We found that several pairwise distances differed between the sexes. For example, the distance from the brow to nasal bridge was found to be more than 5% larger in females than males. We then tested for an interaction between sex and genetic ancestry by testing for differences in the slopes of the ancestry association between males and females. Although the pattern differed slightly between samples, after Bonferroni correction many correlations were the found to be same in both sexes. However, females in all three samples had many additional significant correlations that were not seen in males, while males had very few correlations that were not found in females. The results of these analyses suggest that selection on females is driving the differentiation in facial features among populations. (Liberton et al., 2009)
What is this selective force that acts mainly on female morphology and carries male morphology along in its wake? I suspect Denise Liberton has sexual selection in mind. If so, this finding would support Darwin’s belief that “the races of man differ from each other and from their nearest allies, in certain characters which are of no service to them in their daily habits of life, and which it is extremely probable would have been modified through sexual selection” (Darwin, 1936 , p. 908).
Darwin was puzzled not only by the considerable physical differences separating humans from apes, but also by the considerable physical differences among human populations (Darwin, 1936 , p. 530-531). He concluded that sexual selection was “the most efficient” cause of this differentiation (Darwin, 1936 , p. 908).
Yet sexual selection usually acts on males in other mammals. The females are the ones who normally do the selecting. This is because they must take time out from the mate market for pregnancy, breastfeeding, and infant care. Meanwhile, the males never really leave the mate market, with the result that too many of them are competing for a limited number of available females.
This mammalian ‘law’ has influenced much writing about sexual selection in humans. According to Naomi Wolf, author of The Beauty Myth, "for women to compete with women through 'beauty' is a reversal of the way in which natural selection affects all other mammals" (Wolf, 1990, p. 3). She points to indigenous peoples in sub-Saharan Africa, Australia, and New Guinea as proof that the original human state was one of males vying for the attention of females.
I could cite other writers, but the gist of their argument is always the same. Authentic human nature is represented today by indigenous tropical peoples. They are what we were. Therefore, human nature is about polygynous males who devote little time and energy to raising their progeny and a lot to seducing the limited number of females. Women are thus the ones who have been sexually selecting.
In this kind of argument, we means ‘people of non-tropical origin,’ particularly those of European descent. Yet clearly this argument is false. We were not them for a long time. Europeans have an evolutionary history going back some 35,000 years on their continent. And this was when and where they evolved their current physical appearance: the shape of their face, the color of their skin, hair, and eyes; the length and form of their head hair. To understand why Europeans look the way they do, we should understand how their environment of sexual selection differed from that of tropical humans.
Ancestral humans were exposed to pressures of sexual selection that varied along a north-south axis. In the tropical zone, women could gather food year-round, thus making the cost of a second wife relatively low. With so many being scooped up, female mates were a limited resource. Too many men had to compete for too few women. The pressure of sexual selection was thus on men, with women being the ones who could pick and choose mates.
This situation reversed as humans moved away from the tropical zone. First, it became costlier for a man to provide for a second wife because women contributed less to the family food supply, the longer winters reducing opportunities for food gathering. Second, male mortality increased relative to female mortality because men had to hunt over longer distances. Together, these two trends resulted in too few men competing for too many women. This was particularly so on continental steppe-tundra, where women had almost no opportunities for food gathering and where men had to hunt wandering herds of herbivores over long distances (Frost, 2006; Frost, 2008).
Because of a geographic accident, i.e., a glacial mass over Scandinavia, it was in Europe during the last ice age (25,000 to 10,000 years ago), specifically on the northern and eastern plains, that continental steppe-tundra reached furthest to the south and covered the most territory during the time of modern humans. And this was when and where Europeans came to look European. They did not change in physical appearance because of climatic adaptation. The cause was a change in the direction and intensity of sexual selection: men were now selecting women, and to a much greater degree than elsewhere.
Darwin, C. (1936) . The Descent of Man and Selection in relation to Sex. reprint of 2nd ed., The Modern Library, New York: Random House.
Frost, P. (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4), pp. 169-191.
Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103.
Liberton, D.K., K.A. Matthes, R. Pereira, T. Frudakis, D.A. Puts, & M.D. Shriver. (2009). Patterns of correlation between genetic ancestry and facial features suggest selection on females is driving differentiation. Poster #326, The American Society of Human Genetics, 59th annual meeting, October 20-24, 2009. Honolulu, Hawaii.
Wolf, N. (1990). The Beauty Myth. Toronto: Random House.