Hair: Encyclopedia II - Hair - Human hair

Hair - Human hair

Typically, humans have the longest hair on the top of the head, with shorter hair on the eyelids and eyebrows. Armpit hair and pubic hair serves as lubrication during rubbing.

Sometimes, the term body hair is used, to distinguish it from hair on the head. Individual hairs alternate 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 armpit 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. The rate of growth is approximately 1.25 centimeters, or about 0.5 inches, per month. 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. Another possibility is that long head hair is a result of Fisherian runaway sexual selection, where long lustrous hair is a visible marker for a healthy individual (waist length hair, 1 meter, or 39 inch, long, would take ~80 months, or just under 7 years, to grow - with good nutrition), and this would explain why long head hair (in both sexes) is viewed as "sexy" even now.

Hair grows from 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, the navel and other scar tissue. Some people seem to have less body and facial hair than others, but in fact have shorter and finer body hair while the total number of follicles is relatively constant.

Several theories have been advanced to explain the apparent bareness of human body hair. One suggests that nature selected humans for shorter and thinner body hair as part of a set of adaptations including bipedal locomotion and an upright posture. There are several problems with this savanna theory, not least of which is that cursorial hunting is used by (other) animals that do not show any thinning of hair, and that hair similar to chimpanzees and gorillas also shades the skin from radiant heat and protects it from hot winds, and thus another mechanism for heat loss is not required. Another problem is that bipedal locomotion apparently now predates hominids moving from a forest environment to a savanna environment. 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, possibly occurring 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; see Aquatic ape hypothesis. One problem with this theory is that both chimpanzees and gorillas have the same density and distribution of the eccrine glands, but that they have not been developed for sweat production.

A third theory for the thin body hair on humans proposes that Fisherian runaway sexual selection played a role here (as well as in the selection of long head hair). Possibly this occurred in conjunction with neoteny, with the more juvenile appearing females being selected by males as more desirable; see types of hair and vellus hair. The human female body hair typically has more vellus hair (making the skin appear bare), while the male body typically has more terminal hair (especially on the chest and back). Thus sexual selection can explain the sexual dimorphism in human body hair, with the results of selection being more evident (more extreme) in the female than in the male, a point which the other two theories cannot address without proposing substantially different behavior between males and females. Also, we see that artificially bare (shaved, etc.) legs, arms, etc. on women are seen as "sexy" even today, while body shaving is not nearly as common for men.

Adapted from the Wikipedia article "Human hair", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki