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The Evolution of Animals
Eukaryotes come in two grades of organization: single-celled (protists) and multicellular (plants, animals, and fungi). The world today is full of complex multicellular plants and animals: how, why, and when did they evolve from protists?
A single-celled eukaryote or protist can carry chlorophyll (it can be an autotrophic, photosynthetic, "alga"), it can eat other organisms (it can be an organotrophic, "protozoan" "animal"), or it may do both.
Beginning about 1850 Ma, we find acritarchs, spherical microfossils with thick and complex organic walls. They are probably dinoflagellates that spent most of their life floating in the plankton.
We know that a very diverse array of plankton existed by 800 Ma, because they are known as fossils. But many amoebalike protists do not have cell walls made of cellulose and so do not preserve well. It's possible that while the surface layers of Proterozoic oceans had huge numbers of floating plankton, Proterozoic seafloors were crawling with successful populations of protists consuming the rich food supplies available in bacterial mats.
Evolving Metazoans from Protists: Anatomy and Ecology
A flagellate protist is a single cell with a lashing filament, a flagellum (plural, flagella), that moves it through the water. A sponge is the simplest multicellular variation on this theme. It contains many similar flagellated cells arranged so that they generate and direct water currents efficiently. Sponges are more advanced than simple colonies of choanoflagellates because they also have specialized sets of cells to form a body wall, to digest and distribute the food they collect, and to construct a stiffening skeletal framework of organic or mineral protein that allows them to become large without collapsing into a heap of jelly. Sponges are thus metazoan animals, not protists. Metazoans are not just multicellular, they have different kinds of cells that perform different functions.
Metazoans are most likely a clade, that is, they all descended from one kind of protist. All metazoans originally had one cilium or flagellum per cell, for example. Metazoans also share the same kind of early development. They form into infolded balls of internal cells which are often free to move, and are covered by outer sheets of cells that form an external coating for the animal: a skin, if you like.
The first metazoans were soft-bodied, and we have no fossil record of them. But we can look at the tremendous variety of living animals and at the geologic record to try to reason out what the first metazoans might have looked like and what they might have done.
There are only three basic kinds of metazoans: sponges, cnidarians, and worms. One can imagine scenarios for the divergence of the three great metazoan groups. All of them solved the problem of developing to larger size and complexity, but in different ways.
Sponges evolved by extending the choanoflagellate way of life to large size and sophisticated packaging. They continued to pump water (and the oxygen and bacteria they take from it) through their tissues, in internal filtering modules.
Cnidarians (or coelenterates), including sea anemones, jellyfish, and corals, are built mostly of sheets of cells, and they exploit the large surface area of the sheets in sophisticated ways to make a living. They consist of a sheet of tissue, with cells on each surface and a thickening layer of jellylike substance in the middle. The sheet is shaped into a baglike form to define an outer and an inner surface. A cnidarian thus contains a lot of seawater in a largely enclosed cavity lined by the inner surface of the sheet. The neck of the bag forms a mouth, which can be closed by muscles that act like a drawstring. A network of nerve cells runs through the tissue sheet to coordinate the actions of the animal.
In most cnidarians the outer surface of the sheet acts simply as a protective skin. The inner surface is mainly digestive, and it absorbs food molecules from the water in the enclosed cavity. Because cnidarians are built only of thin sheets of tissue, they weigh very little, and can exist on small amounts of food. They can absorb all the oxygen they need from the water that surrounds them, and they absorb all their food molecules too. Digestive cells lining the cavity then leak powerful enzymes into the water inside the animal. The prey is broken down by these enzymes, and cnidarian absorbs the food molecules through the inner living.
Cnidarians have nematocysts or stinging cells that are set into the outer skin surface. The toxins of some cnidarians are powerful enough to kill fish, and people have died after being stung by swarms of jellyfish. Nematocysts are usually concentrated on the surfaces and the ends of tentacles, which form a ring around the mouth. They provide an effective defense for the cnidarian, but they are also powerful weapons for catching and killing prey, which the tentacles then push into the mouth for digestion in the cavity.
Hardly any sponges can tackle food particles larger than a bacterium, though there are a few exceptions. Living cnidarians routinely trap, kill, and digest creatures that outweigh them many times.
The third and most complex metazoan group contains all other metazoans, including vertebrates. Here I shall simply call them worms. Worms consist basically of a double sheet of tissue that is folded around with the inner surfaces largely joined together to form a three-dimensional animal. In contrast to sponges and cnidarians, worms have evolved complex organ systems made from specialised cells.
