Skin Color Adaptation
Human skin color is quite variable around the world. It ranges from a very dark brown among some Africans, Australian Aborigines, and Melanesians to a near yellowish pink among some Northern Europeans. There are no people who actually have true black, white, red, or yellow skin. These are commonly used color terms that do not reflect biological reality.
Some of the variation in human skin coloration (Sub-Saharan African, Indian, Southern European, and Northern European)
Skin color is due primarily to the presence of a pigment called melanin , which is controlled by at least 6 genes. Both light and dark complexioned people have melanin. However, two forms are produced--pheomelanin , which is red to yellow in color, and eumelanin , which is dark brown to black. People with light complexioned skin mostly produce pheomelanin, while those with dark colored skin mostly produce eumelanin. In addition, individuals differ in the number and size of melanin particles. The latter two variables are more important in determining skin color than the percentages of the different kinds of melanin. In lighter skin, color is also affected by red cells in blood flowing close to the skin. To a lesser extent, the color is affected by the presence of fat under the skin and carotene , a reddish-orange pigment in the skin. Hair color is also due to the presence of melanin.
Melanin is normally located in the epidermis , or outer skin layer. It is produced at the base of the epidermis by specialized cells called melanocytes . These cells have photosensitive receptors, similar to those in the eye, that detect ultraviolet radiation from the sun and other sources. In response, they produce melanin within a few hours of exposure.
Cross section of human skin
(colors are not true to life in this illustration)
Nature has selected for people with darker skin in tropical latitudes, especially in nonforested regions, where ultraviolet radiation from the sun is usually the most intense. Melanin acts as a protective biological shield against ultraviolet radiation. By doing this, it helps to prevent sunburn damage that could result in DNA changes and, subsequently, several kinds of malignant skin cancers. Melanoma in particular is a serious threat to life. In the United States, approximately 54,000 people get this aggressive type of skin cancer every year and nearly 8,000 of them die from it. Those at highest risk are European Americans. They have a 10 times higher risk than African Americans.
Ultraviolet radiation reaching the earth usually increases in summer and decreases in winter. The skin's ability to tan in summertime is an acclimatization to this seasonal change. Tanning is primarily an increase in the number and size of melanin granules due to the stimulation of ultraviolet radiation.
Irish boy who
unable to tan
While skin tanning is often most noticeable on light complexioned people, even those with very dark brown skin can tan as a result of prolonged exposure to the sun. Some Northwest Europeans have substantially lost the ability to tan as a result of relaxed natural selection. Their skin burns and peels rather than tans. This is due to the fact that they produce a defective form of a skin protein Mc1r (melanocortin-1 receptor) which is necessary for the production of melanin. They are at a distinct disadvantage in tropical and subtropical environments. Not only do they suffer the discomfort of readily burning, but they are at a much higher risk for skin cancer. The same is true of albinos.
It would be harmful if melanin acted as a complete shield. A certain amount of shortwave ultraviolet radiation (UVB) must penetrate the outer skin layer in order for the body to produce vitamin D. Approximately 90% of this vitamin in people normally is synthesized in their skin and the kidneys from a cholesterol-like precursor chemical with the help of ultraviolet radiation. The remaining 10% comes from foods such as fatty fish and egg yolks. Simple vitamin D is converted by our bodies into two sequential forms. The last form, commonly referred to as vitamin D3, is needed for the intestines to absorb calcium and phosphorus from food for bone growth and repair. Calcium is also necessary in adults to maintain normal heart action, blood clotting, and a stable nervous system. New evidence suggests that vitamin D may help prevent a wide range of cancers, including those of the colon and breasts. Vitamin D plays an additional important role in promoting the production of cathelicidin, which helps to defend our bodies against fungal, bacterial, and viral infections, including the common flu.
Too much ultraviolet radiation penetrating the skin may cause the breakdown of folate in the body, which can cause anemia. Folate is derived from folic acid (one of the B vitamins) in our food. Pregnant women who are deficient in folate are at a higher risk of having miscarriages and babies with neural tube defects. Because folate is needed for DNA replication in dividing cells, its absence can have an effect on many body processes, including the production of sperm cells. It may be that the ability to produce melanin was selected for in our early human ancestors because it helped preserve the body's supply of folate in addition to reducing the chances of developing skin cancer.
People who live in far northern latitudes, where solar radiation is relatively weak most of the year, have an advantage if their skin has little shielding pigmentation. Nature selects for less melanin when ultraviolet radiation is weak. In such an environment, very dark skin is a disadvantage because it can prevent people from producing enough vitamin D, potentially resulting in rickets disease in children and osteoporosis in adults. Contributing to the development of osteoporosis in older people is the fact that their skin generally loses some of its ability to produce vitamin D. Women who had prolonged vitamin D deficiencies as girls have a higher incidence of pelvic deformities that prevent normal delivery of babies.
