Sexual selection in human evolution

When Charles Darwin initially proposed his theory on sexual selection, there was considerable scepticism on its application to human evolution. More recently, the role of sexual selection in human evolution has begun to gain wide acceptance.

History
Charles Darwin conjectured that the male beard, as well as the relative hairlessness of humans compared to nearly all other mammals, are results of sexual selection. He reasoned that since, compared to males, the bodies of females are more nearly hairless, hairlessness is one of the atypical cases due to its selection by males at a remote prehistoric time, when males had overwhelming selective power, and that it nonetheless affected males due to genetic correlation between the sexes. He also hypothesized that sexual selection could also be what had differentiated between different human races, as he did not believe that natural selection provided a satisfactory answer.

Contemporary theories
Geoffrey Miller, drawing on some of Darwin's largely neglected ideas about human behavior, has hypothesized that human culture arose through a process of sexual selection. He argues that cultural traits such as art, music, dance, verbal creativity and humor are of no survival value. Miller is critical of theories that imply that human culture arose as accidents or byproducts of human evolution. He believes that human culture arose through sexual selection for creative traits. In that view, many human artefacts could be considered subject to sexual selection as part of the extended phenotype, for instance clothing that enhance sexually selected traits.

The zoologist Richard Dawkins pointed out in 1989 that the loss of the penis bone in humans, when it is present in our nearest related species the chimpanzee, demands some form of evolutionary explanation. He speculates that its loss is probably a form of sexual selection by females looking for signs of good health in prospective mates. The reliance of the human penis solely on hydraulic means to achieve a rigid state makes it particularly vulnerable to blood pressure variation. Poor erectile function betrays, not only physical states such as diabetes and neurological disorders, but mental states such as stress and depression.

Runaway Brain
During human evolution, on at least two occasions, hominid brain size increased rapidly over a relatively short period of time followed by a period of stasis. The first period of brain expansion occurred 2.5 million years ago, when homo habilis first began using stone tools. The second period occurred 500,000 years ago, with the emergence of archaic homo sapiens. Miller argues that the rapid increases in brain size would have occurred by a positive feedback loop resulting in runaway selection for larger brains.

The human brain is thought to be a sexually selected trait, because it does not confer enough fitness in itself relative to its high maintenance costs (a quarter to a fifth of the energy and oxygen consumed by a human). Though the human brain does not confer any fitness benefits in itself, it may however have been an indicator of fitness. This in the same way that peacocks tail itself is not has no fitness value, but the quality of the tail is an indicator of the overall health of the peacock.