Chordate evolution and the origin of craniates: An old brain in a new head

The earliest craniates achieved a unique condition among bilaterally symmet rical animals: they possessed enlarged, elaborated brains with paired sense organs and unique derivatives of neural crest and placodal tissues, includ ing peripheral sensory ganglia, visceral arches, and head skeleton. The cra niate sister taxon, cephalochordates, has rostral portions of the neuraxis that are homologous to some of the major divisions of craniate brains. More over, recent data indicate that many genes involved in patterning the nervo us system are common to all bilaterally symmetrical animals and have been i nherited from a common ancestor. Craniates, thus, have an "old" brain in a new head, due to re-expression of these anciently acquired genes. The trans ition to the craniate brain from a cephalochordate-like ancestral form may have involved a mediolateral shift in expression of the genes that specify nervous system development from various parts of the ectoderm. It is sugges ted here that the transition was sequential. The first step involved the pr esence of paired, lateral eyes, elaboration of the alar plate, and enhancem ent of the descending visual pathway to brainstem motor centers. Subsequent ly, this central visual pathway served as a template for the additional sen sory systems that were elaborated and/or augmented with the "bloom" of migr atory neural crest and placodes. This model accounts for the marked uniform ity of pattern across central sensory pathways and for the lack of any neur al crest-placode cranial nerve for either the diencephalon or mesencephalon . Anat Rec (New Anat) 261:111-125, 2000. (C) 2000 Wiley-Liss, Inc.