Humans classify themselves according to observed traits such as skin and hair color. Those with dark skin and course, dark hair are of African descent, while those with fair skin and hair are of European descent. Asian and Middle-Eastern populations have skin color that is somewhere in between. Populations with darker skin color are from areas where UV light exposure is greater.
Skin color variation is related to the amount of skin pigment that is present. This pigment is called melanin, and the presence of melanin is a response to several factors, including vitamin needs and protection from UV light exposure.
Skin Pigmentation is an Adaptation
In their article, "The evolution of human skin coloration," published in the July 2000 issue of the Journal of Human Evolution, Nina G. Jablonski and George Chaplin of the Department of Anthropology of the California Academy of Sciences in San Francisco say that the earliest hominids were most likely covered in dark hair and had skin with little to no pigmentation. As activity levels increased, the need for evaporative cooling also increased. Body hair decreased and the number of sweat glands increased. The decreased hair increased the need for skin protection from UV rays in the form of melanin.
According to Jablonski and Chaplin, skin color has changed many times throughout evolutionary history, so "skin coloration is of no value in determining phylogenetic relationships among modern human groups." In fact, a person of one skin color may be more genetically similar to humans of another skin color than to humans of the same color.
Light Skin Pigmentation Allows Increased UV Light Absorption and Vitamin D3 Production
Jablonski and Chaplin explain that "Vitamin D3 is essential for normal growth, calcium absorption and skeletal development. Deficiency of the vitamin can cause death, immobilization, or pelvic deformities which prevent normal childbirth." UV light exposure facilitates the creation of vitamin D3.
Human populations in northern climates, such as Europe, have adapted to lower UV light exposure by producing less melanin in the skin. Low skin melanin levels allow greater penetration of UV rays which contribute to vitamin D3 synthesis.
Maternal calcium needs increase during pregnancy and lactation because the mother must provide enough calcium for fetal and neonatal skeleton development. Females have lighter skin than males, most likely to facilitate greater vitamin D3 production to aid in calcium absorption.
Skin with greater pigmentation must be exposed to more direct UV light for longer periods of time in order to create sufficient amounts of vitamin D3. Skin that contains too much melanin is unable to synthesize enough vitamin D3, especially if the person lives in a northern climate.
Dark Skin Pigmentation Protects Against Folate Photolysis Caused by UV Light
According to Jablonski and Chaplin, pigmentation serves to regulate "the effects of UV radiation in the contents of cutaneous blood vessels located in the dermis." High skin melanin levels contribute to darker skin color which protects again sunburn, skin cancer, and the destruction of nutrients in the skin. Dark pigmentation also prevents damage to sweat glands which are important for thermoregulation.
Folate is necessary for bone marrow maturation and red blood cell creation, as well as normal development of the neural tube in embryos. This important vitamin undergoes photolysis when exposed to UV light, which can contribute to a folate deficiency. Neural tube defects are more prevalent among people with less skin pigmentation. In fact, there is an increased incidence of neural tube defects with tanning bed use, which exposes the skin to direct UV light resulting in folate photolysis.
Folate deficiency is less prevalent among those of African descent. The increased skin pigmentation protects the folate from photolysis in the presence of UV light.
Skin pigmentation is an adaptation in response to UV light exposure. Pigmentation decreases when vitamin D3 production needs increase. Pigmentation increases to protect against UV damage to folate. This is of biological importance because both vitamins are necessary for reproductive success. The challenge for modern humans who migrate quite frequently compared to their ancestors, this adaptation is a very slow response in terms of a lifetime.
Resources
Brooker, Robert J., Widmaier, Eric P., Graham, Linda E., and Stiling, Peter D. Biology. New York: McGraw-Hill, 2008.
Jablonski, N.G. & Chaplin, G. (2000). "The evolution of human skin coloration." Journal of Human Evolution. 39(1):57-106. Accessed June 22, 2010.
Diane Ursu - Diane Ursu joined Suite101 as a contributing writer in August 2009 and became a Feature Writer in January 2010. She is a freelance writer ...
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