The neural crest is an example of a profound evolutionary innovation in vertebrates. It seems to be something simple—it's a population of cells that are 'left over' at the closure of the neural tube, and that wander out into the other tissues of the body—but it has been co-opted in numerous ways to provide major vertebrate features. You wouldn't have much of a face without the neural crest, for instance; this plastic population of migrating cells get recruited to build much of what lies below your eye sockets, along with some radical reorganization of branchial arches. These cells also form the myelin sheaths of your peripheral nerves, and most visibly, form all of the pigmentation in your skin. Having neural crest in the embryo, obscure as it is to most people, is considered to be one of the hallmarks of being a vertebrate.
One particularly pressing question, then, is where the neural crest came from (and no, divine prestidigitation or alien tweaking aren't under consideration, at least not until someone comes up with actual evidence for such designer intervention.) Where should we look? In lineages that branched off of the chordate line before the evolution of vertebrate neural crest. One such lineage is the urochordata.
Continue reading "Urochordates and neural crest" (on Pharyngula)