Who We Are and How We Got Here

David Reich presents the case that a revolution in ancient DNA science has fundamentally reshaped our understanding of human prehistory. He begins with the work of Luca Cavalli-Sforza, the founder of genetic studies of the human past, whose 1994 synthesis of archaeology, linguistics, and genetics was limited by scarce data. Cavalli-Sforza's influential "demic diffusion" model interpreted gradients of genetic variation in Europe as evidence of Near Eastern farmers mixing with local hunter-gatherers as they expanded westward after nine thousand years ago. Ancient DNA proved this wrong: The first farmers in even the most remote parts of Europe had very little hunter-gatherer ancestry. Reich argues that the deeper problem was Cavalli-Sforza's assumption that people living in a place today descend primarily from people who lived there long ago, which ancient DNA has shown is almost never the case.

Reich describes the explosion of data behind these discoveries. The first five ancient human genomes were published in 2010; by August 2017, his laboratory alone had generated genome-wide data for more than three thousand ancient samples. He credits Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology with inventing much of the foundational technology, and explains how a technique developed by Matthias Meyer and Qiaomei Fu, both researchers in Pääbo's laboratory, uses artificially synthesized DNA as "bait" to isolate human sequences from overwhelmingly microbial ancient bone samples. Reich's lab scaled up and automated this method, reducing costs to under five hundred dollars per sample.

In Part I, Reich traces the deep history of the human species. He explains how mutations, random copying errors in DNA that accumulate at a roughly constant rate, serve as a biological stopwatch. He recounts the 1987 discovery that the deepest branch of the mitochondrial DNA family tree is found only in people of sub-Saharan African ancestry. Mitochondrial DNA, genetic material passed exclusively through mothers, provided early evidence that modern humans originated in Africa. This finding refuted the "multiregional hypothesis," which held that present-day humans descend substantially from Homo erectus populations that dispersed across Africa and Eurasia at least 1.8 million years ago. Reich dismantles the idea that a single genetic mutation could explain the acceleration of modern human behavior after about fifty thousand years ago, showing that apart from mitochondrial DNA and the Y chromosome (the chromosome passed from fathers to sons), no location in the genome has all present-day people sharing a common ancestor within the last 320,000 years.

The sequencing of the Neanderthal genome provides a major case study. Using the "Four Population Test," a statistical method Reich and Nick Patterson, an applied mathematician and Reich's chief scientific collaborator, designed to detect ancient interbreeding, the team found that Neanderthals are equally close to Europeans, East Asians, and New Guineans but closer to all non-Africans than to any sub-Saharan Africans, with odds against this occurring by chance of less than one in a quadrillion. Neanderthals interbred with the ancestors of non-Africans roughly fifty-four to forty-nine thousand years ago, contributing about 1.5 to 2.1 percent of their DNA. Evidence of reduced hybrid fertility appears in the systematic depletion of Neanderthal ancestry on the X chromosome and around genes active in male reproductive tissue.

The discovery of the Denisovans, a previously unknown archaic human population identified from a pinky bone found in a Siberian cave, extended these findings. Despite Denisova Cave lying nine thousand kilometers from New Guinea, Denisovans proved genetically closer to New Guineans than to any mainland Eurasian population, indicating interbreeding that contributed 3 to 6 percent of New Guinean ancestry. Reich also identifies "superarchaic" humans, an ancestral group inferred solely from genetic traces, that separated from the lineage leading to modern humans 1.4 to 0.9 million years ago and contributed ancestry to Denisovans.

In Part II, Reich surveys world population history through the concept of "ghost populations," ancestral groups that no longer exist in unmixed form but can be reconstructed from genetic fragments left in later peoples. The "Ancient North Eurasians," first predicted statistically and then confirmed by a twenty-four-thousand-year-old skeleton from Siberia, contributed ancestry to both Europeans and Native Americans. The "Basal Eurasians," an early-splitting lineage with little or no Neanderthal ancestry, contributed about a quarter of present-day European and Near Eastern ancestry. Around ten thousand years ago, West Eurasia hosted at least four populations as differentiated from one another as Europeans and East Asians are today; the expansion of farming collapsed this diversity.

For Europe, Reich shows that present-day Europeans descend from three highly divergent sources: indigenous hunter-gatherers, Anatolian farmers who arrived after nine thousand years ago, and steppe pastoralists of the Yamnaya culture who expanded after five thousand years ago. Individuals buried with Corded Ware pottery in Germany derived about three-quarters of their ancestry from Yamnaya-related groups, and a later Bell Beaker migration replaced about 90 percent of Britain's population. This mass migration likely spread Indo-European languages, tipping a long-standing debate in favor of the "steppe hypothesis" over the "Anatolian hypothesis," advanced by the scholar Colin Renfrew, which held that farming spread Indo-European languages from Anatolia.

Reich reveals a parallel history in India. Two successive migrations, first of Iranian-related farmers and then of steppe pastoralists, mixed with established populations to form the "Ancestral North Indians" and the "Ancestral South Indians." Mixture dates between four thousand and two thousand years ago coincide with the collapse of the Indus Valley Civilization and the composition of the Rig Veda. West Eurasian-related ancestry correlates with Indo-European language speaking and higher caste status, with disproportionately male inheritance suggesting mixture occurred within a stratified society. Many Indian jati groups, endogamous subgroups within the caste system, have maintained strict endogamy for thousands of years.

In the Americas, Reich traces the "First American" lineage and the surprise discovery of "Population Y," a ghost population with Australasian genetic affinities found in Amazonian groups like the Suruí and Karitiana. For East Asia, he identifies two ghost populations associated with rice and millet farming whose expansion and mixture shaped most of the region's present-day ancestry. In Africa, he presents evidence for a deep mixture of two highly differentiated populations roughly 300,000 years ago that may have been foundational to modern humans, and shows how the Bantu expansion, the spread of Bantu-speaking food producers from west-central Africa beginning around four thousand years ago, and other agricultural movements obscured deeper patterns now being uncovered.

Part III addresses social implications. Reich demonstrates that genomic data reveal a deep history of inequality, particularly through Y-chromosome "Star Clusters," patterns in which large numbers of men share a recent male-line ancestor, indicating extreme reproductive success by elite males. Around five thousand years ago, coinciding with the Bronze Age, such concentration of male power became widespread across Eurasia. Sex-biased mixture, in which males from more powerful groups paired with females from less powerful ones, recurs from the Yamnaya expansion through the colonization of the Americas.

Reich argues that the orthodox position, that biological differences among human populations are too small to matter, is scientifically indefensible, since substantial average differences exist across populations in traits from height to disease susceptibility. Yet he insists that racist alternatives are even more wrong: The genome revolution has shown that present-day population structure does not reflect ancient divisions and that all populations are products of repeated mixture. He proposes treating population differences as analogously to sex differences, acknowledging their existence while according everyone equal rights.

Reich closes by comparing the ancient DNA revolution to the invention of radiocarbon dating, arguing it is even more transformative. He envisions an "ancient DNA atlas of humanity" and concludes that the field's findings overwhelmingly undercut prejudice and highlight connections among peoples not previously known to be related.