Chapter 14: Origin of Life

14-1 Biogenesis

14-2 Earth’s History

14-3 The First Life-Forms

I. Francesco Redi’s Experiment

• 1668; an experiment sought to debunk the hypothesis of spontaneous generation using a control and experimental group of jars containing raw meat.

14-1 Biogenesis

• The dependent variable in Redi’s experiment would be the…

Presence or absence of a lid on the jar

• The independent variable in Redi’s experiment would be the…

Presence or absence of maggots on the meat

(4) People once believed fish could form from the mud in a pond that sometimes dried up. How could you demonstrate that this conclusion is false?

Critical Thinking

II. Lazzaro Spallanzani’s Experiment

• 1700’s; Reworked Redi on a microscopic-scale, using meat broth, two types of flasks (open and closed), and a flame—sought to disprove the “vital force” hypothesis.

• Boiled meat broth in open flasks, then sealed the experimental flask by melting the glass neck shut while the control flask was left open to the surrounding air.

• Spallanzani concluded that the boiled broth became contaminated only when microorganisms from the air entered the flask, however since he used fire “excessively,” critics claimed he had destroyed the “vital force” in the air that could otherwise have generated life in the broth.

III. Louis Pasteur’s Experiment

• 1800’s; the hypothesis of spontaneous generation and the “vital force” is put to death by Pasteur, giving rise to “biogenesis.”

• A swan-necked (curved) and open flask of boiled meat broth was used alongside of a flask with a broken neck (no curve); the result?

• Microorganisms became trapped in the curve of the neck in the swan-flasks, but were able to enter the broken-necked flasks, allowing for contamination and growth.

NOTE: Pasteur avoided the critics by minimizing his use of flame, and allowing the flasks to substantially cool off (thus protection the “vital force.”

(1) What may have happened if Pasteur had tipped one of his flasks so that the broth in the flask came into contact with the curve of the neck? Explain how this result would or would not have supported his conclusion.

Critical Thinking

I. The Formation of Earth

• 5 billion years ago, our solar system was a swirling mass of gas and dust; over time, much material collapsed inward, forming the sun and remaining debris, from repeated collisions, resulted in the planets.

14-2 Earth’s History

• The earth we have today is believed to have taken 400 million years to develop from the beginning of the solar system.

(A) Earth’s Age

• Drilling into the earth reveals the many layers of sediment, providing scientists a picture of its geological history.

(B) Radioactive Dating

• A technique used to establish the age of materials by measuring the amount of a radioactive isotope (the clocks in the rocks) they contain.• This quantity is compared with the amount of some other substance in the fossil that remains constant over time.

(1) Isotopes

• Atoms of the same element that DIFFER in the numbers of neutrons they contain (i.e., # protons does NOT equal # of neutrons); Most elements have several isotopes.

(Ex: Carbon exists as both C-12 and C-14—where 12 and 14 are the mass numbers)

(2) Mass Number• Total number of protons and neutrons in the nucleus of an atom.

(3) Radioactive Decay

• Some isotopes have UNSTABLE nuclei, meaning, the nuclei of these isotopes tend to release particles or radiant energy, or both. (These rates of decay have been determined for several radioactive isotopes)

(4) Radioactive Isotope

• Ex: Relatively young fossils can be dated by measuring the ratio of the amount of C-14 (a radioactive isotope), to the amount of stable C-12.• Organisms taken carbon into their cells constantly, most of which is in the C-12 form; a small proportion however is in the C-14 form, which undergoes decay.

• The C-14 : C-12 ratio is a known quantity for many organisms, upon discovery of remains, this quantity can be compared to a measured quantity to estimate how much decay has taken place since death.

(5) Half-Life

• The period of time it takes for ONE-HALF of any size sample of an isotope to decay. (varies from seconds to billions of years)

Sample Problems…

• Calculate age of two specimens tested for their radioisotope contents:

(2) A meteorite in which 75% (equal to two half-lives) of the thorium-230 has decayed. (Given: T-230 has a half-life of 75,000 years)

• 75% is two half-lives, therefore the meteorite’s age can be approximated to be 150,000 years old.

(1) A human body buried in ice in which 87.5% (equal to three half-lives) of the C-14 has decayed. (Given: C-14 has a half-life of 5,730 years)• 87.5% is three-half lives, therefore the human’s age can be approximated to be 17,145 years old.

(2) Some radioactive isotopes that are used in medicines as tracers in the bloodstream have very short half-lives, often only a few years or less, rather than thousands of years. Would these isotopes also be useful in dating fossils? Why or why not?

Critical Thinking

II. The First Organic Compounds

• How were elements (resulting in early Earth) assembled into the organic compounds found in life?

• Alexander Oparin (1923) suggested that early atmospheric gases, when heated, may have formed simple organic compounds, such as amino acids.• Accordingly, when Earth cooled and water vapor condensed to form lakes and seas, organic compounds collected on the ocean floors and could have been become incorporated into chemical reactions from the energy of lightning and ultraviolet radiation.

(A) The Experimental Synthesis of Organic Compounds

• Oparin hypothesized about the first organic compounds, but Stanley Miller and Harold Urey (1953) were the first to perform experiments to test them.

• Miller and Urey’s apparatus was a model for the atmospheric and temperature conditions of early Earth.

• The experiment produced a variety of compounds, including various amino acids, ATP, and the nucleotides in DNA. Perhaps the beginnings of some of the most vital compounds for life on a young Earth.

(B) Organic Compounds from Beyond Earth• Recently, some scientists are hypothesizing that after the period of Earth’s formation, some organic compounds may have accumulated on the surface of Earth via fallen meteorites rather than originating here.

