page_title: Hair
section_title: Human hairlessness
hierarchical_section_title: Hair / Evolution / Human hairlessness
caption_attribution_description: Red Fox (Vulpes vulpes)
context_page_description: Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals.
  The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types, and hair care, but hair is also an important biomaterial primarily composed of protein, notably alpha-keratin.
  Attitudes towards different forms of hair, such as hairstyles and hair removal, vary widely across different cultures and historical periods, but it is often used to indicate a person's personal beliefs or social position, such as their age, sex, or religion.
context_section_description: The general hairlessness of humans in comparison to related species may be due to loss of functionality in the pseudogene KRTHAP1 (which helps produce keratin) in the human lineage about 240,000 years ago. On an individual basis, mutations in the gene HR can lead to complete hair loss, though this is not typical in humans. Humans may also lose their hair as a result of hormonal imbalance due to drugs or pregnancy.
  In order to comprehend why humans are essentially hairless, it is essential to understand that mammalian body hair is not merely an aesthetic characteristic; it protects the skin from wounds, bites, heat, cold, and UV radiation. Additionally, it can be used as a communication tool and as a camouflage. To this end, it can be concluded that benefits stemming from the loss of human body hair must be great enough to outweigh the loss of these protective functions by nakedness.
  Humans are the only primate species that have undergone significant hair loss and of the approximately 5000 extant species of mammal, only a handful are effectively hairless. This list includes elephants, rhinoceroses, hippopotamuses, walruses, some species of pigs, whales and other cetaceans, and naked mole rats. Most mammals have light skin that is covered by fur, and biologists believe that early human ancestors started out this way also. Dark skin probably evolved after humans lost their body fur, because the naked skin was vulnerable to the strong UV radiation as explained in the Out of Africa hypothesis. Therefore, evidence of the time when human skin darkened has been used to date the loss of human body hair, assuming that the dark skin was needed after the fur was gone.
  It was expected that dating the split of the ancestral human louse into two species, the head louse and the pubic louse, would date the loss of body hair in human ancestors. However, it turned out that the human pubic louse does not descend from the ancestral human louse, but from the gorilla louse, diverging 3.3 million years ago. This suggests that humans had lost body hair (but retained head hair) and developed thick pubic hair prior to this date, were living in or close to the forest where gorillas lived, and acquired pubic lice from butchering gorillas or sleeping in their nests. The evolution of the body louse from the head louse, on the other hand, places the date of clothing much later, some 100,000 years ago.
  The sweat glands in humans could have evolved to spread from the hands and feet as the body hair changed, or the hair change could have occurred to facilitate sweating. Horses and humans are two of the few animals capable of sweating on most of their body, yet horses are larger and still have fully developed fur. In humans, the skin hairs lie flat in hot conditions, as the arrector pili muscles relax, preventing heat from being trapped by a layer of still air between the hairs, and increasing heat loss by convection.
  Another hypothesis for the thick body hair on humans proposes that Fisherian runaway sexual selection played a role (as well as in the selection of long head hair), (see terminaland vellus hair), as well as a much larger role of testosterone in men. Sexual selection is the only theory thus far that explains the sexual dimorphism seen in the hair patterns of men and women. On average, men have more body hair than women. Males have more terminal hair, especially on the face, chest, abdomen, and back, and females have more vellus hair, which is less visible. The halting of hair development at a juvenile stage, vellus hair, would also be consistent with the neoteny evident in humans, especially in females, and thus they could have occurred at the same time. This theory, however, has significant holdings in today's cultural norms. There is no evidence that sexual selection would proceed to such a drastic extent over a million years ago when a full, lush coat of hair would most likely indicate health and would therefore be more likely to be selected for, not against, and not all human populations today have sexual dimorphism in body hair.
  A further
caption: The soft, fine hair found on many nonhuman mammals is typically called fur.
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