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.
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.
This idea, proposed by Otto Bütschli, suggests that metazoa are derived from a planula; that is, the larva of certain cnidaria. Under this hypothesis, the larva became sexually mature through paedomorphosis, so could reproduce without passing through a sessile phase.
The gastraea hypothesis was proposed by Ernst Haeckel, shortly after his work on the calcareous sponges. He proposed that this group of sponges is monophyletic with all eumetazoans, including the bilaterians. This suggested that the gastrulation and the gastrula stage are universal for eumetazoans. It has been perceived as problematic that a gastrulation by invagination is by no means universal among eumetazoans. Only recently has an invagination been confirmed in a Calcarea sponge, albeit too early to form a remaining inner space (archenteron).
This hypothesis was developed by Gösta Jägersten as an adaptation of Ernst Haeckel's Gastraea hypothesis. He proposed that the Bilaterogastraea had a two-stage life cycle, with a pelagic juvenile and a benthic adult stage. The invagination of the original gastrula stage he saw as bilaterally symmetric rather than radially symmetric.
Proposed by Élie Metchnikoff.