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Published: Sunday, 1/19/2003

Skin color evolved for protection

In that famous “I Have a Dream” speech, Martin Luther King, Jr., longed for a time when people “will not be judged by the color of their skin but by the content of their character.”

Among the animal group that includes apes and humans, only people have a mainly naked skin found in a near-rainbow of colors. Ruddy reds, pinks, rich browns, olives, golden yellows, and an almost infinite palate of tints, tones, and shades in black and white.

Why are some people light-skinned and some dark? What's the biology behind skin color?

New scientific evidence is challenging the textbook answer.

Skin color results mainly from melanin, a dark pigment manufactured in the skin's outermost layer. Almost everyone knows that dark-complexioned people make more melanin than light-complexioned people.

But there are other ingredients in the recipe for skin color.

Melanin comes in two forms, for instance. Phenomelanin is red-to-yellow and eumelanin is dark-brown-to-black. People with light skin make mainly phenomelanin, while those with dark complexions make mostly eumelanin.

Carotene, a reddish-orange pigment, also can affect skin color. In people with light skin, blood flowing through deeper layers of the skin lends a pinkish tint.

Scientists agree that “hominids,” our ancient ancestors, got pigmented skin after losing the thick body hair found on apes and chimpanzees, and developing more sweat glands. It happened about 4.5 million years ago, as humans-to-be emerged from the dark rain forests of Africa and began living in open areas.

Getting sweat glands and losing body hair were critical in that giant step toward becoming human.

It protected hominids from overheating while living in the fierce heat of open grasslands. People are very sensitive to temperature. An increase of just a few degrees can mean fatal heat stroke.

Sweat glands helped because sweat cools the body via evaporation of water from the skin. And water evaporates faster from bare skin. That's why skin dries faster than hair after a shower.

Scientists once thought that hominids developed pigmented skin to protect against a danger associated with their new lifestyle in wide open spaces - exposure to ultraviolet rays in sunlight that cause skin cancer.

Now they suspect that dark skin evolved mainly to protect people from loss of folic acid, a B-vitamin essential for normal reproduction.

Studies have shown that exposure to intense sunlight destroys large amounts of folic acid in light-skinned people.

Women need enough folic acid in their bodies at the time of pregnancy or risk bearing children with serious birth defects. Folic acid also is critical for sperm development in men.

Hominids with darker skin had an evolutionary edge. Their higher folic acid levels meant a better chance of reproducing and passing their genes for darker skin on to offspring.

As eons passed, geography determined skin color.

People living in intense sunlight needed the most protection against folic acid destruction, and developed darker skin.

Those living farther away managed with lighter skin.

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