All sponges and most cnidarians are attached to the seafloor and depend on trapping food from the water. But many worms, including the most simple group, flatworms, are mobile scavengers and predators. Worms creep along the seafloor on their ventral (lower) surface, which may be different from the dorsal (upper) surface. They prefer to move in one direction, and a head at the (front) end contains major nerve centers associated with checking and testing the environment.
Probably the mobility of worms on the seafloor led to the differentiation of the body into anterior and posterior (head and tail) and into dorsal and ventral surfaces, as the various parts of the animal encountered different stimuli and had to be able to react to them. A well-developed nervous system coordinates muscles so that a worm can react quickly and efficiently to external stimuli.
The same locomotion that gave a worm a front-to-back axis also gave it bilateral symmetry. Any other shape would have produced an animal that could not move forward efficiently.
The head usually features the food intake, a mouth through which food is passed into and along a specialised one-way internal digestive tract instead of being digested in a simple seawater cavity. No sponge cell or cnidarian cell is very far away from a food-absorbing (digestive) cell, so these creatures have no specialised internal transport system. But the digestive system of worms needs an oxygen supply, and the nutrients absorbed there have to be transported to the rest of the body. Worms therefore have a circulation system, and the larger and more three-dimensional they are, the better the circulation system must be.
Flatworms called acoels are the most primitive living worms and have a body structure that is simply a double sheet of tissue, with a weakly developed head and flattened dorsal and ventral surfaces. They show what an early worm may have looked like as it evolved from a creature that was more two-dimensional and sheet-like. Most other worms are more rounded and 3D. They have an internal fluid-filled cavity, the coelom. All highly evolved metazoans show modifications of the coelomate wormlike body. In humans, for example, the coelom is the sac containing all the internal organs.
R. B. Clark suggested that the evolution of the coelom in some early flatworm allowed it access to a new food supply in organic-rich sediment. Liquid is incompressible, and a flatworm that first evolved any kind of internal fluid pool would have been able to squeeze its internal reservoir by body muscles. Such squeezing would have poked out the body wall at its weakest point, which is usually an end. Such a hydraulic extension of the body could have been used as a power drill for burrowing into the sediment. As well as burrowing for food, a wormlike animal with a coelom would have been able to burrow for safety.
The coelom would have provided another great advantage for wormlike animals. Oxygen must reach all the cells in the body for respiration and metabolism. Single-celled organisms can usually get all the oxygen they need because it simply diffuses through the cell wall into their tiny bodies. Sponges pump water throughout their bodies as they feed, and cnidarians and flatworms are at most two sheets of tissue thick. But larger animals with thicker tissues cannot supply all the oxygen they need by diffusion. Oxygen supply to the innermost tissues becomes a genuine problem with any increase in body thickness or complexity. If the animal evolved some exchange system so that its coelomic fluid was oxygenated, the coelom could act as a large store of reserve oxygen. Eventually the animal could evolve pumps and branches and circuits connected with the coelom to form an efficient circulatory system.
If Clark is correct, respiration problems were particularly serious for early coelomates because they were burrowing for food in rich organic sediments, which are very low in oxygen. Therefore we might expect a successful worm to evolve some special organs to obtain oxygen from the overlying seawater (at one end?) while the main body of the worm can remain safely below the surface for protection and for gathering food (at the other end?). Many worms and more advanced coelomates that live in shallow burrows have various kinds of tentacles, filaments, and gills that they extend into the water as respiratory organs. It is a very short step from here to the point where the coelomate collects food as well as oxygen from the water by filter feeding, as in all bryozoans and brachiopods, in some molluscs, worms, and echinoderms, and in simple chordates.
An alternative solution has evolved in many coelomate worms. They burrow so actively that their body movements inside the burrow pump oxygenated water down the burrow over them. In these worms, respiration through the skin surface is sufficient for their oxygen needs as long as they also have an efficient internal circulatory system to distribute the oxygen (as in some worms and in many arthropods such as burrowing shrimp). Some of these animals have also evolved to collect food from the respiratory currents flowing down into their burrows, but most still are sediment scavengers and predators.
Evolving Metazoans: Regulatory Genes
It is difficult to grow a viable multicellular animal rather than a protist. The DNA has to contain not only the information to build several or many different kinds of cells, but the information to grow them at the right time, to place them accurately within the body, and to drive all the biochemical and biophysical systems that ensure the animal operates as a coherent unit.
The genetic programming that builds an animal works like efficient computer programming. For example, one could instruct a computer to draw a flower, specifying the size, shape, and position of each petal. Given that petals typically have much the same size and shape, however, one could use one shape and size for every petal, and simply tell the computer to move the pen to the right place before drawing the same petal each time.
