I I H

HCH 1

/ C \ H

NH2 xCOOH

HISTIDINE

C — NH 2

1 HCH

| C —H / \

NH2 COOH ASPARAGINE

H H H C H ^ ^ ^ C H

^ C - ^ H

HCH I C—H

NH2 COOH

LEUCINE

H HCH

| HCOH

1 C—H

/ \ NH2 COOH

THREONINE

COOH 1

HCH I

HCH

NH2 COOH

GLUTAMIC ACID

H N C I HCH |

/ C \ H

NH2 COOH TRYPTOPHAN

C — N H 2

• HCH 1

HCH 1

NH2 ^COOH GLUTAMINE

Figure 11.—The twenty amino acids which are the monomer units from which proteins are made. Note that all have a similar structure, shown in hold

faced type, to which different chemical groups are attached.

than one kind of monomer unit, he produces polymer molecules where the order of arrangement of the monomer units is random. In proteins, however, each kind of protein has a fixed sequence


R| H R2 H R, H R2 H

V + V _ V V +», / \ A y\p /\/> /\,p '

HN C-IOH HH C-OH HN C — N C^OH H *· -H H H

Figure 12.—Two amino acids can join together by linking their amino and acid groups and splitting out water, forming a peptide bond, here shown in bold face type. When many amino acids have polymerized to form a long

chain, the chain is called a protein.

I S S

i I gly.ilu.val.gln.glu.cys.cys.ala.ser.val.cys.aer.leu.tyr.gln.leu.glu.asn.tyr.cys.asn

I I 3 S 1 / s s 1 I

phe.Tal.aen.gln.hie.leu.cys.gly.ser.his.leu.val.glu.ala.leu.tyr.leu.val.cys.gly.glu.arg.gly.phe.phe.tyr.thr.pro.lys. Figure 13.—The sequence of amino acids in a very simple protein, insulin. Note that the protein is made up of two chains. These chains are held together by sulphur to sulphur bonds. A sulphur to sulphur bond also

exists between two parts of one of the chains, throwing it into a loop.

in which the amino acids occur in the chain. Figure 13 shows the sequence in a very simple protein, insulin, which is famous because it was the first protein in which the amino acid sequence was determined. One of the great problems of modern biology has been to understand how an organism is able to produce such fixed sequences. Every organism makes several thousand different kinds of proteins; and in each protein, the amino acid sequence is determined with complete accuracy. Recently, we have begun to understand how this is accomplished, and Dr. Watson will tell you what we know of the details of the process.

You might well ask at this point whether the exact sequence of amino acids is really important and why we are so anxious to know how this exactitude is attained. The answer is that the exact sequence differentiates one protein from another, and even a very small change in the sequence can make quite a difference. Let me illustrate. A man may suffer from a hereditary disease called sickle cell anaemia. As a result of his unfavourable hereditary endow-ment, the red oxygen-carrying protein in his blood, hemoglobin, is slightly different from normal. One out of about 300 amino


acids in each molecule of this protein has been replaced by another. As a result, when his hemoglobin has given up its oxygen to the body tissues in his capillaries, it tends to form crystals. The formation of such crystals decreased the life span of the red blood cells which carry the hemoglobin. Because of this, the number of such cells in his blood is below normal, and he suffers from a severe anaemia, or deficiency in hemoglobin. Individuals with this defect are unlikely to live past 30. A very slight change in the amino acid sequence of one protein makes the difference between normal life and sickness and early death. Obviously, the amino acid sequence is important.

We now believe that the differences between different kinds of living things are due to differences in the kinds of proteins they produce. These differences are responsible for the differences in their chemical functions, and, we think, also responsible for their differences in form. This last, it must be admitted, is a matter of faith because as yet we do not understand how differences in protein structure produce differences in the form and behaviour of an organism. Nevertheless, assuming this to be so, it is interesting to enquire how many possible kinds of proteins are there? This question is easy to answer. If we start to make an arbitrary protein, the first amino acid we choose can be any one of 20 different kinds. The second amino acid can again be any one of 20. Thus there are 20 x 20, or 202 different possible combinations of 2 amino acids. If we take a protein to have 100 amino acids (many proteins have more), there are 20100, or very closely 101™ different kinds of proteins possible. This number is so large that if all the carbon and other required elements in the observable universe were used to form proteins, making only one molecule of each kind of protein, there would not be nearly enough material to make them all. Clearly, therefore, the number of kinds of possible proteins, and therefore of kinds of possible organisms, is enormously greater than the number that has ever existed or that will ever exist in the future. There simply is not enough time and material in the universe to exhaust all possibilities.

Even the enormous number 10130 actually understates the possible number of kinds of proteins. Up to this point we have considered proteins to be straight chains of amino acids. In fact,


Figure 14.—A schematic representation of the folding of the protein chain of myoglobin. The structure at the top is the heme group which reacts

with oxygen. (Courtesy of Dr. J. C. Kendrew, F.R.S.)

however, these chains are bent into special configurations. Recently, in a few cases, it has proved possible to determine how such chains are actually arranged in space. Figure 14 shows how the atoms of a relatively simple protein, myoglobin, are arranged. There is no obvious reason why the same sequence of amino acids cannot be bent into a variety of configurations, so the number of possible proteins, taking into account their shape, might well be 10iao multiplied by some large number. The forms of possible life are, presumably, virtually infinite.

What, however, are the functions of the proteins? We shall consider this problem in the next chapter.

CHAPTER 2

The Functions of Proteins

As we have seen, it is the proteins which are the most characteristic components of living matter. Why are proteins important for life? To answer this question, let us examine what the proteins do.

Some proteins have a purely structural function. You are familiar with hair and wool. Hair is made of a special kind of protein, rich in sulphur, called keratin. This protein also forms part of the skin and other animal structures such as scales and feathers. A large part of the protein of our bodies is of a kind called collagen, which forms connective tissues such as the tendons by which the action of the muscles is transmitted to the skeleton. When collagen is boiled, it is slightly degraded and is then known as gelatin. Another example of a structural protein is silk. Caterpillars spin it around themselves as a cocoon tö protect them as they develop into moths. Our more elegant and wealthy women use this protein for clothing.

These structural functions of proteins are important, but in a sense are secondary and do not explain the great importance of proteins for life. The really important property of proteins is that they are chemical catalysts; and to understand the nature of life, it is necessary to understand something of the nature of catalysis.

Let me first present to you an example. In one beaker I have a solution of a special kind of protein called, appropriately, catalase. In another beaker, I have only water. Now I add to both beakers a dilute solution of hydrogen peroxide, or oxygenated water. As may be observed, nothing much happens in the water; but in the beaker containing catalase, there is a rapid evolution

165


of gas. The hydrogen peroxide is breaking down according to the chemical equation:

H202 *+ Η,Ο + O that is, each molecule of hydrogen peroxide is losing one atom of oxygen to give a molecule of water and an atom of oxygen. The atoms of oxygen then combine with each other to give molecular oxygen, a spontaneous reaction, so that to be correct we write the total reaction as

2H202 »-> 2H20 + 02. The protein is making the reaction go, or as we say, catalyzing it. Proteins which act as catalysts are called enzymes; and in biology it is customary to speak not of catalytic activity, as in cheftiistry, but of enzymatic activity.

Is this ability to make a chemical reaction go a mysterious property of proteins, or are there other examples? Indeed there are, and catalysis has long been known in chemistry. Let us take a simple example. Again I have two beakers. This time they contain a colourless compound called an ester, which can be broken down to yield two products by a process called hydrolysis. One of the products is coloured yellow. To one beaker I have added a small amount of hydrochloric acid; and to make the reaction go faster, I have heated both beakers. It can now be observed that the beaker to which I have added hydrochloric acid is turning yellow, showing that the ester is breaking up. Here hydrochloric acid is a catalyst, promoting the reaction.

What exactly is catalysis? To understand it, it is necessary to consider the nature of a chemical reaction. Suppose we take a simple case. As most of you know, there exist a large number of organic compounds which are classified as alcohols and another class which are organic acids. These two compounds can react with one another to form molecules of what is called an ester. The way in which this happens is illustrated in Figure 15. The OH group of the alcohol reacts with a particular hydrogen atom of the acid to form water, while the two molecules in the meantime join together to form a single molecule called an ester. This reaction, in which water and an ester are formed, is called esterification. The reverse action is also possible. An ester can add a molecule of water to give a molecule of the alcohol and the

THE FUNCTIONS OF PROTEINS 167

H H , 0 . H HC — C 0(H H 0 ) - C - C H

H H * - H

ALCOHOL V / ACID

H H 0 . H H20 " x HC —C — 0 - C - C H

WATER H H H ESTER

Figure 15.—Formation of an ester from an alcohol and an organic acid with evolution of water. The reaction can also proceed in the opposite direction, taking up water and splitting the ester into the alcohol and acid. The break-

down is called hydrolysis, a splitting by addition of water.

acid. This reaction is called hydrolysis, which means splitting by the addition of water. Both reactions occur and whichever one predominates depends on the conditions.

To some water let us add an ester, ethyl acetate, which is one made from ordinary alcohol, chemically known as ethanol, and vinegar, technically called acetic acid. The reaction is shown in Figure 15. At first only the ester is present so that the only reaction possible is breaking the ester by hydrolysis. The products of this reaction, ethyl alcohol and acetic acid, begin to accumulate.

Let us now start with the reverse procedure. To some water we add ethanol and acetic acid. Here the only possible reaction is the synthesis of the ester, ethyl acetate, or esterification.

This illustrates what the chemist calls a reversible reaction. Ethyl acetate can disintegrate into ethanol and acetate, and ethanol and acetate can combine to form ethyl acetate. How far do these reactions go?


If we start with a certain amount of ethyl acetate, and wait a few weeks, we find that the reaction appears to come to a stop. The amount of ethyl acetate in our flask gradually diminishes and ethanol and acetic acid accumulate. Eventually, the amount of these compounds reaches a constant amount. If to another flask, under exactly the same conditions, we add an amount of ethanol and acetic acid equivalent to the amount of ethyl acetate in the first flask, ethanol and acetic acid disappear and ethyl acetate accumulates. Again waiting for a few weeks, we find that the ratio of the components reaches a constant figure.

The interesting point is that in both flasks these ratios are the same. We call this the equilibrium concentration, a concentration which is the same whether we approach it by making ethyl acetate from ethanol and acetic acid, or whether we make ethanol and acetic acid from ethyl acetate.

The concept of equilibrium is not at all mysterious. It is due to the fact that at all times both reactions are proceeding simul-taneously: ethyl acetate is hydrolyzing, and is being synthesized. There is a certain probability that one reaction will take place and a certain probability that the other will take place. The ratio of these probabilities determines how much of each compound will be present when equilibrium is reached.

This can be illustrated by a simple game of cards. Take a hundred playing cards, fifty black and fifty red. Now deal fifty hands of two cards each. Let us say that hands which contain one black and one red card are molecules of ethyl acetate, and hands which are all black and all red are molecules of ethanol and acetic acid respectively. To start with, we can stack the pack so that, all hands have a black and red card. We now have a system containing only ethyl acetate. Having dealt this stacked pack, we pick it up and shuffle it. After shuffling it, we deal again. We find now that one-half of the hands have a black and red card, and one-half are of the same colour. Now start by stacking the pack so that all hands are either black or red so that we start with molecules of ethanol and acetate acid only. Again we pick up the cards, shuffle and deal. The result is the same as before; on the second deal we find that one-half of the hands are


all of the one-colour, and one-half are mixed. Going either way, we have approached the same equilibrium concentration.

The equilibrium concentrations of components depends on the ratio of black to red cards in the pack. If to our original pack we add a large number of black cards, shuffle and deal again, we will find that almost all of the red cards are now paired with black; that is, almost all molecules of acetic acid are now in the form of ethyl acetate. The analogy corresponds to the facts. If to a certain amount of acetic acid we add increasing amounts of ethanol, we find that a larger and larger amount of acetic acid is tied up in the form of ethyl acetate. The equilibrium shifts depending on the amount of the components present.

You will notice that whatever the equilibrium is, we approach it because we keep shuffling the cards. This shuffling process corresponds to the fact that the molecules are undergoing reactions back and forth, ethyl acetate breaking down to ethanol and acetic acid and ethanol and acetic acid combining to form ethyl acetate.

There is a further important point which is illustrated by our card analogy. The amounts of the various components are a matter of probability. If we have a pack which has only two hundred cards, the most likely result of a deal is that fifty of the hands will be mixed and fifty will be of the same colour. However, we may get a considerable deviation from this ratio if we use so small a pack. A sixty to forty ratio, for example, is not too improbable. If, however, we increase the size of the pack to a million cards, it will be very improbable that we would deviate significantly from the expected ratio. Now the number of molecules we ordinarily deal with is not a hundred or even a million, but numbers more like a billion times a billion. When dealing with numbers of this size, we expect that there will be no noticeable deviation from the expected figure, just as if we threw a coin a billion times we would expect no noticeable deviation from a ratio of 1:1 in the numbers of heads and tails that would turn up.

When dealing with large numbers of molecules, we can dis-regard random fluctuations. Providing that shuffling is taking place, we can predict in which direction the reaction will go; it will go from the less probable to the more probable state. We


say that molecules in a less probable state have a higher energy content, and those in a more probable state, a lower energy content, so that molecules will move from the less probable state, a state of higher energy, to a more probable state, one of lower energy. To produce molecules in a higher energy state, we have to stack the pack. Chemically this corresponds to doing work on the system. You will notice that so far as an individual molecule is concerned, it can move from a state of lower to a state of higher energy; this reaction is merely less probable than the reverse. It is only the properties of large numbers that make it impossible for many molecules in a larger system to do so simultaneously. Even here, impossible does not mean logically impossible, merely that this is so improbable that we never expect it to happen.

In summary then, we can say that unless work is done on a chemical system, it will spontaneously reach the equilibrium point, which is the lowest energy level of the system. Let us call a reaction proceeding towards equilibrium a "downhill" reaction, when proceeding away from it, an "uphill" reaction.

The next important point about equilibria is the rate with which we get there. Suppose we have dealt a very large number of hands from a stacked pack. We now pick up a certain number of hands, shuffle, deal these out and proceed to the next batch. After each shuffle, the hands on the table are closer to equilibrium. No matter how fast or slow we shuffle, the equilibrium eventually reached is the same, but obviously the rate at which we get there depends on the rate of shuffling. Even if we did not shuffle at all, the concept of equilibrium would still be meaningful.

This analogy, too, corresponds to the facts. Certain chemical reactions go to equilibrium very fast. If you put a match to gunpowder, in a moment it is all smoke and gases. On the other hand, some are very slow. The downhill reaction for iron, in the presence of air and water, is conversion to rust, or ferric hydroxide. This reaction proceeds rather slowly, although still too fast for some people's liking. There are other reactions which are even slower. Thus, there are two crystalline forms of carbon, one diamond, the other a black form called graphite. At ordinary temperatures and pressures the stable form is graphite, so that


we would expect diamonds to turn gradually into black graphite. This is the theoretical expectation. In practice, however, the reaction is so exceedingly slow that this does not happen in millions or even billions of years. Diamond is stable not because it is in the lowest energy state, but because it moves toward such a state so slowly that for practical purposes we can say it does not move at all. We say that it is in a metastable state.

Now the role of catalysts, whether biological or not, is to speed up a reaction. The reaction itself is one that would, in principle, take place anyhow. It must proceed from the less probable to the more probable state, and the equilibrium point that is eventually reached is the same whether a catalyst i$ present or not. The catalyst just makes everything go faster. Although the catalyst participates in a reaction, it is not used up, so that it can increase the rates of reactions of a virtually unlimited number of molecules.

This property of catalysts was considered very mysterious in the early days of chemistry, but we now understand, in principle if not in every individual case, how a catalyst acts.

What is the reason some reactions are fast and some are slow? The reason is that in general, in going from one form to another, a molecule has to pass through several intermediate states. We can imagine that the atoms in a molecule are vibrating, and only when the molecule is in a certain shape can it react. In passing from one intermediate state to another, a molecule may be going either uphill or downhill, although of course the over-all reaction must be downhill. Now as we saw before, there is no absolute prohibition against individual molecules going uphill, i.e., to a less probable state; but the less probable the state, the fewer molecules will reach it. If, in dropping from one state to another, the molecules have to make a climb for part of the way, most of them are stopped here. They are in a metastable state. If the climb is low, eventually they will hop over it; but the higher the hop, the fewer will do it in unit time and the slower will be the over-all reaction (see Figure 16). We call the intermediate climb the activation energy of the over-all reaction. This is analogous to a tank of water some way above the ground. Suppose the tank is not quite full. The water can flow down if we lift it first a little


II 1 1 · 1 *

Figure 16.—An analogue of activation energy. To fall to a lower energy state, the molecules have to jump a barrier. The higher the barrier, the

fewer jump over it in a given time.

way over the edge. The height we have to lift it before it will flow down spontaneously is the analogue of the activation energy.

What a catalyst does is to lower the initial barrier to a reaction, or the activation energy. It forms a compound, or complex, of reactant-catalyst. The energy barrier to form this complex is low, so that the reaction proceeds rapidly. This reactant-catalyst compound is itself unstable and breaks down to the final product. The energy barrier to pass from the catalyst-reactant complex is low, so this reaction also proceeds rapidly. Overall, we can visualize the reaction in the following way. Without the catalyst, the initial compound, in order to drop to a lower energy


state, has to pass through an intermediate improbable state. This makes the reaction slow. When the catalyst is present, it passes through a more probable intermediate state, the reactant-catalyst. This makes the reaction fast. The catalyst is now ready to start over again, which is why it is not used up in the reaction.

Both ordinary catalysts such as are used by the chemist and enzymes are similar so far as speeding up a reaction is concerned. There is, however, a very important difference between them. Ordinary catalysts will catalyze a large number of reactions; for example, hydrochloric acid will catalyze the hydrolysis of all sorts of esters as well as other reactions. Those of you who may have studied a little organic chemistry know that organic reactions almost never proceed to a single product. If in the laboratory you want to make a specific compound, you almost invariably make a number of others. The yield, you say, is low, or at least less than 100%. To get your compound you then have to purify it, and you have wasted a part of your starting material. This is also true if you use an ordinary catalyst, since it will usually catalyze not only the reaction you want, but a number of others.

On the other hand, if you use an enzyme to catalyze some reaction, only one reaction takes place. In the eyes of the organic chemist, enzymes are catalysts of fantastic specificity. Let me illustrate this by a rather extreme, but common, example. Organic compounds differ from each other in the arrangements of their atoms in space. Now it is possible, in certain cases, to have the following situation, illustrated in Figure 17. A carbon atom forms four bonds to other atoms or groups of atoms. We say it is tetrahedral in its chemical geometry. You will notice that if the objects which are attached to such a carbon atom are all different, there are two ways in which they can be arranged. One arrangement corresponds to the other when seen in a mirror. Are such molecules different if they differ only by being mirror-images of each other? The answer is that for ordinary chemical purposes they are not. A compound made of molecules of one kind, as compared with another, the molecules of which are mirror-images of the first, has the same chemical and physical properties, such as the same melting point, the same solubility, density and


Figure 17.—A carbon atom may be surrounded by four different groups, which then form a tetrahedron around it. These groups can be arranged in two different ways, one of which corresponds to the other seen in a mirror and vice versa. One cannot be transformed into the other by rotation. Such molecules are called optical isomers of each other. Note that for this to

happen all four groups attached to the carbon have to be different.

hardness, and undergoes the same chemical reactions. It differs in only one important respect: a solution, having one kind of molecule or another, rotates the plane of polarized light in opposite directions. We call such molecules optical isomers of each other.

When a chemist makes a compound whose molecules can be optical isomers, he invariably makes equal numbers of both kinds of molecules. The result is what he calls a racemic mixture. This is also true if in the course of his preparation he uses some chemical catalyst, say platinum black, but is not so if he uses an enzyme. If there are molecules which are optical isomers of each other the enzyme will catalyze a reaction of one isomer and


leave the other virtually untouched. We say that the optical specificity of enzymes is, in most cases, absolute. It accomplishes with ease what the most skilful organic chemist fails to do. How does it do this?

The answer lies in a remarkable chemical property of living matter. When a chemist synthesizes a compound, it contains the two kinds of molecules (optical isomers) in equal numbers, but the molecules of the body are of one kind only. It is very interest-ing that the same isomer of a given compound occurs in all organisms, plant or animal. Because of this optical purity, as it is called, of living matter, people have considered it one further difficulty in understanding how life could have originated from non-living matter.

The reason for the optical specificity of enzymes is not difficult to understand once we know that enzymes themselves are composed of molecules which are a single optical isomer. This can be readily explained by the following analogy. Let us take two different kinds of compounds. The molecules of compound A, which are optical isomers, can be represented by right hand and left hand gloves, which, like the molecules, are mirror-images of each other. The molecules of compound B are represented by left and right shoes, also mirror-images of each other. Now let us form a compound of A and B. A right glove attached to a right shoe is a mirror-image of a left glove and a left shoe. This means that we have the same compound in both cases. The same is true of a right glove attached to a left shoe with respect to a left glove attached to a right shoe. Again we have a mirror-image, or the same compound. However, a right glove attached to a right shoe is not the mirror-image of a left glove attached to a right shoe. These represent quite different compounds and there is no reason to expect that their properties and reactions will be the same. For example their solubility may be different. This, in fact, is how the chemist resolves his racemic mixtures. He takes a compound which already contains only one type of molecule. Since he cannot himself make such a compound, he isolates it from some living thing, usually an alkaloid from a plant. He then forms a compound between his racemic mixture and the alkaloid. Now he has two types of molecules which are not


mirror-images of each other, corresponding to the two kinds of gloves attached to one kind of shoe. Since they have different properties, he can now separate them. You will notice that he is able to do this only because the plant has done another such separation before him.

The enzyme makes the distinction in the same way. The

/ N H 2 HN = C ^

^NH 1

H C H + H20 HCH HCH

1 1

A H

NH2 COOH

ARGININE

/ N H 2

HN = C ^ X N H

HCH 1 1

HCH + ri20

HCH I |

HN COOH

ENZYME ARGINASE

—̂"̂

ENZYME ARGINASE

^

Ü £ H METHYL ARGININE

NH2

H2N · — < /

* 0 NH2

UREA HCH + 1

HCH HCH

| H2N COOH

ORNITHINE

NO REACTION

Figure 18.—The specificity of an enzyme. The enzyme arginase catalyzes addition of water to the amino acid arginine to split it into urea and ornithine. If some slight change is made in the structure of the arginine molecule, here the addition of a CH3 (methyl) group to the amino nitrogen, the enzyme will no longer catalyze the reaction. Note that this does not involve any change near the site where the addition of water occurs; it is at the other end

of the molecule.


molecules of an enzyme are all of one kind in this respect, so that compounds of the enzyme and the different optical isomers of the substrate are not the same; one reacts, another does not.

All enzymes automatically make the distinction between left and right handed molecules, but they also make a further important distinction. In general, similar molecules undergo similar reactions. An example is shown in Figure 18. Here the amino acid arginine can be split, with the addition of water, to yield a compound called urea. There are a number of other molecules, very similar to arginine, which chemically can also yield urea. The splitting of arginine is catalyzed by an enzyme called arginase. If we present it with arginine, urea is formed; but if we present it with a compound which differs only slightly from arginine, no reaction occurs. We say that the enzyme is absolutely specific for arginine. The reason for this, we believe, is that in order for arginine to react, or in other words to form a compound with the enzyme, it is necessary for arginine to fit in very closely into the enzyme. This is illustrated in Figure 19. The enzyme is supposed to have what we may call a crevice into which the substrate fits, somewhat as a key has to fit into a lock. If the fit is perfect, the lock will turn, or in this case, the reaction will take place. But if the key does not fit into the lock, the door will not open, or the reaction will not take place.

At present, the chemist is very interested in the highly specific catalytic behaviour of proteins. If he could produce such catalysts synthetically, you can readily see that this would revolu-tionize many branches of chemical technology. It would no longer be necessary to use high temperatures and pressures to synthesize many organic compounds. In addition, yields would be virtually 100 per cent, saving both material and much work in purifying the final product. In time, no doubt, this will be accomplished. The exact mechanism of catalysis by enzymes is now a subject of intensive study; and when this will be elucidated, not only biology but also technology will greatly benefit.

Our knowledge at the moment of the detailed factors which determine the specificity of enzymatic reactions is rather deficient. We know that the amino acid sequence of a protein plays an important role. For example, the site at which a variety of


ENZYME

ENZYME-SUBSTRATE COMPLEX

Figure 19.—A schematic explanation of the specificity of enzymes. The enzyme is folded into a configuration which corresponds to the shape of a substrate molecule, a molecule whose reaction the enzyme is catalyzing. If the shape of the substrate molecule is different, it will not fit into the reactive site sufficiently closely and no reaction will occur. The protein is

actually much larger than the substrate molecule.

compounds react on the protein has been determined and for certain substrates is rather similar. However, the mere amino acid sequence is not enough to produce catalytic activity. The protein is folded in a certain manner, and that, too, is important. The site at which the substrate reacts with the protein depends on the way the protein chain is folded. Of course, the chain can be


folded in many different ways; but once it is folded, it is held in that particular configuration by very weak bonds, technically called hydrogen bonds. Each bond is very weak, but there are so many of them that the total force holding the protein molecule in a certain configuration is quite strong. Nevertheless, it is possible to disrupt these bonds. The easiest way of doing so is by boiling a protein. When this is done, these hydrogen bonds are broken; the protein chains uncoil and then take up configurations which are more or less random. We then say that the protein has passed from its native to a denatured state. A good example is putting egg white, a protein, into boiling water. The protein at once becomes a white, insoluble substance. The molecules of egg white have uncoiled and reformed bonds at random. Some of the bonds are no longer between parts of the same molecule, but between one molecule and another, causing, in effect, the formation of larger molecules which precipitate out. A specific type of coiling has been replaced by a random one. If we heat an enzyme, we also

o · · o o o

\ / x / x / N- / - /

/ \ / \ /\ /\ / \ I D O · · O O

\ ,*\ / \ ,"V /*- >' o · o · o

/ V V' V̂ V \ > o · · o o

Figure 20.—A schematic diagram illustrating the way in which the presence of enzymes control the reactions which take place in biological material. Various compounds represented as circles can undergo a large number of possible reactions shown as dotted lines. In practice these reactions are very slow. By catalyzing specific reactions, enzymes (E) speed certain of them up. In practice, therefore, the rate of catalyzed reactions is so much greater than the rate of the uncatalysed that the enzymes effectively determine which

reactions are to take place to any significant extent.


find that its catalytic activity is lost. Apparently, it no longer provides a specific crevice, or lock, into which the key, or substrate molecule will fit. This explains why heat kills living things. Their enzymes are destroyed. We use this to good effect when we heat food to kill bacteria, as in canning.

What is the biological significance of enzymatic specificity? We can explain this readily by a diagram (Figure 20). If we start with a variety of compounds, they can interact spontaneously in a large variety of ways to produce numerous products. As we saw above, however, these reactions in most cases are very slow. Now if we have enzymes present, certain reactions are speeded up. For all practical purposes, therefore, we may say enzymes make it possible for an organism to choose which reactions will proceed and which will not. The biologist, in fact, is so used to the fact that almost every reaction is catalyzed that he tends to say that the organism produces whatever reactions are needed, although of course, what it is really doing is choosing which reaction is going to be speeded up.

How does the organism decide which enzyme to produce and which not? Dr. Watson will discuss this question in consider-able detail. For our purposes, it is sufficient to say the following: the organism is provided with a set of genes, each gene specifying the structure and therefore the existence of some enzyme. It is the battery of genes which decides what reactions are going to be catalyzed by deciding which enzymes are going to be present.

A striking example of this is provided by the phenomena of mutation. Take, for example, a very famous organism, the red bread mould, Neurospora. This organism contains all the enzymes necessary to make whatever compounds it needs if it is supplied with ammonium salts, sugar, and a vitamin called biotin. Now if we irradiate such a mould with ultra-violet light or x-rays, it is possible to destroy some of the genes of this organism. Some-times the gene which is destroyed is the one which is responsible for an enzyme catalyzing some reaction which produces a vitamin. In such a case, the mould will no longer grow in the simple medium in which the original strain did. Nevertheless, it will grow perfectly well if we add the vitamin to the medium. A long series


of investigations have shown that a single gene corresponds to each enzyme.

Incidentally, this explains why we need certain compounds such as vitamins in our foods. There is considerable evidence that originally simple organisms were able to make all the com-pounds that they needed. The reason that we need certain vitamins and other special constituents in our food is that we have lost the genes which specified the enzymes which would make these constituents.

From the above, you see how the organism regulates and decides which reactions are to take place; however, as I pointed out above, an enzyme can only catalyze a downhill reaction. Now we know, of course, that living things can make compounds of a very improbable kind, in other words, they are able to make certain reactions run uphill. The standard example is plants. They start with carbon dioxide and water and make, among other things, wood. From the fact that we can burn wood and obtain energy from it, we know that wood is at a higher energy level than carbon dioxide and water. Clearly plants, and in fact all organisms, have the ability to make certain reactions go uphill.

Now in principle there is nothing very mysterious about making something go uphill. Water spontaneously flows downhill. If we want to make it go uphill we install a pump. The reason it is possible to do this is that we can couple a reaction going downhill to another reaction and cause it to go uphill. For example, falling water can be passed through a turbine to produce electrical power. This power can be transmitted to a motor which operates a pump causing water to rise. Owing to inefficiencies in the system, over-all there will always be more water coming down than going up; but nevertheless, we can cause water to flow uphill.

The living organism is not a heat engine or an electric motor and for a long time it was very mysterious how it could couple certain downhill reactions to make others go uphill. We now know how it does it, and the principle of it is very simple (see Figure 21). Suppose we have a compound A which we want to transform into compound C which is at a higher energy level. We also have available a compound B which is at a higher energy


i

ÜJ

ÜJ

B i \

A

/ ( A B , > ^

y )

\

c

I 1 I 3'

Figure 21.—Coupling of a compound at a high energy level, B, to make it possible to synthesize C from A by an uphill reaction. See text.

level than either A or C. We therefore react A with B to form a compound (AB). This compound is now at a higher energy level than either A or C, although lower than B. It is therefore possible to cause it to react again to form C and another product B1. The Compound C has been formed by a downhill reaction and B1 is at a lower energy level than B. What we have accom-plished here over-all is to degrade B, that is, drop it from a higher to a lower energy level. At the same time we have coupled this degradation to the reaction A to C which goes uphill. The energy for this reaction, of course, comes from the loss of energy in the over-all reaction B to B1. This is quite analogous to the turbines and pumps in our previous example.

Living organisms have developed a single special type of


ATP ADP

H

ADENOSINE.

