When did our ape ancestors first start using symbols to communicate? What evolutionary forces led to (or allowed) this cognitive development to happen?

A team of researchers have identified one possible factor in a new study entitled "Craniofacial Feminization, Social Tolerance, and the Origins of Behavioral Modernity." They note that the great symbolic revolution (when cave paintings and imagery start showing up) happened around the same time as the "feminization" of the human face (lower brow ridges, etc). They draw upon a recent study as an analogy: in Russia, researchers bred foxes by selecting for domesticity (peacefulness, how well they got along with humans). The foxes developed into sweet domestic animals, and their temperament change carried with it physical changes-- floppy ears and soft muzzles. It is possible that a similar change happened in humans; Cieri et al. (authors of the human evolution paper) argue that lower levels of testosterone led to observed physical changes in humans, which accompanied a transition to social tolerance (a necessary prerequisite of symbol use).

Their argument, while fascinating (and perhaps on the right track) may not be the whole story-- or even the right story--for a few reasons. 1) Evolutionary history is complex, and a trait such as the use of symbolism likely cannot be explained simply, 2) did physical traits precede the mental, or vice versa?, and 3) is it really feminization that we should be talking about?

A 2014 paper in the journal Genetics investigates the biophysical mechanism that might explain why domestication leads to "juvenile" traits, such as floppy ears, speckled coats, and soft jaws. They find that interference with the "neural crest," a group of embryonic stem cells that give rise to a variety of traits (such as jawline formation, coat pigmentation, and -- indirectly-- brain formation). When one selects for domesticity, one often often inadvertently selects for animals with mild neural crest deficits!

Could the same pattern be operating in humans? Could physical traits have accompanied a selection process for humans that get along with each other more easily (as the researchers noted, social tolerance is critical!)? Were humans "domesticated" (i.e., were individuals fitter if they had minor neural crest impairments?) As with all prehistoric puzzles, the answer is not immediately clear.

Thanks to Priyanka de Souza for sharing the original article!