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(1)ffi ffiry. * si. l.s:+ tr W 4f'_ ..m#:r. 4. ".ir,a. si*- fu* fu. #"''f* ;* -,. ifteen yearsago, while at the 'W'estern Australia, Universiry of anthropologistNina Jablonski was askedto give a lecture on human skin. As an expert in primate evolution. she decided to discussthe evolution of skin color, but when shewent through the literature on the subject she was dismayed.Some theoriesadvanced beforethe 1970swere racist,and others were lessthan convincing. \Mhite skin, for example,was reported to be more resistant to cold weather.After the 1970s.when. researchers were probably more awareof the controversysuch studiescould kick up, there was very little work at all. "Itt one of thesethings everybodynotices,"Jablonski says,"but nobody wants to talk about." No longer.Jablonskiand her husband, GeorgeChaplin, alsoa scientist,have formulated the first comprehensive theory of skin color.Their findings show skin color is relatedto the strengthof sunlight acrossthe globe. But they also show a deeper,more surprising processat work: Skin color, they say,is largely a matter of vitamins.. /f thesekidsbe/ongto four roces,where slrou/dthe irnesbe drown?. i ..cti. ".6-. ,-,C*,;. f,. ii. G. .t. \r. I j <r: -1D. ai(F. + ,,*t. !-. da.

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(3) f. )y Y ; faY S ;Ju5t 6 rp f.l po yl t lrr i -rrJ,J.,.-c rl te\3fr'n. B t '$. w. !*&. (s\ ,fu. /o.\. 6f l; 1o5r , 1- , C' : lof l. lr nr r r $ ir , e\ , lnr ' '. S/\. ##. tp/. producethe brown pigment Specialcellscolled melanocytes meloninand pockogeit in melonosomes.. to absorbor disperseultr:rvioletlight. But what is it about uitravioletlight that melanin protectsagainst? Someresearchers pointed to the threat of skin cancer.But cancerusuallydevelopslate in life, after peoplehavealreadyhad children, and so doesnot affecttheir abiliry to passon genes t o t h e n e x t g e n e ra t io n . During her preparationfor the lecture in Australia,Jablonskifound a I978 study that exan-rined the effectsolt ultraviolet light on folate,which is one of the B vitamins. An hour of intensesunlight, the study showed,is enoughto cut folatelevels in half if your skin is light. Jablonskimade the next, cn-rcialconnectiononly a few weekslater.At a seminar,sheheard that low folate levelsare linked to fatal birth defects.(To preventthesedefects,the United Stateshas added folate to l1our and other foods peopleeat frequently.) Jablonskilater cameacrossthree documentedcasesin which children's defectswere linked to their mothers'visits. I. Everyone hos the somenumberof melonocytes. But the omountof meloninthey producevoriesfrom personto person..

(4) to tanning salonsduring early pregnancy. She now had some intriguing evidencethat folate might be the driving force behind the evolution of darker skin. But if dark skin is an advantage,why arent we all dark?Why do some people have light skin? As far back as the 1960s, the biochemist'W'.Farnsworth Loomis had suggestedthat skin color is determined by the body's need for vitamin D. The vitamin helps the body absorbcalcium and deposit it in bones,preventing devastatingbone diseases. This is why vitamin D is added to milk. \Thereas folate is destroyedby ultraviolet light, the body usesultraviolet light to produce vitamin D. Loomis believedthat people who live in the north, where daylight is weakest,evolvedfair skin to help absorb more ultraviolet light and that people in the tropics evolveddark skin to block the light, keeping the body from overdosing on vitamin D, which can be toxic at high concentrations. By the time Jablonskidid her research, Loomis'shypothesishad been partially disproved."You can never overdoseon natural amounts of vitamin D," Jablonski says.(In fact, scientistshave recently discoveredthat vitamin D also supports the immune system,helping us fight off infection and cancer,and that most people do not get enoughof this vitamin.) But Loomis'sinsight about fair skin held up, and it made a perfect complement for Jablonskit insight about folate and dark skin. Skin color is a balancingact berween the need to protect vitamin B and the need to make vitamin D. Peopleliving near the equator have dark skin to stop folate from being destroyed,and people living near the poles have light skin so they can make vitamin D.. Dork skinpreventssunlightfrom reachingb/oodvessels whereit con destroyo B vitominthot preventsbirth defeas.. Couldprotecting vitominsbe one reosonPeoPle who livewhere sunlightis strong often cover themse/ves from heod to toe?. muse 29.

(5) -. r :.. { - 1!. t'. : / > ;'l 1a'" r j{., \,rli ''. "-\rtr! \-. \t. \. "i lli./. \.' r ...i .,.. i. :1;: . lY. I. '\,1i-r ..\' ,t I. n ,/. fl- d \--r i. S. I ti.. '"',4-i! ', j, \..^. '''-i. d. I. qo. ts. & kerafin"cyte. *8. 6. e. @ D\. d. = Previtamin 1 6l^1ole":lerol. "EV.lE>. L(ht skin lets sunlight reoch kerotinocytes,cell foctories thot help moke vitaminD, o vitominwith mony heolth benefts.. But so far all Jablonski had was a plausible theory. The next step was to find some hard data matching skin color to light levels.Twelve yearsago, Jablonski and Chaplin took satellite ultraviolet measurementsand compared them with published data on skin color in populations from more than 50 countries. To their delight, there was an unmistakable link The weakerthe 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 barriersin the history of human settlement,"Jablonski says."Only after humans learned fishing, and therefore. 30muse. had accessto food rich in vitamin D, could they settletheseregions." Humans have spent most of their history moving around. To do that, theyve had to adapt their tools, clothes,housing, and eating habits to each new climate and landscape.But Jablonski'swork indicates that our adaptationsgo much further. Peoplein the tropics have developed dark skin to block out the sun and protect their body's folate reserves.Peoplefar from the equator have developedfair skin to drink in the sun and produce adequate amounts of vitamin D during the long winter months. Jablonski hopes her work will begin to changethe way people think about.

(6) J. The lnuit ore on exceptionthot provesthe rule.Their skin is dorker thon Jablonskittheory predictsit should be, givenhow far north they live,but they also eot o vitomin D-rich diet of fish.. skin color. "\(/e can take a topic that has causedso much disagreement,so much suffering, and so much misunderstanding," she says,"and completely disarm it.". journalist, Gina Kirchwegera biologistturnedscience California' in Southern liues now but Aitria in up greu iuhrrr'tht hasto apply mucb moresun blocktoprotect herfolate.. When peoplernove,skin colorcon toke thousondsof yeorsto cotchup' Both the mon (left)ond the Arobman Sudonese (right)liveon the Red Seo,whichis neor the equotor.TheSudoneseore long-term residents,but the Arobsorrivedonly obout2000 yeorsogo.. muse 3 |.

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* si ..m#:r si*- fu* fu #"''f* ;* -,

* si ..m#:r si- fu fu #"''f* ;* -,