A large metazoan phylogenetic analysis published by Dunn et al. last spring sparked a great deal of attention and seemed to both support some long-held beliefs about the evolution of animals, but also interestingly placed ctenophores at the base of the tree. Now, another paper, this one in PLoS Biology and featuring two of my colleagues at the AMNH as authors, sheds some interesting new light on this huge and important question. Central to the new analysis is the inclusion of a really bizarre and shockingly simple metazoan - Trichoplax, an amoeba-shaped creature made up of just three layers of cells without any internal structures at all, which glides along and chomps on bacteria. The inclusion of this taxon, along with a massive analysis that included nuclear and mitochondrial sequences, morphology, secondary structure data, and evo-devo characters now splits animals into two broad clades: the bilateria, which include ecdysozoans, lophotrochozoans, and deuterostomians) and then the diploblasts, which are the cnidarians and ctenophores, the porifera and the placozoans, represented by "cousin Blob", Trichoplax, as the basal lineage to this group. This scenario proposes that these two very different bauplans of animals evolved in parallel, not in the ladder-like progression favored by many other studies.Looks like it's time to re-write some textbooks.
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