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Light skin is a naturally occurring human skin color, which has little eumelanin pigmentation and which has been adapted to environments of low UV radiation.〔(light-skinned ) Princeton University〕〔(【引用サイトリンク】url=http://dictionary.reference.com/browse/light-skinned?s=t&ld=1125 )〕 Light skin is most commonly found amongst the native populations of Europe as measured through skin reflectance. People with light skin pigmentation are often referred to as white〔Oxford Dictionaries. April 2010. Oxford University Press. ''"belonging to or denoting a human group having light-coloured skin"'' ("white" ) (accessed 6 August 2012).〕〔(Dictionary.com: white ) 3.a ''"marked by slight pigmentation of the skin"''〕 or fair, although these usages can be ambiguous in some countries where it is used to refer specifically to certain ethnic groups or populations.〔(【引用サイトリンク】url=http://www.understandingrace.org/lived/global_census.html )〕 It has been hypothesized that dark skin pigmentation was the original condition for the genus ''Homo'', including ''Homo sapiens''. However, as populations migrated away from the tropics between 125,000 and 65,000 years ago into areas of low UV radiation,〔Tim Appenzeller, Nature (Human migrations: Eastern odyssey ) 485, 24–26 doi:10.1038/485024a 2 May 2012〕 they developed light skin pigmentation as an evolutionary selection acting against vitamin D depletion.〔 Based on ancient DNA analysis conducted in 2014 on human skeletal remains from western Europe, this change from dark to light skin pigmentation likely occurred only recently for at least some Europeans. Paleogenomics researcher Carles Lalueza-Fox of the Pompeu Fabra University in Spain and his colleagues observed that a 7,000-year-old hunter-gatherer from the La Braña-Arintero labyrinthine cave in the Cantabrian Mountains possessed the allele for blue eyes but not the European mutations for lighter skin pigmentation. Humans with light skin pigmentation have skin with low amounts of eumelanin, and possess fewer melanosomes than humans with dark skin pigmentation. Light skin provides better absorption qualities of ultraviolet radiation. This helps the body to synthesize higher amounts of vitamin D for bodily processes such as calcium development.〔 Light-skinned people who live near the equator with high sunlight are at an increased risk of folate depletion. As consequence of folate depletion, they are at a higher risk of DNA damage, birth defects, and numerous types of cancers, especially skin cancer.〔 The distribution of indigenous light-skinned populations is highly correlated with the low ultraviolet radiation levels of the regions inhabited by them. Historically, light-skinned indigenous populations almost exclusively lived far from the equator, in high latitude areas with low sunlight intensity; for example, in Northwest Europe. Due to mass migration and increased mobility of people between geographical regions in recent centuries, light-skinned populations today are found all over the world, even in tropical climates.〔 ==Evolution== An abundance of clinical and epidemiological evidence supports that light skin pigmentation developed for the importance of maintaining vitamin D3 production in the skin. Dark skin pigmentation was the original condition for the genus Homo and has been maintained for populations in areas of high UV radiation through human evolutionary history. As a consequence, there must have been a strong selective pressure for the evolution of light skin in areas of low UV radiation.〔 The evidence that dark skin evolved as a protection against the effect of UV radiation is overwhelming, and research shows that eumelanin protects against both folate depletion and direct damage to DNA. This accounts for the development of dark skin pigmentation of people living near the equator but does not account for the increasingly lighter-skinned people living outside the tropics.〔 In the 1960s, biochemist W. Farnsworth Loomis suggested that skin colour is related to the body’s need for vitamin D. The overwhelming positive effect of UV radiation in land-living vertebrates is the ability to synthesize vitamin D3 from it. A certain amount of vitamin D which penetrates the skin helps the body to absorb more calcium which is essential for building and maintaining bones, especially for developing embryos. Vitamin D production depends on exposure to sunlight. People living in latitudes far from the equator developed light skin in order to help absorb more vitamin D. People with light (type II) skin can produce previtamin D3 in their skin at rates 5–10 times faster than dark-skinned (type V) people. People living far from the equator were under evolutionary pressure to develop light skin, which allowed more penetration of UV radiation and helped to produce more of the essential vitamin D.〔 In 1978, NASA launched the Total Ozone Mapping Spectrometer. In 1998, anthropologist Nina Jablonski and her husband George Chaplin collected spectrometer data to measure UV radiation levels around the world, and compared it to published information on the skin colour of indigenous populations of more 50 countries. The results showed a very high correlation between UV radiation and skin colour; the weaker the sunlight was in a geographic region, the lighter the indigenous people’s skin were. Jablonski went on to prove that people living above the latitudes of 50 degrees have the highest chance of developing vitamin D deficiency. "This was one of the last barriers in the history of human settlement," Jablonski states. "Only after humans learned fishing, and therefore had access to food rich in vitamin D, could they settle regions of high latitude." People living far from the equator developed light skin to produce adequate amounts of vitamin D during winter with low levels of UV radiation. Genetic studies suggest that light-skinned humans have been selected for multiple times. Some populations who had diets rich in vitamin D were less affected by the evolutionary selection for light skin. Vitamin D3 is available in low quantities in fish and liver. Populations who lived in coastal areas or areas with access to abundant sources to seafood could get their proportion of vitamin D from food. Some Arctic populations, such as the Inuit, could retain some of their skin pigmentation in areas of low UV radiation. In the winter they receive high levels of UV radiation as reflection from the snow, and their relatively darker skin protects them from the sunlight.〔〔〔 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「Light skin」の詳細全文を読む スポンサード リンク
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