Nakedness and clothing use are characteristics of humans related by evolutionary and social prehistory. The major loss of body hair distinguishes humans from other primates. Current evidence indicates that anatomically-modern humans were naked in prehistory for at least 90,000 years before they invented clothing. Today, isolated Indigenous peoples in tropical climates continue to be without clothing in many everyday activities.
Evolution of hairlessness
The general hairlessness of humans in comparison to related species may be due to loss of functionality in the pseudogene called KRT41P (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. To comprehend why humans have significantly less body hair than other primates, one must understand that mammalian body hair is not merely an aesthetic characteristic; it protects the skin from wounds, bites, heat, cold, and ultraviolet radiation. Additionally, it can be used as a communication tool and as a camouflage. The first member of the genus Homo to be hairless was Homo erectus, originating about 1.6 million years ago. The dissipation of body heat remains the most widely accepted evolutionary explanation for the loss of body hair in early members of the genus Homo, the surviving member of which is modern humans. Less hair and an increase in sweat glands made it easier for their bodies to cool when they moved from living in shady forest to open savanna. This change in environment also resulted in a change in diet, from largely vegetarian to hunting-gathering. Pursuing game on the savanna also increased the need for regulation of body heat. The anthropologist and paleo-biologist Nina Jablonski posits that the ability to dissipate excess body heat through eccrine sweating helped make possible the dramatic enlargement of the brain, the most temperature-sensitive organ in human body. Thus the loss of fur was also a factor in further adaptations, both physical and behavioral, that differentiated humans from other primates. Some of these changes are thought to be the result of sexual selection: by selecting more hairless mates, humans accelerated changes initiated by natural selection. Sexual selection may also account for the remaining human hair in the pubic area and armpits, which are sites for pheromones, while hair on the head continued to provide protection from the sun. Anatomically-modern humans, whose traits include hairlessness, evolved 260,000 to 350,000 years ago.
Phenotypic changes Humans are the only primate species to 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. With the loss of fur, darker, high-melanin skin evolved as a protection from ultraviolet radiation damage. As humans migrated outside of the tropics, varying degrees of depigmentation evolved in order to permit UVB-induced synthesis of previtamin D3. The relative lightness of female compared to male skin in a given population may be due to the greater need for women to produce more vitamin D during lactation. 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.
Sexual selection hypothesis 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 terminal and vellus hair), as well as a much larger role of testosterone in males. Sexual selection is the only theory thus far that explains the sexual dimorphism seen in the hair patterns of males and females. On average, males have more body hair than females. 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.
Water-dwelling hypothesis The aquatic ape hypothesis (AAH) includes hair loss as one of several characteristics of modern humans that could indicate adaptations to an aquatic environment. Serious consideration may be given by contemporary anthropologists to some hypotheses related to AAH, but hair loss is not one of them.
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