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 oVer the paSt F eW D ecaDeS, genetic research has revealed two deep truths about people. The first is that all humans are closely related—more closely related than all chimps, even though there are many more humans around today. Everyone has the same collection of genes, but with the exception of identical twins, everyone has slightly different versions of some of them. Studies of this genetic diversity have allowed scientists to reconstruct a kind of family tree of human populations. That has revealed the sec- ond deep truth: In a very real sense, all people alive today are Africans. Our species, Homo sapiens, evolved in Africa— no one is sure of the exact time or place. The most recent fossil find, from Morocco, suggests that anatomically modern human features began ap- pearing as long as 300,000 years ago. For the next 200,000 years or so, we remained in Africa, but already during that period, groups began to move to different parts of the continent and become iso- lated from one another—in effect founding new populations. In humans, as in all species, genetic changes are the result of random mutations—tiny tweaks to DNA, the code of life. Mutations occur at a more or less constant rate, so the longer a group persists, handing down its genes generation after generation, the more tweaks these genes will accumulate. Meanwhile, the longer two groups are separated, the more distinctive tweaks they will acquire. By analyzing the genes of present-day Africans, researchers have concluded that the Khoe-San, who now live in southern Africa, represent one of the oldest branches of the human family tree. The Pygmies of central Africa also have a very long history as a distinct group. What this means is that the deepest splits in the human family aren’t between what are usually thought of as different races—whites, say, or blacks or Asians or Native Americans. They’re between African populations such as the Khoe-San and the Pygmies, who spent tens of thousands of years separated from one another even before humans left Africa. all non-aF ricanS toDaY, the genetics tell us, are descended from a few thousand humans who left Africa maybe 60,000 years ago. These migrants were most closely related to groups that today live in East Africa, including the Hadza of Tanzania. Because they were just a small subset of Africa’s population, the migrants took with them only a fraction of its genetic diversity. Somewhere along the way, perhaps in the Middle East, the travelers met and had sex with another human species, the Neanderthals; farther east they encountered yet another, the Denisovans. It’s believed that both species evolved in Eurasia from a hominin that had migrated out of Africa much earlier. Some scientists also believe the exodus 60,000 years ago was actually the second wave of modern humans to leave Africa. (See map on page 41.) If so, judging from our genomes today, the second wave swamped the first. In what was, relatively speaking, a great rush, Ancient flows of dark and light Many genes affect how melanin colors human skin. The genes predate humanity; some occur in mice and fish. Variations in four of them—mutations that flip a gene from darkening to lightening or vice versa—explain much of the skin-color diversity in Africa. As our ancestors spread across the Earth, different mutations proved beneficial at different latitudes and were passed on. Genetic mutation Genes randomly mutate over time. Beneficial mutations tend to be passed on to offspring and to spread through a population. 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). ~300,000 years ago Anatomically modern human features emerge in Africa.