The Evolution of Multicellular Plants and Animals

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<ul><li>-' w &gt; w -' a: a: </li><li><p>The Evolution of Multicellular Plants and Animals </p><p>It has been only during the last fifth of the history of life on the earth that multicellular organisms have existed. They appear to have arisen from unicellular organisms on numerous occasions </p><p>The animals and plants one sees on the land. in the air and on the water are all multicellular. made up of millions and in some cases billions of individual cells. Even the simplest multicellular organisms include several different kinds of cells. and the more complicated ones have as many as 200 different kinds. All the multicellular plants and animals have evolved from unicellular eukaryotes of the kind described by J. William Schopf in the preceding article. G. Ledyard Stebbins of the University of California at Davis estimates that multicellular organisms have evolved independently from unicellular ancestors at least 17 times. At least two million multicellular species exist today. and many others have come and gone over the ages. </p><p>Clearly the multicellular grade of construction is advantageous and successful. The chief advantages of multicellularity stem from the repetition of cellular machinery it entails. From this feature flows the ability to live longer (since individual cells can be replaced). to produce more offspring (since many cells can be devoted to reproduction). to be larger and so to have a greater internal physiological stability. and to construct bodies with a variety of architectures. Moreover. cells can become differentiated (specialized for a particular function. as nerve and muscle cells are). with a resulting increase in functional efficiency. The particular advantages that were the keys to the evolution of multicellularity probably varied from case to case. </p><p>The largest categories employed for the classification of organisms are the </p><p>by James W. Valentine </p><p>kingdoms. Multicellular organisms are placed in one or another of three kingdoms on the basis of their broad modes of life and particularly of their modes of obtaining energy. Plants. which are autotrophs (meaning that they require only inorganic compounds as nutrients). utilize the energy of the sun to create living matter through photosynthesis; they make up the kingdom Metaphyta. Fungi (such as mushrooms. which are plantlike but feed by ingesting organic substances) make up the kingdom Fungi. Animals. which are also ingesters. comprise the kingdom Metazoa. Each kingdom includes more than one lineage that evolved independently from the kingdom Protista. consisting of eukaryotic unicellular organisms. </p><p>Much of what is known about the evolution of multicellular organisms comes from the fossil record. The Fungi are so poorly represented as fossils that their evolution is obscure. but the other kingdoms have a rich fossil history. </p><p>The patterns of adaptation that one can observe today amply demonstrate the effectiveness of evolution in shaping organisms to cope with their environment. Each environment contains animals particularly suited to exploit the conditions in it; each kind of organism has been developed by selection to perform a role in the biosphere. When an environment changes. natural selection acts to change the adaptations; sometimes new environmental roles are evolved. The history of life reflects an interaction between environmental change on the one hand and the evolutionary potential of organisms on the </p><p>KINGDOMS OF ORGANISMS are charted according to a concept originated by Robert H. Whittaker of Cornell University. The relatively simple unicellular Monera, which are prokaryotic (Jacking a nucleus), gave rise to the more complex unicellular Protista, from which all three multicellular kingdoms have arisen. Multicellular organisms are placed in the kingdoms Metaphyta, Fungi and Metazoa mainly on the basis of the process by which they obtain their energy. </p><p>other. It is therefore of interest to briefly examine the major causes of the more biologically important environmental changes. </p><p>Notable among them are the processes of plate tectonics. Continents. riding on huge plates of the lithosphere (the outer layer of the solid earth) that move at a rate of a few centimeters per year, break up or collide and can become welded together following a collision. Thus a continent can fragment or grow, the number of continents can increase or decrease and the geographic patterns of a continent can change radically. Ocean basins too alter their sizes. numbers. positions and patterns. The consequences for living organisms can be profound. </p><p>Consider only one of the possible events resulting from plate tectonics: the collision of two continents, once widely separated. to form a single larger continent. The accompanying changes in the biological environment are farreaching. The most obvious change is that the barriers to migration are broken down and the biotas of two continents now compete for existence on one continent. For many land animals the continental interior is now farther from the sea. and