The Urmetazoan is the hypothetical
last common ancestor of all
animals, or metazoans. It is universally accepted to be a multicellular
heterotroph — with the novelties of a
germline and
oogamy, an
extracellular matrix (ECM) and
basement membrane, cell-cell and cell-ECM adhesions and signaling pathways,
collagen IV and fibrillar collagen, different cell types (as well as expanded gene and protein families), spatial regulation and a complex developmental plan, and relegated unicellular stages.[1]
Molecular studies place animals in a supergroup called the
opisthokonts, which also includes 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 as well.
Hypotheses
Several different hypotheses for the animals' last common ancestor have been suggested.
The placula hypothesis, proposed by
Otto Bütschli, holds that the last common ancestor of animals was an amorphous blob with no symmetry or axis. The center of this blob rose slightly above the silt, forming a hollow that aided feeding on the sea floor underneath. As the cavity grew deeper and deeper, the organisms resembled a thimble, with an inside and an outside.[2] This body shape is found in sponges and cnidaria. This explanation leads to the formation of the bilaterian body plan; the
urbilaterian would develop its symmetry when one end of the placula became adapted for forward movement, resulting in left-right symmetry.[2]
The planula hypothesis, proposed by Bütschli, suggests that metazoa are derived from
planula; that is, the larva of certain
cnidaria, or the adult form of the
placozoans. Under this hypothesis, the larva became sexually mature through
paedomorphosis, and could reproduce without passing through a
sessile phase.
The gastraea hypothesis was proposed by
Ernst Haeckel in 1874,[3] shortly after his work on the
calcareous sponges. He proposed that this group of sponges is
monophyletic with all
eumetazoans, including the
bilaterians. This suggests that the
gastrulation and the
gastrula stage are universal for eumetazoans. It has been perceived as problematic that 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).[4]
The bilaterogastraea hypothesis was developed by Gösta Jägersten as an adaptation of
Ernst Haeckel's Gastraea hypothesis. He proposed that the Bilaterogastraea have 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.
Urbilaterian – Possible simple urbilateran candidate
References
^Ros-Rocher Núria, Pérez-Posada Alberto, Leger Michelle M. and Ruiz-Trillo Iñaki. 2021 The origin of animals: an ancestral reconstruction of the unicellular-to-multicellular transition Open Biol. 11:200359. 200359.
http://doi.org/10.1098/rsob.200359
^Haeckel, E. 1874. Die Gastraea-Theorie, die phylogenetische Classification des Thierreichs und die Homologie der Keimblätter. Jenaische Zeitschr. Naturwiss. 8:1-55.