DavidReichWho We Are and How We Got Here: Ancient DNA and the New Science of the Human PastNew York: Pantheon Books, 2018. 368 p. $28.95 (hardcover)

Abstract

The subject is the peopling of the world. Luca Cavalli-Sforza, the founder of genetic studies of human prehistory, is seen as the Moses of the story: “his vision predated the technology needed to fulfill it.” Svante Pääbo, director of the Max Planck Institute for Evolutionary Anthropology in Leipzig, invented much of that technology. And David Reich, at Harvard's Department of Genetics, raised the study of ancient human DNA to an industrial scale, basing it on reconstructions of the whole genome rather than on small fragments of (mostly mitochondrial) DNA. The “new science” of the title refers to this last-mentioned “genomic revolution”—occurring over the past ten years. Much of Reich's book takes the form of a complex detective story, tracing the discoveries of researchers, particularly in Pääbo's and Reich's laboratories, as DNA evidence from around the world has accumulated. It is an absorbing story, astonishing both in the technological capacities achieved and in the pace of discovery. But for demographers, this way of proceeding is also frustrating. The book is full of details of research methods, anecdotes about investigators, discussions of how new results support or (just as often) supplant the findings of traditional archaeology and historical linguistics, and digressions about national and ethnic sensitivities over ancient bones. Intermixed in all this are maps summarizing the state of knowledge of migratory flows: directions and dates; but the flows, of course, refer to genes, not numbers of people. These are the makings of a solidly-founded, if still in many respects sketchy and provisional, picture of the world's deep demographic history, one that fills out and greatly complicates the simple “out-of-Africa” and “land bridges” models of humanity's origins and spread. Some elements of the fuller picture: Anatomically-modern humans emerged in Africa roughly 300,000 years before present (BP). Their main dispersal out of Africa came around 60,000–50,000 BP. (There were earlier dispersals as well.) Modern humans reached Europe, East Asia, and Australia and New Guinea by 50,000–40,000 BP. In the period 54,000–44,000 BP there was some interbreeding of modern humans with archaic strains—Neanderthals (throughout Eurasia) and Denisovians (in eastern and southeastern Asia)—before those strains became extinct. Hunter-gatherers spread through Europe, experiencing “multiple population replacements,” but squeezed southward by glacial ice around 25,000–19,000 BP. Further migration from the Anatolian region circa 14,000 BP “homogenized the population of Europe and the Near East.” Farming (and farmers) spread across Europe from the Near East over 9,000 to 6,000 BP, coexisting with hunter-gatherers. South Asian demographic prehistory was somewhat comparable in nature and timing to Europe's, with farming spreading eastward from Iran. Modern East Asia's population appears to derive from the mixing of two ancient lineages (termed “ghost populations” by Reich) populating the Yellow River and Yangtze valleys, both of which had developed farming by 9,000 BP. Residues of the lineages are found in modern Tibetan- and Tai-speaking groups, respectively. There were likely four prehistoric migrations from Eurasia to the Americas, the earliest from northeast Asia along a late-ice age shoreline route from Alaska that opened 16,000 BP, reaching South America by 14,000 BP. (One of the more remarkable DNA findings is of some common ancestry between populations in the Amazon and aboriginal Australians, New Guineans, and Andamanese.) The southwest Pacific islands were settled over 5,000 to 3,000 BP by migrants from East Asia, bypassing New Guinea; a later migrant stream brought New Guinean ancestry to some of these islands. Reich notes that the ancient DNA revolution has been highly Eurocentric. The African demographic story thus far is particularly sketchy. But he predicts production within the next decade of “an ancient DNA atlas of humanity, sampled densely through time and space”—a resource that “will rival the first maps of the globe made between the fifteenth and nineteenth centuries in terms of its contribution to human knowledge.” References in endnotes; index.