0 0 0 II ' I I II

HO-P-O'VP-O-P-

''··. OH .OH OH

NH2

N/ AACH ADENINE

N

OH OH 'N- - / N H

RIBOSE (A SUGAR)

© © © © © ETC

Figure 22.—ATP acts as a donor and acceptor of phosphate groups from various compounds. Note that in practice it cannot accept phosphate unless

the phosphate is linked to a compound by a high energy bond.

compound at a high energy level which corresponds to B and which, by reacting with innumerable other compounds, can cause them to undergo uphill reactions.

The universal high-energy compound which is used to force reactions uphill is called adenosine-triphosphate or ATP. The structure of this compound is shown in Figure 22. You will notice that it consists of something called a nucleoside which, incidentally, is a component of nucleic acids. To this nucleoside there are attached three phosphate groups. The bond between the second and third phosphate is shown by a curved line to indicate that it is what we call a high-energy bond. If we disrupt


SUGAR V + y ATP /

SUGAR PHOSPHATE

/

- < \

ADP

V >

(

SUGAR PHOSPHATE

STARCH

/

<

Λ PHOSPHATE

Figure 23.—A method of coupling the energy in ATP to perform chemical work. ATP transfers one phosphate to sugar. This sugar phosphate is at a higher energy level than starch and can thus polymerize to starch by a

downhill reaction.

the molecule at this bond, say by putting ATP in a slightly acid solution, the solution will warm up. The energy of this bond has appeared as heat. An organism, however, does not wish to produce heat but rather uses the energy in this bond to do chemical work, that is, to make certain reactions go uphill. Let us take an example.

Suppose that we start with a sugar called dextrose, otherwise known as glucose. From this sugar the organism wishes to make a substance called starch. The structure of glucose and of starch is shown in Figure 23. As you can see from the figure, starch is a polymer whose monomer units are glucose molecules. Now the difficulty in making this reaction go is that starch is at a higher energy level than glucose. The reaction is uphill. What the organism does in this case is as follows: it takes a molecule of the high-energy compound, ATP, and catalyzes the reaction to remove the third phosphate from ATP and attach it to the glucose molecule. The overall reaction is downhill. However, the glucose phosphate compound is now at a higher energy level than the glucose originally was. This has been at the expense of the ATP molecule which has been degraded to a lower energy form, called adenosine-diphosphate or ADP. As a result of this reaction, we now have a compound, glucose phosphate, which


is at a higher energy level than starch and can therefore catalyze a polymerization reaction which links glucose molecules together, incidentally setting free the phosphate groups.

This type of reaction, which uses ATP to do chemical work, is very common. ATP is also used to do other kinds of work. For example, all of us have ingenious devices called muscles, which are able to contract and do work. The energy for this work also comes from the degradation of ATP. As another illustration, suppose I have a solution of an enzyme called luciferase and an organic compound called luciferin. If we darken the room, and add some ATP to this mixture, you will notice that the solution begins to emit light. This is the reaction which causes various insects, deep sea fish, and corals and jellyfish to emit light. The energy for this reaction is again provided by the degradation

Figure 24.—Formation of ATP by oxidation to produce a high energy bond. When A TP does work it releases phosphate which in turn promotes

the breakdown of sugar.


of ATP. The energy for the conduction of nerve impulses, the secretion of acid into the stomach, the separation of blood constituents into urine all comes from the degradation of ATP. ATP is the universal energy-carrying compound in all living cells.

Of course there is very little ATP in the body and it is being degraded all the time. It must therefore be replenished continuously. ATP is indeed the universal energy carrying inter-mediate, but it is not the ultimate source of energy for living things.

Depending on their source of energy, or how they make ATP, living things are classified as animals or plants. Let us first examine the methods used by animals.

The most important energy source for animals is the degra-dation of sugar. In order to trap the chemical energy, the organism degrades sugar not all at once, as we would by burning it in the furnace, but in small steps, some of the steps yielding packets of energy which can be used for the synthesis of ATP. Here is an example (Figure 24). Through a number of steps, all of which are downhill reactions, the glucose (sugar) molecule is cut in half and finally appears as a compound we call glyceraldehyde-diphosphate. This, in our terminology, is a low energy compound. Next glyceraldehyde-diphosphate is oxidized; that is, it loses one hydrogen atom. As a result of this, it is converted to a compound called phosphoglyceric acid. The bond between the phosphate and the acid is now a high-energy bond. Being at a high-energy level it can undergo a downhill reaction which is the transfer of the phosphate to ADP to form ATP; thus, we form our high-energy compound. The hydrogen atom which has been removed eventually combines with oxygen to form water, so that what has happened here is the coupling of the burning of hydrogen, a downhill reaction, to form ATP from ADP, an uphill reaction. There are several points in the course of the degradation of sugai which form ATP in this manner although the exact details diffei in each case. As a result of the conversion of one molecule of sugar to carbon dioxide and water, several molecules of ATP are formed. Every animal cell operates on the chemical energy provided by the degradation of high-energy compounds which, for the most part, are glucose.


H—C = CH2

1 H

\ ' \ >H—CHp

/ \ / \ HC Fc

\ / \

„ / \ CH2 H

HOOC-CH2

\__ H E M E

CH

. /

N — χ

CH2

1

1 CH2-COOH

H - C = C H 2 X 1 H 1

yC 1 £

/ \ ^ c \ c / \ H3C~Cv / \ ^ C " C 2 H 5

/ \ / \ » H-C Mg C-H

\ / \ / H C W N - C

1 1 C H 2 1 1 1 M U Λ/ — U 1 CH2 | 1

Ä ,, COOCH, cooc20H39 3

CHLOROPHYLL

Figure 25.—77ze structures of heme and chlorophyll.

Now obviously, there is only a certain amount of glucose in the world, and if the amount were not replenished, all life would come to an end very rapidly. Fortunately, the living world has discovered a method for making glucose by using a virtually inexhaustible supply of energy from the sun. This, as almost all of you are aware, is done by plants through a process called photosynthesis.

The exact steps in photosynthesis have not as yet been com-pletely elucidated but by now we know the general outline of the process. Briefly, plants possess a substance called chlorophyll. This, as you can see from Figure 25, is a rather complex organic compound which is closely related to the structure of the red substance in our blood and the active group of the enzyme catalase. However, instead of having iron at the centre of the molecule, as does the compound in hemoglobin, it has magnesium. A very important point is that chlorophyll is highly coloured, which means that it absorbs light. Absorption of light means that light loses some of its energy to the substance that absorbs it; or to put it another way, chlorophyll is raised from a low- to a high-energy state by the absorption of light. Through a complicated


series of reactions, chlorophyll sinks back to a low-energy level, but these downhill reactions are again coupled to force the uphill reaction ADP to ATP. Thus the plant is provided with a high-energy compound which it has synthesized using the energy of light. Once ATP is available, the plant uses this to make other reactions go uphill. One such important reaction is the synthesis of sugar from water and carbon dioxide. ATP is also used to provide the energy for the synthesis of other compounds a cell may need.

Thus we see that the living world is an intra-connected system. A number of organisms called plants trap solar energy, first to make ATP, and using ATP, another high-energy compound, a sugar. Animals prey upon plants, using the sugar again to form ATP which is used in turn to make their uphill reactions go. The whole system is a solar engine which keeps the world of life going.

Now we may ask what is the biological point of all these reactions. What do they produce? Briefly, what they do is as follows.

The enzymes act on various low molecular weight materials to produce the building blocks from which the large molecules of living matter are formed. With the help of enzymes, these low molecular weight materials are used to produce copies of genetic materials. The genetic material, in turn, again with the help of enzymes, uses the small molecules that the enzymes have produced to make more enzymes. These again catalyze the reactions which provide the building blocks for the large molecules. This is the essence of the chemistry of living matter, a cyclic process of one compound forming another which keeps going indefinitely. The energy for all this ultimately comes from the sun.

This is a most curious sort of solar engine which has produced a variety of interesting and fascinating objects which differentiate our earth from a lifeless body such as the moon. After many millions of years of operation, this engine has also produced objects, ourselves, who wonder how the whole system could have started in the first place. In our next chapter, we shall consider the problem of the origin of life.

CHAPTER 3

The Problem of the Origin of Life

By studying the fossils in rocks, we learn that species are always changing. With few exceptions, no species exists for more than a few million years, after which it becomes extinct or changes into another species. When seen on a time scale of hundreds of millions of years, there is a certain orderliness to these changes. The most complex animals, the birds and mammals, have appeared most recently. Before them the most complex forms were reptiles, preceded in turn by amphibians, and earlier by fish. In older rocks, only remains of simple invertebrates are found. Before them we find uncertain traces of algae and bacteria, and finally, in the oldest rocks, no signs of life whatever.

The theory of evolution explains this increase in complexity with time. All living things are subject to random heritable changes called mutations. Most of these mutations are harmful, but a certain number of them are useful. Such mutants leave more progeny, so that in time more and more complex organisms arise, better adapted to cope with the environment.

This process accounts in a satisfactory manner for the evolution of life, providing that we have some simple form of life to start with. It does not, however, explain the origin of life itself. We shall now consider this difficult problem.

At one time, the problem of the origin of life presented no more mysteries than ordinary physical or chemical phenomena. This was because people did not understand how complex living matter really is. Mice and frogs were believed to develop spon-taneously in the sunbaked mud of the Nile, and maggots in rotting meat. Redi in Italy was the first to demonstrate that this theory of spontaneous generation was incorrect so far as higher organisms were concerned. He covered pieces of meat with gauze and left others uncovered. Maggots, he was able to show, would only develop in meat if flies layed their eggs in it. This disposed

189

190 LIUHT AND LIFE IN THE UNIVERSE

of the Nile-mud-mouse-frog theory. Almost everybody after Redi admitted that flies, mice, and frogs could not arise spon-taneously; but it was still widely believed that smaller organisms, those which could be seen only under the microscope, could do so. This hypothesis, however, was also disproved by Pasteur. He showed by numerous experiments that even the simplest bacteria could only develop from parents like themselves. Life can arise only from pre-existing life. In spite of this the evidence in the rocks convinces us that at one time the first living thing must have originated from non-living matter.

To the microscopist without chemical knowledge, certain forms of life do appear to be very simple. They are just little blobs of jelly or slime. Some people, therefore, continued to speculate that under special conditions such slimy bits of matter might form spontaneously and by evolution develop into higher forms of life.

From what I have said before you will understand the absurdity of this idea. Even the simplest forms of living matter, far from being unorganized blobs of jelly, are most complex systems of very specifically constructed proteins and nucleic acids which cease to function when even small changes are made in their molecules. How can such a complex system develop "spontaneously"?

Let us consider the problem more closely. From the chemical point of view life is a system of three types of components (Figure 26). There are first small molecules whose breakdown provides the energy to keep the system going, and which also provide the building blocks for constructing larger molecules. The reactions of these molecules are directed by the protein enzymes, which are the second component. The third component is the genetic material, or nucleic acids. The genetic material specifies how enzymes are constructed. Enzymes operate on the small molecules to form copies of the genetic material, and also, under the influence of the genetic material, more enzymes. The entire system is a cycle.

If looked on in this way, the exact nature of the entire system is specified by the nucleic acids, which, by specifying the kinds of enzymes that will be formed, determine which reactions

THE PROBLEM OF THE ORIGIN OF LIFE 191

Figure 26.—Cyclic movement of matter in metabolism. Enzymes act on small molecules to produce genetic material, and, with the help of the genetic

material, more of themselves.

will take place. We may say that the genetic material is a tape, on which is written a series of instructions for replicating enzymes, which in turn replicate the genetic material.

This analogy to a tape is a rather good one, because it illustrates both the power and the limitations of the genetic material. Suppose you do have a tape on which is written some soul-rending melody. The "information" to produce this melody is all in the tape, but you will never hear it from the tape alone; you need a machine which can transform the "information" on the tape into actual sound. In the same way, the genetic material or DNA is quite helpless by itself, and only in a formal sort of way can we say that it contains the "information" necessary to make proteins. In order to do this it requires a machine, which in this case is a system of enzymes and a mass of small molecules on which the enzymes can operate. Even in very simple bacterial cells there are probably a thousand kinds of


enzymes, and at least that number of kinds of nucleic acids. The machine cannot operate below a certain level of complexity, and the minimal complexity is high.

To form even the simplest kind of organism therefore requires the simultaneous synthesis of hundreds or thousands of specific proteins and nucleic acids. You will remember that the number of possible ways of arranging amino acids to form a protein is something well in excess of 10130. Thus even if we had a very large system where amino acids were being randomly polymerized to form proteins, the chances of forming even one molecule of the right protein by such methods would be virtually nil. Furthermore, forming a single protein molecule gets us nowhere. Even though it might have catalytic activity of some kind, after a short time it would disintegrate, leaving the system much as it was before. The same goes for the spontaneous formation of nucleic acids. If, operating at random, some meaningful DNA or genetic material was formed, the DNA alone would be quite helpless to do anything unless it already had the enzymes which would make it possible for it to replicate itself and to control the synthesis of more enzymes.

Enzymes cannot be made without nucleic acids, and nucleic acids cannot be made without enzymes.

Obviously, the spontaneous and simultaneous origin of suit-able genetic material and enzymes by random polymerization is so fantastically improbable that it does not have to be considered. Note that even if such an event were to occur, this would not produce an organism. The molecules of nucleic acids and proteins could not possibly interact at a sufficient rate in even a small volume of liquid. They would disintegrate long before they could replicate the system. In living matter, enzymes and nucleic acids are held in certain configurations and are delineated from their environment by membranes of complex structure which make it possible for them to interact in a suitable manner.

A biochemist will readily appreciate the force of this last argument. Frequently, in order to study the functioning of a cell, he disrupts it to simplify its function. Biochemists have long attempted to study the synthesis of nucleic acids and proteins in a system of disrupted cells. Although in such a system all the enzymes originally present are still there, it has proved quite


impossible to produce the complete cycle of events characteristic of living matter. The best we have been able to accomplish is to find special conditions for one or another reaction to proceed, and that only to a limited extent. Thus you see that even if all the components of living matter were to form spontaneously they would not produce an organism because they would have to be further organized into specific spatial relations to each other.

In spite of this obvious impossibility, some people interested in the origin of life have continued to search for conditions where amino acids or nucleotides could spontaneously form polymers. It has been shown, for example, that concentrated solutions of amino acids, when slowly evaporated and subjected to high tem-peratures, form protein-like materials. True, but in such a system life is no more likely to arise spontaneously than in a can of beef stew which is already full of proteins and nucleic acid but scarcely shows the characteristics of life. It is evident that there is a tremendous gap between ordinary organic compounds and a living system. Is there any way out?

We are often faced with a similar situation when we consider the evolution of complex forms. For example, the eye of a man or an octopus is a tremendously complicated structure, all the parts of which are adapted to form a clear image on the retina and transmit it to the brain. How could such a complex system have developed? The answer is that it developed gradually from a very simple sensory system and that during all stages of develop-ment it always functioned as an eye, gradually getting better and better. Figure 27 shows the various stages of development as we can deduce them by comparing the eyes of different organisms. Originally, it was simply a nerve ending sensitive to light. Then, several such nerve endings were grouped together; and these groupings gradually sank below the surface of the body. The skin which covered them at first was merely transparent, then formed a bulge which acted as a lens to increase the light-gathering power of the eye. By further development, the geometry became such that the lens formed an image on the nerve cell endings, now called the retina. Accessory structures such as the eyelids and iris developed to close the eye and control the amount of light entering it. Muscles developed to vary the curvature of the


Figure 27.—A schematic diagram showing the evolution of the eye from a simple sensory nerve ending to a complex image-forming camera.

lens and thus its focal length. The neural connections in the retina became a very complex computer to sort out and organize the signals received. At the same time, the brain was developing to receive these signals and put them to use controlling the reactions of the entire body. The significant point here is that while the eyes of man and octopus are indeed very complex structures, they evolved by small steps from something very much simpler. The only common factor through all this evolution was that the organ always remained sensitive to light.

I think that this is the way that we must approach the problem of the origin of life. The present systems of specific nucleic acids and proteins, like the eyes of man and octopus, are so complicated and their components are so well adjusted to each other that it is indeed inconceivable that they could have been


present at the beginning. The system to start with must have been more loosely organized out of very much simpler components, but from the beginning it must have possessed certain minimal properties characteristic of life.

As I have stated several times, from the chemical point of view the minimal characteristic of life is that it is a system of catalysts which catalyze reactions which form more catalysts. There are no "self-replicating" molecules, there are only "self-replicating" systems. Each catalyst promotes some reaction which facilitates the formation of other catalysts and thus directly or indirectly of itself. For brevity, let us call such a process reflexive catalysis. The problem, therefore, is whether a process of reflexive catalysis, much simpler than that now found in living things, could have originated spontaneously; and if so, whether it could have continued to develop into more and more complex systems.

The theory of evolution has accustomed us to the idea that from the simpler arises the more complex. As we go back to the origin of things, we expect to reach primeval simplicity and chaos. A priori, therefore, we expect to find that at the beginning there were only the elements. Later the elements formed a variety of compounds, including the compounds of carbon that we call organic. Finally, organic compounds became organized into living things.

There is evidence that this series of events did occur. Our first problem is therefore not the origin of life as we know it, but rather the origin of organic compounds. To understand this problem requires, however, some understanding of organic chemistry. Let me outline to you the salient facts.

Organic chemistry is the chemistry of the element carbon, known to you in pure form as diamond, graphite and charcoal. Carbon is what we call a tetravalent element; that is, one atom of it can form four bonds with other atoms, including other atoms of carbon. As a result, it can form molecules which are long chains or rings, to which, and this is an important point, other atoms are attached. This produces the vast variety of compounds known to organic chemistry.

The simplest type of organic compound is formed when four hydrogen atoms, each having one bond, attach to one carbon


atom. This compound is called methane, and has, when represented in two dimensions, the following structure:

H I

H — C — H

H Methane is the simplest member of the class of compounds

known as hydrocarbons, or compounds containing only the elements carbon and hydrogen. More complex hydrocarbons have single or branched chains which may be quite long. For example:

H H H H H H H H C — C — C — C — C H o r H C — C

H H H H H H H

You are familiar with hydrocarbons in the form of motor fuels. Hydrocarbons are all rather similar in their properties. Those containing only a few carbon atoms have a low boiling point, so they are gases at ordinary temperatures. As the number of carbon atoms increases, the boiling point rises. Ten atoms make a hydrocarbon which is a liquid, forty make one which is a waxy solid or paraffin at room temperature. They are all quite insoluble in water.

By themselves the hydrocarbons form a monotonous series. What gives organic chemistry its variety, and what makes life possible, is that the hydrogens in a hydrocarbon can be replaced by other atoms or groups of atoms. Such a replacement gives the molecule entirely new properties.

For example, a hydrogen can be replaced by metacarbonic acid:

O O H H H_ , / H H H /

H C — C — C.H + H I C — O H - ^ H C — C — C — C — OH 4 H2 H H H H H H

H H C H

C H \ H C H

H


The hydrocarbon now becomes a carboxylic acid. Replace-ment by the OH group of water gives an alcohol:

H H H H H H H C — C — C !_H + H ί OH B^> H C — C — C OH + H2

H H H " H H H

Oxygen can replace in one of two ways. If the replacement is at the end of the molecule, the product is an aldehyde. Note that oxygen has two valences, so it replaces two hydrogens:

H /

■ C = O + H2

H — C -

H

H - C -

H

H - C | H ' i H !

H + 0 2 K > H C -

H

H - C

H

If the replacement is not at the end, the product is called a ketone:

C -H

H !H !H H C —CJ—'C

H !JH__jH

Replacement by

- c —c :~H + H: H H '

Η H C H

H + Oo B ^ C = O + H2

H C H H

ammonia produces an amine:

H H H — N — H »-> H C — C — C — NHo + H2

1 H H H H

So far as biological chemistry is concerned, these are the most important groups which can be attached to carbon atoms, but many others are known. Within certain limitations, more than one group can be attached to the same molecule. The amino acid serine contains three such groups:

NH2 O H | S

HO C — C — C — OH H I

H


Serine is simultaneously a carboxylic acid, an amine and an alcohol.

I should warn you at this point that the "substitution reactions" I have discussed are not real reactions in the sense that they occur in one step. Usually it is necessary to use some indirect means to obtain the desired result. If, for example, we want to obtain acetic acid from the hydrocarbon ethane, we might proceed as follows. Ethane reacts to replace a hydrogen with chlorine:

H H H H HC —C H + C12 *+ HC — C C1+HC1

H H H H

The resulting substituted ethane, monochloroethane, reacts with water to give ethanol, or ordinary alcohol:

H H H H HC — C C l + H O H m^+ HC — C OH + HC1

H H H H

Ethanol, under the right conditions, can react with oxygen to give acetic acid:

O H H H //

HC — C OH + 0 2 »-► HC —C —OH + H20 H H H

The art and science of the organic chemist consists in choosing the proper reaction sequences and the conditions, such as temperature, pressure and catalysts, to obtain the desired product.

Besides carbon compounds which are real or substituted hydrocarbons, referred to as aliphatic compounds, two other


general types of organic molecules exist. Six carbon atoms may form a ring called benzene:

XT

HC CH I II

HC CH c H

Such a ring system has somewhat different chemical properties, and is called an aromatic compound. Like aliphatic compounds, it can carry substituents at various points. The amino acid tyrosine, for example, has the structure:

N N HO <f ^> _ c — C — COOH

H NH2

It can be regarded as a benzene with two groups attached, OH at one point and an aliphatic compound at another. The aliphatic compound itself carries two groups, amino and carboxylic acid.

The third type of organic compounds are rings in which certain carbons are replaced by other atoms, mainly nitrogen and sulphur. Such compounds are called heterocyclic. An important biological example is adenosine, a component of nucleic acids (Figure 22). As adenosine illustrates, various groups, here NH2 and a sugar, can be attached to heterocyclic as well as to aliphatic and aromatic compounds.

We can summarize the above by saying that carbon atoms can form a molecular skeleton to which a variety of other atoms or molecules can be attached. The great variety of organic compounds is due to the fact that the carbon skeleton can be of many different kinds to which other atoms or groups can be attached to many different points. Several hundred thousand organic compounds have already been synthesized and studied. Since there is practically no limit to the size of organic molecules, no definite upper limit can be designed to the number of possible organic compounds. Almost any compound whose formula can be written in accordance with the valence rules can be synthesized by


a competent chemist. Of course, a practical limit is reached if the desired molecule is very large and has a non-periodic structure.

Having loaded our carbon skeletons with various groups, we may well ask what is the biological point of these products? The answer is that we can now reach an even higher level of complexity by causing their groups to react with each other. With these compounds, we can now build our large "super-molecules".

Once we have amino acids, their reactive groups can interact to form polymers. The reaction here is between the amino and acid groups to form an amide (see Figure 12). One amino acid after another can be added to the end of the chain to form a long protein molecule.

Similarly, a nucleoside can react with phosphoric acid to give a nucleotide. This, in turn, can react with other nucleotides to give another kind of polymer, a nucleic acid.

Thus once organic compounds are present, they can interact in a variety of ways to give products of biological importance. At the present time, however, virtually all organic compounds on earth are produced by living matter. Is it possible that organic com-pounds were present on earth before life developed? The results of astronomy, geology, and chemistry indicate that such was indeed the case.

Professors Butler and Messel have discussed with you in their lectures the question of the evolution of our solar system and the evolution of the earth. Recall some of the following points which they pointed out:

Astronomers and geologists believe that the earth was formed about 4500 million years ago, at the same time as the sun and the other planets. Originally, there was supposedly present a mass of gas and dust, the primordial nebula, which in some manner or other condensed to form the sun and planets. The details of this process are still a matter of much dispute among astronomers, but it is sufficient for our purposes to accept the general idea.

What were the conditions when the earth was first formed? We saw that we could obtain some idea of this by studying various astronomical bodies and the geology of our own earth. It appears probable that the earth was then hotter than it is now; in fact, as


pointed out in Chapter 5, it may have been molten — although this hypothesis now appears doubtful. As it cooled, various com-ponents separated out into larger masses. The heavier elements and particularly iron formed the core of the earth. Lighter rocks formed the crust. The oxygen combined with whatever elements it could and especially with carbon and hydrogen. The latter compound is of course water; and as soon as the temperature fell below the boiling point, this water must have condensed to form the oceans.

We strongly believe that life originated in the ocean. Land-living animals and plants are descendants of water-living forms, and the inhabitants of fresh waters are also clearly intruders from the sea. Life on land or fresh water requires a complex set of adaptations. We ourselves, for example, are a sort of aquaria, insulated against drying out and provided with organs for exchanging gases and other materials with the environment. Our cells continue to live in a fluid environment very similar to ocean water. This environment is provided ready-made to simple ocean living forms, which makes life easier and obviates the need for special adaptations. An ocean filled with organic compounds is therefore the place where one would naturally look for the origin of life.

Until rather recently it had been generally believed, however, that the original atmosphere of the earth resembled the atmosphere that we now have. This atmosphere contains about 20% of molecular oxygen, and under these conditions the accumulation of organic compounds in any significant amounts would be impossible.

The reason why this would be so is as follows. In an atmosphere containing oxygen, all compounds that can do so tend to combine with oxygen. The downhill course for organic com-pounds is for the hydrogen to combine with oxygen to form water, and for the carbon to combine with oxygen to form carbon dioxide. Since this is the lowest energy state, no further reactions would be possible, and thus no significant quantities of organic compounds would have been formed.

Within recent times we have realized that this view is probably incorrect. This is partly because we believe that organic compounds must have been present, otherwise life would not have


arisen. There is, however, also more direct evidence for this. In the most ancient strata of the rocks, as well as the most recent, we find iron. In recent strata, as one might expect from the fact that oxygen is present in the atmosphere, the iron is completely oxidized to the trivalent, or ferric state. In the very oldest strata, however, iron is in the less oxidized divalent or ferrous state. This would be expected if there was a dearth of oxygen at that time.

General astronomical considerations also lead us to believe that free oxygen was initially absent. Hydrogen is not only the simplest, but also by far the most abundant element in the universe. The primordial nebula from which the sun condensed must also have had a great excess of hydrogen, since the sun is rich in hydrogen. Thus, as the earth condensed, any oxygen present would have reacted with hydrogen to form water as well as with other elements such as carbon to form C02. If hydrogen was still in excess, as it almost certainly was, it would further react with carbon to form methane, and with nitrogen to form ammonia.

In support of this view is the fact that water, methane, ammonia, and related compounds such as cyanogen are present on Jupiter and in comets. Organic compounds have also been detected in meteorites.

In summary, the excess of hydrogen on the early earth would have ensured a supply of water and at least some simple organic compounds such as methane. You might reread the lectures of Professors Butler and Messel on this matter before proceeding — especially Chapter 6.

While we do not know the exact composition of the mixture of organic compounds which was then present, we can be sure of some of the general chemical characteristics of the system. Unless there was a method of feeding energy into the system, it would be in a state of chemical equilibrium. Now a system at chemical equilibrium is a system which is dead, where nothing can happen. Individual molecules can undergo reactions, but the large scale characteristics of the system cannot change.

Obviously something did happen, so there must have been processes which moved the system away from equilibrium. In moving away from equilibrium, there is reason to suppose that the chemical complexity of the system also increased. Let us consider a few such possibilities.


As mentioned in Chapter 5, chemists have recently taken mix-tures of ammonia, methane, water and carbon dioxide, presumed to have been present in the original atmosphere of the earth, and have passed electrical discharges or illuminated the mixtures with ultra-violet light. Under these conditions, a variety of organic com-pounds are formed. Some of these compounds are exactly those which occur in living matter; for example succinic acid, an important product of the breakdown of sugar, and some of the simpler amino acids. These compounds are at a higher energy level than the ones from which they were made; the driving force for these reactions is provided by light or an electrical discharge.

It must not be supposed that such random syntheses would make only simple organic compounds. The initial products would probably be simple, such as formaldehyde and hydrogen cyanide. These, however, are very reactive, and together give a variety of different compounds. Of special interest are recent observations that adenine (see Figure 22), a component of nucleic acid, is spontaneously formed.

Another complex compound formed by random synthesis is porphyrin, an important constituent of many enzymes and closely related to chlorophy, the substance in green plants which converts light to chemical energy (see Figure 25).

The conditions under which all this happens are not at all unnatural. Ultra-violet light continuously falls upon the atmos-phere from the sun, and electrical discharges occur in the form of lightning. There is therefore every reason to believe that these processes took place during the early history of the earth.

Ultra-violet light and lightning are not the only possibilities. A chemical equilibrium shifts with concentration, temperature, and pressure. Thus, for example, pools of water containing some reactants might partially evaporate, causing some reactions to proceed far in one direction. This would result in the synthesis of compounds which would not occur in the general system. Later rain would wash them out into the ocean. Similarly, vol-canic action would bring to. the surface supplies of reactive com-pounds, such as metal carbides. These would react with water to produce acetylene, a highly reactive compound able to par-ticipate in the formation of many further compounds. In the


early days of the solar system, a considerable amount of such compounds as cyanogen, now found in comets, might have fallen upon the earth from space.

You must remember that in one important respect condi-tions on the earth were different from what they are now. At present, if any organic compounds, even stable ones like wood, are exposed, they are rapidly degraded by one form of life or another. Originally, however, no life was present, so that organic compounds could accumulate.

Most organic compounds are very soluble in water, so that ultimately they would end up in solution. The ocean would also, of course, contain a variety of salts. Before life appeared, the ocean would have had the composition of an organic soup, containing, in various amounts, thousands of compounds.

What would be the end result of all this? Again, we can say very little about the exact composition of the resulting mix-ture, but we can be quite sure of some of its general properties. Certain compounds, even though at a high energy level, are very stable. Such compounds will accumulate. Other compounds break down readily. Their material will recycle through the system; at each cycle, part of the material will be trapped in more stable compounds.

If this were all, we would merely have a cyclic process, driven by a variety of energy sources, which would produce a collection of organic compounds of increasing stability. Reactions would become slower and slower, not because the system approached equilibrium, but because it selected metastable compounds.

The reason this did not happen is because of the existence of catalysis. In organic chemistry there are numerous cases where one organic compound participates in some reaction in the course of which it is regenerated. In the special case where the reaction is speeded up by taking this course, rather than another, catalysis occurs.

Since we believe there must have been thousands of different kinds of organic compounds present in solution, it would be strange indeed if some of these did not catalyze reactions between some of the constituents. Consider what would happen if this


was so. Compound X catalyzes a reaction A-»Y. On occasion, Y will catalyze a reaction B->X. Here we have the simplest example of reflexive catalysis, catalyst promoting a reaction which in turn leads to the production of a catalyst2, which in turn again promotes a reaction leading to the formation of more of catalyst! (Figure 28).

