The domestication syndrome and neural crest cells: a unifying hypothesis
Charles Darwin was the first to notice that all of the mammals domesticated by man show an unusual suite of traits not found in their wild forebears. These include changes in pigmentation (e.g. white spots), short snouts, smaller teeth, and floppy ears. My colleagues Adam Wilkins and Richard Wrangham and I have suggested that the origin of this odd collection of phenotypic variants lies in their embryological origins in the neural crest. The neural crest is a transitory embryonic tissue that, early in development, gives rise to a very diverse set of tissues and organs including pigment cells (melanocytes), bones, muscles and connective tissues in the head, and the adrenal gland. By our hypothesis, selection for tameness during early stages of domestication led to delayed maturation and reduced output of the adrenal component of the “fight or flight” response, via reduced neural crest input. This led, as an unselected byproduct, to other neural crest-derived tissues also being reduced, including melanocytes, the bones of the face, the precursors of teeth and cartilages of the ears. Our hypothesis thus accounts, via a unified selective force and single developmental mechanism, for an otherwise puzzling hodge-podge of traits that have been recognized, but gone unexplained, since Darwin.