Urmetazoan

The Urmetazoan is the hypothetical last common ancestor of all animals or metazoans (hypothetical in this instance means the organism’s affinities and morphology are unknown and subject to debate). 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[2] — but much more information about it is still unknown.

A 2009 maximum likelihood phylogenetic tree of metazoan relationships.[1]

Choanoflagellates

All animals are posited by biologists to have evolved from a flagellated eukaryote. Their closest known living relatives are the choanoflagellates – collared flagellates whose cell morphology is similar to the choanocyte cells 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 as well.

Five hypotheses

Five different hypotheses for the animals' last common ancestor have been put forward. The relationships between the different animal phyla themselves are not well-resolved, so discriminating between the hypotheses is difficult.

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.[1] 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.[1]

Planula hypothesis

This idea, proposed by Otto Bütschli, suggests that metazoa are derived from a 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, so could reproduce without passing through a sessile phase.

Gastraea hypothesis

The gastraea hypothesis was proposed by Ernst Haeckel,[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 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).[4]

Bilaterogastraea hypothesis

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.

Phagocytella hypothesis

Proposed by Élie Metchnikoff.

See also

References
  1. Schierwater, B.; Eitel, M.; Jakob, W.; Osigus, J.; Hadrys, H.; Dellaporta, L.; Kolokotronis, O.; Desalle, R. (January 2009). Penny, David (ed.). "Concatenated Analysis Sheds Light on Early Metazoan Evolution and Fuels a Modern "Urmetazoon" Hypothesis" (Free full text). PLOS Biology. 7 (1): e20. doi:10.1371/journal.pbio.1000020. ISSN 1544-9173. PMC 2631068. PMID 19175291.
  2. 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
  3. Haeckel, E. 1874. Die Gastraea-Theorie, die phylogenetische Classification des Thierreichs und die Homologie der Keimblätter. Jenaische Zeitschr. Naturwiss. 8:1-55.
  4. Sally P Leys; Dafne Eerkes-Medrano (1 April 2005). "Gastrulation in Calcareous Sponges: In Search of Haeckel's Gastraea" (PDF). Integrative and Comparative Biology. 45 (2): 342–351. doi:10.1093/ICB/45.2.342. ISSN 1540-7063. PMID 21676779. Wikidata Q54502332.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.