B

i /

/ /

/ /

/ /

* i

>

·<·-*

1 S

o · ' W X

■ ^ ^ ^ - - ,

^ \ s^ p<" / C A T A L Y S I S ^

A

t \ 1 \ 1

* — * ■

\ \

\ \

\

V * * • o Y Z

Figure 28.—Reflexive catalysis. Compound X catalyzes the formation of compound Y and compound Y that of X. As a result, material is diverted

into the paths X and Y at the expense of W and Z.

The important point about reflexive catalysis is that by its very nature it promotes its own growth. A certain amount of material moves through the system. If there are components which catalyze the formation of each other, the result will be that an increased amount of material will appear in the catalysts at the expense of some other components. The more efficient such catalysts are, the more material will enter them. A system of this kind selects and favours the most efficient catalysts auto-matically.

Since the number of compounds in solution was very large, numerous pairs of reflexive catalysts would be expected to occur. These pairs, of course, would not be independent. By increasing or decreasing the concentrations of certain compounds, one pair could either promote or inhibit the production of other pairs.


Thus not pairs but systems of catalysts would dominate the situation. The catalysts selected would automatically be the most efficient ones in the sense of promoting the maximum flow of material through the system of catalysts as a whole.

You will notice that chemically the end result here is very similar to a living system. A self-catalyzing system of catalysts has appeared. However, there is, as yet, no living organism in the usual sense of the term. The only organism is the ocean itself, which in a restricted but nevertheless very real sense can be considered to be alive. It has, like the living creatures we know, a metabolism of its own.

Reflexive catalysis, incidentally, provides an explanation for the curious fact, on which I have commented before, that only one of the two possible optical isomers, that is right-handed or left-handed molecules of a given compound occur in living matter, and that it is always the same isomer which is found in all plants and animals. In the usual course of events, such as synthesis in the laboratory, equal amounts of right-handed and left-handed molecules are produced. This, of course, would also be true in the primitive ocean. Thus when reflexive catalysis appeared, there would really be two catalytic systems, one which we shall call left, the other right, each a mirror-image of the other, and each present in equal amounts. Unlike the situation in an ordinary chemical system, however, this equality is here unstable. If either system, because of a chance fluctuation from equality in numbers of different molecules, became slightly larger than the other, it would not only keep this lead, but being larger would draw even more material into itself, eventually wiping out the smaller system. A stable state would be either all one isomer or all the other. This is what is actually found in living systems.

Simple organic catalysts are neither very efficient nor very specific. Some of them increase the rate of a reaction only slightly, say two to ten times over the spontaneous rate. In general, they also catalyze the reaction not of one compound, but of a whole class of compounds. Since in a system domi-nated by reflexive catalysis the selection of more efficient cata-lysts is automatic, a catalyst that produces unwanted side-products is less efficient than one which does not do so. So providing that


it is possible to construct more efficient and more specific cata-lysts, the system will do so.

In present forms of life, the metabolism of small organic compounds is organized in a very efficient and interesting manner which suggests how systems of reflexive catalysis might have func-tioned before life itself arose. From the structural point of view, metabolism consists in the transfer of groups or atoms from one molecule to another. As an example we may take adenosine triphosphate, or ATP. We have considered it in the role of a high-energy compound which the organism uses to force reactions uphill. We can also regard it, however, as a phosphate donor and acceptor (Figure 22). Its terminal phosphate can be trans-ferred to other compounds, and when it has lost a phosphate, it can accept a phosphate from other compounds. From this point of view, ATP is a key compound, a carrier or shuttle of phosphate groups from one compound to another.

The same is true of other groups or atoms. There is, for example, the formidable looking but very important compound called diphosphopyridine nucleotide, or DPN. DPN acts as a donor and acceptor of hydrogen atoms. When a compound loses hydrogen, or becomes oxidised, DPN accepts the hydrogen and becomes reduced. Reduced DPN can in turn donate its hydro-

ADENINE

RIBOSE - (?)

/ΥΟΝΗ.

(p) -RIBOSE ^J>M*

i H2

H2 H2 H2 H2 H2

ώ έ έι ώ έ ETC

Figure 29.—Diphosphopyridine nucleotide acts as a donor and acceptor of hydrogen from various compounds in the cell.


gens to the same compounds from which it received them, or to others. Like ATP, it is a universal carrier, this time not of phosphate groups, but of hydrogen atoms (Figure 29).

These are examples of a general biological principle. Groups or atoms are carried from one compound to another not directly, but via a single carrier. This is analogous to a banking system. Instead of one individual paying another, he deposits the money in a bank, which then pays it out to another individual. To con-tinue with the analogy, we could imagine banks which deal in only one form of currency, say francs, pounds, dollars, or yen, the different currencies corresponding to different chemical groups.

The directions in which groups will move via the carrier are determined by two factors, the equilibrium conditions and the presence of enzymes.

There are several other such carriers. Biotin, for example, carries C02 groups, folic acid, methyl groups. These carriers occur in very small amounts, but are vital key compounds. Many animals, including ourselves, are unable to synthesize parts of some of these carriers, so we have to obtain them in our food. From the nutritional point of view, we then refer to them as vitamins.

Because the presence of such carriers is needed to move a group from one compound to another even when enzymes are present, biochemists refer to them as cofactors.

Metabolism is thus organized into subdivisions, each sub-division concerned with a single chemical group. This makes it easy for the student to grasp the principles of biochemistry, but it also has more important consequences. It reduces the com-plexity of living matter to manageable proportions.

Suppose that there are 100 compounds in the cell which can receive or donate phosphate, and that we want to be able to transfer phosphate from any one compound to any other. If all transfers proceed via a single carrier, ATP, we will need a hundred enzymes to catalyze all possible reactions. If, however, transfers are directly from one compound to another, we will need 4,950 enzymes. The saving in the number of enzymes the cell has to make is thus very great. The same principle is used by telephone systems. It is quite impractical to connect every


subscriber by a direct line to every other. What we do is connect everybody to an exchange. This permits any subscriber to call any other with a minimum of installations.

To return to the origin of life, it is an interesting fact that some of these key compounds or carriers are themselves weak catalysts. A particularly striking example is hemin, which is a sort of key compound attached to a variety of proteins. One of these proteins is catalase. These enzymes, as we have seen, catalyzes the decomposition of hydrogen peroxide. Hemin, as shown in Figure 25, is an iron atom enclosed by a complex organic molecule called porphyrin. Now iron, as an ion dissolved in water, also weakly catalyzes the decomposition of hydrogen peroxide. This catalytic activity of iron ions is however increased 1000 times when it is surrounded by a porphyrin molecule. A complex molecule enhances the catalytic activity of iron. Even so, the catalytic activity of hemin is still weak. If now we attach the hemin to the right protein, catalase, its catalytic activity further increases at least 10,000,000 times.

The same is true of some of the more typical carriers. Thus flavine dinucleotide, a special carrier of hydrogen to oxygen, has a weak and non-specific catalytic activity by itself. Even without an enzyme, it can transfer hydrogen from some compounds to oxygen. Attached to the proper protein, its activity rises tremend-ously, as does its specificity.

We can conclude that a carrier molecule, by itself, is not only a carrier but frequently also a weak catalyst. When its would, of course, be favoured by the system, and since this led complexity is increased by attaching it to other compounds, especially to polymers of amino acids, its catalytic activity goes up, and so does its specificity. Such an increase in specificity to an increase in complexity, it also led to an increase in the molecular weights of catalysts.

The reflexive catalysts that arose in the primeval ocean were probably analogous and in some cases identical with the group carriers we now find in living systems. By themselves, they were weak and non-specific catalysts. Precisely because they lacked specificity, they promoted the formation of a large number of compounds, among them, presumably, polymers of amino acids.


These polymers, to some degree, increased the efficiency of the carrier-catalysts. Being themselves catalysts, these polymers were subject to the rules of reflexive catalysts, so that the more effi-cient they were, the more material from the entire system would be trapped in them.

The existence and development of systems of reflexive cata-lysts depends on the presence of metastable compounds at a high-energy level. It is the movement toward equilibrium which makes it possible for some catalysts to increase at the expense of some other compounds.

For this movement to continue, the supply of metastable compounds must be continuously replenished by coupling to it an energy source.

Right from the beginning such energy sources were pro-vided by lightning discharges and ultra-violet light from the sun. These are weak sources because ultra-violet light scarcely pene-trates the atmosphere and ocean and lightning is not too common. Until reflexive catalysis was well developed, the breakdown of metastable compounds was slow, so that considerable concentra-tions of them could nevertheless accumulate. Once reflexive cata-lysis got underway, however, degradation of metastable com-pounds would be rapid, and a coupling to a large and efficient source of energy would be needed to keep the supply of meta-stable compounds at a reasonable level.

If we had been present during the early days when life began, we would not have observed anything very dramatic. The land would have been bare rock and sand, and only a certain frothing and scumminess of the ocean might have attracted ouf attention. At some point, however, a noticeable change would have been observed. The water, slowly but steadily, would develop a green, or reddish colour. This development of colour signified that the system had developed a method of trapping the virtually inexhaustible energy of visible light.

Visible light is in plentiful supply, even to considerable depths of water, but the trouble with visible light is that most organic compounds are colourless; that is, they absorb light weakly. If light is not absorbed, it exerts no chemical effect. There are, however, some organic compounds which are highly


coloured. In a system operating by reflexive catalysis, components which were coloured and therefore would be raised to higher energy levels by absorbing light, would be automatically favoured, if, as a result of this, they produced compounds at a higher energy level that could be used by the system as a whole. We do not know for certain what the original coloured compounds were. At present the compound that serves this purpose is chlorophyl. It may not have been the first light-absorbing com-pound used by life, but it provides a model of what happened.

Chlorophyl is a special type of porphyrin, similar to that found in catalase and in hemoglobin. Its main characteristic is that the metal it surrounds is not iron, as in most porphyrins, but magnesium. One of the groups attached to it is a long ali-phatic "tail". Chlorophyl absorbs light mainly in the red, which explains its green colour (see Figure 25).

The exact structure of chlorophyl may be a later develop-ment, but as I mentioned before, porphyrins very similar to it are formed as a result of random synthesis, say by passing elec-trical discharges through gaseous mixtures of molecules. Porphy-rins are very stable compounds: they have been preserved for hundreds of millions of years in oil deposits. Because of this stability, we might expect that considerable amounts of them might have accumulated in the primitive ocean, even if the rate of their formation had been very low.

Porphyrins are also brilliantly coloured and combine spon-taneously with iron to form hemins. Just recently it has been discovered that if hemin is illuminated in the presence of phos-phate, the phosphate condenses with itself to form a compound called pyrophosphate:

OH OH OH OH | | hemin | |

HO — P — O H + HO — P — O H ^ > HO — P — O — P OH + H 2 0 II II light || || o o o o

Pyrophosphate is a simple analogue of ATP {Figure 22) in that it also is a high-energy compound, so that once it is formed, it can, in theory at least, provide the force to drive reactions


uphill. It is therefore quite possible that very early in the evo-lution of life the driving force of sunlight might have been harnessed to provide the energy to keep the primitive metabolic cycle of the ocean going. If so, the origin of life becomes much easier to understand.

The ultimate result of coupling this energy-source to the metabolic system was also important because it completely altered the environment of the earth. As I said before, we believe that originally there was an excess of hydrogen. Because molecular hydrogen is a very light molecule, any hydrogen not combined with other elements would eventually leak out into space, especi-ally if the earth was initially rather hot. Compounds of hydro-gen would, however, remain. Oxygen would all remain, mostly in the form of water and carbon dioxide. At some point the metabolic system developed a method, called photosynthesis, of using the energy of sunlight to split water into oxygen and hydro-gen and use the hydrogen to reduce carbon dioxide to organic compounds, according to the equation

C 0 2 + H 2 0 »-+ (CHOH) + 0 2

The oxygen is released as molecular oxygen into the atmosphere. Excess hydrogen having already been lost into space, oxygen had nothing to combine with in the atmosphere and began to accumulate.

Once free oxygen appeared, the metabolic system had avail-able an enormous new source of energy. Organic compounds could be oxidised, or burned, to C 0 2 and water, an energy-producing reaction which we carry on in our bodies and in various engines which are the products of our technology. This source of energy was virtually inexhaustible, because by trap-ping the energy of light the metabolic system continued to pro-duce organic compounds and oxygen from carbon dioxide and water. With this energy available, metabolic rates could increase and evolution could progress rapidly. You might ask at this point how oxygen could accumulate if for every oxygen molecule formed by photosynthesis an exactly equivalent amount of organic com-pounds was produced. You would expect that such organic compounds would be quickly reoxidised, so the amount of free


oxygen that could accumulate would be very small. The answer is provided by a non-biological process. Some of the organic material was buried as coal, oil, and shale. The total amount of such fossil organic material is about equivalent to the amount of oxygen in our atmosphere.

At the present time, the green plants of the earth produce, by photosynthesis, enough oxygen to renew all that is present in the atmosphere once every 8,000 years.

Although from the metabolic point of view the system of reflexive catalysis resembled a living organism, individual organ-isms had not yet appeared. This important step was probably the result of the properties of a special class of compounds.

Figure 30.—A fatty acid forms a film on the surface of water, with the hydro-phylic acid groups in the water and the hydrophobic hydrocarbon ends out. The molecules are held together by secondary bonds which give the

film some strength.

There exist an interesting group of substances called soaps. Technically, a soap is the metal salt of a fatty acid (Figure 30). Since there are a large variety of fatty acids, each of which can form salts with many metals, many kinds of soaps exist. The


ones that are used for washing are potassium salts of fatty acids obtained from animal or plant fats. From our present point of view, soaps are of interest because of their solubility properties in water. You will notice from Figure 30 that the fatty acid molecule has two parts, or better two ends. One end is a car-boxylic acid. This acid group is hydrophylic; that is, it has an affinity for and tends to dissolve in water. When this acid group reacts with a base of one of the lighter metals, such as potassium or sodium, its solubility in water is further increased.

On the other hand, the opposite end of the fatty acid has an hydrocarbon structure; that is, it is a compound of atoms of carbon and hydrogen only. Hydrocarbons, when mixed with water, do not dissolve in it, but being lighter, float to the top. The hydro-carbon end of a fatty acid is hydrophobic, or insoluble in water.

A long chain fatty acid in water is thus subject to two opposing tendencies. Its acid end tends to drive it into solution into water, its hydrophobic hydrocarbon end out of solution. As a result you get a compromise. The fatty acid forms an ordered film on the surface one molecule thick, in which all the acid ends are oriented toward the water, the hydrocarbon ends out. When an organized film exists, there is also another force which is important. Hydrocarbon chains, when they lie side by side, are weakly bound to each other by secondary chemical bonds. This gives the film a certain cohesion and resistance to rupture {Figure 30).

Films of this kind have one further important property. If there are other molecules, especially large ones, in solution, they tend to bind to the film. As a result, you get double layers, especially with proteins; such double layers tend to be more soluble. Instead of forming a film on the surface of the water, they round up into tiny particles and form what are called emulsions, which are actually droplets of one liquid in another.

If such double-ended molecules, one end hydrophylic and the other hydrophobic, were present in the primitive ocean, we might expect the course of evolution to be as follows. Because of pressure to increase the efficiency of catalytic compounds, the molecular size of catalysts would increase. These would tend to be absorbed to surface films of fatty acids or similar two-ended molecules. This, by increasing the solubility of the films, would


form emulsions. The ocean would break up into two phases, a continuous non-living phase, as it is now, and a dispersed phase of droplets containing the catalysts.

This break-up of the ocean would have two important con-sequences. By bringing one catalyst into closer proximity to another, the concentration of reactants would be locally increased, speeding up the reactions and improving the efficiency of the whole system.

The second consequences would be equally important. Drop-lets would now be separate individuals, each having a separate fate. Previously, since the entire ocean was one organism, it could only evolve as one entity in one direction. Its evolution would be dictated by chemical kinetics, not natural selection. Once separate organisms were present, the efficiencies of indi-vidual droplets would begin to determine which would increase and multiply faster. Evolution via chemical kinetics would be replaced by evolution by natural selection. When this point had been reached, we might say that evolution had at last definitely passed through the borderland that separates matter that is dead from that which is living.

The hypothetical membrane of protein and fatty material is, indeed, not only the turning point of evolution but still very much with us. A cell is mainly composed of membranes of this sort, curiously contorted and convoluted. If you look at Figure 31 , you can see that starting with a spherical membrane-bounded cell, infolding and pinching off the membranes has produced a large number of the structures of the cell. Studies with the electron microscope show that everywhere these membranes have the same basic structure, a fatty layer covered with protein. At the surface, this membrane regulates the exchange of material with the outside; inside the cell parts of it form the so-called ergastoplasm, on the surface of which, with the participation of nucleic acids, the cell synthesizes its proteins. Other membranes separate enzymes one from another. In nerves, membranes con-duct electrical impulses which convey information from one part of the body to another. There is evidence that such membranes may carry hereditary determinants which determine their own structure. Unfortunately, many of the details of these functions at present escape us. This borderland of biology, chemistry and


UNIT MEMBRANE

MITOCHONDRION

ENDOPLASMIC RETiCULUM

IMAGINATION

GRANULE

RiBOSOMES

GOLGI BODY

Figure 31.—A schematic section of an animal cell, showing how various cell structures are formed by invagination and folding of the surface membrane.

(Courtesy of Scientific American.)

physics is now becoming one of the most active fields of research. The schematic outline of the origin of life that I have pre-

sented to you is of course hypothetical. In detail it may be wrong, but the broad outline, I think, is correct. Life did originate in an aqueous solution of organic compounds, and we have traced evolution to the point where discreet droplets of fatty membranes containing a self-replicating system of catalysts came into being. This is still quite a distance from the finely designed genetic systems that Dr. Watson will describe to you. Because of the darkness that still surrounds this subject, I shall not speculate what the further steps to produce such genetic systems might have been. Although some details still escape us, the evolution of life seen in retrospect appears to have been so inevitable that many of us will ask whether it could have happened also on planets other than the earth. Are we alone in the universe or are there others like us? This is the next question that we shall consider.

CHAPTER 4

Alone in the Universe ?

Two years ago Professor Bracewell, my eminent predecessor here, discussed whether civilizations, some more advanced than our own, might exist on planets circling other stars, and whether it might be possible for us to communicate with them. His answer was a qualified "yes" to both questions*. The argument is as follows. There are about 200,000 million stars in our galaxy. Since there is no reason to suppose that our sun is unusual, many other stars might have planetary systems similar to our own on which life might have developed. Since many stars are older than the sun, life there might also be older, and therefore civilizations might be much further advanced. With our present technology, we can build radio transmitters power-ful enough to reach out to ten or more light years. If other civilizations exist, they might do as well or better, so that it is worth while listening for such signals. It is not impossible that in the near future we might detect them and eventually establish two-way communications.

Professor Bracewell has been more fortunate than his emin-ent predecessor on the same subject. Giordano Bruno, as most of you know, also believed that stars are suns like our own, circled by planets inhabited by intelligent beings. Instead of receiv-ing applause and an honorarium for expressing this opinion, he was burned at the stake in 1600.

If communications could be established with other civiliza-tions, especially with civilizations superior to our own, this would indeed be a most important development. To understand why this would be important, we may start by asking why did Giordano Bruno die?

* Prof. Bracewell's lecture is reprinted in this book. 217


Had you lived in Egypt 5000 years ago, you would have noticed that the Universe was quite different from what it is now. The earth was a flat disc floating on water. It was covered by an inverted bowl, supported on four pillars at the four cardinal points. Lamps hung here and there could be seen as stars. The sun, carried in a boat, crossed this bowl every day. In the evening it reached the western horizon, where it sank beneath the ocean and turned east under the earth. After illuminating the land of the dead, the sun emerged again on the eastern horizon and the astronomical cycle started all over again.

For our purposes the astronomical differences between the universe of the ancient Egyptians and our own are not as import-ant as the biological differences. Man, it was believed, was closely related to the various plants and animals that shared the universe with him. He was a little more intelligent, but this was no cause for self-congratulation. Above him were whole popu-lations of daemons and gods, of various degrees of intelligence and power, ranging from imps no better than man to beings enormously superior to him. These beings were not different from man as matter is from spirit. Everything in the universe was consubstantial, made from the same substance, which you could call matter or spirit as you will. The important point was that man was a part of nature; and since nature included beings much superior to man, he did not occupy a particularly important position in the scheme of things.

One had to live as best one could. Art might give pleasure, and technology might be useful; but there was little point to abstract thought and science. Obviously man could never hope to discover what was not already known to someone else; and this, as any scientist will tell you, takes all the fun out of the game of science. So science developed slowly and created little interest.

Thales of Miletus was the first who began to kill off gods and daemons. He discovered that the sun and moon had, appar-ently, no will of their own, but moved like dead bodies on preset courses. Thus, as Herodotus tells us, he could foretell the solar eclipse on the Halys which halted the famous battle between the Medes and Lydians to within a year of the actual date. Gradually other daemons and gods became extinct.

ALONE IN THE UNIVERSE? 219

Man gained tremendously through the extinction of daemons. Physically the size of the universe increased, so he became a smaller part of it, but intellectually he became unique. One god still remained, but this only accentuated man's unique position. This god had created the universe for the benefit of man, and having done so devoted most of his interest to man's sexual activities. Man ceased to be a part of nature and no longer hesitated to exploit the universe to his exclusive advantage. Knowing that he had no competition, science acquired a special interest.

Giordano Bruno died because he tried to bring back the daemons. His contemporaries felt that the very thought that beings superior to man could exist, even if only on distant stars, would take all the meaning out of life, so he had to be suppressed.

Ideas, as you know, are difficult to suppress, and daemon-ology is coming back in force. If Professor Bracewell is correct, modern man will sink back to a rather mediocre position, sur-rounded, as were the Egyptians, by beings consubstantial with him, but beings who have evolved so much longer that they are vastly superior to man in all respects. Anything we do will become trivial, since it will already have been done by others long before us. Science, in particular, will become pointless as an intellectual activity, although it might survive as technology. If we have any questions, we will get the answer by teletype from some benevolent daemon on a neighbouring star.

All this is not mere science-fiction. The belief in superior beings who live around us is gaining ground rapidly. A fairly large literature has developed on the subject, and astronomers have been able to wring money from a reluctant treasury to build expensive instruments and search for signals that will tell us that we are not unique in the universe. This is as it should be. Astronomy was the mother of the sciences, and it would be fitting if it became the one to make them pointless.

Since the Egyptians managed to live with daemons quite successfully, we may also; but the intellectual and moral basis of our lives will surely suffer a profound change.

Is such a development possible? Until we receive intelligible


signals from other solar systems, we cannot tell. It is neverthe-less of interest to consider whether the idea is reasonable or not.

The problem is best considered in three stages. First, what are the conditions necessary for the appearance of life and its further development to the stage of an advanced civilization? Second, where and how frequently are such conditions found? Thirdly, by what technical means can we communicate with such civilizations?

So far, these problems have been considered mainly by astronomers. I propose to discuss them here from the point of view of a biologist; but since astronomers have not hesitated to express biological opinions, you will perhaps pardon me if now and then I digress into astronomy. My colleagues in that field will not hesitate, I am sure, to correct me where I may be wrong.

If we accept as approximately correct the account of the origin of life I have presented, we must also accept the conclu-sion that, given the same conditions elsewhere, life would also very probably develop there. Its development was not due to the occurrence of a single, very rare event, but rather is the inevit-able outcome of the chemical and physical conditions of the earth. Is it possible, however, that forms of life very different from the ones we know can also exist? Do platinum fish swim in an ocean of molten lead on the planet Mercury, or beings exist which have silicon instead of carbon in their molecules? If our type of life is not the only possible kind, life might be found where conditions are quite unsuitable for us. This would greatly increase the number of places that are inhabited in the universe.

Asking such a question emphasizes what I have said several times before, that life is characterized by its behaviour, not by its composition. Aeroplanes can be built of aluminium, steel or wood, but they are all aeroplanes. Nevertheless, there are limits to this variety, because the material must meet certain specifications.

Considered rather abstractly, living things are characterized by the large amount of "information" they contain. "Information" is here used in a somewhat technical sense, meaning, roughly, the number of choices the organism can make between possible alternatives. By making finer and finer choices, it can specify the result in more and more detail. If, for example, it is


metabolizing sugar, it must distinguish the chemical type of sugar it wants from others. When forming an enzyme, it requires a tremendous store of information to specify exactly which sequence of amino acids it wants out of the enormous number of sequences possible. When an organism begins to be complex, it must create, with great precision, a certain form. The hands of a man, or the feathers of a bird, require a tremendous amount of specification.

Information to specify anything must reside in some material object or process. It may be a book, or a sequence of sound, or light waves, or any number of other things; but it always has a physical basis. The information stored in living objects is, for the most part, chemical information; and the "symbols" in which the specifications of life are written are chemical molecules. Since life is complex, its chemistry must also be complex; and this requires complex molecules.

An organism, therefore, cannot exist as a gas. Only relatively simple molecules are gases. If we try to vapourize a complex compound by heat, it decomposes. It also cannot be a solid, if only because chemical reactions and diffusion in solids are so slow as to be almost negligible. All life must exist, as does the life we know, in solution.

Our form of life is a solution of carbon compounds in water. We might legitimately ask: Is this just an accident, or is it because this is the only way in which life can be constructed? Can a chemical system be made of compounds other than carbon and manifest the properties of life?

If we consider that life has to contain a large amount of information, and thus be of great chemical complexity, the answer is clear. There is no element other than carbon which can form the number of compounds required. One can readily see this from the results of chemistry. The number of carbon compounds known exceeds 200,000, while that of all other elements put together is less than 20,000. The reason for this discrepancy, as I stated before, is that carbon can form long chains by combining with itself, and further can attach a large number of different groups to the molecule.

Some people nevertheless continue to refer to the possibility that some other element might fulfil the same function as carbon.


Silicon is often proposed as a substitute. Silicon is in the same column of the periodic table as carbon and, like carbon, it is a tetravalent element. In theory, one might suppose that there should be a branch of chemistry, the chemistry of silicon, which is as extensive as that of carbon. In fact this is not the case. Because of certain details, the chemistry of silicon is not very similar to carbon. True, it will form a silicon-to-silicon bond, but this bond is much weaker than the carbon-to-carbon bond. Long chains of silicon atoms, even if formed, disintegrate rapidly. Furthermore, silicon does not form double bonds the way carbon does. For example, carbon forms the compound carbon dioxide, O — C = O, which is a gas. This is very convenient for life, since carbon, as a gas, becomes a very mobile element; and furthermore can, in part because of its solubility, participate in many reactions. Silicon forms, in theory, a similar compound, silicon dioxide, Si02. Because of the inability of silicon to form double bonds, this compound is not O — Si -= O, but rather

[ I 0 —Si —O 1 I etc.

which is not a gas but a major constituent of rocks. Silicon is certainly not a carbon analogue. Other elements, when examined in detail, are even worse in this respect. Although it is hard to demonstrate a negative proposition, only carbon appears to have the ability to form compounds complex enough to be the basis of life.

Accepting this conclusion, one can still wonder whether carbon compounds could not be used to form living things in a manner different from that actually found. The answer to this question is undoubtedly yes. In living systems as we know them certain compounds can be replaced by others which do not occur naturally, and the organism will continue to function. For example, a constituent of the genetic material is thymine. In special situations it is possible to replace all the thymine with a similar compound, called bromouracil, where a bromine atom substitutes for the CH3 group of thymine (Figure 32). Thymine, in fact, is normally absent in the genetic material of certain viruses and is replaced by a compound called uracil.


1 ° IHN C

1 N ^

H : - C H

H

:.H

H

THYMINE

0

HN C-Br

1 1 OC CH

N H

BROMOURACIL

0

HN CH

OC CH N H

URACIL

Figure 32.—Thy mine is a component of the genetic material or DNA, but it may be replaced by promouracil or uracil and the DNA can still function. Apparently there are several compounds which can replace each other without

change in function.

As another example, one of the amino acids which compose proteins is methionine (Figure 11). It contains one atom of sulphur. It is possible to substitute an atom of selenium, an element rather similar to sulphur, for the sulphur atom of methionine quite well in proteins, and such unnatural organisms continue to flourish. There are other examples of this kind.

Undoubtedly a large number of somewhat different ways of constructing living things in the carbon-water system are possible. Some of them would be less efficient than others, especially during the early period of reflexive catalysis; and therefore would not be realized in practice. A number might each be equally efficient and which would actually arise would be a matter of chance. Thus at the moment, at least, we can see no reason why only 20 amino acids occur in proteins, and why these particular 20 and not others.

However, while the details would no doubt vary, organisms in the carbon water system would probably be rather similar in a general sort of way. Proteins constructed of different amino acids would still, for example, have similar solubilities in water to the ones we know. If they did not, they could scarcely manifest the catalytic and structural properties essential to life. If of similar solubility, their resistance to heating, or denaturation, would also


be in the range of the proteins we know. Such life would require about the same conditions as we do.

Admitting that carbon is indispensable, we may still ask whether water might not be replaced by another solvent. Let us consider what properties a biological solvent must have. A partial list of these is as follows:

1. It must occur in sufficient abundance so that it can form a significant liquid phase. Most elements are extremely rare, and are thus excluded from consideration. One could scarcely expect liquid neodymium fluoride, for example, to exist anywhere in significant quantities.

2. A related requirement is that the substance occur, in the required abundance, at temperatures and pressures where it is in the liquid state. To illustrate, the distribution of elements in the solar system is not uniform, the inner planets having more, the outer less, of heavy elements. This distribution is undoubtedly related to the mode of formation of the planets. Now carbon dioxide occurs in great abundance on Venus and to some extent on earth, precisely at those points where it cannot, because of too high a temperature, exist as a liquid.

3. The solvent must be stable. An ocean of hydrogen peroxide, for example, would explode spontaneously. Of course many other possible liquids decompose much more slowly, but since the biological solvent has to exist for thousands of millions of years, its stability has to be virtually absolute.

4. The solvent must be a liquid at a relatively low tempera-ture. As we raise the temperature, complex molecules begin to disintegrate. This requirement excludes from consideration such solvents as molten metals and inorganic salts.

5. The liquid must not have too low a boiling point. For life to develop, chemical reactions must proceed with reasonable speed. As the temperature falls, rates of reactions become, in general, less. As we shall see, the length of time required for the evolution of life is a significant fraction of the length of life of a star like the sun, so if chemical reactions are too slow, life will scarcely have time to get started before it is extinguished by the death of its sun. Considerations of this kind exclude solvents such as liquid hydrogen and helium. To reinforce this line of


argument, it should be pointed out that a low temperature implies a low input of energy from the sun. Since a living system is a solar engine, a biological solvent at low temperatures also implies a very low rate of photosynthesis and thus a slow metabolic turnover, quite apart from a slow rate of reaction imposed by low temperature per se.