Structural genes build each piece of the animal, and regulatory genes make sure the piece is built in the right place at the right time. Thus a set of regulatory genes could be used in combination with a set of "segment" genes to build all the segments along a growing worm. The same sort of regulatory genes could easily be used to build a legs on, say, a millipede or a crab, by calling on a "leg" gene the appropriate number of times instead of a "segment" gene. By calling on slight modifications of the "leg" gene as growth developed, regulatory genes could build an animal whose legs were different along its length (as in insects), or build a vertebrate with different bones along the length of a backbone.
Developmental geneticists have now identified regulatory genes that control which way up an animal is formed, which is front and back, and how the animal varies along its length or around its edges. The most thrilling discovery is that much the same control box is used throughout the metazoans. Sets of genes that sit close to one another in the nuclear DNA perform much the same job in developing animals, but because they call in a variety of structural genes in a variety of patterns in time and body areas, the results in terms of anatomy are vastly different.
Hox genes are such clusters. Sponges have one set of Hox genes (and are simple in structure), whereas mammals have 38 sets in four clusters, and goldfish have 48 in 7 clusters. Hox genes control the growth of nerve nets, segments, and limbs throughout metazoans, and their evolution accompanies the divergence in anatomy and physiology and ecology and behavior that gave us all the variety of living animals. Hox genes provide separate, but complementary evidence to accompany the fossil record; however, it is important to remember that we can only study the genes of surviving groups of animals, not those from the 95% of species that have become extinct.
Protists don't need Hox genes, because they don't divide cells in precise patterns to form a multicellular adult. Hox genes evolved in early metazoans, and provided the genetic tool kit to build viable complex animals. Presumably, Hox genes control the lay-out of a sponge that gives efficiency of water currents passing through the body. In the simplest worms, Hox genes lay out the nerve nets that allow the worm to sense the environment all along the body. One can easily imagine that the earliest metazoans, wherever, whenever, and however they evolved, would quickly radiate into a great variety of body shapes and structures, with natural selection acting equally quickly to weed out the shapes that were poor adaptations, and leaving a scrapbook of successful prototypes that proliferated.
We begin to see a reasonable fossil record of animals from rocks around 600 Ma. In South Australia the rocks that were laid down at this time have not suffered much damage since. They bear the traces of soft-bodied animals that are much advanced over any of the protists that dominated the fossil record up to this point. This set of animals is called the Ediacaran fauna, named after rocks near Adelaide in Australia; fossils of the same type and the same age that have been discovered in northern Russia.
Thousands of these fossils have now been collected worldwide in dozens of different localities. Almost all the fossils occur between 565 and 543 Ma, with the highest abundance and diversity during the last few million years from 550 Ma to 543 Ma. After that the Ediacaran animals seem to have become extinct. Most of them probably left no descendants; others gave rise to some of the Cambrian animals that followed.
There are a few Ediacaran sponges, but most Ediacaran fossils are cnidarians of some sort. Jellyfish and other cnidarians floated just like their living relatives. Colonies of sea pens were attached to the seafloor. Sea pens look like plants, but are cnidarians that capture and eat floating animals in the water. Dickinsonia is a very large flattened animal, up to 45 cm long, and there is some debate whether it is a very unusual worm or a very unusual cnidarian. Other Vendian fossils are worms that patrolled the seafloor. Some squirmed through the surface sediment; others walked on the tufts of bristles located on their body segments. Shallow fossil burrows in the sediment show that some worms were a centimeter across and were deposit feeders, leaving fecal pellets behind them. Other smaller worms left trails on the surface as they wriggled across the sediment. Since Vendian animals were soft-bodied and unprotected, there may have been no large carnivores on the seafloor.
The Evolution of Skeletons
One of the most important events in the history of life was the evolution of mineralized hard parts in animals. Various kinds of algal cells and then planktonic protists had evolved tough cell walls and cell coverings, but they were not mineralized. The Ediacaran animals may have had leathery skins, but no Ediacaran animal had true mineralized hard parts.
Beginning rather suddenly, the fossil record contains skeletons: shells and other pieces of mineral that were formed biochemically by animals. Humans have one kind of skeleton, an internal skeleton or endoskeleton, where the mineralization is internal and the soft tissues lie outside. Most animals have the reverse arrangement, with a mineralized exoskeleton on the outside and soft tissues inside, as in most molluscs and in arthropods. The shell or test of an echinoderm is technically internal but usually lies so close to the surface that it is external for all practical purposes. The hard parts laid down by corals are external, but underneath the body, so that the soft parts lie on top of the hard parts and
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