The Inuit people of the American Subarctic are an exception. They have moderately heavy skin pigmentation despite the far northern latitude at which they live. While this is a disadvantage for vitamin D production, they apparently made up for it by eating fish and sea mammal blubber that are high in vitamin D. In addition, the Inuit have been in the far north for only about 5,000 years. This may not have been enough time for significantly lower melanin production to have been selected for by nature.
In the United States and other developed nations, milk is now usually fortified with vitamins D and A in order to prevent developmental problems such as those described above. However, the popularity of carbonated soft drinks and other alternatives to milk along with a decrease in the amount of time spent outdoors has led to a considerable rise in the rate of rickets disease. Not surprisingly, vitamin D deficiency is most acute in the winter in temperate and colder zones, especially among people of African ancestry. There is a growing epidemic of vitamin D deficiency in the United States. The 2007-2008 "National Government Health and Nutrition Examination Survey" found that only 23% of teens and adults had a sufficient amount of this essential vitamin in their blood. It was 45% a decade earlier. The rate for African Americans in particular was far worse. It has dropped from 12% to only 3% having what doctors consider to be an adequate amount of vitamin D for overall good health. Given this difference, it is not surprising that 15% African American women have pelvis deformities, while the rate among European Americans is only 2%.
There is also a strong correlation between the amount of sunlight that children are exposed to and whether or not they will develop multiple sclerosis as adults. Most cases of this degenerative neural disorder are in the temperate regions of the world where the sunlight is rarely intense. Children growing up in tropical and subtropical regions rarely develop MS regardless of where their ancestors came from. This protection apparently continues for those who move to far northern or far southern regions after 16 years of age. What processes are responsible for this protection from MS and its possible relationship to skin color are unknown.
New research by Nina Jablonski and George Chaplin has led to the discovery that women generally produce 3-4% less melanin in their skin than do men in all populations of the world. They suggest that this is probably due to the fact that women have far higher calcium requirements during their reproductive years. Mate selection preference and other cultural practices may also be partly responsible for this gender difference in skin coloration.
Nina Jablonski breaks the illusion of skin color--explanation of why humans have a wide variation
in skin color around the world. This link takes you to an external website. To return here, you
must click the "back" button on your browser program. (length = 14 mins, 46 secs)
Skin Color Distribution Around the World
Before the mass global migrations of people during the last 500 years, dark skin color was mostly concentrated in the southern hemisphere near the equator and light color progressively increased farther away, as illustrated in the map below. In fact, the majority of dark pigmented people lived within 20º of the equator. Most of the lighter pigmented people lived in the northern hemisphere north of 20º latitude, where ultraviolet radiation is much less intense on average.
(Data for native populations collected by R. Biasutti prior to 1940.)
Such a non-random distribution pattern of human skin color was predicted by Constantin Wilhelm Lambert Gloger, a 19th century German zoologist. In 1833, he observed that heavily pigmented animals are to be found mostly in hot climates where there is intense sunshine. Conversely, those in cold climates closer to the poles commonly have light pigmentation. Presumably, the relative intensity of solar radiation is largely responsible for this distribution pattern.
There are exceptions to Gloger's rule in the animal kingdom. In some species the survival value of having a camouflaged body can be more important than the selective pressures of ultraviolet radiation. This is the case with arctic hares, which are white like snow in the winter and mottled black, brown, and gray similar to the rocky ground in their region after the snow melts in the summer. These color changes make it more difficult for arctic foxes to prey on them. Among humans, mate selection preferences may counter some of the evolutionary trend in skin color predicted by Gloger. Ultraviolet radiation is also blocked to some degree by hair, clothing, smog, fog, smoke, clouds, and trees. The Inuit case described earlier suggests that diet may additionally be a significant factor in some societies.
NEWS: In the February 2007 issue of the Journal of Nutrition, a research team led by Lisa Bodnar of the University of Pittsburgh School of Public Health reported that among 400 first-time pregnant women from that city who participated in their study, only 16.1% of African Americans and 52.9% of European Americans had sufficient amounts of vitamin D in their blood to be considered healthy at the end of their pregnancies. This was despite the fact that more than 90% of them had regularly taken standard doses of prenatal vitamin supplements. Tests of umbilical cord blood indicated that only 7.6% of the African American infants and 33.9% of the European American ones had adequate amounts of vitamin D at birth. The authors concluded that prenatal vitamins should include higher doses of vitamin D, especially in the northern regions of the U.S.
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