Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past

by David Reich

we now know, from ancient DNA, that the people who live in a particular place today almost never exclusively descend from the people who lived in the same place far in the past.

the population of northern Europe was largely replaced by a mass migration from the eastern European steppe after five thousand years ago18; that farming developed in the Near East more than ten thousand years ago among multiple highly differentiated human populations that then expanded in all directions and mixed with each other along with the spread of agriculture19; and that the first human migrants into the remote Pacific islands beginning around three thousand years ago were not the sole ancestors of the present-day inhabitants.

The ancient DNA revolution is rapidly disrupting our assumptions about the past. Yet there is at present no book by a working geneticist that lays out the impact of the new science and explains how it can be used to establish compelling new facts. The findings needed to grasp the scope of the ancient DNA revolution are scattered among hard-to-read, jargon-filled scientific papers, sometimes supplemented by hundreds of pages of dense notes on methodology. In Who We Are and How We Got Here, I aim to offer readers a clear view through this extraordinary window into the past—to provide a book about the ancient DNA revolution intended for lay reader and specialist alike.

mixture between highly differentiated populations is a recurrent process in the human past.

A great surprise that emerges from the genome revolution is that in the relatively recent past, human populations were just as different from each other as they are today, but that the fault lines across populations were almost unrecognizably different from today. DNA extracted from remains of people who lived, say, ten thousand years ago shows that the structure of human populations at that time was qualitatively different. Present-day populations are blends of past populations, which were blends themselves. The African American and Latino populations of the Americas are only the latest in a long line of major population mixtures.

In the last few years, the genome revolution—turbocharged by ancient DNA—has revealed that human populations are related to each other in ways that no one expected. The story that is emerging differs from the one we learned as children, or from popular culture. It is full of surprises: massive mixtures of differentiated populations; sweeping population replacements and expansions; and population divisions in prehistoric times that did not fall along the same lines as population differences that exist today. It is a story about how our interconnected human family was formed, in myriad ways never imagined.

One of the profound implications of the Denisovan discovery was that East Eurasia is a central stage of human evolution and not a sideshow as westerners often assume.

more than half the world’s population derives between 5 percent and 40 percent of their genomes from the Ancient North Eurasians.

The genome revolution has taught us that great mixtures of highly divergent populations have occurred repeatedly.6 Instead of a tree, a better metaphor may be a trellis, branching and remixing far back into the past.7

Native Americans derive about a third of their ancestry from the Ancient North Eurasians, and the remainder from East Asians.

There was never a single trunk population in the human past. It has been mixtures all the way down.

the past is not an inevitable march toward the present. Human history is full of dead ends, and we should not expect the people who lived in any one place in the past to be the direct ancestors of those who live there today.

Analysis of ancient DNA data shows that western European hunter-gatherers around eight thousand years ago had blue eyes but dark skin and dark hair, a combination that is rare today.33 The first farmers of Europe mostly had light skin but dark hair and brown eyes—thus light skin in Europe largely owes its origins to migrating farmers.34 The earliest known example of the classic European blond hair mutation is in an Ancient North Eurasian from the Lake Baikal region of eastern Siberia from seventeen thousand years ago.35 The hundreds of millions of copies of this mutation in central and western Europe today likely derive from a massive migration into the region of people bearing Ancient North Eurasian ancestry, an event that is related in the next chapter.36

Surprisingly, the ancient DNA revolution, through its discovery of the pervasiveness of ghost populations and their mixture, is fueling a critique of race that has been raised by scholars in the past, but was never prominent because of a lack of support from hard scientific facts.37 By demonstrating that the genetic fault lines in West Eurasia between ten thousand and four thousand years ago were entirely different from today’s, the ancient DNA revolution has shown that today’s classifications do not reflect fundamental “pure” units of biology. Instead, today’s divisions are recent phenomena, with their origin in repeating mixtures and migrations. The findings of the ancient DNA revolution suggest that the mixtures will continue. Mixture is fundamental to who we are, and we need to embrace it, not deny that it occurred.

Nearly all ancient hunter-gatherers carried one set of mitochondrial DNA types. But the farmers who succeeded them carried no more than a few percent of those types, and their DNA was more similar to that seen today in southern Europe and the Near East. It was clear that the farmers came from a population that did not descend from European hunter-gatherers.

The extraordinary fact that emerges from ancient DNA is that just five thousand years ago, the people who are now the primary ancestors of all extant northern Europeans had not yet arrived.

Ancient DNA has established major migration and mixture between highly divergent populations as a key force shaping human prehistory, and ideologies that seek a return to a mythical purity are flying in the face of hard science.

The Suruí and some of their neighbors in Amazonia harbor some ancestry from a different founding population of the Americas, whose ancestors arrived at a time and along a route we still do not understand.

we need to try to make progress beyond the situation we are facing right now, in which many researchers are reluctant to undertake any studies of Native American genetic variation for fear of criticism, and because of the extraordinary time commitment that would be required in order to accomplish all the consultations that some tribal representatives and scholars have recommended. This has had the effect of putting research into genetic variation among Native Americans into a deep chill—with far less research in this area going on than anyone but the people most hostile to scientific research would like.

