National Geographic : 2018 Apr
East Africans, Hadza, San, South Asians, and Australo-Melanesians Papua New Guineans, Ethiopians, Hadza, and Tanzanians Sub-Saharan Africans (except the San), South Asians, and Australo-Melanesians Europeans, East Asians, Indians, and Native Americans Europeans, San, East Asians, and Africans Europeans, San, and East Asians 29,000 years ago Europeans and South Asians Africans and East Asians Gene variants associated with dark pigmentation Gene variants associated with light pigmentation DDB1 HERC2 MFSD12 996,000 years ago 345,000 years ago 250,000 years ago SLC24A5 Light-to-darkmutationLight-to-darkmutationDark-to-lightmutationDark-to-lightmutation the offspring of all these migrants dispersed around the world. By 50,000 years ago they had reached Australia. By 45,000 years ago they’d settled in Siberia, and by 15,000 years ago they’d reached South America. As they moved into dif- ferent parts of the world, they formed new groups that became geographically isolated from one another and, in the process, acquired their own distinctive set of genetic mutations. Most of these tweaks were neither helpful nor harmful. But occasionally a mutation arose that turned out to be advantageous in a new setting. Under the pressure of natural selection, it spread quickly through the local population. At high altitudes, for instance, oxygen levels are low, so for people moving into the Ethiopian highlands, Tibet, or the Andean Altiplano, there was a premi- um on mutations that helped them cope with the rarefied air. Similarly, Inuit people, who adopt- ed a marine-based diet high in fatty acids, have genetic tweaks that helped them adapt to it. Sometimes it’s clear that natural selection has favored a mutation, but it’s not clear why. Such is the case with a variant of a gene called EDAR (pronounced ee-dar). Most people of East Asian and Native American ancestry possess at least one copy of the variant, known as 370A, and many possess two. But it’s rare among people of African and European descent. At the University of Pennsylvania’s Perelman School of Medicine, geneticist Yana Kamberov has equipped mice with the East Asian variant of EDAR in hopes of understanding what it does. “ They’re cute, aren’t they?” she says, opening the cage to show me. The mice look ordinary, with sleek brown coats and shiny black eyes. But examined under a microscope, they are different from their equally cute cousins in subtle yet sig- nificant ways. Their hair strands are thicker; their sweat glands are more numerous; and the fat pads around their mammary glands are smaller. Kamberov’s mice help explain why some East Asians and Native Americans have thicker hair and more sweat glands. (EDAR’s effect on human breasts is unclear.) But they don’t provide an evolutionary reason. Perhaps, Kamberov specu- lates, the ancestors of contemporary East Asians at some point encountered climate conditions that made more sweat glands useful. Or maybe thicker hair helped them ward off parasites. Or it could be that 370A produced other benefits she’s yet to discover and the changes she has identified were, in effect, just tagalongs. Genetics frequent- ly works like this: A tiny tweak can have many disparate effects. Only one may be useful—and it may outlive the conditions that made it so, the way families hand down old photos long past the point when anyone remembers who’s in them. “Unless you have a time machine, you’re not going to know,” Kamberov sighs. Dna iS oFten coMpareD to a teXt, with the letters standing for chemical bases—A for adenine, C for cytosine, G for guanine, and T for thymine. The human genome consists of three billion base pairs—page after page of A’ s, C’s, G’s, Responding to the sun Dark skin is favored in the tropics because it shields tissue from dangerous UV rays. In regions with less sun, lighter skin allows the body to absorb enough UV rays to synthe- size vitamin D, which is needed for healthy bones and immune systems. JASON TREAT AND RYAN T. WILLIAMS, NGM STAFF SOURCE: SARAH TISHKOFF, UNIVERSITY OF PENNSYLVANIA Light skin has many origins A key gene mutation promoting lighter skin (SLC24A5) occurred 29,000 years ago in Asia and later spread into Europe. But Africa is the source of other gene variants that contribute to lighter skin in populations around the world (DDB1, MFSD12, and HERC2). ~3 0 0,000 years ago Anatomically modern human features emerge in Africa. East Africans, Hadza, San, South Asians, and Australo-Melanesians Papua New Guineans, Ethiopians, Hadza, and Tanzanians Sub-Saharan Africans (except the San), South Asians, and Australo-Melanesians Europeans, East Asians, Indians, and Native Americans Europeans, San, East Asians, and Africans Europeans, San, and East Asians 29,000 years ago Europeans and South Asians Africans and East Asians Gene variants associated with dark pigmentation Gene variants associated with light pigmentation DDB1 HERC2 MFSD12 996,000 years ago 345,000 years ago 250,000 years ago SLC24A5 Light-to-darkmutationLight-to-darkmutationDark-to-lightmutationDark-to-lightmutation Hadza hunter-gatherers of Tanzania, such as Ondoshi Stephano, are among the closest living relatives of the humans who first left Africa.