Human Hair

One theory suggests that nature selected humans for little body hair as part of a set of adaptations including bipedal locomotion and an upright posture. Bipedal locomotion is extremely inefficient, and many animals can outrun human beings for short periods of time; such animals, however, are inefficient radiators of heat, and cannot run for long periods of time (notable exceptions include most cursorial animals, including savannah fauna). Thus, human hunters must be able to chase animals for long periods of time, and must therefore have an efficient mechanism for radiating body heat.

Upright posture exposes less surface area of the body to direct solar radiation, and subcutaneous sweat glands developed, providing such a cooling mechanism. A more recent theory for human hair loss has to do with a possible period of bipedal wading in a salt marsh in the Danakil region of Ethiopia, which occurred in the hominid lineage, between 5 and 7 million years ago. As a wading animal, it was more efficient to develop short body hair and a layer of subcutaneous fat for streamlining and insulation in the aquatic environment; the eccrine sweat glands developed later after the hominids left the water. This is why most hairless mammals are aquatic (dolphins, dugongs, whales), had an aquatic period in their pasts (elephants, rhinoceroses, pigs) or have very short fine fur because of brief periods back out of the water (seals, sea lions). There is a hypothesis that claims humans are no exception to this rule of hairlessness through means of a marine transition (Aquatic Ape Theory).

Typically, humans have more hair on the top of the head, because this region of the body was exposed to solar radiation at all times, even when wading, and also hair where extremities meet the torso (axillary (arm-pit) hair, and pubic hair), on the eyelids and above them (eyebrows). In most societies people shave, style or adorn their hair for aesthetic reasons.

Sometimes, the term body hair is used, to distinguish hair on the body from hair on the head. All hairs alternate regular periods of growth and dormancy. During the growth portion of the cycle, hair follicles are long and bulbous, and the hair advances outward at about a third of a millimeter per day. After three to six months, body hair growth stops (the pubic and axillary areas having the longest growth period). The follicle shrinks and the root of the hair rigidifies. Following a period of dormancy, another growth cycle starts, and eventually a new hair pushes the old one out of the follicle from beneath. Head hair, by comparison, grows for a long duration and to a great length before being shed. Anthropologists speculate that the functional significance of long head hair may be adornment, a by-product of secondary natural selection once other somatic hair had been lost.

Unlike other animals, human beings often have their hair cut or remove it by shaving or other means.

It is important to note that hair grows across all areas of the skin on humans regardless of sex or race except in the following locations: the lips, the nipples, the palms of hands, the soles of feet, certain external genital areas, and the navel.