Animal

:For the Muppet Show character, see Animal (Muppet). For the professional wrestler, see Joseph Laurinaitis.

Origin and fossil record

Animals are generally considered to have evolved from flagellate protozoa. Their closest living relatives are the choanoflagellates, collared flagellates that have the same structure as certain sponge cells do. Molecular studies place them in a supergroup called the opisthokonts, which also include the fungi and a few small parasitic protists. The name comes from the posterior location of the flagellum in motile cells, such as most animal sperm, whereas other eukaryotes tend to have anterior flagella.

Related Topics:
Flagellate - Choanoflagellate - Opisthokont - Fungi - Protist - Flagellum

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The first fossils that might represent animals appear towards the end of the Precambrian, around 600 million years ago, and are known as the Vendian biota. These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all. Aside from them, most animal phyla with known phyla make a more or less simultaneous appearance during the Cambrian period, about 570 million years ago. It is still disputed whether this event, called the Cambrian explosion, represents a rapid divergence between different groups or a change in conditions that made fossilization possible.

Related Topics:
Precambrian - Vendian biota - Cambrian - Cambrian explosion

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