Why descendants of "Black African Eve" look so different? (Evolution biology)

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• published: 13 Nov 2012
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Human skin color is primarily due to the presence of melanin in the skin. Skin color ranges from almost black to white with a pinkish tinge due to blood vessels underneath. Variation in natural skin color is mainly due to genetics, although the evolutionary causes are not completely certain. According to scientific studies, natural human skin color diversity within populations is highest in Sub-Saharan African populations,[2] with skin reflectance values ranging from 19 to 46 (med. 31) compared with European and East Asian populations which have skin reflectance values of 62 to 69 and 50 to 59 respectively. The term "range" is loosely defined in this case, as African albinos have obviously not been taken into consideration when calculating the "range". The natural skin color can be darkened as a result of tanning due to exposure to sunlight. The leading theory is that skin color adapts to intense sunlight irradiation to provide partial protection against the ultraviolet fraction which produces damage and thus mutations in the DNA of the skin cells. Other factors that can modify skin color include protection from ambient temperature, infections, skin cancer or frostbite, an alteration in food, and sexual selection. The social significance of differences in skin color has varied across cultures and over time, as demonstrated with regard to social status and racism. About 70,000--100,000 years ago some modern humans began to migrate away from the tropics to the north where they were exposed to less intense sunlight, possibly in part due to the need for greater use of clothing to protect against the colder climate. Under these conditions there was less photodestruction of folate and so the evolutionary pressure stopping lighter-skinned gene variants from surviving was reduced. In addition, lighter skin is able to generate more vitamin D (cholecalciferol) than darker skin so it would have represented a health benefit in reduced sunlight if there were limited sources of vitamin D. Hence the leading hypothesis for the evolution of human skin color proposes that: From ~1.2 million years ago to less than 100,000 years ago, the ancestors of all people alive were dark-skinned Africans. As populations began to migrate, the evolutionary constraint keeping skin dark decreased proportionally to the distance North a population migrated, resulting in a range of skin tones within northern populations. At some point, northern populations experienced positive selection for lighter skin due to the increased production of vitamin D from sunlight and the genes for darker skin disappeared from these populations. The genetic mutations leading to light skin, though different among East Asians and Europeans, suggest the two groups experienced a similar selective pressure due to settlement in northern latitudes.