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TEN YEARS AGO, WHILE AT THE UNIVERSITY

of Western Australia, anthropologistNina Jablonski was asked to give a lec-ture on human skin. As an expert in pri-mate evolution, she decided to discussthe evolution of skin color, but whenshe went through the literature on thesubject she was dismayed. Some theo-ries advanced before the 1970s tendedto be racist, and others were less thanconvincing. White skin, for example,was reported to be more resistant to coldweather, although groups like the Inuitare both dark and particularly resi tantto cold.After the 1970s,when researcherswere presumably more aware of the con-troversy such studies could kick up, therewas very little work at all. "It's one ofthese things everybody notices;' Iablon-:ski says,"but nobody wants to talk about."

o longer.Jablonski and her husband,George Chaplin, a geographic informa-tion systems specialist, have formulatedthe first comprehensive theory of skincolor. Their findings, published in arecent issue of the Journal of HumanEvolution, show a strong, somewhat pre-dictable correlation between skin colorand the strength of sunlight across theglobe. But they also show a deeper, moresurprising process at work: Skin color.]they say, is largely a matter of vitamins.

Jablonski, now chairman of the an-thropology department at the Califor-nia Academy of Sciences, begins byassuming that our earliest ancestorshad fair skin just like chimpanzees, ourclosest biological relatives. Between 4.5million and 2 million years ago, early

____ humans moved from the rain forest

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DISCOVER FEBR UARY 200 I

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and onto the EastAfrican savanna. Onceon the savanna, they not only had tocope with more exposure to the sun,but they also had to work harder togather food. Mammalian brains areparticularly vulnerable to overheating:A change of only fiveor six degrees cancause a heatstroke. So our ancestorshad to develop a better cooling system.

The answer was sweat, which dissi-pates heat through evaporation. Earlyhumans probably had few sweat glands,like chimpanzees, and those were mainlylocated on the palms of their hands andthe bottoms of their feet. Occasionally,however, individuals were born withmore glands than usual. The more theycould sweat, the longer they could for-

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age before the heat forced them backinto the shade. The more they couldforage, the better their chances of hav-ing healthy offspring and of passing ontheir sweat glands to future generations.

A million years of natural selectionlater, each human has about 2 millionsweat glands spread across his or herbody. Human skin, being lesshairy thanchimpanzee skin, "dries much quicker;'says Adrienne Zihlman, an anthropol-ogist at the University of California atSanta Cruz. "Just think how after a bathit takes much longer for wet hair to dry:'

Hairless skin, however, is particularlyvulnerable to damage from sunlight, Sci-entist long assumed that humansevolved melanin, the main determinant

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of skin color, to absorb or disperse ul-traviolet light. But what is it aboutultraviolet light that melanin protectsagainst? Some researchers pointed to thethreat of skin cancer. But cancer usuallydevelops late in life, after a person hasalready reproduced. Others suggestedthat "SUnburnednipples would have ham-pered breast-feeding. But a slight tan isenough to protect mothers against thatproblem.

During her preparation for the lec-ture in Australia, Jablonski found a 1978study that examined the effects of ul-traviolet light on folate, a member ofthe vitamin B complex. An hour of in-tense sunlight, the study showed, isenough to cut folate levels in half if yourskin is light. Jablonski made the next,crucial connection only a few weekslater. At a seminar on embryonic de-velopment, she heard that low folatelevels are correlated with neural-tubedefects such as spina bifida and anen-cephaly, in which infants are born with-out a full brain or spinal cord.

Jablonski later came across three doc-umented cases in which children'sneural-tube defects were linked to theirmothers' visits to tanning studios dur-ing early pregnancy. Moreover, shefound that folate is crucial to sperm de-velopment-so much so that a folateinhibitor was developed as a male con-traceptive. ("It never got anywhere,"Jablonski says. "It was so effective thatitknocked out all folate in the body.")She now had some intriguing evidencethat folate might be the driving forcebehind the evolution of darker skin.But why do some people have light skin?

As far back as the 1960s, the bio-chemistW. Farnsworth Loomis had sug-gested that skin color is determined bythe body's need for vitamin D. The vi-tamin helps the body absorb calciumand deposit it in bones, an essentialfunction, particularly in fast-growingembryos. (The need for vitamin D dur-ing pregnancy may explain why womenaround the globe tend to have lighterskin than men.) Unlike folate, vitamin D

depends on ultraviolet light for its pro-duction in the body. Loomis believedthat people who live in the north, wheredaylight is weakest, evolved fair skin tohelp absorb more ultraviolet light andthat people in the tropics evolved darkskin to block the light, keeping the bodyfrom overdosing on vitamin D, whichcan be toxic at high concentrations.

Bythe time Jablonski did her research,Loomis's hypothesis had been partiallydisproved. "You can never overdose onnatural amounts of vitamin D,"Jablon-ski says."There are only rare caseswherepeople take too many cod-liver sup-plements." But Loomis's insight aboutfair skin held up, and it made a perfectcomplement for Jablonski's insightabout folate and dark skin. The nextstep was to find some hard data corre-lating skin color to light levels.

Until the 198Os,researchers could onlyestimate how much ultraviolet radiationreachesEarth'ssurface,But in 1978,NASAlaunched the Total Ozone MappingSpectrometer. Three years ago, Jablon-ski and Chaplin took the spectrometer'sglobal ultraviolet measurements andcompared them with published data onskin color in indigenous populations frommore than 50 countries. To their delight,there was an unmistakable correlation:The weaker the ultraviolet light, the fairerthe skin. Jablonski went on to show thatpeople living above 50 degrees latitude

••••••••Abaut 2,500 infants with neural-tulle defects are barn inthe United States eath year. Half those tases prabol,result fram mothers not getting enough folate. The Foadand Drug Administration rel:DMlIlends WOMenof child-hearing age take 400 mitragra.s of f.lic atid e•• ry d.ay.

Jablonski and Chaplin predicted theskin colors of indigenous people acrossthe globe based on how much ultraviolet

liiht different areas receive.

have the highest risk of vitamin D defi-ciency. "This was one of the last barri-ers in the history of human settlement,"Jablonski says. "Only after humanslearned fishing, and therefore had ac-cess to food rich in vitamin D. couldthey settle these regions."

Humans have spent most of their his-tory moving around. To do that, they'vehad to adapt their tools, clothes, hous-ing, and eating habits to each new cli-mate and landscape. But Jablonski'swork indicates that our adaptations gomuch further. People in the tropics havedeveloped dark skin to block out thesun and protect their body's folate re-serves. People far from the equator havedeveloped fair skin to drink in the sunand produce adequate amounts of vi-tamin D during the long winter months.

Jablonski hopes that her researchwill alert people to the importance ofvitamin D and folate in their diet. It'salready known, for example, that dark-skinned people who move to cloudyclimes can develop conditions such asrickets from vitamin D deficiencies.More important, Jablonski hopes herwork will begin to change the way peo-ple think about skin color. "We cantake a topic that has caused so muchdisagreement, so much suffering, andso much misunderstanding," she says,"and completely disarm it." Iiil

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