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long haired girl

Long Haired Girl

Human hair is barely visible as it is thinner, shorter, and more translucent than the hair of other mammals. Humans are part of a trend toward sparser hair in larger animals (fewer follicles per square inch of exposed skin). There are several African mammals that have very sparse fur, including the elephant and the hippopotamus, both at the upper end of this trend. The density of human hair follicles on the skin is about the average for an animal of equivalent size. It still is not clear why so much of human hair is short, under-pigmented vellus hair, rather than terminal hair, and the effect of testosterone on the hair follicles in both human and other mammalian species. Because most human hair is less apparent vellus hair, this gives the appearance of being hairless, especially in females.
Most mammals have light skin that is covered by fur, and biologists believe that early human ancestors started out this way also. Dark skin probably evolved after humans lost their body fur, because the naked skin was vulnerable to the strong UV radiation as would be experienced in Africa. Therefore, evidence of when human skin darkened has been used to date the loss of human body hair, assuming that the dark skin was needed after the fur was gone.
Dr. Alan R. Rogers, an evolutionary geneticist at the University of Utah, used mutations in the MC1R gene to estimate when human skin darkened. He said early humans may have gone through several genetic "clean sweeps" with light-skinned individuals dying off and dark-skinned individuals surviving. He estimates the last of these clean sweeps took place 1.2 million years ago. Therefore, apparent bareness in humans, likely has existed at least since that time. Later when humans began to live in more northerly latitudes, however, light skin became an advantage for the absorption of vitamin D and the recessive gene began to become more prevalent in those populations.
Balding, where terminal hair switches to vellus hair, usually occurs at around thirty to forty years of age. In prehistoric times, most individuals were not as likely to live past 30, with few reaching their fourth decade, and thus it wasn't likely that this trait was subject to selection.

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