6. The solvent must not be so reactive that it decomposes complex molecules in solution. Perchloric acid, for example, would not be suitable on this basis alone. On the other hand, it must have some reactivity, so that solvolysis (breakdown of certain bonds by addition of solvent and the reverse) can occur. This requirement makes substances such as hydrocarbons unlikely candidates.

7. The biological solvent must be a good solvent, able to dissolve a variety of simple and complex molecules. Hydrocarbons would again be unsuitable. They are good solvents for other hydrocarbons, but very poor ones for inorganic salts and organic compounds containing acidic, basic and hydroxyl groups.

When one takes into account that a solvent must meet all requirements simultaneously, this list of requirements, incomplete as it is, already excludes a vast variety of theoretically possible solvents. You will note that the unsuitability of a solvent may be manifested for quite different reasons: It may not be able to exist; it may require too high or too low a temperature; or it may be intrinsically unsuitable on chemical grounds. Taking all this into account, we are left with a small number of possibilities, the compounds that are found in reasonable abundance on planets, meteorites, and comets. The substances that might be candidates, at least at first sight, are listed in Table 1 with some of their properties. You will notice that water shows a unique constellation of properties. It boils, for example, at a remarkably high tem-perature. Its heat of melting and vaporization are uniquely high, as is its dielectric constant. It forms an ice which floats. It is only slightly reactive with most compounds. Furthermore, it is by far the best solvent known, in the variety of compounds it will dissolve without decomposing.

Ammonia runs a rather close second to water in its properties. We exclude ammonia because it is a liquid at too low a temperature.

Water

Ammonia

Carbon Dioxide

Methane

Hydrogen Cyanide

H 2 0

NH3

co2

CH4

HCN

Melting Point

0

— 78

— 57*

—184

— 14

Boiling Point

100

— 33

79**

—162

26

Dialectric Constant

81

22

1.6

1.7

116

Hwat of Vaporization

540

327

87

138

223

Heat of Melting

80

108

45

15

74

Specific Heat

1.0

1.0

0.3

0.8

0.6

TABLE 1.—Some physical constants of several liquids which might be considered as possible biological solvents. Note that in most respects water shows extreme properties. This is mainly the result of the hydrogen bonded structure of water. The uniqueness of water would be

even more evident if more liquids were considered.

* At 5.2 atmospheres. ** Sublimes at atmospheric pressure.

ON

LIG

HT

A

ND

L

IFE

IN

T

HE

U

NIV

ER

SE


This leaves us with water as the only possible solvent, a gratifying conclusion since water is optimum in so many respects.

I conclude that in considering the possibilities of life else-where, we need only consider environments suitable to the carbon-water system, that is, environments very similar to that of the earth. It is, however, not sufficient to have a suitable environment; it is also necessary to have enough time for life to develop.

SPACE TRAVEL SCIENCE -

CIVILIZATION -

MAN-

MAMMALS-

REPTILES-

AMPHIBIANS -

JELLYFISH -

BACTERIA -

ORIGIN -

- Q 1 üJ I ^ 1

- t> 1 < I tu 1

■ tr 1

~ LÜ 1 CD /

^ ^ » ^ ^ ^ ^ ^ ^ ^ ^ ^ * ^ ^ ^ ^

- ~~i " — 1 1 4000 3000 2000 1000

AGE BEFORE PRESENT IN MILLIONS OF YEARS

Figure 33.—The rate of organic evolution.

The only information we have on the rate of biological evolution is that provided by life on earth. The chronology of major events is diagrammed in Figure 33, and except for the earliest events is well established. We may consider life or reflexive catalysis to have started as soon as oceans formed, say not later than 4000 million years ago. Between the start of reflexive catalysis and the appearance of simple bacterial forms, 2000 million years, more or less, elapsed. Developments from these


forms to jellyfish, first found preserved in strata in South Australia, required 1000 million years. From there about 700 million years were required for the development of early land vertebrates, the amphibians. The amphibians developed into the great reptiles of the mesozoic and became extinct in about 200 million years. Mammals began to predominate 100 million years ago. About one million years ago our ancestors, the Australopithecine apes, began to make tools. Ten thousand years ago plants and animals began to be domesticated. Six thousand years ago writing, navigation and cities developed. Geometry and astronomy, first of the sciences, were well started a little over two thousand years ago.

Deliberate application of science to technology is about 500 years old, and man first left the earth's atmosphere six years ago.

Two points are illustrated by this chronology. The evolution of life is very slow, but on the other hand the rate is continuously accelerating. To take the first point, we may ask why is it slow? The answer is that evolution is a statistical process, dependent on the chance co-operation of many improbable events, and that these events must occur in a certain order. Take, for example, the development of fish into amphibians. It is not enough to equip a fish with lungs. Such fish now exist, but they use their lungs not for living on land, but rather to survive in muddy water whose oxygen content is low. To live on land, such a fish requires a variety of interrelated adaptations. In the water, a fish weighs almost nothing; on land its weight is considerable; and it requires strong limbs to movo. about. This in turn requires a reorganization of the nervous system to provide it with the required reflexes. Some of the sensory organs, such as the lateral line and electric sense, are useless on land. Because of the difference in refractive index between water and air, the curvature of the lens of the eye must be altered; and in addition a gland to keep the cornea moist must be developed. The ear must be remodelled, and the organs of smell, adapted to water, must be readapted to air. The internal physiology must meet new requirements. Nitrogenous breakdown products are excreted as ammonia by fish. Ammonia is very soluble in water, but is also toxic. A land animal cannot get rid of ammonia continuously, it must convert ammonia into another


non-toxic product such as urea, and excrete it via the kidneys. The regulation of the salt content of blood, previously done mainly via the gills, is now performed by the kidneys. The physiology of the skin is quite different in fish and amphibians. These are only some of the changes required, and all must be present before the change-over from fish to amphibian is complete. The necessity for numerous simultaneous changes, each more or less useless without the other, limits the rate at which evolution can occur.

It is not often appreciated that evolution is not merely a matter of the evolution of species, but equally important is the evolution of the entire biological system. A biologist does not regard a plant or animal as a separate object, isolated from the rest of the environment. It is a sub-system in a higher system, which in turn is a sub-system of an even higher system. This is true of all organisms, including man. Consider a few examples.

It is generally believed that the oxygen in our atmosphere is the result of photosynthesis. Although bacteria can exist by anaerobic, or oxygen-free reactions, these are very inefficient in producing energy, and cannot support larger and more complex plants and animals. Presumably, therefore, the evolution of higher forms must have been delayed for long ages until enough atmos-pheric oxygen accumulated, and until metabolic processes developed to take advantage of the fact.

Primitive animals feeding on bacteria would have no advan-tage in being large and complex. Such larger and more complex forms would develop only when their prey also reached some size and complexity. Animal life on land was obviously impossible until land plants appeared. This occurred, apparently, in the Silurian period. Once this happened, land animals developed rather rapidly.

These considerations explain why the overall rate of evolution is slow. They also explain why the rate has accelerated with time. As the number of components in a system increases, the possible ways of interaction between components increase not in proportion to their number, but rather to something more like their product. From two letters we can form four two-letter words, but from three, the number is nine. Once you have a certain complexity,


further progress is exponential. It was vastly more difficult to invent the hand-axe than to invent gunpowder, and much more difficult to invent gunpowder than television.

As we see, evolution to civilization on earth has required about 4500 million years. Since evolution is a statistical process, it should require about the same amount of time elsewhere, of the order of a few thousand million years.

Since we are unable to travel outside our solar system (at least not yet), from the practical point of view we are not interested in any form of extraterrestrial life, but only in forms of life which we could contact. If such forms of life exist, we might be able to do this by teletype, as Professor Bracewell has so well explained. Can we assume that if life has once developed, it will inevitably progress to intelligence if we give it enough time?

To judge from what happened on earth, I think the answer to this is no. Very special conditions are needed to enable a complex brain to evolve and to manifest its full capacity.

For reasons which I shall discuss immediately, complex brains cannot, apparently, develop in the ocean. This implies that the amount of water on a planet is a critical factor in the evolution of higher forms. If the earth had only a little more water, the continents would be submerged; and land, if it existed at all, would consist of small islands such as New Zealand. On such small bodies of land, air breathing forms might evolve but would be unlikely to develop into more complex forms in any reasonable period of time. For rapid biological progress, it is necessary to have very large land masses, presumably to allow large populations to interact and be subjected to varied conditions. Australia is a fairly large land mass; but from this point of view, it is small. When it was cut off from Asia ages ago, the Australian mammals evolved much more slowly than elsewhere, and remained at the more primitive marsupial stage. That this is no accident is shown by the similar biological history of South America. Until 20 million years ago, it was a continent like Australia, separated from the other continents by oceans. The mammals that were there also evolved slowly and inefficiently. When the land bridge to North America appeared, the inferiority of South American mammals became evident. Most of them, such as the giant sloth,


armadillo, and horse-like forms, rapidly became extinct when North American and Asiatic mammals, such as dogs, cats and deer, moved in.

Clearly large populations and thus large and varied land masses are needed for evolution at a reasonable rate.

A somewhat smaller quantity of water would produce larger bodies of land, but such land would be mainly desert. This again would vastly decrease the rate of evolution.

Why do we think that the development of landforms is a necessary condition for the development of a complex brain? Is this not merely prejudice because we ourselves are land dwellers? I think not. It is because the brain requires a constant internal environment which cannot develop in the ocean.

The eminent English physiologist Barcroft was interested in the mechanisms which the body uses to maintain constant the various parameters of the internal environment. To investigate these mechanisms, he often subjected himself to extreme conditions where his body could no longer maintain its internal environment constant. One example was a famous series of experiments where Barcroft lay nude in a cold room, so cold that his metabolism could no longer maintain a constant body temperature. As his body cooled, he described his subjective sensations via a micro-phone to his assistants. The significant result of such experiments, as summarized by Barcroft, was that any deviation from constancy in the internal condition of the body, in so far as it produced an effect, first affected the higher mental faculties. Other functions of the body were affected much later, if at all. It is this fact, of course, which makes anaesthesia possible. Consciousness is obliterated without harming other physiological functions. Diabetic coma precedes death. Absence of oxygen, as in drowning, may not harm other organs; but if it lasts even a few minutes, it causes permanent damage to the brain. The brain, and especially its higher functions, are the most sensitive to any deviation from constant conditions.

Conversely, animals which lack some of our higher mental functions are not bothered by large deviations from constancy. For example, we "black out" if the acidity of our blood changes ever so slightly, and we have elaborate mechanisms to prevent


this. Crocodiles, however, experience violent acidity changes in their blood after every meal without harm, because the higher mental functions are not there to be disturbed.

The conclusion is that the brain is the most complex of organs and that it cannot function unless its environment is held virtually absolutely constant. One might argue, however, that this is a peculiarity of man and might not apply to other organisms.

I believe that this is not a peculiarity of man and that the development of any kind of brain requires a remarkably close regulation of the internal environment. Temperature regulation cannot develop in the ocean because heat is lost too rapidly to water. To me it appears significant that no true marine animal has achieved a high mental level. The highest level achieved has been that of the fish and the octopus, who are about equal in this respect.

An octopus, as you know, is in one respect rather similar to man. He with his tentacles, as we with our hands, can manipulate material and even constructs some primitive shelters for himself. A better brain should be useful, but it has not developed.

On the other hand, the ancestors of whales and porpoises did develop temperature regulation on land. Even though these animals cannot manipulate objects, their brains continued to develop when they entered the ocean, showing that even if your only occupation is catching fish and plankton, natural selection finds good brains advantageous. The octopus did not develop a better brain, not because it would not be useful to him, but because lack of temperature regulation prevented him from doing so.

This conclusion is not particularly remarkable. It is to be expected that the more complex the mechanism, the more readily it might be thrown out of kilter by changes in the environment. The brain is by far the most complex thing we know, so naturally it requires the most closely regulated conditions to function.

Granting all this, does a complex brain and a suitable level of intelligence imply the development of an advanced culture? By no means. Until recently the opinion has been universal that we are the most intelligent animals that have developed on this earth. To some extent this depends on the definition of intelligence. There is no doubt that we have achieved the most in changing


and controlling our environment, so in this sense we are indeed the most intelligent. The reason why some people have begun to doubt whether we are potentially the most intelligent is related to the size of our brains.

In a general sort of way, the size of the brain determines the intelligence of an animal. Within a species the correlation is not, or course, perfect. The brain of the average modern human being weighs about 1450 grams. Some famous writers, such as Anatole France, had brains of about 1300 grams, while those of several idiots had 1800 grams. Some Neanderthal men had average brain-weights somewhat larger than ours. Whether they were more intelligent than modern man is not definitely known. It is not improbable that they were, since Neanderthal man is credited with the invention of fire and various tools, inventions which have been taken over by our species when we exterminated the Neanderthals, partly, no doubt, by hybridization.

Now some people have noticed that our brains are by no means the largest among the mammals. The family of whales have brains which are much larger than our own, up to 9000 grams. Until recently this has been dismissed by saying that since whales have much larger bodies than our own, they need larger brains to control them; and that so far as intelligence goes their brains are really smaller. This now appears doubtful. The number of muscles and other organs is no larger in whales than in ourselves, so that it is hard to see why larger brains would be required. Large sharks and the giant reptiles of the Mesozoic have or had very small brains, weighing only a few tens of grams. While there is some correlation between brain-size and body-size, it would appear that so far as intelligence is concerned the absolute size of the brain is also important. Since the brains of Cetaceans (members of the whale family) are so much larger than ours, it is a priori a fair guess that these animals may have an intelligence superior to our own.

This subject is now an active field of research. As you can well imagine, it is not easy to investigate the intellectual life of whales, and the few studies have been confined to the smaller whales, or porpoises, whose brains weigh about 1800 grams.

These mammals certainly display an astonishing ability to


perform complex actions when taught in captivity. It is known that they communicate by sounds; and there is some evidence that they can describe and not merely point to objects; but the nature of their language, and its content, has so far been difficult to elucidate, because it is at a very high pitch and very rapid. At the moment, therefore, we cannot be certain whether they are, as would be reasonable to expect, more intelligent than we are, but it is certain that they are very intelligent.

Irrespective of whether whales and porpoises are more or less intelligent than we are, we definitely show our technical superiority by rending them into blubber, making about 4% on our capital. Several of their species are extinct or nearly so. This, by itself, does not necessarily demonstrate our intellectual superiority. The organism responsible for syphilis, Treponema pallidum, has long preyed on man, although it probably has a lower intellectual capacity than we do.

What the porpoise and whale demonstrate is that a large brain and perhaps a very high intelligence is not sufficient by itself to produce a culture and technology. We may be less intelligent than the Cetaceans, but we manipulate objects. I think you will concede that if you had no arms or legs, and spent your entire life chasing squid or mackerel, no amount of innate intelligence would make you an intellectual.

If a big brain does develop and produces a culture, we may still not be able to establish communication with it, even if the technical capacity to do so exists on both sides. People tend to take it for granted that an extraterrestrial culture, especially if more advanced than our own, will want to communicate with us. This is by no means obvious. One may readily imagine that precisely because they have reached a level superior to our own they may have exhausted the possibilities of science and have lost their curiosity and interest in a lot of things that currently interest us. Science and technology have been growing at an ever-accelerating pace since early Sumer and Egypt. It would be naive, however, to assume that this will continue indefinitely. The usual curve of growth for almost anything is shown in Figure 34. Growth at first accelerates, then slows and gradually stops. Since our science and technology is still accelerating, no


O

<

TIME Figure 34.—Number of bacteria increasing with time in a test tube with a limited amount of food. This curve represents not only the growth of bacteria

but also almost anything else.

doubt it is not yet at the half-way mark, but it would be remark-able indeed if a limit did not exist. Superior civilizations, if they have long existed, would have reached such a limit long ago and probably ceased to interest themselves in science and technology.

Another obvious factor that might reduce the number of extra-terrestrial civilizations is that some might become extinct, especially by suicide. Man, as we know, is distinguished from other animals not only by a higher intellect, but also by a peculiar ferocity. Few if any other animals kill their own species, nor do they seem to enjoy killing anything else for the mere fun of it. We, presumably because of our higher intelligence, are different. To take a trivial example, the oryx, a member of the antelope family, has just been exterminated by being run down by cars mounted with machine guns. The only reason for this was that the oryx, like Mount Everest, was there to be killed. Game is


becoming scarce; but fortunately we can still find an outlet for this urge. Recent history is no doubt well known to you, so I will mention only a few incidents. The Indians killed off about a million of themselves in a few weeks to celebrate the success of non-violence in terminating British rule. During the last war, the Germans managed to kill civilians at the rate of about 2 million per annum, not counting military casualties. This is a rather slow rate, but Americans, again using civilian targets, improved this to about 100,000 per minute. Technical methods are still improving and so are psychological ones. When the British burnt Copenhagen down to the ground in 1807, this barbarity shocked the world, but now we consider such acts a mere trifle.

The situation is to some extent mitigated by the fact that we are motivated not only by a desire to kill, but also by fear of being killed. This induces a certain caution. Even so, we recognize that our situation is precarious. Since there is no reason to suppose that we are extreme in this respect, it is possible that a large fraction of extraterrestrial civilizations may have succumbed in this manner.

In summary, each step in evolution requires special conditions. A planet does not guarantee life, nor life a brain, nor does a brain guarantee a civilization. The higher the level that is reached, the fewer reach it.

For life to develop to the stage of a technological culture, we therefore need a planet that meets at least three requirements. It must be similar in chemical composition to the earth. Its distance from its sun must be such that its surface temperature is between the freezing and boiling point of water. Finally, the sun must exist long enough for intelligent life to develop, a matter of a few thousand million years. Do such planets exist, and if so, are they common?

Until recently it has been believed that our planetary system was the result of a close encounter between our sun and another star. Such encounters are so excessively rare that not more than a handful of systems like our own could exist in our galaxy. Recently, however, astronomers have discarded this hypothesis and have concluded that planetary systems arise as the by-product


of the condensation of a star from a mass of gas and dust. If so, the number of planetary systems is enormous, since there are about 200,000 million stars in our galaxy alone.

This conclusion is based on a hypothesis. Is there any observational evidence that other planetary systems exist? At the present time it is quite impossible to expect to observe planets around another star by optical methods. In two or three cases, however, faint and near stars have been observed to move in a manner indicating that their motion is perturbed by an invisible body having a mass somewhat greater than the planet Jupiter. To some degree, this supports the idea that other planetary systems may exist, although it must be admitted that it is debatable whether such bodies should be classified as planets or as very small stars of a double star system.

The second observation which to some degree argues for the existence of other planetary systems is the slow rotation of the sun. When a body is rotating freely, there is a quantity called angular momentum which remains constant. This quantity is pro-portional to the product of mass, radius of rotation, and velocity. The sun rotates around its axis once in 27 days. In our system most of the angular momentum resides in the planets, so that if the planets were to fall into the sun, the speed of rotation of the sun would increase enormously, from days to hours.

It is possible to determine the speed of rotation of stars from spectroscopic data. The general result is that stars much more massive than the sun rotate very rapidly, in a matter of hours, while stars with masses like the sun or less rotate slowly, about like the sun. One interpretation is that when large stars were formed, the primordial nebula all condensed into the star, so that all of the original angular momentum of the primordial nebula resides in the star, which thus rotates rapidly. The slower rotation of less massive stars could be explained by assuming that here a planetary system was formed, the planets retaining most of the angular momentum and causing the star to rotate slowly.

We might thus regard slow rotation as evidence for a planetary system. It is by no means certain, however, whether this line of reasoning is correct.


Observation thus gives little indication of the presence or absence of other planetary systems, and our belief in their existence rests on theory and analogy. Provisionally, however, let us assume that such systems do exist.

If such systems exist, how much time does a suitable planet have to develop life? A star converts its hydrogen into helium, with production of light, at a rate which is approximately pro-portional to the fourth power of its mass. When most of the hydrogen is gone, the star temporarily increases its radiation enormously, and then becomes a white dwarf. The final increase in brightness is so great that it would melt or vaporize its planets. The time available for biological evolution therefore depends critically on the mass. Our own sun has about 6-8 thousand million years, so we are somewhat past the half-way mark. Faint red stars, however, have life-spans of perhaps 100,000 million years. On the other hand, stars a little more massive than the sun have only a couple of thousand million years, and very massive stars may have a life-span of only a million years. So far as any prolonged evolution of life is concerned, we can discard all stars slightly more massive than the sun.

A suitable planet must be at a suitable distance from its star for life to develop. Its entire orbit must be in what has been called the "habitable zone". This zone is broad around a bright and massive star, and very narrow around a faint star (Figure 35). For life to develop, a planet must also have the right chemical composition.

Even if we assume that planetary systems are common, we have no detailed knowledge that would permit us to calculate quantitatively the probable distributions of sizes, compositions and distances of planets. To illustrate our ignorance, we may consider a few of the possible factors that might influence the nature of any planetary system formed.

We find that very old stars are very poor in elements heavier than hydrogen and helium. The galaxy is thought to have partially condensed into stars as a mass of almost pure hydrogen. Heavier elements were formed, it is believed, when the first massive stars exploded and scattered their material into space. This material condensed into the next generation of stars, the more massive


TOO COLO

M5

Figure 35.—Thermally Habitable Zone of various types of stars is represented by hachured area around star. Here the A5 spectral type has largest zone, but the star does not remain stable long enough for evolution to take place on a planet near it. Habitable zone of our sun (a G2 type) extends from orbit of Venus to orbit of Mars. Tiny M5 star has the smallest zone. (Courtesy of

Scientific American.)


members of which repeated the process. Each successive genera-tion of stars was formed from material which contained more heavy elements. If so, early stars might have no planets like the earth, which consists mainly of heavy elements. In somewhat later stars an insufficiency of heavy elements might produce only a small planet like Mercury or the moon in the habitable zone, unable to retain water and an atmosphere. Too much of heavy elements, on the other hand, might make such a planet so massive that it would retain an excess of hydrogen, the reducing properties of which would halt the development of complex forms of life. A suitable planet in a habitable zone might be confined to stars that originated at some special stage of the evolution of our galaxy.

About half or more of all stars are double or multiple. It is not impossible that a double star is an alternative to a planetary system. Even if this is not so, in a binary system the planetary orbits would be complicated and liable to leave the habitable zone.

No habitable planet is likely to exist around a variable star, which presumably would alternately scorch and freeze it.

Stars are believed to condense from a nebula in clusters. This in fact is why double stars are numerous; they have been formed in the vicinity of each other. The mere fact that stars in a single cluster vary tremendously in mass might indicate that their planetary systems, if any, would also show a tremendous range of variation, very few of which might have a chemically suitable planet in the habitable zone.

All this is a pure guess. Some astronomers like to stress the vast number of planetary systems that might exist, and from this infer that our galaxy is teeming with intelligent life. Since there is no evidence for this, it is just as reasonable to suppose the opposite. Each step in evolution toward a higher culture requires special conditions. Once you have a gaseous nebula, it has a certain probability of condensing into stars. A fraction of such stars may have planets. A fraction of such planets may have the right chemical composition, and a fraction of these may have orbits in a habitable zone. On these, primitive forms of life may develop. In many cases, the lifetime of the star will be too short


for life to reach our own level. A relatively small deviation from optimum conditions might inhibit the development of complex brains. Given a complex brain, not all species might develop a culture. Of those that did, some might have no interest in com-municating with us, and others would be too far away to com-municate even if they wanted to. We do not know.

We shall not know until there are observational methods which will make it possible to observe other planetary systems and to detect any intelligible signals that might originate there. Our present astronomical instruments are limited in size and effectiveness by the fact that they have to be situated on the surface of the earth. Because of the presence of the atmosphere, extraneous light and radio waves limit the resolution and sensitivity of our instruments. In addition, gravity, temperature changes and wind strongly limit the size of the instruments we can build.

No optical telescope much larger than the 200 inch mirror of Palomar is likely to be ever built on Earth; and if it were built, it would not be very useful because of atmospheric effects. The largest steerable mirror which radio astronomers now have is the 250-foot dish at Jodrell Bank in England. Americans have tried, and failed, to build one of 600 feet. Obviously we are near the limit of what can be done. In any case, radio astronomy on earth is slowly but surely being choked by the proliferation of interference from our own transmissions.

Such limitations would not exist if observatories could be built in space. Perhaps we could then have optical mirrors many times larger than the 200 inch, and dishes to detect radio signals many miles, instead of many feet in diameter. If we do not become extinct, such things might become reality within the next century. We would then learn whether the Egyptians were right in thinking that man is a rather mediocre creature and that many beings exist who vastly surpass him. What the effect of such information would be on the human race is difficult to predict, but it will certainly be interesting.

CHAPTER 5

Reason and Purpose in Biology

In my first lecture I defined a living thing as a material object with a purpose. I should now explain why I chose such a definition and what I mean by purpose, a concept which is fundamental to biology.

Living things show purpose because they are goal-seeking mechanisms. What such mechanisms are and how they operate is best explained by considering a few examples.

Suppose you are an artillery officer whose purpose is to direct the fire of a battery on to a stationary target. To start with you have only a vague idea of the relative positions of the target, your observation post and your battery. You guess that the range is 8000 yards and fire one gun. You observe that the point of impact is in line with the target, but it is "over", i.e., the elevation of your gun is too great. You do not know by how much it is too great, but you guess it is 2000 yards. You order the elevation lowered to correspond to a range of 6000 yards and fire again. This time it is "under", or falls short. The correct range is therefore between 6000 and 8000 yards. You try 7000, it is over. Since 6500 is also over, you fire at 6250. Over again, you try 6125 and hit the target. This process of determining the correct elevation of the guns by systematic trial is called bracketing.

You now know the correct elevation. Some time later you receive an order to repeat your fire on the same target. You had previously "zeroed in" when the air was still, but now you have a wind of 20 miles an hour blowing directly from the target toward your battery; and it is raining. Both of these effects impede the flight of your shells. You therefore consult your firing tables to find out how much your elevation should be raised to compensate

243


for this, order the necessary change, and fire. With some luck, you are again on target.

The system here is a typical goal-seeking mechanism; it moves from any state whatever to a predetermined state, which in this case is an impact on the target.

The way in which it does this is as follows. The point of impact is determined by a variety of factors, such as the weight of the shell, the length of the gun barrel, the amount and quality of the propellant charge, the elevation of the gun, etc. In addition to the gun, there is a sensing and computing device, in this case the observer. The gun nres and the shell impacts at some random point. The observer measures the deviation of the point of impact from the target. Notice that the measurement does not have to be quantitative; it is sufficient to know whether it is over or under. Having observed the deviation, the sensing device reacts back on one of the factors which affect the point of impact in such a way as to decrease the deviation. For practical reasons the factor which is changed is the elevation of the gun barrel, but theoretically you could also change the quantity of the propellant charge. This in fact is done with howitzers, when you want to change the range by a large factor or change the angle at which the shell approaches the target. In any case the deviation is reduced. The process is repeated and each time the deviation, as measured, corrects itself by reacting on the factor, the elevation, which produces it. The impact point thus oscillates back and forth, gradually approaching the target. When on target, the goal has been reached.

The change in atmospheric conditions illustrates a somewhat more complex application of the same principle. Here the sensing device signals the presence of conditions which would produce a deviation. Since the guns have not fired, this deviation is virtual, but it produces the same result as if it had actually been observed. Using his firing tables, the observer calculates the magnitude of the virtual deviation and again changes the parameters of his system to compensate for it.

To summarize, the deviation from the goal reacts back on a factor producing the deviation to nullify it. A goal-seeking system of this kind is called a negative feedback mechanism. You

REASON AND PURPOSE IN BIOLOGY 245

will note the ingenuity and beauty of this simple principle. It will operate no matter what is the reason for the initial deviation from the present goal. The guns may be worn, the ammunition may be defective, there may be an unusual wind and barometric pressure and any number of other reasons for the deviation. It is only necessary that the deviation react back to some factor which produces it, and the goal can be achieved. A mechanism of this kind makes the system independent of the external environ-ment by automatically compensating for variations in external conditions.

Goal-seeking is the most fundamental characteristic of living things. It is not something that is added as an afterthought or refinement to a living thing, but is the living thing itself. Even to carry on the simplest biochemical functions a feedback mechanism is required, otherwise the result is merely a jumble of unco-ordinated reactions. What I have previously told you about autocatalytic systems and what Dr. Watson will tell you about the genetic code is a necessary first step to understanding their function, but is only important in so far as it contributes to our understanding of goal-seeking, which is the real essence of life. Let me present some examples of biological goal-seeking mechanisms at increasing levels of complexity.

A bacterium, in order to grow at an optimal rate, has to maintain some amino acid at a specified level inside the cell. To do this it produces an enzyme which makes the amino acid. The rate at which the enzyme is made is not, however, constant. If the amino acid level is low, it produces the enzyme at a maximal rate, which in turn causes a rise in the level of the amino acid. Now things are so arranged that the synthesis of the enzyme is inhibited by the presence of the amino acid, so that as more enzyme is made, there is more amino acid and therefore the rate of enzyme formation falls. The system finally reaches its goal, the maintenance of the requisite level of amino acid irrespec-tive of the amount of amino acid that was initially present. The feedback signal which controls the amount of amino acid present is the level of the amino acid itself. (Figure 36).

Let us take another example. When a muscle performs work, the energy for this comes from the breakdown of sugar. When


ΓΜ ENZYME ^AMINO

GENE ACID

Figure 36.—An example of a feedback mechanism. A bacterium produces an enzyme which makes amino acid. The amino acid in turn inhibits the production of the enzyme which makes it. Eventually this mechanism

produces a constant level of amino acid in the bacterial cell.

you drive a car, you press on the accelerator to admit the proper amount of fuel to the engine for the task it has to perform. In the same way, the muscle needs some kind of mechanism to regulate the breakdown of sugar according to the amount of work the muscle is doing.

Sugar is broken in a series of reversible steps to yield a compound called glyceraldehyde phosphate (Figure 24). This product then picks up another phosphate to give glyceraldehyde diphosphate. As I mentioned before, this is a downhill reaction. Glyceraldehyde diphosphate is oxidized to phosphoglyceric acid, where the phosphate is now a high energy bond. The phosphate is transferred to ADP to form ATP. When muscle contracts, the energy for the contraction comes from the breakdown of ATP to ADP. This breakdown regenerates phosphate.

When muscle is not working, it is not using ATP. As a result, phosphate is depleted by conversion to ATP. Since phosphate becomes low, the formation of glyceraldehyde diphos-phate slows down, and since reactions back of it are reversible, the breakdown of sugar also slows or comes to a stop. Thus when muscle is not working sugar is not used.

When muscle starts to work the situation is reversed. ATP breaks down and phosphate is liberated. This in turn produces glyceraldehyde diphosphate, which in turn causes sugar to break down. More ATP is now produced, which is exactly what is required to keep the muscle working.

