Science Fair Project Encyclopedia
Human skin color
Human skin colour 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 colour 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 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 colours.
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.
While dark skin protects vitamin B, it can lead to a vitamin D deficiency. 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 from the higher calcium needs of women during pregnancy and lactation.
The evolution of the different skin colours 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 colour 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.
Research on skin colour variability
The colour of human skin varies from dark brown to pale pink. In attempting to discover the mechanisms that have generated such a wide variation in human skin colour, Nina Jablonski and George Chaplin (2000) discovered that there is a high correlation between the coloration of the human skin of indigenous peoples and the average annual ultraviolet (UV) radiation available for skin exposure where the indigenous peoples live. Accordingly, Jablonski and Chaplin plotted the whiteness (W) of skin coloration of indigenous peoples who have stayed in the same geographical area for the last 500 years versus the annual UV available for skin exposure (AUV) for over 200 indigenous persons and found that whiteness W of skin coloration is related to to the annual UV available for skin exposure AUV according to
(Jablonski and Chaplin (2000), p. 67, formula coefficients have been rounded to one-figure accuracy) where the whiteness W of skin coloration is measured as the percentage of light reflected from the upper inner arm at which location on humans there should be minimal tanning of human skin due to personal exposure to the sun; a lighter skinned human would reflect more light and would have a higher W number. Judging from the above linear fit to the empirical data, the theoretical maximum whitest human skin would reflect only 70 per cent of incident light for a hypothetical indigenous human-like population that lived where there was zero annual UV available for skin exposure (AUV = 0 in the above formula). Jablonski and Chaplin evaluated average annual UV available for skin exposure AUV from satellite measurements that took into consideration the measured daily variation in the thickness of the ozone layer that blocked UV hitting the earth, measured daily variation in opacity of cloud cover, and daily change in angle at which the sunlight containing UV radiation strikes the earth and passes through different thicknesses of earth's atmosphere at different latitudes for each of the different human indigenous peoples' home areas from 1979 to 1992.
Jablonski and Chaplin proposed an explanation for the observed variation of untanned human skin with annual UV exposure. By Jablonski and Chaplin's explanation, there are two competing forces affecting human skin color:
- the melanin that produces the darker tones of human skin serves as a light filter to protect against too much UV light getting under the human skin where too much UV causes sunburn and disrupts the synthesis of precursors necessary to make human DNA; versus
- humans need at least a minimum threshold of UV light to get deep under human skin to produce vitamin D, which is essential for building and maintaining the bones of the human skeleton.
Jablonski and Chaplin note that when human indigenous peoples have migrated, they have carried with them a sufficient human gene pool so that within a thousand years, the skin of their descendants living today has turned dark or turned white to adapt to fit the formula given above--with the notable exception of dark-skinned peoples moving north, such as to populate the seacoast of Greenland, to live where they have a year-round supply of food, such as fish, rich in vitamin D, so that there was no necessity for their skin to turn white to let enough UV under their skin to synthesize the vitamin D that humans need for healthy bones.
In considering the color of human skin in the long span of human evolution, Jablonski and Chaplin note that there is no empirical evidence to suggest that the human ancestors six million years ago had a skin color different from the skin color of today's chimpanzees—namely pale-skinned under black hair. But as humans evolved to lose their body hair a parallel evolution permitted human populations to turn their base skin colour dark or white over a period of less than a thousand years to adjust to the competing demands of 1) increasing eumelanin to protect from UV that was too intense and 2) reducing eumelanin so that enough UV would penetrate to synthesize enough vitamin D. By this explanation, in the time that humans lived only in Africa, humans had dark skin to the extent that they lived for extended periods of time where the sunlight is intense. As some humans migrated north, over time they developed white skin, though they retained within the gene pool the capability to develop black skin when they migrated to areas with intense sunlight again, such as across the Bering Strait and south to the Equator. 
Ashley Robins (1991) has argued that the white skin of indigenous peoples living north of 40° north latitude need not arise necessarily from the natural selection pressures on those humans to synthesize enough vitamin D. Robins argues that people with dark skin can get enough vitamin D if they spend enough time in the spring and summer sun, and he hypothesizes that the fatty tissues of humans would store enough vitamin D to maintain human health during the northern-most winters. 
- Jablonski, Nina G., and George Chaplin. 2000. "The evolution of human skin coloration." Journal of Human Evolution 39: 57-106. (in pdf format)
- Jablonski, Nina G., and George Chaplin. 2002. "Skin Deep." Scientific American 287 (4) (October): 74-82.
- Robins, A.H. 1991. Biological Perspectives on Human Pigmentation. Cambridge: Cambridge University Press. 
- Nicholas Wade. 2003. "Why Humans and Their Fur Parted Ways." New York Times (Science Times) (August 19). Summary of clues to the saga in which humans evolved to lose their hair and had to adjust, including turning from white skin to black skin, together with an estimation of the time at which humans invented clothing.
The contents of this article is licensed from www.wikipedia.org under the GNU Free Documentation License. Click here to see the transparent copy and copyright details