An End to the Skin Color Controversy?

A. Ten years ago, while at the University of Western Australia, anthropologist Nina Jablonski was asked to give a lecture on human skin. As an expert on primate evolution, she decided to discuss the evolution of skin color, but when she went through the literature on the subject she was dismayed. Some theories advanced before the 1970s 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 (Eskimos) are both dark and particularly resistant to cold.

Jablonski and her husband set about formulating the first comprehensive theory of skin color. Their findings, published in a recent issue of the Journal of Human Evolution, show a strong and 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, they say, is largely a matter of vitamins.

Jablonski begins by assuming that our earliest ancestors had fair skin just like chimpanzees, our closest biological relatives. Between 4.5 million and 2 million years ago, early humans moved from the rain forest and onto the East African plains. There, they not only had to cope with more exposure to the sun, but they also had to work harder to gather food. Mammalian brains are particularly vulnerable to overheating: A change of only five or six degrees can cause heatstroke. So our ancestors had to develop a better cooling system.

B. The answer was sweat, 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 soles of their feet. Occasionally, however, individuals were born with more glands than usual, and more patches of hairless skin which allows the sweat to exude. 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 of natural selection later, each human has about two million sweat glands spread across his or her body. Human skin, being less hairy than chimpanzee skin, “dries much quicker,” says Adrienne Zihiman, an anthropologist at the University of California at Santa Cruz. “Just think how after a bath it takes much longer for wet hair to dry.”

Hairless skin, however, is particularly vulnerable to damage from sunlight. 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.

C. Jablonski found a 1978 study that examined the effects of ultraviolet light on folate, a member of the 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 later came across three documented cases in which children’ s neural-tube defects were linked to their mothers’ visits to tanning studios during early pregnancy. Moreover, she found that folate is crucial to sperm development—so much so that a folate inhibitor was developed as a male contraceptive. (“It never got anywhere,” Jablonski says. “It was so effective that it knocked out all folate in the body.” ) She now had some intriguing evidence that folate might be the driving force behind the evolution of darker skin. But why do
some people have light skin?

As far back as the 1960s, the biochemist W. Farnsworth Loomis had suggested that skin color is determined by the body’s need for vitamin D. This vitamin helps the body absorb calcium and deposit it in bones, an essential function, particularly in fast-growing embryos. Unlike folate, vitamin D depends on 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 tropic; evolved dark skin to block the light, keeping the body from overdosing on vitamin D, which can be toxic at high concentrations. Loomis’s insight about fair skin complemented Jablonski’s insight about folate and dark skin perfectly. The next step was to find some hard data correlating skin color with light levels.

Until the 1980s, researchers could only estimate how much ultraviolet radiation reaches the Earth’s surface. But in 1978, NASA launched the Total Ozone Mapping Spectrometer. Jablonski took the spectrometer’s global ultraviolet measurements and compared them with published data on skin color in indigenous populations from more than 50 countries. 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. “This was one of the last barriers in the history of human settlement,” Jablonski says. “Only after humans learned fishing, and therefore had access to food rich in vitamin D, could they settle these regions.’

D. Humans have pent most of their history moving around. 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 further. 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 produce adequate amounts of vitamin D during the long winter months.

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. “We can take a topic that has caused so much disagreement, so r much suffering; and so much misunderstanding,” she says, “and completely disarm it.”

Questions 1—5
The reading passage has four sections (A—D). In boxes 1—5 on your answer sheet write the appropriate letter A, B, C or D to show in which section you can find a discussion of the following points. You may use any letter more than once.

1. The effect of ultraviolet light on skin color.
2. The role of folate levels in ensuring healthy births.
3. Early theories of skin color were flawed.
4. Fish and the spread of human settlement.
5. How hairless skin is a mechanism of the body’s cooling system.

Answer: 1-C, 2-C, 3-A, 4-C, 5-B

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