The first bilaterian organisms: Simple or complex? New molecular evidence

The quest for the first bilaterian organisms is the biggest riddle in metazoan evolution and in understanding the evolution of developmental mechanisms. Recent molecular work has regrouped the bilaterian phyla into three superphyletic clades: the Deuterostomia, the Lophotrochozoa and the Ecdysozoa. In these trees, Platyhelminthes, for a long time considered basal bilaterians, have a more derived position among the Spiralia. However, a recent 18S rDNA analysis showed Platyhelminthes to be polyphyletic with one of its orders, the Acoela, as the earliest extant bilaterian. To corroborate such position, we have sequenced new 18S and other nuclear genes, two mitochondrial genes, and examined the number and type of Hox cluster genes in acoels, nemertodermatids and other platyhelminthes and metazoans. Results confirm acoels and nemertodermatids as the earliest extant bilaterians. These results imply that the last common bilaterian ancestor was a small, benthic, direct developer without segments, coelomic cavities, nephridia or a true brain. In addition this argues for an extended pre-Cambrian period within which different simple bilaterian stem lineages emerged from which more complex ones diversified during the Cambrian explosion.