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 humanskin. As an expert in primate evolution, she decided to discuss theevolution of skin color, but when she went through the literature onthe subject she was dismayed. Some theories advanced before the 1970stended to be racist, and others were less than convincing. White skin,for example, was reported to be more resistant to cold weather, althoughgroups like the Inuit are both dark and particularly resistant to cold.After the 1970s, when researchers were presumably more aware of thecontroversy such studies could kick up, there was very little work atall. "It's one of these things everybody notices," Jablonski says, "butnobody wants to talk about."
No longer. Jablonski and her husband, George Chaplin, a geographicinformation systems specialist, have formulated the first comprehensivetheory of skin color. Their findings, published in a recent issue of the Journal of Human Evolution, show a strong, somewhat predictablecorrelation between skin color and the strength of sunlight across theglobe. But they also show a deeper, more surprising process at work:Skin color, they say, is largely a matter of vitamins.
Jablonski, now chairman of the anthropology department at theCalifornia Academy of Sciences, begins by assuming that our earliestancestors had fair skin just like chimpanzees, our closest biologicalrelatives. Between 4.5 million and 2 million years ago, early humansmoved from the rain forest and onto the East African savanna. Once onthe savanna, they not only had to cope with more exposure to the sun,but they also had to work harder to gather food. Mammalian brains areparticularly vulnerable to overheating: A change of only five or sixdegrees can cause a heatstroke. So our ancestors had to develop abetter cooling system.
The answer was sweat, which dissipates heat through evaporation.Early humans probably had few sweat glands, like chimpanzees, and thosewere mainly located on the palms of their hands and the bottoms of theirfeet. Occasionally, however, individuals were born with more glands thanusual. The more they could sweat, the longer they could forage before theheat forced them back into the shade. The more they could forage, thebetter their chances of having healthy offspring and of passing on theirsweat glands to future generations.
A million years of natural selection later, each human has about2 million sweat glands spread across his or her body. Human skin, beingless hairy than chimpanzee skin, "dries much quicker," says AdrienneZihlman, 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 fromsunlight. Scientists long assumed that humans evolved melanin, the maindeterminant of skin color, to absorb or disperse ultraviolet light. Butwhat is it about ultraviolet light that melanin protects against? Someresearchers pointed to the threat of skin cancer. But cancer usuallydevelops late in life, after a person has already reproduced. Otherssuggested that sunburned nipples would have hampered breast-feeding.But a slight tan is enough to protect mothers against that problem.
During her preparation for the lecture in Australia, Jablonskifound a 1978 study that examined the effects of ultraviolet light onfolate, 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 skinis light. Jablonski made the next, crucial connection only a few weekslater. At a seminar on embryonic development, she heard that low folatelevels are correlated with neural-tube defects such as spina bifida andanencephaly, in which infants are born without a full brain or spinalcord.
Jablonski and Chaplin predicted the skin colors of indigenouspeople across the globe based on how much ultraviolet light differentareas receive. Graphic by Matt Zang, adapted from the data of N.Jablonski and G. Chaplin
Jablonski later came across three documented cases in whichchildren's neural-tube defects were linked to their mothers' visitsto tanning studios during early pregnancy. Moreover, she found thatfolate is crucial to sperm development -- so much so that a folateinhibitor was developed as a male contraceptive. ("It never gotanywhere," Jablonski says. "It was so effective that it knocked outall folate in the body.") She now had some intriguing evidence thatfolate 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 Loomishad suggested that skin color is determined by the body's need forvitamin D. The vitamin helps the body absorb calcium and deposit itin bones, an essential function, particularly in fast-growing embryos.(The need for vitamin D during pregnancy may explain why women aroundthe globe tend to have lighter skin than men.) Unlike folate, vitamin Ddepends on ultraviolet light for its production in the body. Loomisbelieved that people who live in the north, where daylight is weakest,evolved fair skin to help absorb more ultraviolet light and that peoplein the tropics evolved dark skin to block the light, keeping the bodyfrom overdosing on vitamin D, which can be toxic at high concentrations.
By the time Jablonski did her research, Loomis's hypothesishad been partially disproved. "You can never overdose on naturalamounts of vitamin D," Jablonski says. "There are only rare caseswhere people take too many cod-liver supplements." But Loomis'sinsight about fair skin held up, and it made a perfect complementfor Jablonski's insight about folate and dark skin. The next stepwas to find some hard data correlating skin color to light levels.
Until the 1980s, researchers could only estimate how muchultraviolet radiation reaches Earth's surface. But in 1978, NASAlaunched the Total Ozone Mapping Spectrometer. Three years ago,Jablonski and Chaplin took the spectrometer's global ultravioletmeasurements and compared them with published data on skin color inindigenous populations from more than 50 countries. To their delight,there was an unmistakable correlation: The weaker the ultravioletlight, the fairer the skin. Jablonski went on to show that peopleliving above 50 degrees latitude have the highest risk of vitamin Ddeficiency. "This was one of the last barriers in the history of humansettlement," Jablonski says. "Only after humans learned fishing, andtherefore had access to food rich in vitamin D, could they settlethese regions."
Humans have spent most of their history moving around. To dothat, they've had to adapt their tools, clothes, housing, andeating habits to each new climate and landscape. But Jablonski'swork indicates that our adaptations go much further. People in thetropics have developed dark skin to block out the sun and protecttheir body's folate reserves. People far from the equator havedeveloped fair skin to drink in the sun and produce adequate amountsof vitamin D during the long winter months.
Jablonski hopes that her research will alert people to theimportance of vitamin D and folate in their diet. It's alreadyknown, for example, that dark-skinned people who move to cloudyclimes can develop conditions such as rickets from vitamin Ddeficiencies. More important, Jablonski hopes her work will beginto change the way people think about skin color. "We can take atopic that has caused so much disagreement, so much suffering,and so much misunderstanding," she says, "and completely disarm it."
(From Discover, Vol. 22, No. 2, February, 2001. Gina Kirchweger © 2001. Reprinted with permission of Discover. )
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