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Urmetazoan
The Urmetazoan is the hypothetical last common ancestor of all animals. It was undoubtedly marine, but beyond this its form is difficult to determine; five different hypotheses have been put forward, and discriminating between them is difficult because the relationships of animal phyla are not completely resolved.
Animals are considered by biologists to have evolved from a flagellated eukaryote. Their closest known living relatives are the choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges. Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few small parasitic protists. The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella.
==Placula hypothesis==
Under the placula hypothesis, the last common ancestor of animals was a somewhat amorphous blob, lacking any form of symmetry or axis. The centre of this blob became raised slightly from the sediment, to create a cavity which assisted feeding on the sea floor below. As this cavity developed, it became deeper and deeper, until the organisms resembled a thimble, with an inside and outside. Some sponges and cnidaria have this kind of body form.
The development of the bilaterian body plan follows from this hypothesis; the urbilaterian would develop its symmetry as one end of the placula became adapted for moving forwards, which would result in a left-right symmetry.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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