Cross references: Ediacaran Period
Neoproterozoic Metazoa
Abstract: Parazoa Porifera Placozoa Pre-Bilateria Locomotion Coelenterata Cambrian Explosion Bilateria Protostome Deuterostomes Early origin of the bilaterian developmental toolkit (Goog) http://www.ncbi.nlm.nih.gov/pubmed/19571245 "Whole-genome sequences from the choanoflagellate Monosiga brevicollis, the placozoan Trichoplax adhaerens and the cnidarian Nematostella vectensis have confirmed results from comparative evolutionary developmental studies that much of the developmental toolkit once thought to be characteristic of bilaterians appeared much earlier in the evolution of animals. The diversity of transcription factors and signalling pathway genes in animals with a limited number of cell types and a restricted developmental repertoire is puzzling, particularly in light of claims that such highly conserved elements among bilaterians provide evidence of a morphologically complex protostome-deuterostome ancestor. Here, I explore the early origination of elements of what became the bilaterian toolkit, and suggest that placozoans and cnidarians represent a depauperate residue of a once more diverse assemblage of early animals, some of which may be represented in the Ediacaran fauna (c. 585-542 Myr ago)." Full length PDF available online for free. "Thus,
the last common ancestor of all metazoa was able to specify multiple
cell types, establish body axes and array different cell types along
these axes and produce multicellular structures (Larroux et al.
2006), but evidently lacked the regulatory complexity and depth of
transcription factors and microRNAs required to produce complex gene
regulatory networks (GRNs), and thus more complex morphological
structures." "The presence of many ‘bilaterian’ developmental tools in morphologically simpler organisms reinforces the view that the original role of these genes and regulatory networks was in the formation of specialized cell types in specific regions of the body, not necessarily in producing complex multicellular structures" or Early origin of the bilaterian developmental toolkit http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2873006/ |