The Evolution of Animals

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...

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... in animals that did not have strong skeletons. However, many of these animals had outer coverings that were tough, but lightly mineralized: the Burgess arthropods are particularly good examples.

The common factor along successful groups of Cambrian animals is larger body size. All of this suggests that in some way the world had become ready for large animals, and in turn that tells us that the Cambrian event was driven by worldwide ecological factors, but we do not yet know what they were. They could have been related to a change in food supply in the sea, which in turn depends on upwelling, which in turn depends on climatic and geographic patterns on a global scale. We don't yet know enough about Cambrian geography and climate to say anything sensible about these factors, but it's here that the answer probably lies and where future research should be focused.