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Human skin color can range from almost black to nearly colorless (appearing pinkish white due to the blood in the skin) in different people. In general, people with ancestors from sunny regions(places closer to the equator) have darker skin than people with ancestors from regions with less sunlight. However, this is complicated by the fact that there are people with ancestors from both sunny and less-sunny regions, and whose skin color may have any shade of the spectrum of possible tones. On average, women have slightly lighter skin than men. On a cultural level, color metaphors for race have evolved based upon genetic variations in human skin color and changing customs or traditions of what arbitrary criteria and the amount of categories to use.
Skin color is determined by the amount and type of the pigment melanin in the skin. Melanin comes in two types: pheomelanin (red to yellow) and eumelanin (dark brown to black). Both amount and type are determined by four to six genes which operate under incomplete dominance. One copy of each of those genes is inherited from the father and one from the mother. Each gene comes in several alleles, resulting in a great variety of different skin colors.
Dark skin protects against those skin cancers that are caused by mutations in skin cells induced by ultraviolet light. Light-skinned persons have about a tenfold greater risk of dying from skin cancer under equal sun conditions. Furthermore, dark skin prevents UV-A radiation from destroying the essential B vitamin folate. Folate is needed for the synthesis of DNA in dividing cells and too low levels of folate in pregnant women are associated with birth defects.
While dark skin protects vitamin B, it can lead to a vitamin D deficiency. The advantage of light skin is that it does not block sunlight as effectively, leading to increased production of vitamin D3, necessary for calcium absorption and bone growth. The lighter skin of women may result from the higher calcium needs of women during pregnancy and lactation.
The evolution of the different skin colors is thought to have occurred as follows: the haired ancestors of humans, like modern great apes, had light skin under their hair. Once the hair was lost, they evolved dark skin, needed to prevent low folate levels since they lived in sun-rich Africa. (The skin cancer connection is probably of secondary importance, since skin cancer usually kills only after the reproductive age and therefore does not exert much evolutionary pressure.) When humans migrated to less sun-intensive regions in the north, low vitamin D3 levels became a problem and light skin color re-emerged.
Dark-skinned people who live in less sun-intensive regions often lack vitamin D3, one reason for the fortification of milk with vitamin D in some countries.
The Inuit and Yupik are special cases: even though they live in an extremely sun-poor environment, they have retained their relatively dark skin. This can be explained by the fact that their traditional animal-based diet provides plenty of vitamin D.
Albinism is a condition characterized by the absence of melanin, resulting in white skin and hair; it is caused by a genetic mutation.
Skin color has sometimes been used in an (often controversial) attempt to define human races.
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