the moderating effects of marine air and temperature are diminished. Mountains rising along the suture will diversify the environment further. creating rain "shadows" and perhaps deserts if they happen to interrupt major flows of moisture-laden wind. </p><p>Entirely new environmental conditions may thus appear. They create opportunities for new modes of life. as does the general diversification of conditions following the collision. The biotas of the two former continents are subjected to competition and to environmental conditions for which they have not been adapted. and at the same time they are presented with novel opportunities. Evolution can be expected to produce a considerable change in the flora </p><p>141 </p><p> 1978 SCIENTIFIC AMERICAN, INC</p></li><li><p>and fauna. The marine organisms of the shallow continental shelves, where 90 percent of marine species live today, will also be affected. For many of them the increased continental area leads to a lowering of environmental stability, requiring new adaptive strategies. In general it is expected that fewer marine species could be supported around one large continent than could be supported around two separate smaller ones. </p><p>As oceans widen or narrow, as continents drift into cooler or warmer zones, as winds and ocean currents are channeled in new directions, the pattern and the quality of the environment change. The pace of change may often be slow, because of the low rate of sea-floor spreading, the phenomenon that drives the drifting of the continents and the </p><p>ERA PERIOD EPOCH </p><p>QUATERNARY PLEISTOCENE 0 PLIOCENE (5 N MIOCENE 0 TERTIARY OLIGOCENE z </p><p>0 </p><p>W EOCENE 0 PALEOCENE 50 </p><p>100 CRETACEOUS 0 (5 N 0 150 (/) JURASSIC W ::E </p><p>200 TRIASSIC </p><p>250 PERMIAN </p><p>0 , </p><p>0 0 PENNSYLVANIAN lila:: </p><p>a::w (!) &lt; (/) 300 a:: &lt; W &gt;-u.. 350 0 (/) DEVONIAN </p><p>0 (5 N 0 SILURIAN W </p><p>z 0 :::; --' 400 :E </p><p>--' &lt; a.. 450 </p><p>ORDOVICIAN </p><p>500 </p><p>CAMBRIAN 550 </p><p>z &lt; 600 a: III EDIACARAN ::E &lt; 0 650 W a:: a.. </p><p>700 </p><p>opening of the oceans. At other times more rapid and dramatic changes can be expected, as for example when continents finally collide after millions of years of approaching each other or when an ocean current is finally deflected from an ancient path. </p><p>Just as changes in the environment can affect organisms, so can the activities of organisms affect the environment </p><p>to create new conditions. A fateful example is the rise of free oxygen in the atmosphere, owing chiefly to photosynthesis. Early organisms could not have existed with free oxygen, whereas most contemporary organisms cannot exist without it. Another point to bear in mind is that organisms form part of one another's environment and interact in </p><p>EVENTS EVOLUTION OF MAN </p><p>MAMMALIAN RADIATION </p><p>LAST DINOSAURS FIRST PRIMATES </p><p>FIRST FLOWERING PLANTS </p><p>DINOSAURS FIRST BIRDS </p><p>FIRST MAMMALS THERAPSIDS DOMINANT </p><p>MAJOR MARINE EXTINCTION PELYCOSAURS DOMINANT </p><p>FIRST REPTILES </p><p>SCALE TREES, SEED FERNS FIRST AMPHIBIANS </p><p>JAWED FISHES DIVERSIFY </p><p>FIRST VASCULAR LAND PLANTS </p><p>BURST OF DIVERSIFICATION IN METAZOAN FAMILIES </p><p>FIRST FISH </p><p>FIRST CHORDATES </p><p>FIRST SKELETAL ELEMENTS </p><p>FIRST SOFTBODIED METAZOANS </p><p>FIRST ANIMAL TRACES (COELOMATES) </p><p>MAJOR EVENTS in the evolution of multicellular organisms over the past 700 million years are depicted chronologically. The data are based principally on what is revealed by fossils. </p><p>142 </p><p>numerous ways: as predators, competitors, hosts and habitats. As populations of organisms increase, decrease or change, the environment changes too. </p><p>In the course of the diversification of the multicellular kingdoms over the past 700 million years major new types of organisms have appeared and several revolutions have taken place within established groups. In many instances it is possible to identify the kind of environmental opportunity (or, with extinctions, the environmental foreclosure) to which the biota is responding. The history of animals is known best. They arose at least twice: sponges from one protistan ancestor and the rest of the metazoans from another. The major categories into which animals are divided are phyla; over the aeons at least 35 phyla have evolved, of which 26 are living and nine are extinct. </p><p>The fossil record that reveals something of the circumstances of the early members of the phyla varies in quality according to the type of fossil. Certain trace fossils are the most easily preserved, particularly burrows and trails left behind in sediments by the activity of organisms. Next come durable skeletal remains, such as seashells and the bones of vertebrates. Finally, the fossils of entirely soft-bodied animals turn up on rare occasions, usually as impressions or films in ancient sea-floor sediments. </p><p>The earliest animal fossils are burrows that begin to appear in rocks younger than 700 million years, late in the Precambrian era. Both long horizontal burrows and short vertical ones are found, comparable in size to the burrows of