A 2008 study by scientists from the 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 continued searching for the mechanisms that allowed mammals to use the keratins of animal claws to produce hair.

Human "hairlessness"

Human hair is barely visible as it is thinner, shorter and more translucent than the hair of other mammals. Historically, some ideas have been advanced to explain the apparent hairlessness of humans, as compared to other species.

Human hair under 200-times magnification

Most mammals have light skin that is covered by fur, and biologists believe that 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 would be experienced in Africa. Therefore, evidence of 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.

Dr. Alan R. Rogers, an evolutionary geneticist at the University of Utah, used mutations in the MC1R gene to estimate when human skin darkened. He said humans may have gone through several genetic "clean sweeps" with light-skinned individuals dying off and dark-skinned individuals surviving. He estimates the last of these clean sweeps took place 1.2 million years ago. Therefore, humans, in part, have been hairless at least since that time, as body hair does still remain in human populations.

The soft, fine hair found on many nonhuman mammals is typically called fur.

The savanna hypothesis suggests that nature selected humans for shorter and thinner body hair as part of a set of adaptations to the warm plains of the African savanna (in addition to bipedal locomotion and an upright posture). Some counter this argument by noting that among the most hairless people are Northern Europeans who live in a cold and relatively low sun environment. However, abundant genetic and archaeological evidence indicates that the hairlessness of those current-day modern humans whose immediate ancestors came to occupy Northern latitudes is attributable to the relatively recent origin of these people in equatorial, sub-Saharan Africa approximately 200,000 years ago, followed by an even more recent departure from Africa that was initiated approximately 60,000 years ago. Hence it is highly likely that the ancestors of Northern Europeans (et al. Northern groups) failed to develop fur due to a) their relatively recent entry into the area, and b) the fact that the high levels of intelligence that had evolved in the human lineage while in Africa enabled them to survive in the cold European climate by way of the practice of wearing animal furs. Hence the development of fur was rendered effectively unnecessary.

Nevertheless, other species likely migrated to Africa by way of a gradual process. This provided them with time to adjust to the intense UV and sunlight by way of other means (such as panting). Hominids, on the other hand, originally possessed fur, but, due to a relatively sudden change in behavior 2.5 million years ago (due to hominid inventiveness/technological innovation) that involved intense hunting during the day, they developed sweat glands that enabled them to perspire. This change necessitated the loss of most body hair in order to facilitate sweat evaporation (i.e. cool the body).

Balding usually occurs at around 30 – 40 years of age. In prehistoric times, most individuals were not as likely to live past 30. Hence it wasn't as common a trait. Also, dark pigmentation of the skin could have partially compensated for premature baldness. There are other African mammals that have lost fur due to equatorial heat. These include the elephant and the hippopotamus. Thus it is arguable that the Savanna Theory model provides the best explanation for the loss of fur experienced by the human lineage given the available evidence.

Another theory for the thin body hair on humans proposes that Fisherian runaway sexual selection played a role (as well as in the selection of long head hair), (see types of hair and vellus hair), as well as a much smaller role of testosterone in women.

The aquatic ape hypothesis posits that sparsity of hair is an adaptation to an aquatic environment, but the theory has little support among scientists.

Humans, are part of a trend toward sparser hair in larger animals. The density of human hair follicles on the skin is actually about the average for an animal of equivalent size. It is still not clear why so much of human hair is short, underpigmented vellus hair, rather than terminal hair and the effect of testosterone on the hair follicles in both human and other mammallian species.