The Agnathan ark: the origin of craniate brains.

An outgroup analysis of head organization of tunicates, lancelets and craniates indicates that the origin of craniates involved the development of a new muscular pharyngeal pump, which was a key innovation underlying the evolution of a more metabolically active group of chordates. This innovation involved the genesis of new embryonic tissues, neural crest and neurogenic placodes, whose derivatives formed skeletal elements of the new pump as well as new receptors advantageous to the more mobile craniates. These changes also involved the evolution of a more complex brain by elaboration and reorganization of a lancelet-like cephalic neural tube. All of these events appear to be linked to a number of homeobox genes that were duplicated in the ancestral chordates leading to craniates. An outgroup analysis of the life histories of lancelets, hagfishes and lampreys suggests that hagfishes and lampreys have life histories that are substantially derived in comparison to that of ancestral craniates and that the highly derived features of the brain of hagfishes evolved in conjunction with their invasion of a bathyal benthic environment.