Evolution of the bilaterian germ line: lineage origin and modulation of specification mechanisms.

A key focus of evolutionary developmental biology (evo-devo) in recent years has been to elucidate the evolution of developmental mechanisms as a means of reconstructing the hypothetical last common ancestors of various clades. Prominent among such reconstructions have been proposals as to the nature of the mysterious "Urbilateria," originally defined as the last common ancestor of the extant Bilateria (protostomes and deuterostomes). Indeed, drawings of this animal can now be found, as well as detailed information on the genetics and morphological processes that it used to construct its gut, heart, eyes, appendages, segments, and body regions. Perhaps surprisingly, however, no explanations have yet been offered as to how this animal might have achieved the successful reproduction that must have been necessary for it to give rise to those lineages that are ancestral to today's diverse clades. The present article examines the comparative data available to date on the specification of the only cells containing the genetic hereditary material, the germ cells, and speculates on the possible evolutionary and developmental origin of the Urbilaterian germ line.