Neurophylogeny: Retracing Early Metazoan Brain Evolution

Abstract

The current view of early metazoan phylogeny suggests that the bilaterian body plan arose only once during evolution. This first urbilaterian animal was most likely equipped with an anterior condensation of nerve cells – a brain – from which all brains of modern animals have diverged. Until recently, the ancestor of all bilaterian phyla was viewed as a very simple animal with an accordingly simple brain. Molecular studies, however, demonstrate a multitude of homologous genes that are expressed in similar patterns in the developing brains of vertebrates, insects, and annelids. Taken together, these findings imply that the anatomy of the urbilaterian cerebrum might have been more elaborate than previously assumed. If true, ancient architectural features might have been conserved during evolution and should be identifiable in distantly related modern animal phyla. Comparative studies on representatives of arthropods, onychophorans, and annelids suggest that this is indeed the case. This chapter summarizes recent neuroanatomical surveys that aim to retrace the early evolution of the metazoan brain and to use neuroanatomical data to test conflicting hypothesis on phylogenetic relationships between major animal phyla.