Early central nervous system evolution: An era of skin brains?

The central nervous system (CNS) in different animals develops under the control of genes that are often conserved both structurally and functionally. The question of whether such conservation means that the common ancestor of all bilateral animals had an anatomically complex CNS is controversial. Recently, a hemichordate was found to express homologues of many of these genes in a rostrocaudal order resembling that in Drosophila and vertebrates. Surprisingly, however, the hemichordate genes are expressed not in a localized CNS, but in widespread epidermal domains pervaded by a basiepidermal nerve net. Here, I discuss the implications of this discovery for CNS evolution.