"This manuscript represents a redrafting of an earlier paper posted at BioRxiv following two rounds of review at Evolution and Human Behaviour. This redrafting will require further changes and review before being resubmitted elsewhere.The current draft MS contains significant changes from the earlier draft. Valuable feedback provided by previous reviewers suggested the original article was likely to be of interest to readers and, therefore, worthy of eventual publication, but was too wide-ranging, contained sections of excessive speculation, and required further clarification of hypotheses and testable predictions. I acknowledge these concerns and have continued to develop this line of research.
In response to reviewers’ comments:

  • The previous MS will now be treated via three separate texts. These will:

    • Explore links between masculinity and the human self-domestication process, including mechanisms of selection on masculine behaviour and morphology (the present draft MS).
    • Explore possible influences of the self-domestication process upon human beards, i.e. the operation of neural crest cells in the formation of the facial dermis of human males.
    • Outline links between female preferences for neural crest cell-derived signals of masculinity and vertebrate immunity due to their contributions to the thymus and other immune system components.

  • The present MS, therefore, has had several sections of the earlier draft removed, including the section on beards, which prompted a change of title.
  • A key mechanistic hypothesis of the present MS is that changes to human masculinity, as related to human self-domestication, are likely to involve interaction between testosterone and neural crest cell-derived tissues [a combination of hypotheses provided by Cieri et al. (2014)and Wilkins et al. (2014)]. Existing observational and experimental research is used here to demonstrate the influence of testosterone on growth and development in several key features of domestication syndrome.
  • The counterintuitive observation that males of domesticated non-human species display relatively diminished ‘masculinity’ but also show relatively elevated testosterone levels, prompts the suggestion that lower androgen receptor density within neural crest cells may drive domesticated reductions in masculinity.
  • These hypothesised interactions between testosterone and neural crest cell-derived tissues require further elaboration and will be a primary avenue for productive future investigation.
  • The next draft of this MS will include a more detailed presentation of previous findings on relevant human evolutionary changes, including: reduced sexual dimorphism; skeletal gracility; brain and tooth size reductions, and altered cranial morphology (brow ridges, jaw size, and the evolution of the human chin).

Abstract Recent fossil analysis points to ‘feminisation’ of human cranio-facial morphology as evidence of human self-domestication leading to the emergence of behavioural modernity. Research regarding the biophysical nature of the domestication process more generally, suggests that traits associated with domestication syndrome emerge via hypoplasia of neural crest cell (NCC) derived features. The present article offers an integration of these insights by showing that multiple traits indicative of human masculinity are commonly either directly derived from embryonic NCCs, or are significantly influenced by structures with NCC origins. Based on this observation, a logical expectation is that differential selection on indicators of relative masculinity may moderate processes of human self-domestication. I present a model of human self-domestication based on reductions in mean masculinity and review selective pathways that might achieve this reduction using existing research on selection for or against human masculinity. I suggest female mate choice trade-offs between masculine traits and elevated paternal investment may be a primary driver of human self-domestication, but also show how self-domestication might relate to other theories of recent human evolution—especially regarding increased human sociability and cooperative capacity. Finally, I discuss potential limitations to the hypothesis and suggest avenues for further empirical investigation of links between masculine traits and the physiology of human self-domestication."