In this system a deviation from the goal, which is the


required rate of breakdown of sugar, produces a signal, the phosphate concentration, which increases or decreases the rate of sugar breakdown.

Take another example. Mammals and birds keep their body temperatures approximately constant. They are able to do this in spite of very large fluctuations in the production of internal heat, as when we engage in violent exercise or rest, and large fluctuations in the external temperature. There is a sensing device in the brain such that if the temperature rises, a large number of different reactions are brought into play which promote the loss of heat. The blood vessels of the skin dilate, so that more of the blood flows close to the surface of the body and can cool. The sweat glands begin to operate, and the evaporation of sweat again decreases the body temperature. You feel reluctant to undertake muscular exercise, which also decreases the production of body heat. On the other hand, if the body cools too much, you shiver, which increases heat production. Secretion of sweat stops, and blood vessels near the surface contract. This raises the body temperature. The signal which sets these mechanisms in motion is the difference between the goal, a certain body temperature, and the actual temperature. The greater this difference, the greater the corrective response.

There are innumerable other examples of living things regu-lating conditions so as to produce a certain state, irrespective of perturbations. Simple processes, such as these I have presented above, are called by physiologists homeostasis or maintenance of constant internal conditions.

The study of goal-seeking mechanics has recently been given a new name, cybernetics. I am afraid that the biologist is not sufficiently respectful of this new science, since he claims that this principle goes back to the work of the French physiologist Claude Bernard a century ago, if not to earlier works, and has been a part of biological thinking ever since. Cybernetics developed when engineers and mathematicians noticed that they were building goal-seeking machines, and that these machines, in their functions, resembled living things. Take for example the regulation of the temperature of a house. Here a sensing device, called a thermo-stat, measures the internal temperature. If the temperature falls


below a preset level, the furnace is turned on. In principle it is very similar to the method our own bodies use. Once the engineer grasped the biological principle of feedback, very complicated goal-seeking devices have been built. I gave you an example in my first lecture, a target-seeking anti-aircraft missile. The missile incorporates a device which computes its trajectory at any given moment. It also has a sensing device, radar, which provides the data to compute the trajectory of the target. The two trajectories are compared, and the deviation from the desired goal, a collision, is measured. The amount of deviation then determines a change in the trajectory of the missile. In principle all this can be built into the missile itself, but in practice it is more convenient to locate these devices on the ground and merely feed in the final result, the corrections of the missile's trajectory, to the missile by radio.

In spite of what the biologist tends to think, cybernetics has made a contribution not only to the construction of goal-seeking inanimate mechanisms but also to biology. It has formalized the concepts of feedback and goal-seeking and made them amenable to quantitative treatment. The operation of a goal-seeking mechanism depends on the quantative aspect. There is a certain time-delay in the transmission of the feedback signal and also a certain optimum relation between the size of the deviation and the resulting correction. If these quantities are not within the right range, the system will not seek its goal. For example, the system may be so arranged that when a deviation occurs, the correction is too great. In such a case you get what is called an overshoot. The system will then oscillate, or "hunt". Or the feedback may change from negative to positive. In this case, instead of a deviation reducing the cause which produces it, it increases it. When this happens, the machine inverts its goal. Instead of approaching it, it seeks to deviate from it as much as possible.

Breakdowns of goal-seeking occur in biological mechanisms. Let me give you two examples.

When you want to pick up a pencil, your sense organs give you the position of the pencil and the position of your hand. You compute the difference between the two and signal the appropriate


muscles to produce a certain movement which will reduce the difference to zero. You may however suffer from a disease of the nervous system which changes the amount of response to a given deviation. As a result, you signal a muscle movement which is too great, so you overshoot the pencil. You then go back, over-shooting again by a less amount, finally, with some difficulty, reaching the pencil. This symptom, due to incorrect quantitative relations in the feedback mechanism, is called "intention tremor".

Take another example. There are a number of mechanisms which regulate the rate at which the heart pumps blood, depending on the needs of the body. Normally the heart has a very large excess capacity; and when it receives the signal to work harder, as in exercise, it has no trouble in meeting the demand. Suppose, however, that for some reason it is damaged, so its capacity is reduced. It also receives the signal to work harder and tries to do so. However, it is now strained to the limit, and responding to the signal further weakens it. As a result its output falls. This results in signals to work even harder. The heart again makes the attempt, which further weakens it. Eventually the heart fails. The patient has died of cardiac decompensation. This illustrates a phenomenon common in diseases, a vicious cycle. The goal-seeking mechanism, because of incorrect quantitative relations, drifts away from the goal instead of approaching it.

The development of cybernetics has made it possible to study such cases in a more quantitative way and understand the phenomena more fully.

From the examples I have presented, you will note that goal-seeking can be directed to achieving some internal state, such as maintaining body temperature constant, or may involve external behaviour, such as picking up a pencil. There is no essential difference between the two, and frequently both may be used to perform the same function. We regulate our body temperature mainly by elaborate internal feedback mechanisms. Reptiles lack these mechanisms. Nevertheless, some snakes and lizards are able to control their body temperature quite closely. In a hot desert they would quickly die from overheating if exposed to the full sun. They therefore dig, or find, deep burrows which are cool. During the day they move to that part of the burrow which


is at the optimum temperature, leaving the burrow only after sun-down. Bees can also regulate the temperature of their hive to some extent, by body movements which produce heat, or cooling the hive by evaporating water they bring in.

So far the goal-seeking mechanisms I have discussed have been of a type whose detailed functioning we more or less under-stand. As we reach more complex levels, the details escape us. So far as we can tell they appear to be extensions of the sort of thing we find at lower levels. Although we do not understand the details, the more complex goal-seeking mechanisms are so important that we cannot fail to discuss them.

More complex external behaviour which is goal-seeking and which is innate, so the animal does not have to be taught to react in a certain way, is called an instinct. Thus spiders build webs not because their parents teach them how to do so, but because the neural connections in their brains are genetically determined to produce this result. All animals have a variety of instincts which make their survival possible. The instinctive drive to find food and engage in copulation are obvious examples. Most remarkable, perhaps, is an untaught ability to recognize and inter-pret forms. Thus goslings, if presented with a black cardboard object like that shown in Figure 37, react quite differently depending on the way it is moving. If it moves to the left, they pay no attention to it. If it moves to the right, they scurry for shelter. The utility of these reactions is obvious. If the object moves with the long end forward, it is the silhouette of a swan, a harmless object. If it moves with the short end forward, it is the silhouette of an eagle, which is obviously to be avoided. There is some evidence, though as yet no con-clusive proof, that birds which regularly migrate long distances have an innate knowledge of the constellations. In conjunction with a biological clock, this makes it possible, so some people think, for the birds to compute their position on the earth's surface and thus arrive at their correct destinations by astronomical navigation. This, by itself, would be remarkable enough; but it also raises an even more interesting problem. Because of the precession of the equinoxes, which has a cycle of 26,000 years, the genetically determined computer in the bird's brain must change some parameter to keep in step with the cycle. Whether


EAGLE Figure 37.—Goslings and other young birds of the duck family recognise a cardboard figure of this shape as harmless when it moves to the left, and dangerous when it moves to the right. Presumably it is a swan in the first case, and an eagle in the second. (Courtesy of Dr. N. Tinbergen and

Oxford University Press.)

this be so or not, remarkable feats of navigation are a fact. The Golden Plover, hatched in Alaska in the spring, flies non-stop in the autumn to Hawaii, a mere pinpoint in the ocean. In case this feat fails to impress you, imagine what your own insurance company would think if you were born in Alaska, had never seen a map, had no navigational instruments, had never heard of Hawaii and took off in a light plane in a generally southern direction. The Golden Plovers hit it on the nose. Best wishes, and good-bye, to you.

If we compare different animals, we notice that the same instinctive drive can be gratified by an increasingly complex behaviour. Consider the following example. On one side of a fence is a chicken, a dog, and a man, all hungry. On the other side is food. The chicken runs back and forth but cannot reach the food. Both the dog and the man notice that the fence is open at one end, so they make a detour and reach the food. (The


dog, being faster, gets there first; but this is irrelevant to my point.) Instead of bumping into the fence, the dog and the man mentally construct a symbolic or virtual representation of the process and notice that the direct route is blocked by an obstacle. They then symbolically construct alternative routes and notice that one is feasible. The dog and man are, in this respect, more intelligent than the chicken; but this is a matter of degree. If the fence were shorter, the chicken would also detour around it. If the fence were replaced by a very complicated maze, even the most intelligent man would fail to reach the food.

To illustrate another aspect of the increasing complexity of behaviour, consider another example.

If a small monkey sees a banana it wants, it reaches out its hand and takes it. If, however, the banana is hung by a string from the ceiling, the monkey hops in frustration. Enters, a chimpanzee. He also wants the banana, and he also cannot reach it. Instead of hopping about frantically, he sits down and examines the situation carefully. All at once the solution comes to him. He takes some boxes stacked along the wall, piles one on top of another, climbs up and triumphantly grabs the banana.

Now enters a man. At first sight, the situation is hopeless; the banana has already been eaten. One should not, however, underestimate his ingenuity. He flies to Australia, delivers some lectures ön "Light and Life in the Universe", receives ä sum of money for doing so and then buys a banana.

In all these cases the drive, or motive, is the same: to get a banana. However, the monkey can gratify this drive only if the banana is immediately available, while the chimpanzee and man can also do so by indirection, the man more so than the chimpanzee.

"Indirection" here means that to achieve a primary goal, you construct a series of secondary goals, which, when reached, will achieve the primary goal. The secondary goal for the chimpanzee was building a pile of boxes. The man used the same general idea, but his secondary goals were very numerous. We say that the man was more intelligent, not because his goal was different but because he used more complex methods to achieve it. By


using more complex methods, he could achieve success where the chimpanzee would conclude that the situation is hopeless.

We tend to think that instincts are something confined to lower animals and that we operate by reason, not by instinct. The behaviour of the beaver who builds dams becomes mysterious to us, in spite of the fact that we build dams ourselves. This harks back to the idea that man is not a part of nature and operates by methods which are different from that of other living creatures.

It is perfectly correct that our behaviour is different from that of ants, but the difference is not due to the fact that they operate by instinct while we do not. The difference stems from the fact that we gratify our instinctive drives in indirect and complex ways which are much more effective. The goal of our actions remains, however, to gratify our primary instinctive drives. Because our methods of doing this are so complex, we may become confused and not realize that we are doing this.

Hens in a barnyard, as well as many other animals, have an instinctive drive to establish what is called a pecking order. When it comes to roosts, nesting sites, or food, a hen always defers to another hen higher in the pecking order. Initially the order is established by fighting or bluff and remains more or less stable thereafter. Among hens this instinct is rather weak, but in man it is very strong. The method of gratifying this instinct, however, is often much more indirect and subtle, and varies depending on the group to which he belongs. If a man is engaged in some business, his ostensible aim is to make money for food and shelter and the support of his family. If he makes only a little money, this is indeed his dominant motive. Often, however, he makes much more money than he can spend for these purposes. You might suppose that he would then take it easy and cease working. Nothing of the kind happens. The more money he has, the harder he works. He can buy a flashier car and deck out his wife with diamonds. This will cause his friends envy, and he will rise in the pecking order, which of course is his real reason for working. In economics this is called conspicuous consumption. If he is very successful, he may use more subtle methods. He may drive a second-hand car and wear shabby clothes. This


signifies that he is now so superior that he no longer has to demonstrate it, thus vastly increasing the envy of his colleagues and his own gratification.

Consider another group. It is popularly supposed that scien-tists are primarily interested in discovering how the universe is constructed and how it operates. This opinion is held only by those who have never attended a scientific meeting. Having both organized and attended scientific meetings, I can describe to you what actually takes place. Invitations to attend are issued to people you want to impress with your organizing ability, selecting those whose opinions agree with yours. However, your status improves if you have a reputation for being objective, so you deliberately invite a sprinkling of those you do not like, either personally or scientifically. Once the meeting gets under way, facts and theories are reported. The purpose of such reports is to impress the audience. The audience, of course, does not want to be impressed, as this lowers them in the pecking order. Since there is safety in numbers, the participants break up iixta groups, each trying to get the better of the other. They applaud members of their own group, and gang up on others. There are certain rules to this game. Theoretically, you bow to the evidence; but the evidence is often ambiguous, so there is a lot of leeway. If the facts do not agree with your point of view, you can usually find some way of explaining the facts away. Max Planck, the founder of quantum theory, remarked that new ideas progress not because they convince the opposition, but because the opposition dies off.

To continue with our meeting. The most vociferous group now flies to Washington or Moscow, as the case may be, and informs the government that it is urgent to determine if there are earthquakes on the moon. There is a rumour that the French or British are working on a new ram-jet operating at Mach 20, which will make it possible to put much larger payloads into space more cheaply, so that a delay might result in better data at less cost. Thus there is no time to be lost. This report throws the government into consternation and results in a large appro-priation of money for the project and for a new scientific meeting.

The real motive here is to gratify the instinctive drive to


obtain the best possible position in the social order. While hens do so by fighting and bluff, man creates various sub-goals, science, money, or whatnot, the achievement of which promotes, sometimes quite indirectly, the gratification of instinctive drives. The instinctive drives remain basic, however.

One should not sneer at instinctive drives. The role of instinctive drives in life is absolutely indispensable; and no organism, including man, could exist for a moment without them. The reason they are indispensable is that they are non-rational. Notice that I did not say irrational, or contrary to reason. Reason, by itself, does not provide a motive for doing anything. If you try to find a rational motive for the idea that it is better to be alive than dead, you will always end up by saying that this is the way you feel. If you operated purely rationally, you would never do anything. Instinctive drives, in biology, are analogous to the system of axioms in geometry. You cannot prove the axioms; your only reason for accepting them is that if you did not, geometry would be impossible. Of course you can choose different systems of axioms, but some system has to be chosen. In the same way, unless some basic, non-rational drives exist, no organism can operate.

The role of reason is twofold. On the one hand, it can indicate more effective methods of gratifying our instinctive drives. On the other, if we are motivated by several instinctive drives simultaneously, it can indicate whether they are compatible or not. Suppose, for example, that you have a cardiac condition which contra-indicates violent exercise. Reason, in the form of physiology, will tell you that you may refrain from such exercise, and risk living, or engage in it, and risk dying. It does not tell you which you should choose, only what the result of your choice will be.

When I presented business men and scientists as examples of men striving to gratify an instinctual drive, in this case a rise in the pecking order, I presented somewhat of a caricature of the actual situation. Such men are not driven exclusively by one instinctive drive, but by complex motives. It is true that originally all our actions are dominated by a desire to gratify some instinct. In order to gratify our instincts, we create secondary goals, the


achievement of which makes it possible to gratify our primary drives. Once, however, such secondary goals are established, they achieve a life of their own and are done for their own sake. To many business men and to many scientists, making money or investigating the universe does become a goal in itself.

There is, however, a fundamental difference between instinctive drives and drives derived from secondary goals. Each instinctive drive has associated with it a specific emotional quality. Instinctive drives which are continuous, such as breathing, have an emotional accompaniment when they are interrupted. If you start to drown, the interruption of breathing is very unpleasant. If the instinctive drive is gratified intermittently, a pleasant sensation is associated with its gratification. Examples are eating and copulation. There are apparently as many emotions as there are instinctive drives, and vice versa.

On the other hand, secondary goals, which are artificially created, have no specific emotional quality attached to them. There is no specific type of pleasure attached to solving quadratic equations. Only a rather faint emotional quality may attach to secondary goals in so far as they may be associated with primary instinctive drives.

The fact that a secondary drive does not have a specific emotional quality does not mean that it is necessarily weak. A secondary goal can become a habit, and a habit can become very strong. In fact it can be so strong that it is able to suppress by replacement many instinctive drives. It is interesting to read some of the autobiographical writing of Charles Darwin with this in mind. As he tells us, he developed a secondary drive which was almost overwhelming, a drive to work in science and the theory of organic evolution. This drive reduced his normal instinctive drives to a minimum, and certainly achieved its goal, since Darwin was undoubtedly one of the greatest scientists of all time. Darwin tells us he obtained no satisfaction from all this. His whole personality was taken over by a secondary goal which had no emotional quality attached to it, so that subjectively he felt somewhat as one might imagine a computer to feel. This I believe is a not uncommon experience of people dominated too exclusively by intellectual or secondary goals. If we demolish


our instinctive drives, life becomes empty. The intellect is invalu-able as a slave of instinct, but kills the soul when it becomes the master.

I have now come to the end of my discussion of biology as a theoretical science. We started at the most primitive level, a level called molecular biology, and in ascending order considered higher and higher levels of organization. The highest level that we know is our own personality, the most important object in biology. What can we say that is significant about this problem? What are the problems in biology that remain?

The problems that remain unsolved in biology are numerous and important, so numerous that I will not list them here. Never-theless, most of them are of the same type as those which have been solved; and there is no doubt that given enough time and effort, they too will be solved. The most intractable appears to be the physiology of the central nervous system, and especially the mode of functioning of the brain. This will probably require at least a few more generations of workers. Here too, however, the construction of machines which are able to imitate some of our higher mental functions, while throwing little or no light on how our brain actually functions, indicates that a solution in terms of some kind of circuitry will become available.

At one time biologists were divided into two schools of thought, the vitalists and the mechanists. Vitalists postulated that there exists a special force or principle which directs living matter, not found in inanimate phenomena. Mechanists believed that all functions of life will be explained by physical and chemical principles.

The motives for the division between mechanists and vitalists were not really scientific. It was widely believed by both sides that a victory by one or another would have theological implica-tions. Thus if all life could be reduced to mechanism, man would be deprived of a soul, free will and other spiritual attributes, while if it could not be so reduced, such attributes might remain possible. Thus the acerbity of the discussion. How does this controversy stand today?

There is no question that, up to a point, the mechanists have achieved an overwhelming victory; one phenomenon after another


has become comprehensible in purely physical terms. There remain, however, some situations which are puzzling and important, and which require careful consideration.

The most obvious problems are subjective phenomena. To take an example, when light of a certain wavelength falls on your retina, you see the colour blue. The physicist informs you that you see blue, not red, because the wavelength of incident light is 4000 angstroms, not 6000. This is perfectly correct, but does not explain why you see blue, not red, nor why you see colour at all, rather than a quantitative change corresponding to a difference between the numbers 4000 and 6000. In fact, there is no explanation for the existence of sensations in general. We do not attribute a sensation of a photoelectric device which can distinguish one colour from another, so how do we come to have sensations?

Again the physicist has an answer. The answer is given in terms of a scientific method called operationalism. Roughly, this method accepts as scientifically real any phenomenon which can be objectively described and thus communicated by one person to another. Thus, for example, a length is real, because I can describe to you the procedure for measuring it; and you can then repeat it. On the other hand, there is no way I can describe to you my sensation of blue. I can of course point to a blue object, but how do I know that it does not look red to you? I can only proceed by assuming an analogy between you and me. Since, even in principle, there are no objectively describable operations which could resolve this problem, the operationalist states that this is a pseudo-problem, i.e., no problem really exists. So far as science is concerned, there is no sensation of the colour blue. Note that it is only the sensation that does not exist in the operational sense. There is no difficulty in distinguishing blue and red light by objective effects, say with filters and photocells.

Operationalism pushed to this point gives even a physicist pause. When he sees a blue light, he will not hesitate to admit that the sensation of seeing blue is much more real and certain to him than his most strongly held belief in Newton's laws of motion. Why then is this sensation not real for science as well as for himself?


We can answer this question only by considering what physics and biology, jointly, are actually doing. Let us begin with physics. A physicist is concerned with producing a sort of map of the world. This map, being a map, is not the world itself, but a representation of it. In fact the physicist is interested not in the actual world but in generalizations about it; but this need not concern us here. The map is filled with a variety of entities behaving in a certain way, the details of which again need not concern us. What does concern us is that the physicist stations all over his map other entities which he calls observers. This point is quite funda-mental for him, since if he is not permitted to do so his phenomena become unobservable and thus, operationally, unreal or non-existent.

Now what are these observers? They are a certain class of biological objects, to wit, ourselves. Of course, not every observer need be physically a biological object. We can station a camera at some point, but unless we can eventually read the resulting photograph, the observer has not been there. Now the nice thing for the physicist is that he does not have to consider the nature of the observer. He takes that for granted. In this sense the observer is outside his system of study, outside his map. The observer is nothing but the physicist looking at the map from the outside.

Now enters the biologist. His interest is in biological objects, and a physicist is as good a biological object as any other, so why not study him?

Noting that physics has been remarkably successful, the biologist attempts to "reduce" the biological object to chemistry and physics. (Chemistry is merely a branch of physics.) This means that he wants to map the physicist (and himself, since he has now become one) on the physicist's map.

Now what does "reducing" a physicist to physics imply? It means that if the mapping is successful, it must not only map the physical universe, but also the map that the physicist has of the physical universe. Unlike the situation that existed before the biologist appeared, the observer has now become part of the system.

Maps have an interesting property. A perfect map can only


be constructed if it is not a part of the object mapped. Suppose you want to draw a perfect map of Australia. In principle such a map is possible if you are not in Australia. But if you are in Sydney, you fail. The reason is obvious. Your map, to be perfect, must include, as part of it, a map of your map, which in turn includes a map of your map and so on in infinite regression; a perfect map is here not possible; it can be defined but not actually constructed.

Note that this difficulty does not arise if we include merely the body of the physicist and all objectively describable behaviour of his on our physical map. It is only when we try to map the mental map of the physicist that we run into trouble. We have to choose between the two courses. Either we keep the mental map outside the system, and then we can hope to produce a perfect but incomplete map, or we can include it, and then our map is complete but imperfect. Going back to the mechanist-vitalist controversy, we see that in different ways both were right. Although this yet remains to be demonstrated in detail, all objective, describable aspects of biological objects, and thus of ourselves, can presumably be reduced to physics. On the other hand, the subjective aspect of ourselves cannot be so reduced, since to do so implies a contradiction.

The Greeks, without our knowledge of physics and biology, demonstrated a knowledge of this argument. Suppose, they said, logic is universely applicable. Now Epimenides the Cretan had said that all Cretans are liars. What does logic have to say as to the truth of his statement? If he lies, he is telling the truth, but if he is telling the truth, he is lying. At first sight this is quite puzzling. What logicians have done is very similar to what I have suggested above. They state that logic is not applicable to certain reflexive statements, that is, to statements which apply to themselves. If we remove Epimenides outside the system, so that his statement does not apply to himself, the paradox vanishes. The price for this is the non-universality of logic. If we insist on universality, we can only do so by conceding the existence of paradoxes, i.e., problems with no solutions. I believe this is a close analogue of the relation of physics and biology. If we insist on the universal applicability of physics to biology, certain


problems, which are the subjective phenomena of human per-sonality, cannot be understood, so physics is imperfect. By removing such phenomena outside physics, we make physics perfect, but not universal.

The most important object in biology, the personality which constructs all of science, is, I believe, of necessity outside science. If you are theologically included, you may say that man does have a soul, although its exact nature is not specified by this statement. What some of the properties of it are, we know by immediate introspection. Introspection, far from being an un-reliable guide, is on the contrary the source of the most immediate and useful knowledge.

This indeed would be the common-sense view if we were not misled by ancient metaphysical preconceptions. In the ordinary course of events, we construct an external world which is a symbolic representation or analogue of reality. In a more developed form this is science. In this analogue our sensations are repre-sented by a series of impulses flowing over objects called nerves. Having constructed this analogue, we are astonished that such series of impulses lack those properties of which we are most certain, pleasure, pain, desire, colour, etc. Some have come to identify the analogue so closely with reality that they conclude from this that it is not the analogue which is imperfect, but that our sensations, and thus we ourselves who have constructed the analogue, are unreal. This absurd belief arises from a naive confusion of the map for the thing. The confusion goes back to magicians who, by pronouncing names or burning dolls, could change the fate of things and people, since the name or the doll, they thought, was not just a symbol for the thing, but the real essence of it.

Science is an admirable tool that we have developed to manipulate our environment, both intellectually and physically. Like all good things, it has limits; and to use it to convince ourselves that we, its creators, are in any sense less significant than the tool we have made, is a procedure of dubious validity.

*

CHAPTER 6

The Practical Effects of

Biology on our Lives

Science, as I have described it to you, is a form of human activity we have developed to gratify, sometimes indirectly, our primary instinctive drives, among which, of course, curiosity is included. It is only one activity among many, but has a peculiar interest because it has produced such profound changes in our mode of life. The changes produced by the physical sciences no doubt are well known to you. I would like to conclude my lectures with a discussion of the practical, rather than the intellec-tual, effects biology has had on our affairs. These effects illustrate rather well both the power and the limitations of science in general.

About one hundred years ago, four men published works which still dominate our thinking in biology. Darwin and, inde-pendently, Wallace conceived the theory of evolution by natural selection. In what is now Czechoslovakia, a monk in the monastery of Brno, Gregor Mendel, laid the foundations of the science of genetics. This, in conjunction with advances in chemistry, led to what is now known as molecular biology, a currently popular field which will be described to you by Dr. Watson. As far as practical applications go, the theory of evolution and molecular biology have remained academic subjects. They provide mental stimulation to all of us and salaries to professors who train students to replace them when they retire. Genetics of the classical sort has had some practical applications in the field of animal and plant breeding. It can scarcely be claimed, however, that these achievements have been of overwhelming practical importance.

Not so with the work of the Frenchman, Louis Pasteur. By training, he was neither a biologist nor a medical man, but a chemist. Because his discoveries have become so much a part of our own lives and seem so obvious, for some of us it is now

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difficult to appreciate the originality of his genius. Pasteur's work produced profound changes in human biology. To understand these changes, and to get a reasonable idea of what we may expect from biology in the future, we must examine the charac-teristics and behaviour of populations, particularly human popula-tions. Populations consist of individuals who are born and die, so for a start let us study the nature of death.

All of us know that, in practice, no organism lives forever. Psychologically we regard this as a disaster which personally threatens us all; but a more rational consideration assures us that death is not merely inevitable, but also the source of all biological progress. Because organisms die, it is possible for evolution to select the better from the worse. Only because old men die off is it possible for new ideas to take over. If the ancient Egyptians had never died, all of us, no doubt, would still be building pyramids. Nothing could be more disastrous to biological and social progress than immortality.

Why does every living thing eventually die? Curiously, it is not easy to answer this question. The trivial answer is that if you live long enough, you are certain, eventually, to be run over by a motor car or to meet with some other accident. However, ordinary experience tells us that even if no accident occurs, life will come to an end. The difficulty in understanding the finite span of life is that the material in every cell of our bodies is in a constant state of flux. Except for the genetic material, all the molecules of every cell are constantly renewed, so in this respect the cell is always new. Perhaps it is the genetic material which ages? There are good reasons, too lengthy to discuss here, for doubting this, so that we must admit we do not understand in any fundamental sense why an organism ages and eventually dies. However, we can get some understanding of this problem on a less fundamental level from a comparison of the life spans of different organisms.

Some organisms are popularly supposed to be immortal. The classic example is a small single celled animal called amoeba. After living for a while as a single cell, it divides into two. Each of the daughter cells has the same growth potential as the mother cell, eventually growing in size and again dividing into two.

THE PRACTICAL EFFECTS OF BIOLOGY ON OUR LIVES 265

Because, barring accidents, there is never any corpse, the amoeba is said to be immortal. However, in the process of division the mother cell disappears, so it can scarcely be said to be immortal in the usual sense of the term. In fact, quite recently it has been shown that the life span of the amoeba is finite, like other organisms. Under special conditions it is possible to induce amoebae to enter into a state where they are unable to divide, but where they otherwise appear to be quite normal. In this case, the cells will survive for a long time, but then quite suddenly, for no apparent reason, they die. Without understanding why this is so, our biological experience tells us that without exception any individual cell has only two possible fates open to it: it can either divide into two cells, or it can live for a while and then must die.

The concept of a finite life span of an individual cell explains why our own life span is limited. As you all know, our bodies are highly organized colonies of cells of various types. Some of these cells, like amoebae, are always dividing, for example in the skin, the intestinal epithelium and the bone marrow. On the other hand, there are some cells which never divide. The most notable of these are muscle cells and the cells of our brains. The number of these cells never increases after birth and they are never replaced.

In principle death in organisms such as ourselves is due to the fact that we enter life with a certain number of cells which are essential to life but are unable to reproduce. For reasons which we do not understand, with the passage of time these cells begin to wear out and die. These cells cannot be replaced, so that the functioning of various organs is impaired, weakening the whole body and eventually leading to death. Cells not only die, but also seem to deteriorate with age. The frequency of malig-nancies, for example, increases strikingly with age. Certain parts of us are potentially immortal, since they are continually re-juvenated, but some parts are not, and it is the latter which lead to our death.

You are all familiar with an example which illustrates this point. Most men, when they reach a certain age, begin to lose hair. This is due to the fact that there are a limited number of


hair follicles in the skin covering our heads and with the passage of time these gradually die and are not replaced. A similar phenomena accounts for loss of colour in the hair. Certain cells in the hair follicles produce a pigment, melanin, which gives our hair a more or less dark shade. When these cells die, the hair becomes silvery white because no more pigment is produced.

This provides a graphic picture of biological death. It also illustrates, however, that there are numerous factors which deter-mine the rate at which death of cells occurs. For example, baldness is much more common among men than women. We know that one factor here is the presence in men of a sex hormone called testosterone which decreases the life-span of the hair follicles. On the other hand, loss of hair pigment occurs with about equal frequency in both men and women. Senescence, clearly, is determined by various factors and these factors are different for different types of cells. Baldness or grey hairs, while due to the death of hair follicles or pigment cells, does not mean that other body cells are dying at the same rate. Some people become bald or have grey hair very early in life, but other cells of their body are not necessarily dying faster; such people do not necessarily have shorter lives than others.

Are there any organisms which are really immortal? The answer is yes. I think you are all familiar with a rather lowly organism called a sea anemone. Some of them are very beautiful, and many of the most striking occur in profusion along the Australian coasts. Although their organization is quite simple, they have most of the various types of tissues that are found in other organisms, such as connective tissue, muscle, nerves and various sensory cells. How long does a sea anemone live?

Since they do well in captivity, people have kept them in aquaria to determine their life span. Almost invariably they have outlived the investigator. In Scotland some individuals have been kept alive for seventy or eighty years, and through this time they showed no loss of vigour. Eventually, it is true, they died, but this was because someone forgot to feed them or to change the sea water. So far as we know, the sea anemone is immortal. How does it do it?


Figure 38.—Hydra, a simple form of a sea anemone. Cells around the mouth part, which have been stained black, eventually move to other parts of the body and finally disappear. This demonstrates that new cells are being formed around the mouth part which continually replace the cells dying in other

parts of the body.

