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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?

Proterozoic 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