Why Does Skin Color Vary?
People vary in the color of their skin because of a network of pigment-forming cells called melancoytes. This network is interspersed between, and lies underneath, the cells of the deepest layer of the epidermis, or outer skin, which is called the stratum basale.
The melanocytes have slender, branchlike extensions which touch one another and also extend upwards between the cells of the deeper portions of the epidermis. There are about 1,000 to 3,000 melanocytes in each square millimeter of skin, and each one produces the dark pigment melanin formed as a result of oxidation.
This oxidation is catalyzed by a copper-containing enzyme called tyrosinase, which gives the reddish spectrum of color changes. Various stages of formation produce pale yellow, tawny, orange, reddish, brown and, finally, intense black. Human skin contains greater or lesser amounts of melanin. In fair-skinned races the deep skin layer of melanocytes contains very little pigment. In the darker races, the deposits are heavy, and other melanocytes are to be found in the upper layers of the epidermis.
Melanin is a natural protection from harmful sun-rays and, on exposure to sunlight, man’s skin normally undergoes gradual tanning. This increase of melanin pigment helps to safeguard underlying tissues. In blondes and redheads the pigment cells respond only slightly and rather unevenly. The consequence of this may be a “freckling” effect rather than a sun-tanned look.
Variations in human skin color are adaptive traits that correlate closely with geography and the sun’s ultraviolet (UV) radiation. As early humans moved into hot, open environments in search of food and water, one big challenge was keeping cool. The adaptation that was favored involved an increase in the number of sweat glands on the skin while at the same time reducing the amount of body hair.
With less hair, perspiration could evaporate more easily and cool the body more efficiently. But this less-hairy skin was a problem because it was exposed to a very strong sun, especially in lands near the equator. Since strong sun exposure damages the body, the solution was to evolve skin that was permanently dark so as to protect against the sun’s more damaging rays.
Melanin, the skin’s brown pigment, is a natural sunscreen that protects tropical peoples from the many harmful effects of ultraviolet (UV) rays. UV rays can, for example, strip away folic acid, a nutrient essential to the development of healthy fetuses.
Yet when a certain amount of UV rays penetrates the skin, it helps the human body use vitamin D to absorb the calcium necessary for strong bones. This delicate balancing act explains why the peoples that migrated to colder geographic zones with less sunlight developed lighter skin color. As people moved to areas farther from the equator with lower UV levels, natural selection favored lighter skin which allowed UV rays to penetrate and produce essential vitamin D.
The darker skin of peoples who lived closer to the equator was important in preventing folate deficiency. Measures of skin reflectance, a way to quantify skin color by measuring the amount of light it reflects, in people around the world support this idea. While UV rays can cause skin cancer, because skin cancer usually affects people after they have had children, it likely had little effect on the evolution of skin color because evolution favors changes that improve reproductive success.
There is also a third factor which affects skin color: coastal peoples who eat diets rich in seafood enjoy this alternate source of vitamin D. That means that some Arctic peoples, such as native peoples of Alaska and Canada, can afford to remain dark-skinned even in low UV areas. In the summer they get high levels of UV rays reflected from the surface of snow and ice, and their dark skin protects them from this reflected light.