A simple experiment provides the answer. Here (Figure 38), we have a very simple form of fresh water sea anemone called Hydra, often studied by biologists. It is possible to stain cells with dyes which do not harm the cells, but make it possible to identify the cells wherever they are. We stain the region around the mouth part of Hydra with such a dye and observe what happens. As time goes by, the stained cells move up the tentacles and down toward the base of the organism. Evidently new cells are being produced all the time around the mouth part, and when they reach the extremities they die. This explains the immortality of the sea anemone. Each of its cells, like our own cells, has a finite life span. Unlike us, however, there are no cells in the sea anemone which cannot be replaced. The entire organism is thus immortal, somewhat like that legendary sailor's knife, whose blade and handle had been replaced many times, but which remained both new and the same.

Let us now consider a population of sea anemones. In principle, any one of them can live forever. In practice, however,


the life of the sea anemone is beset by hazards. Some fish eat them. A sudden flood of fresh water may kill them. Even if nothing else happens, the coastline or coral reef is silted up, eroded or elevated above water. Eventually all sea anemones die one way or another, and in all probability one which is a hundred years old is rare or non-existent in nature.

Suppose we start to observe a large number, say a million, of young sea anemones. Although potentially immortal, they are subject to random accidents. Now it is of the essence of a random accident that it can happen to anybody, so that no matter who you are, or how long you have lived, you are just as subject to have it happen to you as to anybody else. If the chance that a random fatal accident will happen within one year to a sea anemone is 1/2, the history of our original one million sea anemones will be as illustrated in Figure 39. After the first year one-half will be left. After two years, one-half of the remainder will still survive, or one-quarter of the original population, after three years one-half of the quarter, or one-eighth, and so on. This mortality curve is identical with the mortality curve of the atoms of a radioactive substance and is due to the same cause, that death or disintegrations are pure accident.

Now imagine a hypothetical population which is never subject to accidents, but whose members, unlike sea anemones, have a fixed life span. At birth a clock starts ticking, and after a certain time, say three years, each individual dies. The mortality curve of this sort of population would be as shown in the same Figure. The biological difference between the two is considerable. In the population of sea anemones, older individuals get rarer and rarer, but statistically there is no absolute upper age limit for an individual. In the second population the number of individuals of any age is equal to the number of any other age, but no individuals beyond the upper age limit are found at all.

If we, like sea anemones, were physiologically immortal, how long would we live? The only cause of death would be various accidents. Assuming that the probability of a fatal accident occurring to an individual in a year's time is one in a thousand, which is a fairly realistic figure, our mean life span would be about 700 years, or about ten times longer than it is now.


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TIME

Figure 39.—The sloping line shows the history of a population, born at time O, one-half of whose members dies every year. The flat curve represents a population, all members of which live to be three years old and then all

die simultaneously.

In actual fact, of course, our own curve of death is not at all like that of the sea anemones. A rather minor cause of death is due to random accidents. Infectious diseases are another cause. To some degree, such diseases can also be considered accident, since they can strike both young and old. Beyond the age of thirty or so, however, the major causes of death are the so-called degenerative diseases, such as circulatory disturbances,


cancer, diabetes, and so on. As we age, the loss by ageing of our cells does not produce a simple additive effect. Weakening of one organ puts a strain on another, which in turn puts a strain on on the first. Thus ageing, once it starts, proceeds faster and faster.

Figure 40 shows the increasing incidence of fatal degenera-tive diseases. Instead of the probability of death increasing linearly with time, it increases approximately as the sixth power. There appears to be a maximum cut-off point beyond which man

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Figure 40.—The death rate from cancer and heart disease at different ages (men in Holland). (Courtesy of Academic Press.)


cannot live, at an age of about 100 years. Of course, because of the very great increase in mortality as our age increases, very few people ever reach this maximum age.

Somebody has illustrated the course of human life by the following graphic model. Every morning we approach an urn that is filled with black and white balls and draw a ball at random. If the ball is white, we survive through that day; if it is black, we die before nightfall. Now when we are young, most of the balls in the urn are white, so our chances of survival are good. Every morning, however, there are a few more black balls placed into the urn, so that as we age our chances of drawing a black ball increase. Finally, there are so many black balls in the urn that it is almost certain we will draw a black. Of course this is not the actual mechanism of death, but for any population everything happens as if this were so.

The above illustrates that it is the mode of death which determines the age composition of a population. We may ask also what determines the total size of a population. Let us start to investigate this problem by considering some simple example.

Suppose we fill a test tube with nutrient broth and inoculate it with a few bacteria. At intervals we determine the number of bacteria present. What we find is that for a short time the number of bacteria remains constant. During this time they are adjusting their physiological processes to the new medium, or as we say, adapting to it. After they are adapted, they begin to increase. Reproduction in bacteria is a process of a cell growing, reaching a certain size and then dividing into two cells. A generation or the time between divisions in bacteria is something from 20 minutes to a few hours. Once they start to grow in our test tube, the population of bacteria increases in geometrical progression. One bacterium gives rise to two, the two to four, the four to eight and so on. The absolute number of bacteria in our test tube therefore increases at first slowly and then almost explosively. The interesting question is what brings the increase to a stop.

The answer in this case is almost obvious. Bacteria, like every other living thing, require food. There is only a limited amount of food in the test tube, and therefore a limit to the


number of bacteria which can exist there. We observe the effect of the food supply on the following graph (Figure 37). At first, when there is plenty of food, the bacteria grow at a maximal rate, doubling say every half-hour. We call this half-hour the generation time. While there is plenty of food, this generation time remains constant. However, as the food supply begins to be exhausted, the bacteria find it more difficult to live and grow. There is still some food available, but their growth rate begins to decrease, or the generation time becomes longer. Eventually the generation time becomes infinite, i.e., the bacteria cease to grow. They have exhausted their food supply.

We say that the growth of the bacterial population is brought to a stop by a density-dependent effect, meaning that the rate of increase of the population is a function of the size of the popula-tion at that instant. The density-dependent effect is here produced by a limited food supply.

All this is so simple and obvious, that you may well ask what all the fuss is about. Everybody knows that you cannot have a larger population than the food supply will support. The point in starting with so simple a case is that many populations, including man, show a more complicated behaviour, and it is always well to start with the simplest cases. Consider the following.

In Canada there live two animals which are very important to each other. One is the snowshoe rabbit and the other is the Canadian lynx. Canada has been a great fur-producing region for several centuries, and the Hudson's Bay Company, which has had a virtual monopoly of this trade, has kept careful records of the trappers' annual take. When these records were examined by biologists, a curious fact emerged. The number of rabbits, as determined by the number trapped, was not constant from year to year, but increased and decreased in a cycle of about 11 years. The same was true of the lynxes, but their cycle was somewhat out of phase with that of the rabbits. The explanation of these periodic fluctuations is as follows.

Let us start at a point where the numbers of rabbits and lynxes is low. The rabbits now begin to increase, both because the number of their enemies, the lynxes, is low and because their


food supply is adequate. Rabbits, as you probably know, can multiply faster than lynxes because their litters are more numerous and come more frequently. However, since rabbits are the main food of lynxes, the lynxes also start to increase. This is an example of an information feedback mechanism; the signal which causes the lynxes to multiply is the number of rabbits available as food. For a while, the rabbits continue to increase more rapidly than the lynxes, but eventually their numbers begin to press on their food supply and, like the bacteria in our first example, their rate of increase slows down. The lynxes continue to increase because rabbits are numerous, but then the inevitable happens. There are so many lynxes that the rabbit population is decimated. Lynxes begin to be hungry and redouble their efforts at the hunt, but there are just not enough rabbits. The forests are now filled with lynxes dying from starvation. A few survive. There are also a few rabbits which survive, and the cycle starts all over again.

Here we have a more complicated case than that of the bacteria, but the same basic principle applies. The number of rabbits is again determined by density-dependent effects, one being the food supply, which is more or less constant, and the other of lynxes, which is, somewhat indirectly, also a function of the number of rabbits. Because the feedback signal, which is the population-density of rabbits, influences the number of lynxes with a certain time-delay, an oscillation in the populations occurs. The average populations over the years remains fairly stable, however, so we say that lynxes and rabbits are well adjusted to each other. Neither becomes extinct.

There is another important, but rather long-range effect which is illustrated here. When the populations of rabbits and lynxes are increasing, even poor specimens of both species find it relatively easy to survive. These paradisical conditions, however, never last long. When times get hard, as they always do, a premium is placed on ability to withstand starvation and disease, to catch prey or to avoid being caught. Selection now operates with ruthless efficiency against the weak. To the extent that these qualities are inherited, the population is kept in a state of maximum physical and mental efficiency.


We might well ask what would happen to the rabbits if we exterminated the lynxes. Usually, some other predators would replace them, for example, owls, weasels or foxes. However, it is possible to exterminate almost completely all significant predators of a species. Misguided sportsmen and agriculturalists in America, in an effort to obtain a larger yield of deer, have killed off wolves, mountain lions and other controls on the deer population. If the deer are adequately hunted, this has merely resulted in replacing one predator, the mountain lion, by another, man. In some sections of the American south-west, however, predators were virtually exterminated and not replaced by man. In such cases the numbers of deer increased enormously. The deer first multiplied to a level which could be supported by the amount of vegetation at that moment. However, the food supply, or vegetation could not renew itself as fast as it was eaten. As a result, a vicious cycle developed. The less food, the more deer. New shoots were eaten before they could develop. The deer then dies off from starvation. Since the ground now lacked cover, the soil was washing off, making the situation permanent. While there were plenty of deer when predators were present, they became virtually extinct when these were removed. When the population was allowed to increase indefinitely, it destroyed its own environment.

A similar situation is familiar to all who have visited North Africa. There, goats and camels, protected by man, have changed most of the area to barren desert.

Let us now consider human populations. At first sight, we might conclude that man, like some other large animals such as the crocodile, the elephant or the shark have no predators which keep his numbers down. A shark or tiger may occasionally eat a man, but statistically this is an insignificant cause of death. The idea, however, that man and other large animals have no predators is misleading in the extreme.

No large animals prey on man, but there are innumerable small ones that do so. Over vast areas of Asia, Africa and South America a small flatworm infests aquatic snails. From these it moves into the human body, producing a serious disease called Schistosomiasis, leading to general debility and not infrequently


to liver cancer. Over 70% of Egyptians harbour this worm; and at present, it is rapidly spreading in the Near East as more areas are irrigated and thus provide more habitats for aquatic snails. Such small predators of man, which we call parasites, are not necessarily worms nor are they confined to tropical countries. We are all familiar with them as the bacteria or viruses which cause disease. Man, like other animals, has relations to innumerable organisms which live with him, no less so than lynxes or snowshoe rabbits. These relations are sometimes beneficial to us, as for example with the wheat plant which provides us with food, and at other times fatal, as with the bacterium which produces diphtheria.

A rather simple example of a host-parasite relation is the interaction of man and the influenza virus. Periodically, an epidemic of influenza sweeps over the world. Some people are killed by the disease, but the majority survive. The epidemic is self-limiting. This is because once a person has had the disease, he develops an immunity to it, so that we may say that the parasite, in this case the influenza virus, has destroyed its environ-ment. To produce a further epidemic, the virus either has to wait until the immunity wears off, or has to mutate to a new form to which the immunity does not apply. The result is a semi-periodic rise and fall in the virus population not unlike that of the snowshoe rabbit.

A more interesting and complex example is provided by the malaria parasite. Malaria is a biological system requiring the participation of at least three organisms. The cause of the disease is a small protozoan which, for part of the life cycle, inhabits human blood and destroys the red cells. Like all parasites, however, it requires some method of getting from one victim to the next, and for this purpose it uses a mosquito. When a mosquito, which in a minor way is also a predator on man, bites someone who has malaria, the parasite gets into the mosquito and multiplies there. When the mosquito bites a healthy person, the parasite again re-enters man. In this way it moves from one host to the next. From our point of view we call the mosquito a vector or transporter of the malarial parasite. Malaria is a serious disease, producing much debility, reducing fertility and


often directly causing death. When it is common, it is an important factor in controlling human populations, just as the lynx controls the snowshoe rabbit population.

Now let us consider this biological relation more closely. Mosquitoes, in general, feed not only on man but also on other mammals and birds, although each species has its own individual preferences. The malaria parasite, however, can live only in man and the mosquito, and must pass from one to the other in order to survive.

From the point of view of the parasite, a success is scored when an infected man is bitten by a mosquito which then in turn bites an uninfected man, causing an increase in the population of the parasite. If the mosquito is eaten by a bird, or if instead of biting an uninfected man it bites a cow, the sequence is a failure, since then the population of the parasite does not increase.

The entire system operates in a manner very analogous to a nuclear reactor. As you know, fission of a uranium atom produces neutrons. If such a neutron is absorbed by another uranium atom, fission again results. Whether the reaction is self-propagating depends on whether neutrons released by the fission of one atom induce on the average at least one further fission. Some neutrons will always be lost to the system, say by absorption by other material. If we increase the concentration of uranium, the proba-bility of capture increases and the reaction may become explosive. If we decrease the concentration sufficiently, the reaction will cease to be self-propagating and will die out.

The ecology of malaria is similar but more complicated. Infected men and mosquitoes have a finite lifetime, so they are always leaving the system. The rate of transmission of the parasite to new hosts must therefore at least equal this rate of loss. Now the rate of transmission depends on (a) number of infected men, (b) number of mosquitoes, (c) total human popula-tion and (d) ratio of men to other mosquito-bitable organisms. Other things being equal, the incidence of malaria will increase with the size of the human population, so here we have a typical density-dependent effect limiting a human population. The larger the population, the higher the ratio of infected to uninfected indi-viduals. From this point of view, a small population is preferable


to a large one, and since malaria is a serious disease, a certain upper limit is imposed automatically.

You will notice something interesting if you examine this system closely. If the probability of transmission for some reason becomes too low, the number of new cases of malaria may be less than the number of infected individuals leaving the system, and the parasite will eventually become extinct. This is the prin-ciple on which anti-malarial campaigns are based. Ideally, three things are done simultaneously. First, the mosquito population is reduced by insecticides and drainage of their breeding places. Second, drugs are used to make known malaria-carriers uninfec-tive. Third, drugs are again used to make uninfected people unsuitable hosts. All these measures decrease the probability of transmission, and the malaria parasite may begin to decrease spontaneously and become extinct, even though not all mosquitoes are eradicated nor every individual harbouring the parasite is treated. By this method, malaria has been eradicated from large parts of the world, and some are hopeful that it may eventually become totally extinct.

So far we have been considering the effect of death on populations. Our main conclusion has been that, as a population increases, a density-dependent effect comes into play to limit the population. This density-dependent effect may be of various kinds, food, predators or diseases, but it always exists. Depending on its nature, it may limit a population at a constant level, cause a cyclic fluctuation in number, or lead to an increase followed by a decrease and finally extinction.

We have taken it for granted that a population will increase as much as possible. Why is this so? The reason is that every natural population maintains a birth rate which is higher than that actually required to maintain a stable population. If it did not, a slight worsening of environmental conditions, by increasing the death rate, would lead to a rapid decrease and extinction.

A corollary of this is also important. Since more individuals are born than the environment can support, natural selection operates against the less fit, thus maintaining or improving the genetic endowment of the populations.


We may now consider what the biological discoveries of Pasteur, and their further development, have done to human populations. The effects have been three-fold; on the age dis-tribution, on population size, and on the human genetic endow-ment.

As most of you know, Pasteur discovered three facts that are significant in this connection. Firstly, that a variety of diseases are caused by bacteria and other small parasites. Secondly, that these parasites in all cases can only develop from parents like themselves, never spontaneously. Thirdly, he discovered a prac-ticable and systematic way of producing immunity to certain diseases.

Until very recently, infectious and parasitic diseases have been the main causes of death. When it was realized that such diseases were caused by parasites, even very simple methods proved highly

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Figure 41.—A mortality curve showing per cent surviving as a function of time after birth. Curve A is a hypothetical population all of whose members reach the age of 100 years and then die. Curve B is an actual mortality curve for the population of New Zealand, Curve C, the mortality of Indians.

(Courtesy of Scientific American.)


effective in practically wiping many of them out. Chlorination of water supplies, eradication of mosquitoes, and general sanitary methods made an enormous difference in the death rate. Such measures can be applied on a large scale and are more of an engineering than a medical measure. They could have been instituted long ago, but were not until the nature of infectious diseases was discovered by Pasteur. Ultimately, his discovery led to such drugs as penicillin and anti-malarials, and fairly directly to vast advances in surgery. The average length of life rose from about 30 to about 70 years.

The effect is quantitatively shown in Figure 4L Curve A is a hypothetical mortality curve which assumes that all individuals born live to be 100, the maximum possible, and then die. It is the theoretical optimum, against which we can measure the actual mortality. Curve B is that of modern New Zealanders, one of the healthiest populations in the world, and curve C, of Indians. You can observe the striking difference that sanitary measures have made by comparing B and C.

It is also worth considering what has not been done, and this is shown in Figure 42, which shows the mortality at different ages of two United States' populations, those of 1900 and 1950. During this time, there has been a very great fall in mortality at ages under 65, but after this age there is little improvement. The reason for this is that at individual ages, non-infectious diseases become the most important primary cause of death. Medicine has done little to diminish mortality from such diseases.

You will also note from the figure that infectious diseases are no longer a serious cause of death from the point of view of the population as a whole, although of course individuals still die from them. The line of advance initiated by Pasteur has about run its course, not because it has failed, but because it has been so successful that there remains but little room for improve-ment. This applies, of course, only to the so-called "developed" countries.

The major causes of death are now circulatory disorders and malignancies. Since these ultimately involve subtle malfunctions of individual cells, one may hope that molecular biology, now


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Figure 42.—Death from all causes at various ages in three human populations. Note the logarithmic scale of the ordinate. (Courtesy of Academic Press.)

a purely academic subject, may provide an entering wedge for their solution.

By making us healthier and enabling us to live longer, the work of Pasteur has been an unmitigated gain. Of more doubtful benefit has been another result of his work, the enormous increase in human populations.

If you take any natural population, say a population of rats, and improve its conditions of life by freeing it of diseases and providing it with food, the population will increase, until it again


runs up against some limiting factor. Note that this is of no benefit to the individual rat. When the food supply was limited rats were starving. When the food supply was increased, they multiplied to compensate for this increase, so again they ended up starving. The only difference is that when the food supply was low, fewer rats starved. This illustrated what I said before, that a population tends to increase to the maximum possible.

Rats would consider me a pessimist, and might bring for-ward an argument to refute me. Suppose rats were starving on a dump, and suddenly a large amount of garbage appeared. The rats would no longer starve, nor would their children, nor their grandchildren. They could legitimately claim that life had really become better; and by multiplying they made it possible for more rats to enjoy life. This argument is valid, providing you pay no attention to what happens to their great-grand-children. If they multiply by geometrical progression, they will finally, and very soon, produce a population which will starve, no matter how much garbage is available.

So much for the rats. Let us now consider human popu-lations. In part, because of technical advances in raising food, but mainly because the death rate has been drastically lowered by medical advances, the population of the Earth has begun to increase at an ever-increasing pace. It is not just that there are more people all the time as a result of a constant rate of growth; the rate of growth itself is accelerating. Over the entire earth the human population increased at the rate of 0.3% per annum between 1650 and 1750. Between 1900 and 1950, the rate was 0.9% and now stands at about 1.7%. The curve of population growth is shown in Figure 43.

As a result of this population increase, food, among other things, has become scarce in many parts of the world. About 10,000 people die every day of malnutrition or outright starva-tion, and about 1500 million are underfed by any reasonable standard.

Although this is not my main theme, let me consider briefly the significance of large populations. If a population is so large that it presses hard on its resources, and is also doubling every 20-30 years, the obvious and traditional methods of action are


Figure 43.—Growth of the world's population, in thousand millions.

inapplicable. Thus attempts to provide more food are futile. The population merely rises to wipe out the surplus, and there are more people to starve. The Aswan dam on the Nile is an example. It is one of the most remarkable engineering works in the world and will increase by 30% the irrigated land in Egypt. However, by the time it is completed, the number of ill-fed Egyptians will have increased by more than that percentage.

Acquisition of territory, or emigration, is also futile. Thus China would have to export about 10 million people a year merely to stay where it is. This would overpopulate Australia in a decade, at the end of which time everyone would be worse off than when they started.


Importing food is impossible except as a temporary act of desperation, because such populations are so poor they have very little to export in exchange. Besides, nobody has as much food as that to export. Needless to say, unrest, revolution and disorder are rife. South America is a notorious example. Its rate of popula-tion growth is the largest in the world. Almost everywhere, the situation is getting worse, and the rate of worsening is increasing.

The question is often asked what is the population the Earth, or some country, can support. So phrased, the question is meaningless. It all depends on the level at which you want to support it. When you begin to press on your resources, you can always get more by putting in an extra effort, but the yield per unit effort becomes less and less. If there were 50 million Indians, they could no doubt live very comfortably. India can obviously also support over 400 million, since it is doing so right now, at a completely miserable level.

This is not merely an Asian problem, nor always a matter of a shortage of food. In the United States, about one-tenth of all the water that falls upon the country is used for human purposes. Water shortages are developing everywhere, and the country is foolish enough to have begun to use an irre-placeable resource, oil, to distill sea water to sprinkle the lawns of San Diego. In less than a century the population of the United States is scheduled to exceed 1000 million. More than fifty per cent of all water will then have to be utilized, a rather fantastic figure. Of course it can be done. By reutilization, economy, and distillation, you can get it. The point is merely that water, which cost nothing when the population was low, now becomes something you have to work for.

Nevertheless, there is still plenty of water. Perhaps we can distill it by developing a new energy source, fusion of deu-terium. What we cannot develop is more space. Many Americans spend several hours every day in traffic jams to get to and from work, to shop and visit friends. To relieve this situation super-highways are built. As a result, people live further and further from each other and from their places of business and have to buy additional cars for their wives and teenage children. Traffic jams get worse, and this is only the beginning. There are now


serious proposals to prohibit all motor traffic in the centre of New York City. Such a proposal has actually been put into effect in a part of Copenhagen. There is no very definite imme-diate limit to the number of people you can support, provided only that you are willing to put up with more and more incon-venience, and eventually more and more misery of one sort or another.

All this, one might suppose, is obvious. There is clearly an ultimate upper limit to the population of the earth, and since human populations are doubling every thirty years, this limit, no matter how great, will be quickly reached. Whatever the limiting factor will prove to be, space, food, water, or the sheer exasperation of overcrowding, it will be a limit involving misery. One would therefore expect that human populations would choose to limit themselves by the least painful method, by regulating their birthrate to just compensate for their death rate. Appar-ently, not so. The United Nations solemnly convenes confer-ences to discuss the maximum population the Earth can support, and how to achieve this. You will note that in such conferences there is no mention of the optimum population. It is tacitly assumed that a large population living in misery is better than a smaller population living in comfort.

So far, then, everything about the human population resembles the rats on the dump. More food has been provided, and diseases partially wiped out. As a result, the human population has expanded to wipe out these benefits. Of course it is unflattering to compare ourselves with rats. Men are not rats; this cannot be doubted. The important question is: In what way are we different?

The way we differ from rats can best be explained by an example. Take two human populations, say those of India and Sweden. The population of Sweden is increasing very slowly. That of India, on the other hand, is increasing rapidly. If we had studied only rats, we would draw, from this, the following con-clusion: Sweden must be a country which is greatly overcrowded, very short of food and swept by large epidemics. India, on the other hand, must be sparsely inhabited, well fed, and very healthy. The absurdity of this conclusion demonstrates that we are very


different from rats. What is the cause of this most unratlike behaviour?

The difference is in the way man regulates his reproductive rate. Among higher animals, the birth rate is maintained at the requisite level by innate tendencies or instincts to engage in sexual intercourse and to take care of the young. Depending on the species, the male may or may not participate in the latter activity. Animals of course are not aware of the consequences of their sexual activities and reproduction proceeds without much fuss or bother.

In man the same innate tendencies are present; but there is one important difference. Man is aware that sexual activities lead to conception and child-bearing. This is a fairly recent discovery. Until quite recent times, many of the Australian Aborigines and some of the Melanesians were not aware that there was any relation between sexual intercourse and pregnancy. Apparently, man in general became aware of this fact in upper paleolithic or early neolithic times. This was probably one of the first important discoveries in biology.

Realizing this, human populations deliberately determine their own birth rate. The traditional behaviour of the Swedish population is to have a low birth rate, while that of the Indian is to have a high one. This is what makes populations of rats and men different.

The next question, of course, is why different human popu-lations have decided to have different birth rates. Three or four centuries ago conditions of life in Sweden and in India were about the same. Mortality, mainly from infectious diseases, was high; and a high birth rate was traditional and necessary in both countries. Now in Sweden conditions changed slowly. Gradually modern technology, education, and sanitary measures were introduced. The death rate also decreased gradually. Since the change was slow, lasting many generations, cultural traditions and behaviour could change without upsetting people too much. Among such changes was a fall in the birth rate. Education, of course, played a great role here. The conditions of life in Sweden improved steadily, and gains were not wiped out by a rapid increase in population.


In India, however, the changes were sudden. Dams, rail-roads, and primitive sanitary measures resulted in a significant fall in the death rate. People, however, because of tradition and lack of education, continued to maintain a high birth rate. The population increase wiped out any gains from science and technology and actually reduced the inhabitants to a worse con-dition than they were a century before. The Indians did not have time to adjust, and a vicious cycle developed. How rapidly science can change the conditions of life is illustrated by Ceylon. Malaria was wiped out in three years, cutting down the death rate 30%.

As you see, science and technology have produced diametri-cally different effects at different points of the globe. In the so-called western countries, including the Soviet Union and Japan, they have improved the lot of mankind. Other countries are growing poorer and poorer, not merely by comparison with the wealthier, but in many cases in absolute terms. The life of an Asian peasant is definitely worse than that of a pygmy in the Congo jungle.

Let us now consider another effect of the developments initiated by Pasteur, that on the human genetic endowment.

As Dr. Watson will explain to you in more detail, all of us possess a collection of genes which determine our functions and capacities. The major, or perhaps the sole constituent of the genes is a substance called deoxyribonucleic acid—DNA. Physically, a gene is mortal, since it disappears when the individual carrying it dies, but it produces exact copies of itself which are transmitted to the offspring.

Some, if not all, genes can occur in alternative forms. Thus a gene can determine that your blood group will be A. The same gene, slightly modified, will produce a blood group B. These are alternative active forms of the same gene. However, a gene can also occur in an inactive form, or may be missing altogether. If an individual carries an inactive gene, the particular function the gene determines is absent. Let me give you two examples.

One I have already mentioned. There is a gene which in the normal form determines the production of hemoglobin, the oxygen-carrying substance of our blood. The gene can also occur


in an alternative form and produce abnormal hemoglobins, such as the so-called sickle-cell form. This leads to a serious anaemia, but is not immediately fatal. Here the gene is partially functional; but it is nevertheless defective.

If you cut yourself slightly, you will bleed, but not usually to death. This is because your blood clots and seals off the injury. The clotting of blood is a very complex phenomenon and requires the participation of a number of substances, most of them proteins. The structure and presence of these proteins is also determined by genes, one gene to each protein. There is a condition called haemophilia, where the blood clots very slowly and ineffectively. A small cut can cause a person suffering from this condition to bleed to death. The cause of this disease is the absence of a protein necessary for clotting, which in turn is due to the fact that the gene producing this protein is in an inactive form.

Haemophilia and sickle-cell anaemia are examples of hereditary diseases. A person suffers from them because he has received defective genes from his parents.

We can consider a population to be a collection of genes travelling through time. The individuals of this population are carriers of the genes and transmit them from one generation to another. However, the transmission of genes is subject to accidents. Genes are very stable, but nevertheless can undergo mutation, either by being exposed to radiation or chemicals, or spontaneously. The normal mutation rate is of the order of once every million replications.

Usually, when a mutation occurs, it produces an inactive gene. Furthermore, the inactivation is permanent, that is, the chances that it will mutate back to an active form are negligible. Mutation is essentially a unidirectional process destroying active genes.

The process is graphically represented in Figure 44. If you take this picture literally, you will see that eventually every gene will mutate to an inactive form and the population will inevitably become extinct. The process is not as slow as it looks. So far as an individual gene is concerned, it is true that the probability of its mutating is small, but since man has at least 10,000 genes,


Figure 44.—A collection of genes can be represented as travelling in time (dark arrows). At all times they are subject to inactivation by mutation. In the absence of selection, all genes would eventually become inactivated and

the population carrying them would become extinct.

even this rate will insure that in a short time every individual in the population carries at least one defective gene.

The reason populations do not usually become extinct in this manner is because defective genes are removed from the gene pool. The details of how they are removed are as follows.

As some of you know, all of us carry not one copy of each gene but two, one which we inherit from the father and one from the mother. Our parents, of course, also had a pair of each gene. When eggs and sperm are produced, each carry only one copy of the gene. Suppose that both the father and the mother had one gene which is normal, let us call it A, and one mutant, or a. At mating, the assortment of genes is random, so the offspring may receive the following pairs of genes: aa, aA, or A A. If the father is AA, and mother is aa, all offspring are A a.

We have two copies of every gene, so what happens if we carry one inactive gene? Usually nothing. An active gene A is sufficient to make up for the inactivity of a. Thus individuals having A a get along almost as well as those having A A.

Now consider the numbers of different kinds of individuals in a population with respect to the gene A and its mutant form a.


The total frequency of such genes in the population is one. The number of kinds of different individuals in a large population is then given by the expression:

a2 + 2aA + A2 = 1

Figure 45.—A geometrical explanation of the expression for calculating the relative frequencies of genetically different individuals. In a large population, the frequency of a given gene and its alternative form is A and a, and eggs and sperm carrying this gene and its alternative form are also produced with frequencies A and a. In a large population, matings are essentially random with respect to any given gene. Therefore offspring essentially (carrying two doses of a given gene or its alternative form) are produced with a relative frequency proportional to the product of the relative frequencies of different

forms of a given gene in the population.

A geometrical derivation of this expression is given in Figure 45. To illustrate, if 10% of all genes in a population are mutant a,


the frequency of a is 0.1 and of A, 0.9. The relative frequencies of individuals will be as follows:

(aa) — 0.12 — 0.01 (aA) = 2 x 0.1 x 0.9 — 0.18 (AA) = 0.92 = 0.81

If the absence of the normal gene is fatal, individuals (aa) die, which removes one-tenth of the genes a from the gene pool each generation. Suppose however that the frequency of a is lower, say 0.01. Then we have:

(aa) — 0.012 — 0.0001 (aA) — 2 x 0.01 x 0.99 — 0.00918 (AA) = 0.99 = 0.9801

Here only 0.01% of the deleterious genes are removed each generation. This is what you might expect. If a deleterious gene is rare, the chances of two carriers mating and producing aa offspring is low; if it is frequent, the chances are much higher. Thus a deleterious gene of this sort will be quickly reduced to a low level; but once there, further removal by selection will be slow. New deleterious genes are always appearing by mutation, so the gene frequency will reach an equilibrium level, depending on the rate of mutation and on the rate of removal.

