A celebration of the new head and an evaluation of the new mouth

Abstract

Twenty years ago now, Carl Gans and Glen Northcutt proposed that the main invention of vertebrates was a new head, with its full array of sensory organs involved in an active predatory lifestyle. Tracing back the embryological origin of these structures, they showed how all are primarily derived from the neural crest and the placodes, two transient ectodermal cell populations in the embryo. These cell types were then used for further innovations, such as a new mouth in jawed vertebrates. The interplay between patterning and plasticity of the neural crest is largely responsible for the endless variation of vertebrate craniofacial features in evolution.