Fig. 6.

Phylogeny of basal metazoa and independent origin of neurons in Ctenophora. The relationships among five basal metazoan groups (Ctenophora, Porifera, Placozoa, Cnidaria and Bilateria) is based upon [20, 34]. Although the exact placement of Ctenophora and Porifera is not currently resolved (dotted lines), these two lineages were branched before Placozoa, Cnidaria and Bilateria (a clade that was named ParaHoxozoa [72]). The topology of this tree and our recent analysis of Ctenophore gene complements suggest that neurons evolved independently in ctenophores. Sponges either primarily lack neurons or they might have lost them secondarily. Placozoa might also be viewed as a secondarily simplified group