Skin colour is de­ter­mined by genes, not race

The Times of India (New Delhi edition) - - TIMES TRENDS - Carl Zim­mer

For cen­turies, skin colour has held pow­er­ful so­cial mean­ing — a defin­ing char­ac­ter­is­tic of race. “If you ask some­body, ‘ What are the main dif­fer­ences be­tween races?’, they’re go­ing to say skin colour,” said Sarah A Tishkoff, a ge­neti­cist at the Univer­sity of Penn­syl­va­nia.

On Thurs­day, she and her col­leagues showed this to be a pro­found er­ror. In ‘Sci­ence’, the re­searchers pub­lished the first large-scale study of the ge­net­ics of skin colour in Africans. The re­searchers pin­pointed eight ge­netic vari­ants in four nar­row re­gions of the hu­man genome that strongly in­flu­ence pig­men­ta­tion — some mak­ing skin darker, and oth­ers mak­ing it lighter.

These genes are shared across the globe, it turns out; one of them, for ex­am­ple, light­ens skin in both Euro­peans and hunter-gath­er­ers in Botswana. The gene vari­ants were present in hu­man­ity’s dis­tant an­ces­tors, even be­fore our species evolved in Africa 300,000 years ago. The wide­spread dis­tri­bu­tion of these genes and their per­sis­tence over mil­len­ni­ums show that the old colour lines are es­sen­tially mean­ing­less, the sci­en­tists said. The re­search “dis­pels a bi­o­log­i­cal con­cept of race”, Tishkoff said.

Hu­mans de­velop colour much as other mam­mals do. Spe­cial cells in the skin con­tain pouches, called melanosomes, packed with pig­ment mol­e­cules. The more pig­ment, the darker the skin.

Skin colour also varies with the kind of pig­ments: Melanosomes may con­tain mix­tures of a brown-black called eu­me­lanin and a yel­low-red called pheome­lanin.

To find the genes that help pro­duce pig­ments, sci­en­tists be­gan by study­ing peo­ple of Euro­pean an­ces­try and found that mu­ta­tions to a gene called SLC24A5 caused cells to make less pig­ment, lead­ing to paler skin. “We knew quite a lot about why peo­ple have pale skin if they had Euro­pean an- ces­try,” said Ni­cholas G. Crawford, a co-au­thor of the study. “But there was very lit­tle known about why peo­ple have dark skin.” Since the early 2000s, Tishkoff has stud­ied genes in Africa, dis­cov­er­ing vari­ants im­por­tant to every­thing from re­sis­tance to malaria to height. African pop­u­la­tions vary tremen­dously in skin colour, and she rea­soned pow­er­ful ge­netic vari­ants must be re­spon­si­ble.

Study­ing 1,570 peo­ple in Ethiopia, Tan­za­nia and Botswana, Tishkoff and her col­leagues dis­cov­ered a set of ge­netic vari­ants that ac­count for 29% of the vari­a­tion in skin colour. (The re­main­ing vari­a­tion seems tied to genes yet to be dis­cov­ered.)

The eight gene vari­ants dis­cov­ered in Africans turned out to be present in many pop­u­la­tions out­side the con­ti­nent. By com­par­ing the DNA of these peo­ple, the team was able to es­ti­mate how long ago the genes ap­peared.They turned out to be im­mensely old. A vari­ant for light skin — found in both Euro­peans and the San hunter-gath­er­ers of Botswana — arose roughly 900,000 years ago, for ex­am­ple.

Even be­fore there were Homo sapi­ens, then, our fore­bears had a mix of genes for light and dark skin. Some pop­u­la­tions may have been dark-skinned and oth­ers light-skinned; or maybe they were all the same colour.

In all, the new study pro­vides “a deeper ap­pre­ci­a­tion of the ge­netic pal­ette that has been mixed and matched through evo­lu­tion”, said Nina Jablon­ski, an ex­pert on skin colour at Penn­syl­va­nia State Univer­sity.

Getty Im­ages/Dig­i­tal Vi­sion


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