Humans have dramatically reduced body hair even though it is important for protection against the climate and the sharp teeth and claws of other animals. Body fur can also be used to signal with color and patterns and also through piloerection, or raising the hairs to puff up and look big and aggressive.
Not only are human functionally "naked" but we are sweaty too. We have more sweat glands than any other primate and are some of the sweatiest mammals as well. It follows that the probable reason for our loss of body hair was due to the need for increased sweating. There is no fossil evidence for the evolution of human body hair so genetics and adaptations of other human traits guide the hypotheses.
Nina Jablonski argues that the need for thermoregulation while walking around with a big hot brain in the sunny hot periods of the day (presumably while foraging, hunting, and scavenging) led to the selection for body fur loss in early members of the genus Homo about 2 Mya. The loss of body fur was accompanied by the evolution of more sweat glands for copious sweating (and hence body cooling) and the evolution of darker skin for protection from harmful ultraviolet radiation (UVR). The combination of body fur loss, increased heat tolerance through sweating, and darker skin enabled hominins to travel further and spend longer periods of time under the hot sun. The MCIR gene associated with skin pigmentation points to a date of 1.7 Mya for the emergence of dark skin, which is consistent with the hypothesis that body fur loss and dark pigmentation evolved in concert around 2 Mya.
Hair is abundant on the top of the head where the sun hits the body directly and in the underarm and pubic regions, presumably for protection against chaffing and rubbing, or for storing scents that signal fertility and identity. All the hair on the body is actually only one of two types: terminal hair occurs on the head, eyebrows, and eyelashes but vellus hair is everywhere else. These types of hair differ in their lasting ability: they all grow at about a half an inch per month but they have different growing durations before they drop out of the skin.
Comparative genomics will someday illuminate the genetic history behind our unique body hair distribution. To investigate the loss of body fur we can look for candidate genes in the genomes of hairless mice, naked mole rats, hairless cats and dogs (with furry out-groups for comparison, of course) to see if humans have anything similar.
One candidate gene for long hair is known from mice (called "angora mice") with a mutation in FGF5, a gene that in its normal state functions to stop hair growth. Perhaps humans have a genetic mechanism for ignoring this hair-growth-halting gene as well. There are also several genes for keratin, the substance that makes up hair. Many of these genes are identical in humans, chimpanzees, and gorillas but one is different in humans. It has no apparent function in humans, but it codes for proteins in the other primates. The mutation in humans occurred about 250 Kya which could be the time when human head hair took its current ever-growing form. More studies on these genes will need to be done to understand this process better.
If body hair was reduced, why was head hair increased? It could be protection for the brain from the direct rays of the sun. It could be sexually selected for because of beauty. It could be a fitness indicator for health and social standing. Human hair needs extra grooming and taking the time to groom one's hair or to have someone else groom it could signal importance or status. The Venus figurine from Willendorf, Austria (Figure 5.4) had an elaborate hairdo, showing us an early glimpse at the importance of good hair by 23 Kya.
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