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Androgenic hair, colloquially Body hair, is the terminal hair on the human body developed during and after puberty. It is differentiated from the head hair and less visible vellus hair. Androgenic denotes its growth is related to the level of androgens (male hormones) in the individual. Due to a normally higher level of androgens, men tend to have more androgenic hair than women.
From childhood onward, regardless of sex, vellus hair covers the entire human body. Exceptions include the lips, the backs of the ears, the palms of hands, the soles of the feet, certain external genital areas, the navel and scar tissue. The density of hair (number of hair follicles per area of skin) varies from one person to another.
Development and growth
Rising levels of androgens during puberty cause vellus hair to transform into terminal hair over many areas of the body. Hair follicles respond to androgens, primarily testosterone and its derivatives. Different areas respond with different sensitivities. The rate of hair growth and the weight of the hairs increase. The sequence of appearance of androgenic hair reflects the gradations of androgen sensitivity. Since the pubic area is most sensitive to androgen, heavier hair usually grows there first. Genetic factors determine both individual levels of androgens and the hair follicle's sensitivity to androgens, and thus hair development.
Most noticeable in both sexes are the development of axillary hair and pubic hair. Thus it is sometimes viewed as a secondary sex characteristic. There is a sexual differentiation in the amount and distribution of androgenic hair, with males having more terminal hair in more areas. This includes facial hair, chest hair, abdominal hair, leg and arm hair, and foot hair. Females retain more of the less visible vellus hair, though leg, arm, and foot hair is noticeable.
Evolution
Evolutionary and genetic evidence suggests human androgenic hair is the remnant of body hair of the type found on today's great apes. Full-on body hair phased out in the course of human evolution roughly 2-3 million years agocitation needed. Based on the numbers and spread of variations in the MC1R nucleotide sequences between humans and chimpanzees, modern hair patterns were seen by 1.2 million years ago. However, hair densities and patterns on humans differ for populations in differing climates.citation needed
Three theories are prevalent as to why people lack fur, all of which relate to the loss conferring some evolutionary advantage.[1] One theory relates to the aquatic ape hypothesis and suggests that humans traded body hair for an increase in body fat. A second theory concerns thermoregulation through sweating and suggests that tree dwelling apes had less of a problem with overheating than those who walked around in the sun. The third theory depends upon the notion that a hairy body provides a better habitat for ectoparasites than a naked one, and that loss of hair led to an overall decrease in diseases spread by these parasites. None of these theories have overwhelming evidence supporting or refuting it.
References
- ^ Staff (2007-06-04), "Ask the Experts: Q—What is the latest theory of why humans lost their body hair? Answer provided by Dr. Mark Pagel", Scientific American (Scientific American, Inc.) 297(3): 124, September 2007, <http://www.sciam.com/article.cfm?id=latest-theory-human-body-hair>. Retrieved on 3 August 2008
Wikipedia content modification information:
- This page was last modified on 3 September 2008, at 09:52.
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