Also see:
Domain Eukaryota

Kingdon Animalia comprises, the metazoans, that are multicellular organisms having eukaryotic cells with a nucleus, a cell membrane, and organelles such as mitochondria. They are heterotrophic (obtain food from external sources) and utilize oxygen for energy catabolism. Some of the oldest known fossils of these organisms are found in rocks ranging from 540-650 million years old corresponding to the Vendian (also called the Ediacaran Period), but the paucity of these fossils make their association with those of the Cambrian almost impossible, and the possibility exists that they are not even animals.

Except for the Ediacaran biota, most animal phyla (some that persist and some that did not) appear in the fossil record rapidy and essentially simultaneously at the base of the Cambrian period, some 570 million years ago, an event known as the Cambrian Explosion. Still in dispute is whether the Cambrian Explosion really was a rapid divergence between different groups or was instead a change in size and the emergence of protective armor such as exoskeletons and shells that facilitated fossilization.

It is widely accepted that animals evolved from flagellate protozoa. If so, the closest living relatives are the choanoflagellates, which are heterotrophic, single-called eukaryotes having the same structure as some sponge cells. Phylogenetic 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.

Also see: Eumetazoa