Masculinity and the Mechanisms of Human Self-Domestication

Objectives: Pre-historic decline in human craniofacial masculinity has been proposed as evidence of selection against reactive aggression and a process of ‘human self-domestication’ thought to have promoted complex capacities including language, culture, and cumulative technological development. This follows observations of similar morphological changes in non-human animals under selection for reduced aggression. Two distinct domestication hypotheses posit developmental explanations; involving dampened migration of embryonic neural crest cells (NCCs), and declining androgen influences, respectively. Here, I assess the operation and potential interaction of these two mechanisms and consider their role in human adaptation to a cooperative sociocultural niche. Methods: I provide a review and synthesis of related literature with a focus on physiological mechanisms affecting domesticated reductions in masculinity and sexual dimorphism. Further, I examine several modes of pre-historic sociosexual selection against aggressive reactivity which are proposed to have driven human self-domestication. Results: I show that pluripotent NCCs provide progenitors for a wide range of vertebrate masculine features, acting as regular targets for sexually driven evolutionary change. This suggests hypoplasia of NCC-derived tissues due to dampened NCC migration is sufficient to explain declines in lineage specific masculine traits and features under domestication. However, lineage-specific androgen receptor variability likely moderates hypoplasia in NCC-derived tissues, and may influence NCC migration, though this latter influence requires further investigation. Conclusions: These findings synthesise and extend theorised physiological mechanisms of domestication and human self-domestication. Self-domestication under sociosexual selection for dampened reactive aggression and correlated masculine physiology enabled human adaptation to an increasingly complex sociocultural niche. The analysis highlights several avenues for further productive investigation.