many modern marine organisms. The ability to burrow implies that the animals had evolved hydrostatic skeletons. that is. fluid-filled body spaces that work against muscles, so that the animal could dig in the sea bed. Although some simple animals such as sea anemones manage to employ their water-filled gut as a hydrostatic skeleton and so to burrow weakly, long horizontal burrows suggest a more active animal. probably one with a coelom, or true body cavity. This is quite an advanced grade of organization to find near the base of the fossil record of multicellular forms. Trace fossils are rather rare until about 570 million years ago, when they increase remarkably in kind and number. </p><p>The next animal fossils, surprisingly, are soft-bodied remains from between 680 and 580 million years ago, called the Ediacaran fauna from the region in southern Australia where they are best known. The ones that are clearly identifiable with modern phyla are all jellyfishes and their allies. which are at a simple grade of construction. The other fossils are more enigmatic; a few may be allied with living phyla (one resembles annelid worms) and some may not be. </p><p> 1978 SCIENTIFIC AMERICAN, INC</p></li><li><p>SOFT -BODIED ANIMAL of middle Cambrian times left a record in the Burgess Shale of British Columbia. The creature was a polychaete worm with a number of setae, or bristlelike parts, that appear clearly </p><p>PLANT FOSSILS include a tree fern from Jurassic times (left) and a birch leaf from the Miocene epoch (right). The ferns were among the </p><p>SKELETAL ANIMALS represented in the fossil record include a bed of crinoids (left) from the upper Cretaceous period and a pterodactyl (right) of late Jurassic times. The crinoids were marine echino-</p><p>here. The photograph of the fossil, wbich is enlarged about five diameters, was made in ultraviolet radiation by S. Conway Morris of the University of Cambridge. Lamp was set at a high angle to specimen. </p><p>earliest land plants, part of the dominant land flora in Devonian times. Birch leaf represents a more advanced group, vascular land plants. </p><p>derms, a group represented today by such organisms as starfishes and sea urchins. The pterodactyl was a flying reptile that had a membranous, featherless wing. This specimen was found in Germany. </p><p>143 </p><p> 1978 SCIENTIFIC AMERICAN, INC</p></li><li><p>It is probable that some are coelomates. Durable skeletal remains finally ap</p><p>pear in the fossil record in rocks that are about 580 million years old. The earliest ones are minute scraps, denticles and plates of unknown affinity that were parts of larger animals. Then, starting some 570 million years ago and continuing over the next 50 million years or so, nearly all the coelomate phyla that possess durable skeletons appear in what is in evolutionary terms a quick succession. The exceptions are the phylum Chordata (which nonetheless does appear as a soft-bodied fossil) and the phylum Bryozoa, which finally appears less than 500 million years ago. These durable skeletonized invertebrates seem to have one thing in common: they all originally lived on the sea floor rather than burrowing in it, although one group (the extinct Trilobita) probably grubbed extensively in the sea floor, digging shallow pits or perhaps burrows in search of food. </p><p>A highly valuable fossil assemblage from near the middle of the Cambrian period is found in British Columbia in a rock unit named the Burgess Shale. Much of the fauna from this shale is soft-bodied, trapped by rapidly deposited muds and preserved as mineral films by a process that has yet to be determined. Here is an array of more or less normal invertebrate skeletons associated with such soft-bodied phyla as the Annelida (the group that includes the living earthworms), the Priapulidae (possibly pseudocoelomate worms) and the Chordata. The Burgess Shale also contains several animals that represent phyla not previously known. Only one of the phyla could be ancestral to a living phylum; the others are entirely distinct lineages that arose from unknown late Precambrian ancestors and have all become extinct. </p><p>From these facts and from the wealth of accumulated evidence on the comparative embryology and morphology of the living representatives of these fossil groups it is possible to build a picture of the rise of the major animal groups. The animals for which the fewest clues exist are the first ones, the founding metazoans, although there is no shortage of speculation on what they may have been like. </p><p>Since the evolution of novel organisms involves adaptation to new or previously unexploited conditions, one can imagine the adaptive pressures the earliest multicellular forms would have faced and can thereby derive a variety of plausible animals radiating from a trunk lineage into the available modes of life. Bottom-dwelling animals can eat deposited or suspended food. Suspension feeders must maximize the volume of water on which they can draw. Body sapes such as domes that jut up into the water and create turbulence in the ambi-</p><p>144 </p><p>ent currents would increase the volume of water washing the ani...</p></li></ul>