You will notice that the deleterious gene a can only exist because it can hide, so to speak, in an individual who also carries a normal gene. It produces its fatal effect only when it is paired with itself. In theory, if we could recognize "carrier" individuals aA and prevent them from breeding, we could exterminate the deleterious gene a in one generation. In man it is now becoming possible to recognize such carriers.

Note that the gene, when aa, does not have to be literally fatal to be removed; it merely has to produce an individual who cannot reproduce, or at least reproduce less effectively. This has an important consequence. If the mutation produces its pernicious effect later in life, past the age of reproduction, natural selection can do nothing to remove such a gene from the gene pool. It can therefore build up to a high level. Selection is indifferent to the fate of individuals past reproductive age. To take one example. The Weddell seal of the Antarctic is proved with strong teeth.


He uses his teeth to make holes in the ice so he can come up for air. As Weddell seals get older, their teeth wear out, and the seal eventually drowns. This appears to be a major cause of death in this species. From the point of view of the individual seal, it would be a great advantage to develop teeth like those of the beaver which grow continuously. Evolution, however, cannot provide this. The seal drowns at an age where he has already finished reproducing.

It is not improbable that many diseases typical of old age in man are not a primary result of senescence, but are really due to genes which produce their result only in later life. Such genes would have no effect on the number of offspring produced, so selection against them can not operate.

We can summarize the above as follows. The genes of any population are subject to deterioration by mutation. To maintain the genetic endowment and prevent extinction, selection against individuals carrying such genes has to operate. Selection may involve death of the individuals or merely non-participation in the reproductive activities of the population.

What has science done to this process? So far as the most deleterious types of mutations are concerned, nothing. Such mutations produce fatal results so early in life that the child is not born, so he is neither noticed nor missed. There are, however, a large class of mutations which are not immediately fatal; and with sufficient knowledge, the individual, to a greater or lesser extent, can be kept alive. Haemophiliacs, for example, can be aided by blood transfusions. Or take another example. Gamma globulin is a protein which produces immunity to bacterial infec-tions. There is a genetic condition which results in the absence of gamma globulin in the blood, technically called aggamaglo-binaemia. Until quite recently, this condition was not known, not because such individuals did not exist, but because they always died shortly after birth from infections. Now, however, such individuals can be pulled through with antibiotics. Medical science has created a new disease, previously unknown. For the first time in history, individuals of this kind are now reaching reproductive age, and may be expected to transmit their genes to the gene pool.


The effect of such medical advances is to decrease the effec-tiveness with which deleterious genes are removed from the popula-tion. What will be the logical result of this? The result is obvious: the frequency of deleterious genes will rise to much higher levels. The greater the number of individuals carrying such genes that are enabled to reproduce, the higher the level. The number will of course also increase with time. The frequency of each such hereditary disease may not become very great, but there will be cripples, each requiring injections of hormones, blood proteins, so many of them that very few individuals will fail to suffer from at least one. By definition such diseases will not be fatal. The population will consist of a miscellanous collection of biochemical antibiotics or special diets.

At the moment, admittedly, this problem is not nearly as serious as the increase in population. These effects can only be produced over many generations, and the medical advances that will make them possible are only beginning. But one can already see the dawning of a new age.

Let me now summarize the effect that the line of investigation initiated by Pasteur has had on human populations.

The birth rate, the death rate, and the genetic integrity of the human population form an interacting system; any change in one factor produces an effect on another. The work of Pasteur has prolonged human life. This effect is so immediately desirable that it acts as a scientific trap. Once it has occurred, there is no going back. A proposal to organize an international commission for the dissemination of cholera and bubonic plague would scarcely meet with the approval of most people.

On the other hand, since death is only one factor in human biology, prolongation of life has resulted in two serious effects, an increase in population that has already led to much human misery, and an eventual deterioration of the genetic endowment of the population which will lead to much more. Is all this inevitable?

The answer is, "No", but only if you are willing to face the facts. Imagine an aeronautical engineer in the first World War who decided that it would be very desirable to increase the speed of aeroplanes. He invents and produces a very powerful jet engine which he fits to a Sopwith Camel. His plane does indeed take


off with unprecedented speed, but unfortunately disintegrates shortly after takeoff. Does this mean that a speed of more than 200 miles an hour is impossible? Obviously not. What the designer has overlooked is that speed does not depend merely on the engine. If you increase the power of the engine, you must also change the fuselage to keep in step.

Human biology presents an analogous situation. If we change only one factor, in this case the death rate, and complacently proceed as before, we shall end up in a disaster which is already very close.

On the other hand, we are presented with an opportunity such as mankind has never had before. For the first time, we can free ourselves of numerous causes of disease and death. If we are willing to control our birth rate, we can avoid the miseries of having our population limited by external factors. If, in addition, we are willing to exercise a rational selective control on our reproduction, our growing knowledge of human genetics will make it possible to exterminate, not merely to prevent the increase, of the numerous burdens of disease which heredity has imposed on us.

You will note that our problems are not really scientific. By now, science is only a minor factor. The problem is the inertia of our social institutions, which have developed, like the Sopwith Camel, to cope with the situations of the past. Such institutions are hard to change, no matter what the facts may require. Changes in our mode of life have inverted the meaning of important and valuable concepts. The meaning of compassion and humanity used to be clear. You fed the starving and tended the sick. Now if you feed the starving, more will starve in the future; and if you tend the sick, more will be sick later. Only by acting on the total biological situation can you hope to achieve your goals. But if you do act, you may produce unprecedented results.

Science, as I said before, is only one human activity among many. Our major problems are not scientific, but a matter of will and determination. I have lived in a generation which has failed. You are going to decide which way fate will turn.

CHAPTER 1

Introduction

It is very easy to consider man unique among living organisms. He alone has developed complicated languages allowing meaningful and complex interplay of ideas and emotions. Great civilizations have developed and changed our world's environment in ways in-conceivable for any other form of life. There has thus always been a tendency to think that something special differentiates man from everything else. These beliefs often find expression in man's reli-gions, which try to give an origin to our existence and, in so doing, to provide workable rules for conducting our lives. Just as every human life begins at a fixed time, it was natural to think that man did not always exist, but that there was a moment of creation, perhaps occurring at the same time for man and for all other forms of life.

These beliefs, however, were first seriously questioned just over 100 years ago when Darwin and Wallace proposed their theories of Evolution based upon selection of the most fit. They stated that the various forms of life are not constant, but are constantly giving rise to slightly different animals and plants, some of which are more adapted to survive and multiply more effectively. At that time, they did not know the origin of this continued variation, but they cor-rectly realized that these new characteristics must persist in the progeny if they were to form the basis of Evolution. At first there was a great deal of furore against Darwin, most of it coming from people who did not like to believe that man and the rather obscene-looking apes could have a common ancestor, even if he occurred some 50-100 million years in the past.

There was also initial opposition from many biologists who failed to find Darwin's evidence convincing. Among these was the famous Swiss-born naturalist Louis Agassiz, then at Harvard, who spent many years writing against Darwin and his champion, T. H. Huxley, the most successful of the popularizers of Evolution. But

297

·.<·.;·

β ^ 4:

* > > '

Figure 1.1.—A thin section of a rat liver cell as revealed by the electron microscope. The round object is the nucleus. The magnification is 8100x.

INTRODUCTION 299

by the end of the nineteenth century, the scientific argument was almost finished; both the current geographical distribution of plants and animals and their selective occurrence in the fossil records of the geological past are explicable only by postulating continuously evolving groups of animals and plants descended from a common ancestor. Today, the Theory of Evolution is an accepted fact for all except a fundamentalist minority whose objections are based not on reasoning but are the consequences of doctrinaire adherence to religious principles.

An immediate consequence of the acceptance of Evolution is the realization that life first existed on our Earth some one-to-two billion years ago in a simple form, possibly resembling the bacteria, the simplest variety of life now existing. The mere existence of the very small bacteria, of course, by itself tells us that the essence of the living state is found in very small organisms. Nonetheless, evolutionary theory profoundly affects our thinking by suggesting that the basic principles of the living state will be the same in all living forms.

The same conclusion is independently given by the second great principle of nineteenth century biology, the "cell theory". This theory, first put forward convincingly in 1839 by the German microscopists Schleiden and Schwann, proposes that all the larger plants and animals are constructed from small fundamental units called cells. All cells are surrounded by a membrane and usually contain an inner body, the nucleus, also surrounded by a membrane (the nuclear membrane) (Figure 1.1). Most importantly, cells were observed to arise only from other cells by the process of cell divi-sion. Most cells are capable of growing larger, followed by roughly equal splitting to give two daughter cells. At the same time, division of the nucleus also occurs so that each daughter cell also contains a nucleus.

Each nucleus was found to contain a fixed number of linear bodies called the chromosomes. Before cell division, each of the chromosomes divides to form two chromosomes identical to the parental body. This process, first accurately observed by Fleming in 1879, doubles the number of nuclear chromosomes until, during nuclear division, one of each pair of daughter chromosomes moves into each daughter nucleus. As a result of these events (now


termed mitosis), the chromosomal complement of daughter cells is usually identical to that of their parental cells.

One important exception was found to the mitotic process. During the formation of the sex cells, the sperm and egg, the number of chromosomes was found to be reduced to one-half its normal number. At the same time, these late nineteenth-century cytologists observed that although most cells contained a number of morpholo-gically different chromosomes, each specific type was present in two copies (the diploid state). During sex cell formation (the process of meiosis), the resulting sperm and egg usually contain only one of each type (the haploid state). Union of the sperm and egg during fertilization results in a fertilized egg containing one haploid chromo-some from the male parent and another from the female parent, thus restoring the normal diploid chromosomal constitution.

The cell theory thus tells us that all cells come from pre-existing cells. All the cells in adult plants or animals are derived from divi-sion and growth of a fertilized egg, itself formed by the union of two other cells, the sperm and the egg. All growing cells contain chromo-somes, usually two of each type, and here again new chromosomes always arise by division of previously existing bodies.

Although the cell theory developed from observations of higher organisms, it holds with equal force for the more simple organisms like protozoa and bacteria. Each bacterium or protozoan is a single cell whose division usually produces a new cell identical to its parent, from which it soon separates. This contrasts with higher organisms, where the daughter cells not only often remain together, but also often differentiate into radically different cell types, like nerve or muscle cells. Here, new organisms arise by the production of the highly differentiated sperm and egg, whose union initiates a new cycle of division and differentiation.

Thus, although a complicated organism like man contains a very large number (up to 1012) of cells, they all initially arise from a single cell. The fertilized egg thus contains all the information necessary for the growth and development of an adult plant or animal. Again this tells us that the living state per se does not de-mand the complicated interactions of complex organisms, but that its essential properties can be found in single growing cells.

The most striking attribute of a living cell is its transmission

INTRODUCTION 301

of hereditary properties from one cell generation to another. The existence of heredity must have been known to early man as he witnessed the passing of characteristics like eye or hair colour from parents to their offspring. Its physical basis, however, was not understood until the first years of the twentieth century, when in a remarkable period of creative activity the chromosomal theory of heredity was established.

Hereditary transmission through the sperm and egg became known by 1860, and already in 1868 Haeckel, noting that sperm consisted largely of nuclear material, postulated that the nucleus was responsible for heredity. Then almost twenty years passed before the chromosomes were singled out as the active factors. Before this could happen, the details of mitosis, meiosis, and fertilization had to be worked out.

Then it could be seen that, unlike other cell constituents, the chromosomes were equally divided between daughter cells. More-over, the complicated chromosomal changes observed during meiosis by which the sperm and eggs are produced were understandable if the chromosome number had to be kept constant. These facts, however, were merely suggestive that chromosomes carry heredity.

Proof came at the turn of the century with the effective dis-covery of the basic rules of heredity. These rules, known after their discoverer, Mendel, were in fact first proposed in 1865 but the climate of scientific opinion was not yet ripe for their acceptance. They were completely ignored until 1900, despite some early efforts on Mendel's part to interest the prominent biologists of his time. Then de Vries, Correns, and Tschermak, all working independently, realized the great importance of Mendel's forgotten work. All three were plant breeders doing experiments related to Mendel's, and each reached similar conclusions before they knew of Mendel's work.

Mendel's original experiments followed the results of genetic crosses between strains of peas differing in characteristics like size or flower colour. Each of these traits behaved as if they were con-trolled by two independent factors which we now call genes. One of each gene pair is derived from the male parent, and the other from the female. When the sex cells are formed, only one randomly chosen gene from each pair is included in a given sperm or egg.


Three years later, the physical basis of Mendel's laws was pointed out when the young American cytologist Sutton showed that Men-del's genes distributed themselves during genetic crosses exactly like chromosomes and correctly inferred that the simplest hypothesis was to believe that genes were located on the chromosomes. This theory tells us that normally each cell contains two copies of every gene because each chromosome is present twice. Likewise, each sperm or egg contains only one copy of each gene because the meiotic divisions which form the sex cells reduce the chromosome number to one-half the normal diploid complement. Although Sut-ton's hypothesis was only tentatively believed at first, many ingenious supporting experiments were quickly done, and by 1915 the great American biologist T. H. Morgan and his young collaborators, the geneticists Bridges, Müller, and Sturtevant, were able to publish their definitive volume, The Mechanism of Mendelian Heredity. This classic book showed the general validity of the chromosomal basis of heredity, a concept which ranks with the theories of evolu-tion and the cell as the main achievements of the biologist's attempt to understand the nature of the living world.

It now became possible to understand the nature of the here-ditary variation which is found throughout the biological world and forms the basis of the Theory of Evolution. Genes are normally copied exactly during chromosome duplication. Rarely, however, changes (mutations) occur in the genes, giving rise to altered genes (mutant genes) which are then copied in the altered form. This process is necessarily very rare; otherwise many genes would be changed during every cell cycle, and offspring would not ordinarily resemble their parents. There is, instead, a strong advantage for a small but finite mutation rate. For it provides the constant source of new variability necessary to allow plants and animals to adapt to a constantly changing physical and biological environment.

Already in Darwin's time chemists asked whether living cells utilized the same chemical rules found in non-living systems. Cells were quickly found not to contain any atoms distinctive to living material. Also recognized early was the predominant role of carbon which occurs in a very large variety of different molecules. An initial tendency to distinguish between carbon compounds like those in living matter and all other molecules is still reflected in two major

INTRODUCTION 303

divisions of modern chemistry, organic chemistry (the study of most compounds containing carbon atoms) and inorganic chemistry. Now we know that this distinction is artificial and has no biological basis. There is no purely chemical way of deciding whether a compound comes from a cell or was synthesized in a chemist's laboratory.

Nonetheless, through the final quarter of the last century, there existed a strong feeling in many biological and chemical labora-tories that some vital force outside the laws of chemistry was the difference between the animate and inanimate. Part of the reason for the persistence of this "vitalism" was the still-restricted success of the biologically oriented chemists (now usually called bio-chemists). Although the techniques of the organic chemists were suf-ficient to work out the structures of relatively small molecules like glucose, there was increasing awareness that many of the most important molecules in the cell were very large—the so-called macromolecules—and much too big to be pursued by even the best of organic chemists.

The most important of the macromolecules was first believed to be the proteins. Already by the end of the nineteenth century, there was evidence that virtually all chemical reactions occurring between cell compounds are enormously speeded up by special catalytic molecules now called enzymes. Enzymes are highly specific and generally catalyze only one of the very large number of .cellular chemical reactions. Initially, controversy existed as to whether enzymes themselves were small molecules or macromolecules. By 1925, the point was virtually established when the enzymatic nature of a crystalline protein was first demonstrated. But even then, their very complex structures were undecipherable by available chemical tools, and it was still possible to believe that their structures would eventually be shown to have features unique to living systems.

There also existed the general belief that the genes, like the enzymes, would be proteins. There was no direct evidence, but their very high degree of specificity suggested to most people who specu-lated on their nature that only proteins, known to occur in the chromosomes, could possess the necessary specificity. Another class of molecules, the nucleic acids, were also found to be a common chromosomal component. But then the nucleic acids were thought to be relatively small and incapable of carrying sufficient genetic information.


Besides this general ignorance of the structures of the large molecules, there was often expressed the feeling that there was something unique about the three-dimensional organization of the cell which gave it its living feature. This argument was sometimes phrased in terms of ignorance of the exact chemical interactions, but more frequently in the expectation that some new natural laws, as important as the cell theory or evolution, would have to be dis-covered before the essence of life would be understood. But these almost mystical ideas never led to meaningful experiments and in their vague form could never be tested. Instead, the only way for-ward has seen chemists and biologically-oriented physicists patiently attempting to devise new ways of solving more and more complex biological structures. But for many years there were no triumphs to shout, and the chemists and biologists usually moved in different and sometimes hostile worlds, often with the biologist denying that the chemist would ever provide the real answers to the important riddles of biology. Always not too far back in some biologists' minds was the feeling, if not hope, that something more basic than mere com-plexity and size separated biology from the bleak, inanimate world of a chemical laboratory.

During the last ten years, however, new techniques, arising largely from the world of the physicist, for the first time gave mean-ingful answers about macromolecules. The complete structure of a protein has been achieved, and the general structural features of the nucleic acids, now realized to be the carriers of hereditary infor-mation, are known. Most importantly, this structural data has provided a framework by which almost all the significant features of heredity, always the most mysterious of life's characteristics, can be understood in molecular terms.

Now there exists complete certainty that other characteristic features of living organisms, for example, selective permeability across the cell membranes, muscle contraction, nerve contraction, and eventually the hearing and memory processes, will all be com-pletely understood in terms of the co-ordinative interactions of small and large molecules. Already much is known about their less com-plex features, enough to give us confidence that further research of the intensity recently given to genetics will eventually give man the ability to describe completely the essential features of life.

CHAPTER 2

A Chemist's Look at the Living Cell

The most striking aspect of living cells is their tendency to grow and divide. In this process food molecules are absorbed from the external environment and transformed into cellular constituents. The rates of cell growth vary tremendously but in general the smal-lest cells grow the fastest. Under optimal conditions some bacteria double their number every 20 minutes while most larger mammalian cells can divide only about once every 24 hours. But independently of how long the time interval, growth and division necessarily de-mands that the number of cellular molecules doubles each cell generation. One way, therefore, of asking the question What is Life? is to ask how does a cell double its molecular content; that is, how are biological molecules replicated as a cell grows.

But at first sight, the problem of soon, if ever, understanding the essential chemical features of a living cell should seem insuper-able to an honest chemist first faced with this challenge. For he will immediately grasp the fact that on the chemist's scale, even the smallest cells are fantastically large. One such "very big" small cell is the bacterium Escherichia coli. E. coli cells are rod-shaped and average about 2μ length and 1μ in diameter. Because of their small size and also since they are easily grown under laboratory conditions, they are now, except for man, the most intensively studied living organism. A typical E. coli cell like that shown in my lecture, weighs approximately 2 x ICH2 gram (MW ^ 1012). This number which initially may seem very small is on the chemist's scale immense since it is 6 x 1010 times greater than the weight of a water molecule (MW = 1 8 ) . Furthermore this mass results from the highly complex arrangement of a large number of different carbon-containing molecules.

305


There is a seemingly infinite variety in the chemistry of these molecules. But fortunately (for our memory), it is possible to place most of them into several well-defined classes possessing common arrangements of some of their atoms. These are the carbohydrates, lipids, amino acids, nucleotides, proteins, and nucleic acids. There are, moreover, many other molecules possessing chemical groups common to several of these classes and whose classification is neces-sarily arbitrary. In addition there are small molecules like 0 2 and C0 2 and numerous electrically charged inorganic ions like Na+, K+, and PO % Finally, there is H 2 0, the most common molecule in all cells and which is a solvent for most biological molecules, allowing them to quickly diffuse from one cellular location to another.

A hint into cellular organization is shown in an elec-tron micrograph of a very thin section cut from a rapidly growing E. coli cell. On the outside is the rigid cell wall, a 100 A thick mosaic structure built up from protein, polysaccharide, and lipid molecules. Just inside the cell wall is a flexible, thin cell mem-brane, composed largely of protein and lipid. This membrane is semipermeable and controls which molecules enter and leave the cell. Of vital importance is its ability to maintain a concentration gradient—most molecules, both small and large, are present at much higher levels inside than outside the cell membrane. This is true for both inorganic ions like Na+ and Mg++ and for most important organic molecules. Within the membrane there is a lighter staining region (sometimes referred to as the bacterial nucleus), largely containing deoxyribonucleic acid (DNA). This is the compound re-sponsible for the transmission of genetic information from one cell to another. Outside the nuclear region there occur some 10,000-20,000 200Ä thick spherical particles, the ribosomes. These particles, which contain approximately 50% protein and 50% ribonucleic acid (RNA), are the cellular sites of protein synthesis. The remainder of the cell-inside is filled with water and a very large number of different molecules.

Now we can make only an approximate guess of the number of chemically different molecules within a single E. coli cell since each year many new molecules are discovered. This best guess is that between 3000 and 6000 different types of molecules are present

H~Ö

1 Inorganic Ions |

(Na+, K+, Mg++·, Ca+, Fe + + +

Cl", P04 = , S 0 4 ~ , etc.)

Carbohydrates and Precursors

lAmino Acids and Precursors

Nucleotides and Precursors

1 Other small molecules

1 Lipids and Precursors

1 Proteins

Nucleic Acids:

DNA

RNA

% of total weight

75

1

3

0,4

0,4

0,2

2

12

1

5

average MW

ΠΓ 40

150

120

300

150

500

40,000

2,000,000,000

750,000

Total Number per cell

4 x T Ö ^

2 5xl08

io8

3xl07

12x1ο7

1 5xl07

4xl07

3xlOe

1

6x10*

1 Number different kinds

Ϊ

20

300

150

200

200

200

1000 - 3000

1

1000 - 2000

Figure 2.1.—Approximate Chemical Composition of an Escherichia coli Cell. Is

A C

HE

MIST

'S LO

OK

AT

TH

E L

IVIN

G C

EL

L


H 1

HO-C - H

\ c— o

H / H \ H \ / C C

HO \ θ Η H / C — C

1 i i

H OH

Glucose

H H

N

1 C N

i ii H C C

V N, | H

1 Adenine

\ OH

\ C-H

/

HO

H

H

H

H 1

- C - H

c c

H '^H HNl OH

C C

II il OH OH

Ribose

H

— C — OH

1 — C — O H

1 — C — OH

1 H

Glycerol

O OH

c

1 HO — C — H

I H — C — H

| 1 H

Lactic Acid

0

1

^ N C — H

N

1 1

H

Uracil

Figure 2.2.—Chemical formula of several relatively small biologically important molecules. Molecules in this size range can be studied easily by

the organic chemist.

(Figure 2.1). Some of these, like H20 and C02, are chemically simple. Others, like the common sugar glucose or the nitrogen con-taining compound adenine (Figure 2.2), are more complex but nonetheless rather easily studied by current chemical techniques. But very many other cellular molecules, in particular the proteins and nucleic acids, are very large and even today their chemical structures are still immensely difficult to unravel. Only very few of these macromolecules are being actively studied as their com-plexity forces chemists to concentrate on relatively few out of many different ones. Thus we should immediately admit that the structure of a cell will never be understood in the same way as the H20 or glucose molecules. Not only will the exact structures of most macromolecules not be solved but, in addition, their relative location within cells can only be vaguely known.

A CHEMIST'S LOOK AT THE LIVING CELL 309

It is thus not surprising that many chemists, after brief periods of enthusiasm for studying "life", silently return to the world of pure chemistry. Others, however, are more optimistic since they focus attention on the following generalizations which can be drawn from patient observation of the living cells' multitudinous and highly diverse molecules. (a) All the molecules in a cell can be derived from each other by

chemical reactions occurring within cells. This point is directly seen by allowing the bacterium

E. coli to grow in a simple, well defined medium {Figure 2.3) con-taining the sugar glucose. Under these conditions glucose is the only source of cellular carbon and thus all the carbon atoms of all E. coli molecules must eventually result from chemical transformations by which glucose molecules are either broken down to smaller frag-ments or added to each other to form larger molecules like starch or glycogen. The exact steps and rates by which all these trans-formations (commonly known as intermediary metabolism) occur is enormously complex and most biochemists concern themselves with studying (or even knowing about!) a very small fraction of the total interactions. Now most experiments utilize the availability of

NH4CI

MgS04

KH2P04

Na2HP04

Glucose

Water

1.0g

0.13g

3.0g

6-Og

4-Og

1000ml

Figure 2.3.—A simple synthetic growth medium for E. coli. Traces of other ions (e.g., Fe++)are also required for growth. Usually these are not separately added since they are normally present as contaminants in either the added inorganic salts, or the water itself. In this medium, cell division occurs every 60 minutes. Addition of other organic molecules (e.g., the amino acids, and purine and pyramidine bases) reduces the division time to 20 minutes. This effect is due to direct incorporation of these components into proteins and nucleic acids, thus sparing the cell the task of carrying out their synthesis.


H H

V 1 II

HC C — X. / N

H H

|

1 1 HC C -

H H

1

1 1 HC C

^ / N

- N % C H

- I K H

Adenine (a purine base)

- N

C H | j i i I ^ " " Adenosine

\ O C (a nucleoside)

„- s/Jt N, 1 1 ' 1 OH OH

CH

\ C

H '

OH H H 1

o X C — O — P - 0

\ H H S

' i OH OH

Adenosine monophosphate (AMP) (a nucleotide)


H H \ /

N

A N C - N ^ .. .. O >CH H H |,

H C C — N ^ ^ - O V — O - P - O - P - O H

H C- V OH OH I I

OH OH

Adenosine diphosphate ( A D P ) ( a nucleotide)

H H O O O

\ / ι II n ^ Ο ^ C - O - P - 0 - P - O - P - O H

C—C OH OH OH I I

OH OH

Adenosine triphosphate ( A T P ) (a nucleotide)

Figure 2.4.—Adenine and some of its phosphorylated derivates. When the pentose sugar (ribose) is added to adenine, it forms a new compound called a nucleoside. When phosphate is added, it forms the nucleotide. There are a number of such nucleosides and nucleotides that differ in the nature of the

base attached to the sugar.

molecules labelled with radioactive isotopes. For example, when we very briefly grow E. coli upon glucose in which all its carbon atoms were labelled by C14 atoms, we see that the radioactive atoms soon appear in chemically similar molecules. Glucose is initially transformed by reacting with the phosphate group donor adenosine-triphosphate (ATP) {Figure 2.4) to form the related molecule glu-cose-6-P04 {Figure 2.5). Then with time the C14 is no longer present in glucose-6-P04 but instead we see radioactivity in molecules con-taining 3 carbon atoms like phosphoglyeerie acid. Similarly if we


H Λ----^, ~~--̂ .

H-r'C— 6[H) \ H

C 0 \ H-O^Pi=0 H / H \ H ·^ ,.i;

OH \OH H / OH H-0-P=0

? — ? o H OH ;

(ATP)

H OH 1 1

H-C-O — P — O H \ o c<—o

Hs / "H \ / H

OH \OH H/ xOH +

c c 1 1 H OH

o H - 0 - P = O

0

A

(ADP)

Figure 2.5.—Formation of Glucose-6-phosphate from Glucose and ATP.

label oxygen atoms with O18 or nitrogen with N15 we again can follow their movement from one type of molecule to another. In Figure 2.6 are shown some of the first transformations of glucose after it enters the cell. (b) The construction of macromolecules by the linear linking of

small molecules. When bacteria grow on glucose, the carbon atoms of all mole-

cules from the very small to the largest proteins and nucleic acids derive from atoms originally present in glucose. Macromolecules, like small molecules, form by the gradual addition of new atoms. We might guess, however, that their relatively immense size would make insuperable even a superficial analysis of their synthesis. Fortunately, however, there exist three simplifying structural genera-lizations which reduce this problem to a difficult but not impossible task.

(1) First, all macromolecules are polymeric molecules formed by the condensation of small molecules. Macromolecule synthesis thus occurs in two stages. To begin, the smaller subunits are put together, then the subunits are systematically linked together. An analogy can be made with the building of a house from precon-structed bricks.

(2) Secondly, the building blocks for a given macromolecule have common chemical groupings. This is shown in Figure 2.7 which describes the main structural features of several important macro-molecules. For example, proteins form by the condensation of nitrogen containing organic molecules called amino acids. The chemical bond linking the amino acids together is the peptide bond (Figure 2.8). These are 20 important amino acids, each of which

A CHEMIST'S LOOK AT T H E LIVING CELL 313

Glucose + ATP

IT Glucose-6-phosphate It

Fructose-6-phosphate

ATP

Fructose-1, 6-diphosphate

Glyceraldehyde-3-phosphate ^_

H3 pa

1,3 diphosphoglyceric acid

ADP v

ATP *

3-phosphoglyceric acid T

)

Glucose

2-phosphoglyceric acid

>> H20

ψ Phosphoenol pyruvic acid

ADP

ATP

Pyruvic acid +- ATP

- C0 2

Acetaldehyde

Alcohol

+ 2 ADP + 2H3 P0 4

+Dioxyacetone phosphate

2 Alcohol + 2 C02 4- 2 ATP + 2 H2 0

Figure 2.6.—Some initial metabolic transformations of glucose following cellular entry.

1 Macromolecule

iGlycogen |(a polysacharide)

DNA 1 (deoxyribonucleic 1 acid)

RNA (ribonucleic acid)

Protein

Monomeric Unit

g l u c o s e H 0 H 2 C 6

H V -

c; H

/ \ 0 H

| H

deoxynuc leo t ides

ribonucleotides

amino acids

H>. ^ N -

H ^

General Monomer Chemical Formula

0

V H A

? 0 / 0 H

\ OH

B a s e - deoxyr ibose — phosphate

B a s e - r ibose -phosphate

R (side groups)

- c - c 1. "0H

Number of different monomers

j

1

4 (adenine guanine thymine cy tos ine )

4 (adenine guanine uracil cytosine)

20 (qlycine alanine serin e etc.)

Linear or

Branched

Branched

Linear

Linear

Linear

F ixed or irregular chain length

indefinite — may b e > 1 0 0 0

Cl

to

3c :S

genet i ca l ly f ixed, may b e > 1 0 7

gene t i ca l l y f ixed, often

> 3 0 0 0

genetically fixed, usu-ally varies between 100 and 1000

General ized Polymer 1 Formula

* ^ ~ *°r °

H 0 ' \

;cNH 5c

4C 4C

^ ° 2C/°20/°

Base-deoxyribose-£h,osphate

^-^-^"^ 1 Base -deoxyr ibose -phosphate l

-̂-— 1 Base -deoxyr ibose -phosphate l

Base -r ibose -phosphate 1

^-^^ I Base-ribose-phosphate J

Base-ribose-phosphate

R, : 0 R2 0 H | ; II ·. / // \ N - C - : C - N * - C - C ά I *. 1 · 1 M H ·. H · H

S ' * P E P T I D E B O ND

Figure 2.7.—Structural Organization of Several Important Biological Macromolecules.

