Hair

:Hair is also a musical: see Hair (musical) and Hair (movie)

Human hair

Typically, humans have more hair on the top of the head, and also hair where extremities meet the torso (axillary hair, and pubic hair), on the eyelids and above them (eyebrows). In most societies people shave, style or adorn their hair for aesthetic reasons.

Related Topics:
Pubic hair - Eye - Shave

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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. 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.

Related Topics:
Anthropologist - Adornment - Natural selection

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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

Related Topics:
Hair cut - Remove it

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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. Some people may seem to have less body hair than others. In fact the difference is that some people have shorter and thinner body hair than others. Overall coverage in terms of number of folicles is relatively constant.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Several theories have been advanced to explain the unique features of human hair. One theory 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 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 possibly 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 proposes that sexual selection played a role, possibly in conjunction with Neoteny, with the more juvenile appearing females being selected by males as more desirable; see Types of hair and Vellus hair. This would also explain the sexual dimorphism in human body hair. At this point the evidence is inconclusive as to the cause of the unique features of human hair.

Related Topics:
Bipedal - Cursorial hunting - Chimpanzees - Gorillas - Ethiopia - Aquatic ape hypothesis - Sexual selection - Neoteny - Types of hair - Vellus hair - Sexual dimorphism

~ ~ ~ ~ ~ ~ ~ ~ ~ ~