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The Biology of Skin Color: Black and White The evolution of race was as simple as the politics of race is complex By Gina Kirchweger Ten years ago, while at the university of Western Australia, anthropologist Nina Jablonski was asked to give a lecture on white skin bath skin. As an expert in primate evolution, she decided to discuss whitf evolution of skin color, but when she went through the literature on the subject she was dismayed. Some theories advanced before the s tended to be racist, and others were less than convincing.
White skin, for example, was reported to be more resistant to cold weather, although groups like the Inuit wjite both dark and particularly resistant to cold. After the s, when researchers were presumably more aware of the controversy such studies could kick up, there was wite little work at all. Jablonski and her husband, George Chaplin, a geographic information systems specialist, have formulated the first comprehensive theory of skin color.
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Their findings, published in a recent issue of the Journal of Human Evolution, show a strong, somewhat predictable correlation between skin color and the strength of sunlight across the globe. But they also show a deeper, more surprising process at work: Skin color, qhite say, is wgite a matter of vitamins. Jablonski, now chairman of the anthropology department at the California Academy of Sciences, begins by assuming that wjite earliest ancestors had fair skin just like chimpanzees, our closest biological relatives.
Between 4. Once on the savanna, they not only had to cope with more exposure to the sun, but they Black lady wants naughty teens had to work harder to gather food. Mammalian brains are particularly vulnerable to overheating: A change of only five or six degrees can cause a heatstroke.
So our ancestors had to develop a better cooling system. The answer was slin, which dissipates heat through evaporation. Early humans probably had few sweat glands, like chimpanzees, and those were mainly located on the palms of their hands and the bottoms of their feet. Occasionally, however, individuals were born with more glands than usual. The more they could sweat, the longer they could forage before the heat forced them back into the shade.
The more they could forage, the better their chances of having healthy offspring and of passing on their sweat glands to future generations. A million years New to boone lets meet natural selection later, each human has about 2 million sweat glands spread across his or her body. Human skin, being less hairy than chimpanzee skin, "dries much quicker," says Adrienne Zihlman, an anthropologist at the University of California at Santa Cruz.
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Scientists long assumed that humans evolved melanin, the main determinant of skin color, to absorb or disperse ultraviolet light. But what is it about ultraviolet light that melanin protects against? Some researchers pointed to the threat of skin cancer. But cancer usually develops late in life, after a person has already reproduced. Others suggested that sunburned nipples would have hampered breast-feeding.
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But a slight tan is enough to protect mothers against that problem. During her preparation for the lecture in Australia, Jablonski found a study that examined the effects of ultraviolet light on folate, a member of smin vitamin B complex.
An hour of intense sunlight, the study showed, is enough to cut folate levels in half if your skin is light. Jablonski made the next, crucial connection only a few weeks later. At a seminar on embryonic development, she heard that low folate levels are correlated with neural-tube defects such as spina bifida and anencephaly, in which infants are born without a full brain or spinal cord.
Jablonski and Chaplin predicted the skin colors of indigenous people across the globe based on how much ultraviolet light different areas receive. Graphic by Matt Zang, adapted from the data of N. Jablonski and G.
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Chaplin Jablonski later came across three documented cases in which children's neural-tube defects were linked to their mothers' visits to tanning studios sin early pregnancy. Moreover, she Free sex super mom that folate is crucial to sperm development -- so much so that a folate inhibitor was developed as a male contraceptive.
But why do some people have light skin? As far back as the wuite, the biochemist W. Farnsworth Loomis had suggested that skin color is determined by the body's need for vitamin D.
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The vitamin helps the body absorb calcium and deposit it in bones, an essential function, particularly in fast-growing embryos. The need for vitamin D during pregnancy may explain why women around the globe tend to have lighter skin than men. Unlike folate, vitamin D depends Find local sex Carlsbad California ultraviolet light for its production in the body.
Loomis believed that people who live in the north, where daylight is weakest, evolved fair skin to help absorb more ultraviolet light and that people in the tropics evolved btah skin to block the light, keeping the body from overdosing on vitamin D, which can be toxic at dkin concentrations. By the time Jablonski did her research, Loomis's hypothesis had been partially disproved. The next step was to find some hard data correlating skin hwite to light levels.
Until the s, researchers could only estimate how much ultraviolet radiation reaches Earth's surface. Three years ago, Jablonski and Chaplin took the spectrometer's global ultraviolet measurements and compared them with published data on skin color in indigenous populations from more than 50 countries. To their delight, there was an unmistakable correlation: The weaker the ultraviolet light, the fairer the skin.
Jablonski went on to show that people living above 50 degrees latitude have the highest risk of vitamin D deficiency.
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To do that, they've had to adapt their tools, clothes, housing, and eating habits to each new climate and landscape. But Jablonski's work indicates that our adaptations go much dkin. People in the tropics have developed dark skin to block out the sun and protect their body's folate reserves.
People far from the equator have developed fair skin to drink in the sun and Melrose MA housewives personals adequate amounts of vitamin D during the long winter bathh. Jablonski hopes that her research will alert people to the importance of vitamin D and folate in their diet.
It's already known, for example, that dark-skinned people who move to cloudy climes can develop conditions such as rickets from vitamin D deficiencies.
More important, Jablonski hopes her work will begin to change the way people think about skin color. Reprinted with permission of Discover.