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Dark skin is a naturally occurring human skin color rich in eumelanin pigments and having a dark colour.〔(dark-skinned ) Princeton University ''"naturally having skin of a dark color"''〕 People with relatively dark skin are referred to as brown,〔(Dictionary.com: black ) 3.a ''"a member of any of various dark-skinned peoples"'' 21.a''"pertaining or belonging to any of the various populations characterized by dark skin pigmentation"''〕 and those with very dark skin are often referred to as black,〔Oxford Dictionaries. April 2010. Oxford University Press. ''"belonging to or denoting any human group having dark-coloured skin"'' ("black" ) (accessed 6 August 2012).〕 although this usage can be ambiguous in some countries where it is also used to specifically refer to different ethnic groups or populations.〔(Dictionary.com: black ) 3.a ''"a member of any of various dark-skinned peoples"'' 21.a''"specifically thedark-skinned peoples of Africa, Oceania, or Australia."''〕〔(【引用サイトリンク】url=http://www.understandingrace.org/lived/global_census.html )〕〔Oxford Dictionaries. April 2010. Oxford University Press. ''"especially of African or Australian Aboriginal ancestry"'' ("black" ) (accessed 6 August 2012).〕 Evolution of dark skin pigmentation began around 1.2 million years ago in light-skinned early hominid species after they moved from the equatorial rainforest to the sunny savannas. In the heat of the savannas, better cooling mechanisms were required, which were achieved by the loss of body hair and development of more efficient perspiration. The loss of body hair led to the development of dark skin pigmentation, which acted as a mechanism of natural selection against folate depletion, and to a lesser extent, DNA damage. The primary factor contributing to the evolution of dark skin pigmentation was the breakdown of folate in reaction to ultraviolet radiation; the relationship between folate breakdown induced by ultraviolet radiation and reduced fitness as a failure of normal embryogenesis and spermatogenesis led to the selection of dark skin pigmentation. By the time modern ''Homo sapiens'' evolved, all humans were dark-skinned.〔 Humans with dark skin pigmentation have skin naturally rich in melanin (especially eumelanin), and have more melanosomes which provide a superior protection against the deleterious effects of ultraviolet radiation. This helps the body to retain its folate reserves and protects against damage to the DNA.〔 Dark-skinned people who live in high latitudes with mild sunlight are at an increased risk – especially in the winter – of vitamin D deficiency. As a consequence of vitamin D deficiency, they are at a higher risk of developing rickets, and numerous types of cancers, and possibly cardiovascular disease and low immune system activity.〔 However, some recent studies have questioned if the thresholds indicating Vitamin D deficiency in light-skinned individuals are relevant for dark-skinned individuals, as they found that on average dark-skinned individuals have higher bone density and lower risk of fractures than lighter-skinned individuals with the same levels of Vitamin D. This is attributed as possibly due to lower presence of Vitamin D binding agents (and thus higher bioavailability) in dark-skinned individuals. The global distribution of generally dark-skinned populations is strongly correlated with the high ultraviolet radiation levels of the regions inhabited by them. These populations almost exclusively live near the equator, in tropical areas with intense sunlight: Australia, Melanesia, New Guinea, South Asia and Sub-Saharan Africa. Studies into these populations indicates that dark skin is a retention of the pre-existing high UV adapted state of modern humans before the Out of Africa migration and not a later evolutionary adaptation.〔 Due to mass migration and increased mobility of people between geographical regions in the recent past, dark-skinned populations today are found all over the world.〔 == Evolution == Due to natural selection, people who lived in areas of intense sunlight developed dark skin colouration to protect against ultraviolet light and to protect their body mainly from folate depletion. Evolutionary pigmentation of the skin was caused by ultraviolet (UV) radiation of the sun. As hominids gradually lost their fur (between 4.5 and 2 million years ago) to allow for better cooling through sweating, their naked and lightly pigmented skin was exposed to sunlight. In the tropics, natural selection favoured dark-skinned human populations as high levels of skin pigmentation protected against the harmful effects of sunlight. Indigenous populations’ skin reflectance (the amount of sunlight the skin reflects) and the actual UV radiation in a particular geographic area is highly correlated, which supports this idea. Genetic evidence also supports this notion, demonstrating that around 1.2 million years ago there was a strong evolutionary pressure which acted on the development of dark skin pigmentation in early members of the genus Homo. The effect of sunlight on folic acid levels has been crucial in the development of dark skin.〔 The earliest primate ancestors of modern humans most likely had light skin, like our closest modern relative – the chimpanzee. About 7 million years ago human and chimpanzee lineages diverged, and between 4.5 and 2 million years ago early humans moved out of rainforests to the savannas of East Africa.〔 They not only had to cope with more intense sunlight but had to develop a better cooling system. It was harder to get food in the hot savannas and as mammalian brains are prone to overheating – 5 or 6 °C rise in temperature can lead to heatstroke – so there was a need for the development of better heat regulation. The solution was sweating and loss of body hair.〔 Sweating dissipated heat through evaporation. Early humans, like chimpanzees now, had few sweat glands, and most of them were located in the palms of the hand and the soles of feet. At times, individuals with more sweat glands were born. These humans could search for food and hunt for longer periods before being forced back to the shades. The more they could forage, more and healthier offspring they could produce, and higher the chance they had to pass on their genes for abundant sweat glands. With less hair, sweat could evaporate more easily and cool the body of humans faster. A few million years of evolution later, early humans had sparse body hair and more than 2 million sweat glands in their body.〔 Hairless skin, however, is particularly vulnerable to be damaged by ultraviolet light and this proved to be a problem for humans living in areas of intense UV radiation, and the evolutionary result was the development of dark-coloured skin as a protection. Scientists have long assumed that humans evolved melanin in order to absorb or scatter harmful sun radiation. Some researchers assumed that melanin protects against skin cancer. While high UV radiation can cause skin cancer, the development of cancer usually occurs after child bearing age. As natural selection favours individuals with traits of reproductive success, skin cancer had little effect on the evolution of dark skin. Previous hypotheses suggested that sunburned nipples impeded breastfeeding, but a slight tan is enough to protect mothers against this issue.〔 A 1978 study examined the effect of sunlight on folate – a vitamin B complex – levels. The study found that even short periods of intense sunlight are able to halve folate levels if someone has light skin. Low folate levels are correlated with neural tube defects, such as anencephaly and spina bifida. UV rays can strip away folate, which is important to the development of healthy foetuses. In these abnormalities children are born with incomplete brain or spinal cord. Nina Jablonski, a professor of anthropology and expert on evolution of human skin coloration, found several cases in which mother’s visits to tanning studios were connected to neural tube defects in early pregnancy. She also found that folate was crucial to sperm development; some male contraception drugs are based on folate inhibition. It has been found that folate may have been the driving force behind the evolution of dark skin.〔 As humans dispersed from equatorial Africa to low UVR areas and higher altitudes sometime between 120,000 and 65,000 years ago, dark skin posed as a disadvantage.〔Tim Appenzeller, Nature (Human migrations: Eastern odyssey ) 485, 24–26 doi:10.1038/485024a 2 May 2012〕〔 Populations with light skin pigmentation evolved in climates of little sunlight. Light skin pigmentation protects against vitamin D deficiency. It is known that dark-skinned people who have moved to climates of limited sunlight can develop vitamin D related conditions such as rickets, and different forms of cancer.〔〔(【引用サイトリンク】url=http://www.culturechange.org/cms/content/view/174/65/ )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「dark skin」の詳細全文を読む スポンサード リンク
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