Hair is one of the defining characteristics of mammals[1] which may have evolved from hair reptilian scales, since it provides a similar protection from solar radiation, but appears to have evolved a more complex structure that involves several distinct components.
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Hair has many survival functions beyond protection from solar radiation including its compliment, the retention of heat, sexual dimorphism, sexual attraction and possibly the elicitation social grooming[2].
A recent study by scientists from Medical University of Vienna traced the origins of hair to the common ancestor of mammals, birds and lizards that lived 310 million years ago. The study found chickens, lizards and humans all possessed a similar set of genes that was involved in the production of alpha keratin. In chickens and lizards, the α-keratin produced was found in their claws, but in mammals it was used to produce hair. The scientists involved were still searching for the mechanisms that allowed mammals to use the α-keratins of animal claws to produce hair.[3][4]
Humans retain significant hair on the scalp. Jablonski [5] postulates that increasing body size, in conjunction with intensified hunting during the day at the equator, gave rise to a greater need to rapidly expel heat. As a result, humans evolved the ability to sweat: a process which was facilitated by the loss of body hair. She studied patas monkeys and concluded that, although their hair is thin, their quadrupedal locomotion required retention of solar protection.
In contrast, she avers, the bipedalism of human ancestors led to the retention of hair on the scalp, where the absorption of solar radiation is greatest and the generation of heat from muscular activity is low. The brain of a modern human produces about 20 Watts whereas the active human body as a whole produces around one Kilowatt of heat energy which must be discarded[6]
Prof David Reed studied the DNA of human head lice which he considers have infested our ancestors for some 6 mya, genital lice which are related to gorilla infestations and date from about 3 mya (homo erectus. Most animals harbor only one type of louse. Finally clothing lice - a variant of head lice - lives only in clothing (all other types of louse live only on hair) and so must indicate the adoption of apparel which he dates to 650tya. He concluded that loss of body hair occurred soon after the genera homo diverged from pan some 4 mya and clothing was probably used by several extinct species of the genus Homo[7].
Much of the above information was sourced from a documentary Horizon (BBC TV series) 'What's the Problem with Nudity? made by evolutionary psychologist George Fieldman in 2007-2008
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