I finally got a chance to finish this post about my recent article on Human Self-domestication in the ‘American Journal of Physical Anthropology’ The paper was written with my supervisor, Dr Geoff Kushnick. In it we explore one of three proposed evolutionary mechanisms behind the phenomenon of human self-domestication.
Domestication syndrome involves a range of correlated changes that occur within domesticated populations of vertebrate taxa. It is thought to be triggered by selection against reactive aggression of the type involved in autonomic stress responses–like the ‘fight or flight’ response–as demonstrated in Dmitry Belyaev’s fox experiments.
The possible selective mechanism we considered in humans was female mate choice in favour of less aggressive males. Two other proposed mechanisms are: (1) generalised social benefits (hence enhanced reproductive fitness) derived from being a more capable cooperator; and (2) group ostracism, whereby groups of more sociable individuals avoid or eliminate excessively aggressive group members.
An important current hypothesis suggests that the recent rise of human civilisations following the dawn of the Neolithic required ongoing selection for more sociable individuals, especially more cooperative and sociable males. Analysis of fossil evidence appears to support this process. As such, self-domestication provides a biophysical explanation for multiple aspects of our physical and cultural evolution.
Domestication syndrome involves correlated trait changes regularly seen in domesticated animals and in some wild species (as suggested for bonobos). Recent authors have proposed that self-domestication may be evidenced in hominin lineages as far back as Ardipithecus ramidus. If accurate, such a longstanding trend could only have been maintained via repetitive aspects of hominin social and sexual interaction, and related processes of self-selection for certain behavioural traits.
For the bulk of human existence such self-selection must have taken place in small-group hunter-gatherer social settings. Related trends among modern urban-industrial human groups are harder to diagnose and predict, but the rise of complex civilisations likely only occurred after a long period of self-domestication and acculturation among earlier populations.
In our study, which used ethnographic and morphological data from a range of groups listed in the Standard Cross Cultural Sample, we found smaller differences in stature between males and females (one of several known outcomes of domestication) which were associated with an interaction between female social status and food resource availability.
This suggests that in cultures with higher female social status, women are better able to choose reproductive partners and, under conditions where food is relatively secure, they exercise this capacity as a preference for less-dominant, more cooperative and sociable male partners. Effectively, where women have both higher status and secure access to resources, they choose less-masculine men to settle down with. Over time, this leads to relatively less difference in height between the average male and female within that particular cultural group.
However, in populations where food resources are insecure, women with higher status appear to choose taller, more dominant and masculine males who can compete successfully for resources and may pass on their masculine genes to sons.
In situations where women have relatively low social status they have less choice of who they partner with, so preferences one way or the other should have minimal effect. Under these conditions male-male competition would be more influential; and, in accord with standard evolutionary theory, we found differences between male and female stature are larger.
These results appear to provide an indication of evolutionary changes moderated by norms affecting women’s social status and autonomy. The fact that we used data for genetically unrelated populations, and that most of the trend involves differences in male height, strongly suggests our observations are not simply due to improved nutrition in higher status women and girls. Furthermore, our results contradict known nutritional effects whereby undernourished populations reliably show the lowest sexual stature differences.
For me, one of the fundamentally interesting aspects of our study is that it suggests varying levels of female social status can affect evolutionary outcomes. This finding is not unexpected given longstanding observation of different reproductive strategies among males and females.
Taken together with social scientific analysis regarding female autonomy and its association with societal outcomes like democracy and less armed conflict, our study points to further productive anthropological research in areas including human behavioural ecology and gene-culture co-evolution.
I’d hope our article may prompt further investigation into the evolutionary effects of female social status on human self-domestication, and on human sociability more generally. Such investigation might expand our knowledge regarding the evolution of human hyper-cooperation and the expansion of complex and cooperative human civilisations.