The scalp hair of modern people is variable: hair fibres can be fine or thick, straight or tightly coiled. The evolutionary reasons for this variation are unknown. Because the scalp is one of the few regions of the human body that has not evolved to be effectively hairless, it is important to understand whether natural selection has shaped human hair variation to the extent that it has shaped the fur (hair) of other mammalian species. It has been suggested that tightly coiled hair, as is found in many African populations today, has evolved to facilitate evaporative cooling and protect a thermogenic and thermosensitive brain from solar radiation. Likewise, the thick hair fibres found in many East Asian populations have been hypothesized to be an adaptation to cold climates. However, to date, there has been no attempt to empirically test the functional significance of human hair variation. We propose to use principles from environmental ergonomics to evaluate the hypothesis that variation in human hair morphology evolved as a thermoregulatory adaptation. Information on optimization of materials for various thermal environments drawn from environmental ergonomics can be used to inform appropriate experiments and simulations to test this hypothesis. Here we present information on the expected differences in insulating properties of different hair types, and data on the environmental conditions under which human hair types are thought to have evolved, as well as what is currently known of the evolutionary genetics of human hair variation. Our aim is to understand the insulating properties of hair and the extent to which insulation afforded by scalp hair affected human physiology and hence selective pressures on human hair morphology.
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