East Asia—the vast region encompassing China, Japan, and Southeast Asia—is one of the great theaters of human evolution. It harbors more than one third of the world’s population and a similar fraction of its language diversity. Pottery was first invented there at least nineteen thousand years ago.1 It was the jumping-off point for the peopling of the Americas before fifteen thousand years ago. East Asia witnessed an independent and early invention of agriculture around nine thousand years ago. East Asia has been home to the human family for at least around 1.7 million years, the date of the oldest known Homo erectus skeleton found in China.2 The earliest human remains excavated in Indonesia are similarly old.3 Archaic humans—whose skeletal form is not the same as that of humans whose anatomically modern features begin to appear in the African fossil record after around three hundred thousand years ago4—have lived in East Asia continuously since those times.

the people “left behind” in Africa changed just as much as the descendants of the people who emigrated.

In 1942, the anthropologist Ashley Montagu wrote Man’s Most Dangerous Myth: The Fallacy of Race, arguing that race is a social concept and has no biological reality, and setting the tone for how anthropologists and many biologists have discussed this issue ever since.

It is now undeniable that there are nontrivial average genetic differences across populations in multiple traits, and the race vocabulary is too ill-defined and too loaded with historical baggage to be helpful. If we continue to use it we will not be able to escape the current debate, which is mired in an argument between two indefensible positions. On the one side there are beliefs about the nature of the differences that are grounded in bigotry and have little basis in reality. On the other side there is the idea that any biological differences among populations are so modest that as a matter of social policy they can be ignored and papered over. It is time to move on from this paralyzing false dichotomy and to figure out what the genome is actually telling us.

If as scientists we willfully abstain from laying out a rational framework for discussing human differences, we will leave a vacuum that will be filled by pseudoscience, an outcome that is far worse than anything we could achieve by talking openly.

Wade’s Troublesome Inheritance ran with the theme again, suggesting that a politically correct alliance of anthropologists and geneticists has banded together to suppress the truth that there are significant differences among human populations and that those differences correspond to classic stereotypes. One part of the argument has something to it—Wade correctly highlights the problem of an academic community trying to enforce an implausible orthodoxy. Yet the “truth” that he puts forward in opposition, the idea that not only are there substantial differences, but that they likely correspond to traditional racial stereotypes, has no merit. Wade’s book combines compelling content with parts that are entirely speculative, presenting everything with the same authority and in the same voice, so that naive readers who accept the parts of it that are well argued are tempted to accept the rest. Worse, when compared to Wade’s previous writing, in which the rebels speaking the truth were scholars of creativity and accomplishment, he does not identify any serious scholarship in genetics supporting his speculations.38 And yet by celebrating those who have opposed the flawed orthodoxy, he implies wrongly that their alternative theories must be right.

The right way to deal with the inevitable discovery of substantial differences across populations is to realize that their existence should not affect the way we conduct ourselves. As a society we should commit to according everyone equal rights despite the differences that exist among individuals. If we aspire to treat all individuals with respect regardless of the extraordinary differences that exist among individuals within a population, it should not be so much more of an effort to accommodate the smaller but still significant average differences across populations.

Because of the multidimensionality of human traits, the great variation that exists among individuals, and the extent to which hard work and upbringing can compensate for genetic endowment, the only sensible approach is to celebrate every person and every population as an extraordinary realization of our human genius and to give each person every chance to succeed, regardless of the particular average combination of genetic propensities he or she happens to display.

The real offense of racism, in the end, is to judge individuals by a supposed stereotype of their group—to ignore the fact that when applied to specific individuals, stereotypes are almost always misleading.

We now know that nearly every group living today is the product of repeated population mixtures that have occurred over thousands and tens of thousands of years. Mixing is in human nature, and no one population is—or could be—“pure.”

The centrality of mixture in the history of our species, as revealed in just the last few years by the genome revolution, means that we are all interconnected and that we will all keep connecting with one another in the future.

Our particular ancestors are not the point. The genome revolution provides us with a shared history that, if we pay proper attention, should give us an alternative to the evils of racism and nationalism, and make us realize that we are all entitled equally to our human heritage.

The measure of a revolutionary technology is the rate at which it reveals surprises, and in this sense, ancient DNA is more revolutionary than any previous scientific technology for studying the past, including radiocarbon dating. A more apt analogy is the seventeenth-century invention of the light microscope, which made it possible to visualize the world of microbes and cells that no one before had even imagined. When a new instrument opens up vistas onto a world that has not previously been explored, everything it shows is new, and everything is a surprise. This is what is happening now with ancient DNA. It is providing definitive answers to questions about whether changes in the archaeological record reflect movements of people or cultural communication. Again and again, it is revealing findings that almost no one expected.