Evolutionary Origins of the Neural Crest and Neural Crest Cells

Abstract

I evaluate the lines of evidence--cell types, genes, gene pathways, fossils--in putative chordate ancestors--cephalochordates and ascidians--pertaining to the evolutionary origin of the vertebrate neural crest. Given the intimate relationship between the neural crest and the dorsal nervous system during development, I discuss the dorsal nervous system in living (extant) members of the two groups, especially the nature, and genes, and gene regulatory networks of the brain to determine whether any cellular and/or molecular precursors (latent homologues) of the neural may have been present in ancestral cephalochordates or urochordates. I then examine those fossils that have been interpreted as basal chordates or cephalochordates to determine whether they shed any light on the origins of neural crest cell (NCC) derivatives. Do they have, for example, elements of a head skeleton or pharyngeal arches, two fundamental vertebrate characters (synapomorphies)? The third topic recognizes that the origin of the neural crest in the first vertebrates accompanied the evolution of a brain, a muscular pharynx, and paired sensory organs. In a paradigm-breaking hypothesis--often known as the 'new head hypothesis'--Carl Gans and Glen Northcutt linked these evolutionary innovations to the evolution of the neural crest and ectodermal placodes (Gans and Northcutt Science 220:268-274, 1983. doi:10.1126/science.220.4594.268; Northcutt and Gans The Quarterly Review of Biology 58:1-28, 1983. doi:10.108…