The relatively hairless skin of humans compared with the thick coats seen on other primates is a difference that has long interested paleoanthropologists. Though we have maintained the same amount of hair folicles, our ancestors lost their functional body hair and evolved a fine hair covering or what is often referred to as a naked skin. The loss of functional body hair is usually thought to be an adaptation to thermoregulation (keeping the body at a constant temperature).

All warm-blooded mammals must maintain a homeostatic (internally balanced) body temperature that does not vary by more than a couple of degrees to insure proper bodily functions. One way to accomplish this is by panting, the rapid intake and exhalation of air through the mouth. Many animals rely upon panting -- watch your cat or dog outside on a hot day. Humans, however, are limited in their ability to pant. The rapid intake of air through the mouth causes hyperventilation if done for an extended time period. Thus, panting is an adaptation for animals that engage predominately in short bursts of activity. Another way to keep body temperature constant is to avoid solar radiation by staying out of the sun. Some animals are nocturnal, avoiding exposure to the sun entirely. However, this is not feasible for many animals, many of which rely upon the daylight hours for finding food.

Other adaptations to thermoregulation involve complex biological changes. One way is to reduce the surface area exposed to direct sunlight. This has been proposed as a factor in the evolution of bipedality. As early humans adopted an upright posture, they decreased the surface area that would be exposed to the sun's rays during the hottest hours of the day. The surface area to volume, or body mass, ratio (SA/BM) helps individuals maintain homeostatic temperatures by increasing or decreasing the area from which heat can escape. A lower SA/BM ratio benefits individuals living in colder climates and those living in extremely humid environments (such as rainforests). The relatively low ratio found among populations inhabiting humid environments occurs because humidity decreases the effectiveness of evaporative cooling. The higher ratio allows greater amounts of heat to escape and is more advantageous in hot, dry climates. Increasing peripheral vasodilation, bringing blood vessels to the skin surface to cool blood down, is another adaptation. Nasal morphology was also changed to moisturize the inhaled air (essential before it enters the lungs) and then to extract and retain the moisture inside our bodies as the air is exhaled.

Mammalian body hair acts as an insulator, allowing a constant temperature to be maintained. When the external temperatures fall, body hair helps trap heat. When environmental temperatures are higher than internal temperatures, body hair provides a filter for solar radiation, protecting the body's surface from direct exposure that would raise internal temperatures. However, in hot weather, the thick fur can make it difficult for excess body heat to be dissipated. In humans, exposed skin allows heat to escape easily from the body through sweat and evaporation. The cost of exposed skin is that the body has no way to trap escaping heat when the external temperature is lower than internal temperatures.

Although it is impossible to find evidence of exactly when functional hairlessness occurred in human evolution, it is possible to infer the timing of other thermoregulatory changes. Members of the genus Homo have modern body proportions (longer legs and shorter arms) whereas earlier hominins had more ape-like bodies. The taller and thinner hominins would have been able to diffuse heat more effectively in an open environment. Increasing aridity and the spread of open habitats would have placed a premium on relatively hairless, evaporative skin as a way of regulating body temperature.