LIG

HT

A

ND

L

IFE

IN

T

HE

U

NIV

ER

SE

side group of

alanine

[ HO O i

H 3 C r C - H

side group of

valine

■VH0/ 0

L> C

H3C-CJ-C-H

/

•™< x H *

side group of O^ \ | | aspartic acid jC)- C - C - H

— * > ' A J

carboxyl group

,^ Η;

amino'group

Free Amino Acids

the formation of a peptide

bond is accompanied by

■ —— ^~

release of a water molecule.

*\ r - C - H

;(H — N N

polypeptide backbone

,Ο

Η3ς

I \ \

H 3 C - C - j - C - H

H3C/ J

{(w N V s^, c o ΓΗ

C —C ί - C - H / | I

HO H

+ 2 H 2 0

pept ide bond

N

/ \ H H

Polypeptide Chain

Figure 2.8.—Linking of free amino acids to form a polypeptide chain.

A C

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MIST

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OK

AT

TH

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IVIN

G C

EL

L


has part of its structure identical to that of comparable regions in the other amino acids. Attached to the regular region are "side groups" specific for each amino acid {Figure 2.9). Each amino acid thus has a specific (the side group) and a non-specific region.

NAME

Alanine

Arginine

Asparagine

Aspartic Acid

Cystine

Glutamic Acid

Glut a mine

Glycine

Histidine

Isoleucine

Leu cine

Lysin

ABBREVIATION

Ala

Arg

Asp=NH2

Asp

Cy~SH

Glu

Glu-NH2

Gly

His

lieu

Leu

Lys

FORMULA

H O O C - C H - C H 3

NH 2

HOOC- C H - ( C H 2 ) 3 - N H - C - N H 2

NH 2 NH

H O O C - C H - CH2 — C — NH 2

1 H NH 2 0

H O O C - C H - C H 2 — COOH

NH 2

H O O C - C H - C H 2 — SH 1 N H 2

HOOC- C H - (CH 2 ) 2 -COOH

N H 2

HOOC- CH - (CH 2 ) 2 - CO - NH 2

N H 2

HOOC- C H 2 1

N H 2

H O O C - C H - H o C — c = CH i l l 1

L· NH N N H 2 - * Q *

H 1

H O O C - C H - C H - C H 9 C H 0 1 I 3

NH 2 CH 3

HOOC- C H - CH 2 -CH- C H 3

N H 2 C H 3

H O O C - C H - ( C H 2 ) 4 - N H 2

N H 2


NAME

Methionine

Phenylalanine

Proline

1 Serine

1 Threonine

1 Tryptophane

1 Tyros ine

1 Valine

ABBREVIATION

Met

Phe

Pro

Ser

Thr

Try

Tyr

Val

FORMULA

H O O C - C H - C H 2 - C H 2 - S - C H 3

N H 2

c - cvN H O O C - C H - C H 2 C C

1 N ' N H 2

C ~ C

C H 2 — C H 2 H O O C - i H C'H2

H O O C - C H - CHo 1 1

N H 2 OH

H O O C - C H - C H - C H o 1 ' N H 2 OH

C H O O C - C H - C H 2 - C CT^C

' II I 1

H c-c / w

H O O C - C H - H 9 C - C C - O H 1 W N H 2

C " c

H O O C - C H - C H - C H o 1 I N H 2 C H 3

Figure 2.9.—Chemical Formula of the 20 Important Amino Acids.

(3) Our third generalization is that most macromolecules (all nucleic acids, proteins, and some polysaccharides) are linear aggre-gates in which the subunits are chemically linked together by a chemical bond between atoms in the non-specific regions. The linear shape follows from the fact that most subunits possess only two atoms capable of forming bonds to another subunit. Moreover, since the same two atoms are generally used to link all monomers, a large fraction of most macromolecules consists of a repeating series of identical chemical groups (the backbone) (Figure 2.8).


Figure 2.7 also reveals an important difference between the polysaccharides, like glycogen, and the proteins and nucleic acids. Polysaccharides are usually constructed by regular (or semi-regular) aggregation using one or two different kinds of building blocks. In contrast, proteins contain 20 different amino acids while four dif-ferent nucleotides are found in the nucleic acids. Moreover, in the nucleic acids and proteins, the order of subunits is highly irregular and varies greatly from one specific molecule to another. Thus the synthesis of nucleic acids and proteins demands not only the making of the correct backbone bond but, in addition, a highly efficient mechanism must exist for ordering the correct subunits.

(c) Almost all the chemical reaction rates in living cells are speeded up by enzymes.

Most biological molecules contain atoms linked together by relatively strong chemical bonds. Outside of cells they are essentially inert. Only when we raise their temperature much above normal physiological conditions (0-40° C) do they decompose, often by reacting (burning) with molecular oxygen. For example, pure glucose is very stable unless it is treated with strong acid or alkali or heated above 150° C when it breaks down, by reaction with oxygen. Though many important biological molecules are less stable than glucose, an essential aspect of almost all important intermediary metabolism reactions is that under normal cellular conditions they do not occur spontaneously at detectable rates.

Instead the relatively fast reaction rates which occur in living cells are due to the presence of highly specific molecules called enzymes. Each cell contains thousands of different enzymes and in general each specific biochemical reaction is speeded up by a specific enzyme. For example, glucose-6-P04 is formed when glucose and the phosphate group donor ATP interact (Figure 2.4) on the surface of the enzyme hexokinase. The combination of this enzyme with its substrates (molecules whose reactivity is catalyzed by the enzyme) weakens normally strong covalent bonds in both glucose and ATP, allowing transfer of the terminal ATP phosphate to the glucose molecule (Figure 2.10),

This ability of a third molecule to speed up (catalyze) reactions between two highly different ones is not limited to biological systems. A very well known example involves the interaction ot

Glucose-6-phosphate^^v^V

Enzyme substrate complex

Figure 2.10.—The formation of an enzyme-substrate complex, followed by catalysis.

A C

HE

MIST

'S LO

OK

AT

TH

E L

IVIN

G C

EL

L


sulphur dioxide (S02) and oxygen (02). This reaction is speeded up by the presence of finely ground platinum which absorbs SOL> and Oo molecules and brings them in the necessary close contact. A very important characteristic of catalysts is that they are never consumed in the course of reaction but once the reaction is com-plete the catalytic surface is free to absorb new molecules and function again (Figure 2.10). On a biological time scale (seconds to years), enzymes can work very fast, some being able to function as many as 106 times per minute. On the contrary, if they are absent, often not one successful collision would occur in this same time interval.

All enzymes are now known to be proteins. All proteins, how-ever, are not enzymes; some serve purely structural roles such as the keratin molecules found in skin and hair. The vast majority of proteins are, nonetheless, enzymes and now it seems likely that enzymes are both large and complicated because they must be able to both specifically combine with the correct substrates and to in-crease their chemical reactivity. As yet the complete structure of not one enzyme is known, though there is good reason to believe this will be accomplished in the next 3-10 years. Then it will be possible for a chemist to ask on a chemical level the exact mechanism of enzyme action. Already there are numerous hypothetical schemes, suggesting how a protein enzyme may work. All of these are based on reactions known to occur in the organic chemist's test tube, and there is no good reason to believe that new chemical concepts must be developed to understand how enzymes act. Instead the primary task remains the development of techniques for protein structure analysis.

(d) Energy conserved during degradative transformation is used to drive biosynthesis reactions.

All biochemical reactions are theoretically reversible with the final equilibrium conditions determined by the nature of the react-ing molecules. Thus in a biochemical reaction of the type (1),

(1) A + B enzy ey (x) C ± energy ± entropy (thermodynamic disorder)

< the final concentrations of A, B, and C are independent of the presence of enzyme (x). Only the rate at which equilibrium is reach-


ed is determined by the enzyme concentration. The equilibrium values depend on the changes in free energy and entropy (amount of disorder) which accompany the changes in chemical bonds. Ther-modynamic arguments tell us the most common final reactants are those which accompany the maximum release of free energy and disorder. For example, when glucose reacts (burns) at high tem-perature with molecular oxygen, the appearance of the final reaction products (2)

(2) Glucose + 02 ( H20 + C02 + heat (free energy) + entropy

leads to generation of heat (energy) as well as an increase in the total atomic disorder. The final equilibrium thus favours a great excess of H 2 0 and C 0 2 over glucose. Nonetheless, this reaction is unimportant in normal cellular existence for no enzymes are present to speed up its normal insignificant rate of physiological tempera-tures. Instead we normally find that most degradation of foodstuffs is not accompanied by release of significant free energy in the form of heat. Instead the bond energy of the C-C or C-O covalent bonds of glucose is efficiently transformed into new chemical bonds whose total energy is almost the same as the initial bonds. Thus the energy used to make the new molecules necessary for growth (biosynthetic reactions) can come from roughly equivalent bonds in small molecules like glucose. Most frequently the energy released by the degradation of a food molecule is not directly transferred to a chemical bond made in a biosynthetic reaction. These energy transfers are usually mediated by intermediate transfer to P-O-P bonds in phosphated compounds like ATP (Figure 2.4). For ex-ample, during the complete enzymatic degradation of glucose to C0 2 and H 2 0 , approximately 40 new P-O-P bonds in ATP are generated. These are then available for any of a large number of energy-requiring biosynthetic steps.

The growth of a cell thus does not violate the classical roles of thermodynamics by which the total free energy and energy must increase. The highly organized synthesis of new cellular molecules is only possible because new energy is being added from the out-side either in the form of molecules like glucose, or in the form of new ATP molecules generated by light quantum emanating from the sun. When this outside energy is taken into account, the


total of free energy and entropy increases as in any other closed system.

The most frequent biochemical reactions are thus in theory predictable if we know (1) the thermodynamic equilibrium and (2) the concentration of the relevant enzymes. This latter factor is of major importance despite the fact that the equilibrium concentra-tions are not determined by the enzyme. This follows from the fact that the rate of uncatalyzed reactions is generally slow, even on the biological time scale of seconds to days. Thus, even if the thermodynamic conditions are highly favourable, the occurrence of almost all biochemical reactions demands the participation of specific enzymes. (e) Molecules are restrictively sticky.

For example, a given enzyme specifically combines with its substrate and shows little affinity for other small molecules. Like-wise, a given small molecule generally only has high combining power with its specific enzyme(s). Enzyme-substrate interactions usually do not involve the covalent bonds, but instead depend upon the much weaker electrostatic bonds between positive and negative atoms, Van der Waals' forces, and/or hydrogen bonds. Though all these interactions are weak in comparison to covalent bonds, they are strong in comparison to random thermal forces and sufficient to assure that cells are not hopelessly inefficient because of unfunc-tional interactions. The extremely elaborate structure of proteins should be viewed as a means of achieving very accurate interaction specificity. Moreover, the structures of existing small molecules-have evolved so that they also cannot become stuck to another small molecule.

Restricted stickiness also seems to be the mechanism by which the cell membrane and walls are put together. Both are mosaic sur-faces made up of regular arrangements of smaller molecules which most likely only form stable contacts with other molecules in the cell membrane (cell wall). This is easy to see in the case of the lipid containing molecules. Lipids (fats) are extremely insoluble in water and so these are not likely to be found in the largely aqueous environment of the cell's interior. Instead, a newly syn-thesized lipid will have a strong tendency to attach to other lipids in the cell wall (or membrane).


(f) A tendency for cell expansion exists because of osmotic pressure on the cell surface.

The higher concentration of small molecules inside cells means that there are relatively fewer water molecules. Since water can diffuse quickly through cell membranes, there is always a thermody-namic tendency for more H 2 0 to move into cells than to diffuse out-wards. If cell membranes were infinitely expansible, cells would expand until the internal and external water concentration became equal. On the contrary, however, membranes are only slightly elastic and can be only slightly stretched in the absence of synthesis of new membrane material. As a result the tendency of water to enter is counterbalanced by an inward pressure exerted by the cell mem-brane. The pressure exerted by the solution is called the osmotic pressure. In many animal cells, the cell membrane is strong enough to contain the osmotic pressure erected by its concentrated supply of small molecules. In most bacterial and plant cells, however, the cell membrane by itself is not sufficiently strong. Here we find that the osmotic force is contained by the rigid cell walls against which the cell membrane is tightly packed. This point can be shown by experiments which selectively remove the cell wall. When this hap-pens, the naked cells quickly expand and burst unless the osmotic pressure is reduced by suspending the wall-less cells in solutions containing high concentrations of small molecules.

The continuous presence of this osmotic force may provide the explanation why increase in cell size goes hand in hand with intake of food molecules and corresponding synthesis of new cell con-stituents. Under conditions of normal physiological growth, small regions of the cell surface may temporarily rupture, erecting tiny gaps. If at the same time there are present nearby newly synthesized cell surface components, they will quickly diffuse out of the un-favourable aqueous environment of the cells' interior and fill the gap.

CHAPTER 3

The Concept of Template Surfaces

By now our chemist knows that there is not one but at least several "key secrets of life" upon which the ability of a cell to grow and divide depends. First, there must exist a highly organized surface membrane capable of maintaining, through selective per-meability, a high concentration of internal molecules. Secondly, enzymes must exist which catalyze the movement of the atoms in food molecules into new cellular building blocks. Thirdly, useful energy must be derived from food molecules or the sun to ensure that the thermodynamic equilibrium favours biosynthetic rather than degradative reactions.

All these properties are intimately dependent upon the existence of proteins. Only these very large proteins with their 20 different building blocks possess sufficient specificity to build selectively permeable membranes or to catalyze highly specific chemical trans-formations. We must thus add to the list of "key secrets of life" the ability to synthesize the physiologically correct amount of specific proteins. This requirement at first might seem to fall under the more general prerequisite of enzyme catalyzed biosynthesis. But, as we shall soon learn, the synthesis of a protein does not proceed accord-ing to rules governing the synthesis of small molecules. This point becomes clear when we look at the way enzymes are used to con-struct increasingly large molecules.

The synthesis of small molecules. Let us first look at how the amino acid serine is normally

put together in E. coli cells growing upon glucose as its sole energy and carbon source. Figure 3.1 illustrates that serine is formed in several steps from 3-phosphoglyceric acid, a key metabolite in the normal degradation of glucose {Figure 2.6). Some serine molecules are then broken down in several more steps (whose exact chemistry has yet to be worked out) to give the simplest amino acid glycine. Each of these steps requires a specific enzyme, with a characteristic

325

COOH

HCOH I

Glucose ►CH2OPO3H2

COOH

I c = o

2H ^ CH 2 OP0 3 H 2

3-Phosphoglyceric

acid

3- Phosphohydroxy

pyruvic acid

H 2 0

H3PO4

COOH

I C - 0 I

CH2-OH (NH3)

COOH

I CH-NHo I

-^ C H 2 - O H

Hydroxypyruvic

acid

Serine

Figure 3.1.—Biosynthesis of serine.

as

LIG

HT

AN

D

LIF

E IN

TH

E U

NIV

ER

SE

THE CONCEPT OF TEMPLATE SURFACES 327

surface capable of combining only with its correct substrate. Each of the other 18 amino acids are synthesized by the same principle. In every case, a metabolite derived from glucose metabolism serves as the starting point for a series of specific enzymatically mediated reactions leading finally to a given amino acid(s). Likewise the purine and pyrimidine nucleotides, the building blocks out of which the nucleic acids DNA and RNA are constructed, are synthesized by a series of consecutive reactions starting with smaller molecular units whose carbon atoms are derived from glucose molecules. Some of the reactions leading to the synthesis of the pyrimidine nucleotide, uridine-5'-phosphate, are seen in Figure 3.2. The syn-thesis of the larger purine nucleotides requires more steps, since more covalent bonds must be built. Again, however, the same basic

►CO« ATP NH 2 C00P0 3 H 2 c (acetylglutamatey carbamyl-

phosphate

H ^ P O C r ^ O C * 0 ™

N /O-P-O-P-OH LA O H Ö H OH OH O H «

aspartic acid

aspartate carbamyl transferase

2H

NHo

I CO

K C H 0

H carbamyl-aspartate

CH

- C 0 2 H

5- phosphor i bo sy I-pyrophosphate

pyrophosphate

orotic acid

dihydroorotic " \ dehydrogenase

C02H

dihydro-orotase

NH CH 2

CO JM

H C 0 2 H dihydroorotic

acid

orotidine-5-phosphate pyrophosphorylase

QH - C Q 2

orotidine-5-phosphate ι ' decorboxylase (2

C 0 2 H

HgOgPOCKO

OH OH orot id ine-5-phosphate

OH OH ur id ine-5 -phosphate

Figure 3.2.—Biosynthesis of uridine-5'-phosphate.


principles govern: (1) each reaction requires a different specific enzyme and (2) the reactions result in an increase of free energy.

C3POCH2 ATP AMP

Mg+ +

H P O |

o p 2 ° 5

5-Phosphoribosyl -pyrophosphate

Glutamine + H 2 0

Glutamate

+ H P 2 0 7 05POCH2

Mg++ OH OH

5-Phosphor ibosylamine

Figure 3.3.—Initial steps in Purine formation.

This free energy release (usually as heat) means that the thermodynamic equilibrium favours the generation of the biosyn-thetic reaction products necessary for cell growth. It is often accom-plished by having one of the substrates react with the "energy rich molecules ATP to form activated substrates in which a phosphate (P04), pyrophosphate (PP), or adenylic acid (AMP) group is attach-ed to an atom involved in the formation of the desired biosynthetic bond. A typical ATP driven synthesis is shown in Figure 3.3. Here is shown how ribose-5-P04 is transformed into 5-phosphoribosyla-mine (PRA). This transformation, one of the initial steps in purine nucleotide formation, occurs in two enzymatic steps. In the first ribose-5-P04 and ATP combine to form ADP and 5-phosphori-bosylpyrophate (PRPP). The second step involves the reaction of PRPP with glutamine to yield PRA, PP, and glutamic acid. The equilibrium of the first reactions favours PRPP synthesis because there is more energy in a pyrophosphate bond than in the phosphate ester (C-O-P) bond attaching PP to ribose-5-P04 (Figure 3.4). Like-wise the second equilibrium favours PRA formation because the C-N bond in PR has less energy than the alternative COP phosphate


high energy bonds

ojfi - O - P - O - P - O "

Ό- 6-

(pyrophosphate)

The bond joining the two phosphate groups in pyro-phosphate is energy

rich.

high energy bonds

p^pX^p Adenosine - O - P - O - P - O - P - O "

. . i - '- '_ phosphate ester bond

( A T P )

In ATP, the two terminal phos-phates are attached by "high energy" bonds. There is less energy in the bond to the proximal phosphate since it con-nects a phosphorus atom to a

non-resonating C-O group.

high energy bonds

?y*?~ CHJ-C-O-P-O

6~

(Acetyl phosphate)

This molecule con-tains high energy bonds since the phosphate is at-tached to a C-O resonating group.

Figure 3.4.—Examples of "energy rich" phosphate bonds.

ester. Both biosynthetic steps are thus accompanied by the release of free energy as heat. In contrast there is little energy difference between the initial CiO (H) bond of ribose-5-P04 and the final C-N linkage. Hence activation by an energy donor is a necessary pre-requisite for this biosynthetic step. Activation, however, is not an obligatory feature of all biosynthesis. In these examples, the relevant covalent bonds in a necessary cell constituent have signi-ficantly less energy than those in the metabolites from which they derive.

Synthesis of a large "small" molecule. The construction of chlorophyll (Figure 3.5) is a good example.

Here is a molecule (MW=892) which looks very complex even to a first-rate organic chemist and whose total laboratory synthesis has just been recently achieved. It contains a very complicated porphyrin ring to which is attached a long unbranched alcohol (phytol). As yet only the broad outlines of its biosynthesis are known. The porphyrin and phytol components are most likely synthesized separately and later joined together. Most of our current hard facts concern the putting together of the porphyrin ring (Figure 3.6). Here a very large number of different enzymes are used to rearrange the C, N, O, and H atoms found initially in the much smaller glycine and acetate precursors. No new qualitative features thus appear to distingush the synthesis of molecules with chlorophyll-


9H2 COOMe xO COOPhytyl

Figure 3.2.—The structure of Chlorophyll.

like complexity from the construction of small organic molecules. In both cases, enzymes and favourable thermodynamic equilibria are found. There is only the quantitative difference that the biosynthesis of large complex molecules needs more different enzymes and usually more externally added energy.

Synthesis of a regular very large polymeric molecule (glycogen).

Here is a macromolecule whose molecular weight is often above a million. Yet only four different enzymes are necessary to build glycogen up from glucose. This is because it is a polymeric molecule built up by the repetitive linking together of glucose units. Figure 3.7 shows the specific chemical steps by which glucose is activated at its number 1 carbon atom to a "high energy" compound and then polymerized. Only one enzyme is required for the final polymerization because each polymerization step makes the same type of chemical bond. Almost all the linkages are glucosidic bonds

8Glycine

8Succinyl-CoA

pyridoxal P 0 4 , biotin (?)

- 8 C 0 2

8-S-aminolevulinic acid

coproporphyrinogen 111," 2

-2C02r4H

coproporphyrin111

uroporphyrinogen 111

-6H

uroporphyrin 111

(?) Protoporphyrinogen - 6 H Protoporphyrin

hemes, chlorophylls

-4NH3

■8H20

4 porphobilinogen

uroporphyrinogen 1

- 6 H

uroporphyrin 1

Figure 3.6.—The path of tetrapyrrole synthesis.

THE CONCEPT OF TEMPLATE SURFACES


1) Glucose 4- ATP > Glucose-6-phosphate

2) Glucose-6-phosphate > Glucose-1-phosphate

3) Glucose-1-phosphate + uridine-triphosphate (UTP) > uridinediphosphate glucose (UDPG) - pyrophosphate

4) Glycogen (glucosen) + UDPG-» Glycogen (glucose η + χ )

Figure 3.7.—Biosynthesis of glycogen.

(C-O-C) between carbon atoms number 1 and 4, Much less com-monly another enzyme catalyzes the formation of 1-6 glucosidic bonds and as a result glycogen is often branched.

We thus see that the number of enzymes necessary to synthesize a molecule is not necessarily related to its size but instead to its chemical complexity. Thus glycogen, which is an easy molecule for the organic chemist to understand, also poses no fundamental pro-blems to the biochemist.

A deeper look into protein structure. Before we go into the problems involved in the synthesis of

protein, we must first look more closely into their structure. They are immensely more complex macromolecules since they are poly-mers built up from 20 different building blocks (the amino acids). Thus the organic chemist must determine both how the amino acids are linked together and their order within a given linear poly-peptide chain. Likewise the biochemist's wish to know both how the backbone linkages are connected and what trick is used to order the amino acids during synthesis. In both types of work, the ques-tions involving sequence have proved to be much more difficult. In fact, it was not until 1953 that the first complete amino acid sequence became known. The protein studied was the hormone insulin, a relatively small protein containing 53 amino acids. More recently the sequences of several additional proteins have been solved, the largest containing 158 amino acids. This is the protein found on the outside surface of the intensively studied virus of tobacco plants, tobacco mosaic virus (TMV). Figure 3.8 shows this sequence which required almost 10 years' work by a large group of

TH

E C

ON

CE

PT

O

F T

EM

PL

AT

E

SUR

FA

CE

S 333 Figure 3.8.—Amino acid sequence in Tobacco Mosaic Virus Protein.


talented chemists. Now there are available new experimental tech-niques which make sequence determinations easier but nonetheless even today at least several years' hard work is usually required to solve a relatively small protein structure.

There is also the problem of how the polypeptide chains assume their final 3-dimensional configuration. For we know that the func-tional activity of virtually all proteins depends not only upon the possession of the correct amino acid sequence but also upon their exact arrangement. The polypeptide backbone, however, is not a rigid structure as many of its atoms can freely rotate and assume diiferent relative locations. Nonetheless, there is very good indirect evidence that, under a given environmental situation, all protein molecules with identical sequences have the same "native" 3-D form. Very recently this belief has received direct support from the complete 3-D structural determination of the protein myoglobin. In Figure 3.9 is shown the structure as revealed by X-ray diffraction analysis. Though it is immensely complex, detailed inspection shows the important simplification that the chain has folded to bring to-gether atomic groupings which attract each other.

For example, the side groups of several amino acids, like valine and leucine, are very insoluble in water, while others, like those of glutamic acid or lysine, are highly water soluble. It thus makes chemical sense that the water insoluble side groups are found stacked next to each other in the centre of myoglobin, while the external surface contains groups which mix easily with water. The 3-dimen-sional configuration thus represents the energetically most favour-able arrangement of the polypeptide chain. Each specific sequence of amino acids takes up the particular "native" arrangement which makes maximum favourable atomic contacts between it and its nor-mal environment. This is strongly supported by very striking experi-ments in which first high temperature or other unnatural conditions break down the native 3-D form (denaturation) to give randomly oriented, biologically inactive, denatured polypeptide chains. Then the denatured chains are carefully returned to their normal environ-ment. Some of the denatured chains can then reassume their native conformation (renaturation) with full biological activity.


Enzymes cannot be used to order amino acids in proteins.

We thus come back to the ordering dilemma with the realiza-tion that it is the heart of the matter of protein synthesis. The prob-lem of how the connecting links form is, in comparison, minor. For this process involves the synthesis of only one type of covalent bond (the peptide bond) which hints that one or at most several enzymes are needed. On the other hand, the ordering itself cannot be accom-plished by recourse to enzymes specific for each amino acid in a protein. This device requires as many ordering enzymes as there are amino acids in the protein. But since all known enzymes are them-selves proteins, still additional ordering enzymes are necessary, and so on. This is clearly a paradox unless we assume a fantastically interacted series of syntheses in which a given protein can alter-natively have many different enzymatic specificities. It might be then just possible to visualize a workable cell (and then with great diffi-culty). It does not seem likely, however, that most proteins do in fact have more than one task. All our knowledge in fact points towards the opposite general conclusion of one protein — one function.

It is, therefore, necessary to throw out the idea of ordering with enzymes and to predict instead the existence of a specific surface, the template, which attracts the amino acids (or their ac-tivated derivatives) and lines them up in the correct order. Then a specific enzyme common for all protein synthesis attaches and makes the peptide bonds. It is furthermore necessary to assume that the templates must also have the alternative capacity of serving (either directly or indirectly) as templates for themselves (self-duplication). That is, in some way their specific surfaces must be exactly copied to give new templates. Again we cannot invoke the help of specific enzymes for that immediately leads us back to the enzyme cannot make enzyme paradox.

Template interactions are based on weak bonds acting over short distances.

The existence of proteins thus simultaneously demands the coexistence of highly specific template molecules. Moreover, the templates themselves must be macromolecules of size at least as large as their poly peptide products. We see this when we

Figure 3.9.—Three-dimensional model of myoglohin based upon X-ray analysis. A number of helical struc-

tures can be recognized (black).

338 LIGHT AND LIFE OF THE UNIVERSE

examine the rules which must govern the selective binding of small molecules to their templates. We first see that the binding is not done using strong covalent bonds. Instead the attraction is based on relatively weak bonds which can form without enzymes. These are (1) electrostatic salt linkages between positively and negatively charged atoms, (2) hydrogen bonds in which a covalently-linked hydrogen atom is attracted to electronegative atoms like oxygen or nitrogen and (3) Van der Waals' or dispersion forces (Figure 3.10).

All these forces operate only over very short distances ( < 5 A) and so templates can order small molecules only when they are in close contact on the atomic level. Thus it is to be expected that the size of the specific (attracting) regions of the template will be in the same size range as the collective size of the amino acid side groups in the protein product. Attraction of opposites is easier to visualize than self-attraction.

Here we pose the obvious question: Can a polypeptide chain serve as a template for its own synthesis? This would make possible a great reduction in the chemical prerequisites for life. For then the problems of protein synthesis and template replication would be the same, and the additional biochemical complexity required to main-tain a special class of template molecules would be unnecessary. This conceptual possibility finds no support, however, from close inspec-tion of the amino acid side groups. For there is no chemical reason why, for example, the occurrence of valine on a template should preferentially attract the specific side group of another valine mole-cule. In fact, none of the amino acid side groups have specific af-finities for themselves. Instead it is much easier to imagine molecules with opposite or complementary features attracting each other. Negative charges obviously attract positive groups while hydrogen atoms can only form hydrogen bonds to electronegative atoms. Similarly, the Van der Waals' forces can specifically attract only when they possess complementary shapes allowing a cavity in one molecule to be filled with a protruding group of another molecule.

A formal way remains, however, to save the possibility of protein templates. We might imagine the existence of 20 different specific molecules which we could call connectors. Each would possess two identical surfaces complementary in charge and/or

(a) Electrostatic bond (between ammonium ion and car boxy I group of amino acid gly-cine).

(c) Van der Waa ls ' interactions (between two a l i -phatic hydrocarbons).

(b) Hydrogen bond (between tyrosine and uraci l ) .

Figure 3.10.—The important weak chemical bonds in biological systems.

THE CONCEPT OF

TEMPLATE SURFACES


shape to a given amino acid. They would then have the capacity for the lining up of identical amino acids along a polypeptide chain. No evidence, however, exists for such molecules. Instead, as we can show, a specific template class does in fact exist.

A chemical argument against the existence of protein templates. This failure of proteins ever to evolve a template role may

have its origin in the composition of the amino acid side groups. The argument can be made that no template whose specificity de-pends upon the side groups of closely related amino acids, like valine, or alanine, could ever be copied with the accuracy demanded for efficient cellular existence. This follows from the fact that some amino acids are chemically very similar. For example, valine and isoleucine differ only by the presence of an additional methyl group in isoleucine. Likewise, glycine and alanine also differ by only one methyl group. This immediately poses the question whether any copying process can be highly accurate which must distinguish between such closely related molecules. Our answer depends in part upon what we mean by highly accurate. It is clear from amino acid sequence study that an accuracy of at least 99.9% efficiency is achieved. But it also seems chemically unlikely that a methyl group difference could ever be the basis of a copying process with errors less than 1 in a million.

This means that if proteins were the templates for their own replication, the informational content of the templates would be in constant flux. At the same time their protein products would also show great variation. Now, even though it is impossible to give a good quantitative argument either for the error level or its con-sequences for the maintenance of co-ordinated cellular metabolism, there seems no valid reason for the development of such an obviously borderline cellular system if a more efficient template system is possible.

Documents

Light and Life in the Universe. Selected Lectures in Physics, Biology and the Origin of Life