III. From Molecules to Cell-Like Structures

• Sidney Fox (1960s) has researched cell-like structures that have formed spontaneously from solutions of simple organic solutions.

(1) Microspheres

• A type of microscopic droplet enclosed by a membrane composed of organic molecules; formed under laboratory conditions similar to that of early Earth (possibly forerunners to the first prokaryotic cells)

(2) Coacervates

• Collections of microspheres that are composed of molecules of different types, including linked amino acids and sugars.

NOTE: Besides coacervates and microspheres possessing an ability to spontaneously form under certain laboratory conditions, they also possess two life-like properties of modern cells, including…

(1) An ability to take up certain substances from their surroundings.(2) An ability to grow and bud to form smaller microspheres

• However, more research in evolutionary cell biology is needed to bridge the gap between nonliving organic compounds and cellular life.

I. The Origin of Heredity• RNA vs. DNA, why have both? Research is striving to prove that RNA was perhaps the first hereditary molecule.

14-3 The First Life-Forms

• Unlike DNA, RNA can take on a greater variety of shapes due to bonding, which has led to speculation of RNA behaving as a catalyst (like an enzyme), capable of running chemical reactions in a cell.

II. The Roles of RNA

• Thomas Cech (1986) isolated a type of RNA found in some unicellular eukaryotes that was able to behave as an enzyme (“ribozyme”).

(1) Ribozyme• An RNA that by acting as an enzyme, could hypothetically, have the ability to replicate itself—led to speculation that life may have started with self-replicating molecules of RNA.

II. The First Prokaryotes (approx. 3.5 b.y.a.)• Due to a lack of O2 in early Earth’s atmosphere, the first living cells had to be anaerobic (fermenting) and heterotrophic, gathering spontaneously formed organic compounds on the ocean floor for food.• As this group of pioneering, anaerobic, heterotrophic bacteria multiplied, CO2 began to accumulate in the atmosphere, creating strong selective pressures towards an organism that could harvest the CO2 during metabolism…leading possibly to the first autotrophs.

(A) Chemosynthesis

• A metabolic pathway of a primitive group of autotrophic bacteria (Archebacteria) that produce carbohydrate through the use of energy from inorganic molecules (including CO2) instead of sunlight.

(1) Archaebacteria• A kingdom of ancient bacteria that thrive under extremely harsh conditions and run chemosynthesis to manufacture carbohydrates and other organic compounds. (also known as chemoautotrophs)

(2) Chemosynthesis• Autotrophic process often resulting from the oxidation of various inorganic substances, including sulfur. (deep sea vents)

(B) Photosynthesis and Aerobic Respiration• Both are a series of many chemical reactions that are essentially opposite to each other in terms of reactants and products. Nearly all life forms depend on the complementary relationship between these two forms of cellular metabolism.

(1) Cyanobacteria (first appeared approx. 3 b.y.a.)

NOTE: As CO2 accumulated in the seas and atmosphere, a new breed of autotrophic cells adapted to harness its abundance to run a metabolic process resulting in carbohydrates and O2. The chemoautotrophs are speculated to have led to our traditional autotrophs, of which also add in a factor of sunlight to their metabolism.• The accumulating O2 led to some of our first “aerobic” cells, of which found a way to prevent free oxygen from doing cellular damage by causing it to bond to other compounds.

• First autotrophic and photosynthetic bacteria to evolve; resulted in an O2 revolution, with a billion or more years of pumping O2 into our atmosphere today.

(2) Ozone (O3)

• When oxygen reached the upper-atmosphere, it was bombarded with sunlight, of which, split some O2 into single O atoms that were highly reactive with O2 nearby. •The result was ozone (O3), a poison to cells but safely away in the upper atmosphere, it absorbs intense UV radiation, allowing cellular DNA to remain safe and stable and fostering new life-forms on land.

(3) Some forms of air pollution damage Earth’s ozone layer. How might such damage affect life?

Critical Thinking

III. The First Eukaryotes (Heterotrophic and Autotrophic)

• Lynn Margulis (1960s)—Evidence suggests that between 1.5-2.0 b.y.a., two types of small aerobic prokaryote entered and began to live and reproduce INSIDE larger, anaerobic prokaryotes.

(1) Endosymbiosis

• Theory of a mutualism between large anerobic prokaryotes and two types of aerobic prokaryotes…

(a) The first invading aerobe evolved to become a mitochondrion.

(b) The second invading aerobe evolved to become a chloroplast.

• This theory is backed up by BOTH of these organelles having their OWN DNA as well as replicating on their own.

Extra Slides AND Answers for Critical Thinking Questions

(1) If Pasteur had tipped his flask so that the broth inside had come in contact with microorganisms trapped in the curve of the neck, the broth would have been contaminated and microorganisms would have grown in the flask. This would have supported Pasteur’s conclusion that microorganisms came not from the air but from other microorganisms.(2) Radioactive isotopes with very short-half lives would not be useful for dating fossils because after a relatively short period, the amount of undecayed isotope would be too small to give an accurate indication of age.(3) Damage to the ozone would allow more ultraviolet radiation to reach Earth. UV radiation damages living things, in particular, their DNA.(4) Students’ answers will vary but should be logical. For example, remove a large sample of mud from a dry lake that regularly has fish during the wet season. Place the mud in an aquarium with water and see if fish develop. If they do, repeat the experiment, but divide a second sample of mud into two parts. Put one part in a water-filled aquarium and cook the other part in an oven before placing it into a separate water-filled aquarium. The cooked mud cannot give rise to fish, while the uncooked mud should.