Craniofacial Feminization in Canine and Human Evolution

Anatomically modern humans are recognized in the fossil record primarily by retraction and diminution of the facial skeleton compared to pre-modern “archaic” humans. Several explanatory models for this facial “gracilization” shift have been proposed, all of which have theoretical or empirical shortcomings. The last 200,000 years of human cultural evolution have also witnessed initially ephemeral occurrences of innovation, planning depth, and abstract and symbolic thought (i.e., “behavioral modernity”) which became persistent sometime after 80,000 years ago. A promising model for the advent of facial diminution argues that anatomically modern humans represent a ‘self-domesticated’ species where selection for increased social tolerance led to growth and developmental alterations producing craniofacial “feminization,” which itself results in a phenotypic signal of reduced aggressiveness. Higher levels of social tolerance was likely a necessary prerequisite to increased human population densities, and/or extended cooperative social networks among relatively small groups leading to persistent behavioral modernity. A consideration of key parallel craniofacial changes occurring during both the archaic-to-modern human and wolf-to-dog evolutionary transitions, considered in light of previous experimental work in other animal models (especially domesticated foxes), provides key insights and support for the modern human ‘self-domestication’ model.