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Human skin color can range from almost black to pinkish white in different people. In general, people with ancestors from sunny regions have
darker skin than people with ancestors from regions with less sunlight. (However, this is complicated by the fact that there are people whose ancestors come from both
sunny and less-sunny regions; and these people may have skin colors across the spectrum.) On average, women have slightly lighter
skin than men.
Skin color is determined by the amount and type of the pigment melanin in the
skin. Melanin comes in two types: phaeomelanin (red to yellow) and eumelanin (dark brown to black). Both amount and type are
determined by 4-6 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 skin cancer, 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.
The advantage of light skin is that it lets more sunlight through, which leads to increased production of vitamin D3, necessary for calcium
absorption and bone growth. The lighter skin of women results either from sexual preference
or from the higher calcium needs of women during pregnancy and lactation (also
possibly from both).
The evolution of the different skin colors is thought to have occurred as
follows: the haired ancestor 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 doesn't exert much evolutionary pressure.) When humans migrated to sun-poorer regions in the
north, low vitamin D3 levels became a problem and light skin color evolved.
Dark-skinned people who live in sun-poor regions often lack vitamin D3, one reason for the fortification of milk
with vitamin D in some countries.
The Inuit are a special case: 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; see also racism.
See also:
Human physical appearance, Sunshine, Tanning, History, Tom
Irwin.
References:
- Nina G. Jablonski, George Chaplin, "Skin Deep", Scientific American, Vol 287 No 4, October 2002
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