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

by David Reich

In humans, the profound biological differences that exist between the sexes mean that a single male is physically capable of having far more children than is a single female. Women carry unborn children for nine months and often nurse them for several years prior to having additional children.13 Men, meanwhile, are able to procreate while investing far less time in the bearing and early rearing of each child, a biological difference whose effects are amplified by social factors such as the fact that in many societies, men are expected to spend little time with their children. So it is that, as measured by the contribution to the next generation, powerful men have the potential to have a far greater impact than powerful women, and we can see this in genetic data.

The time around five thousand years ago coincides with the period in Eurasia that the archaeologist Andrew Sherratt called the “Secondary Products Revolution,” in which people began to find many uses for domesticated animals beyond meat production, including employing them to pull carts and plows and to produce dairy products and clothing such as wool.19 This was also around the time of the onset of the Bronze Age, a period of greatly increased human mobility and wealth accumulation, facilitated by the domestication of the horse, the invention of the wheel and wheeled vehicles, and the accumulation of rare metals like copper and tin, which are the ingredients of bronze and had to be imported from hundreds or even thousands of kilometers away. The Y-chromosome patterns reveal that this was also a time of greatly increased inequality, a genetic reflection of the unprecedented concentration of power in tiny fractions of the population that began to be possible during this time due to the new economy. Powerful males in this period left an extraordinary impact on the populations that followed them—more than in any previous period—with some bequeathing DNA to more descendants today than Genghis Khan.

The archaeologist Marija Gimbutas has argued that Yamnaya society was unprecedentedly sex-biased and stratified. The Yamnaya left behind great mounds, about 80 percent of which had male skeletons at the center, often with evidence of violent injuries and buried amidst fearsome metal daggers and axes.22 Gimbutas argued that the arrival of the Yamnaya in Europe heralded a shift in the power relationships between the sexes.

Instead, it is clear that the Y chromosome was a nonrepresentative part of the genome where certain genetic types were more successful at being passed down to later generations than others. In principle, one possible explanation for this is natural selection, whereby some Y chromosomes gave a biological advantage to those who carried them, such as increased fertility. But the fact that this genetic pattern manifested itself around the same time in multiple places around the world—in a period coinciding with the rise of socially stratified societies—is too striking a pattern to be explained by natural selection at multiple independently occurring advantageous mutations. I think a more plausible explanation is that in this period, it began to be possible for single males to accumulate so much power that they could not only gain access to large numbers of females, but they could also pass on their social prestige to subsequent generations and ensure that their male descendants were similarly successful. This process caused the Y chromosomes these males carried to increase in frequency generation after generation, leaving a genetic scar that speaks volumes about past societies. It is also possible that in this period, individual women began to accumulate more power than they ever had before. Yet because it is biologically impossible for a woman, even a very powerful one, to have an extremely large number of children, the genetic effects of social inequality are much easier to ...

genetic data are truly ancient. Take for example the founding of the ancestral population of non-Africans. Any genetic analysis of non-Africans reveals evidence of a population bottleneck dating to some time before fifty thousand years ago—that is, a small number of individuals giving rise to many descendants today. In 2009, I worked with Alon Keinan, a postdoctoral scientist in my laboratory, to compare genetic variation on the X chromosome, the larger of our two sex chromosomes, to the rest of the genome. To our surprise, we found much less genetic variation in non-Africans on chromosome X than would be expected from the level of variation in the rest of the genome, assuming that males and females participated equally in the founding of the ancestral population of non-Africans. The pattern was too extreme to be explained by a simple scenario of more men than women participating in the founding of the ancestral population of non-Africans. But we discovered that one scenario that could explain the pattern is that after the initial budding off of the ancestral population of non-Africans, there was genetic input into this population from males of other groups. Since males carry one copy of chromosome X for every two copies of other chromosomes, a process of repeated waves of male immigration would decrease X-chromosome diversity (meaning that there would be less genetic variation in the population) compared to the rest of the genome, producing the pattern we observed.

Now that the genome revolution has arrived, with its power to reject long-standing theories, we need to abandon the practice of approaching questions about the human past with strong expectations. To understand who we are, we need to approach the past with humility and with an open mind, and to be ready to change our minds out of respect for the power of hard data.

But I think the lesson is just the opposite. Constant effort to struggle against our demons—against the social and behavioral habits that are built into our biology—is one of the ennobling behaviors of which we humans as a species are capable, and which has been critical to many of our triumphs and achievements. Evidence of the antiquity of inequality should motivate us to deal in a more sophisticated way with it today, and to behave a little better in our own time.

But this consensus view of many anthropologists and geneticists has morphed, seemingly without questioning, into an orthodoxy that the biological differences among human populations are so modest that they should in practice be ignored—and moreover, because the issues are so fraught, that study of biological differences among populations should be avoided if at all possible.

The concern is so acute that the political scientist Jacqueline Stevens has even suggested that research and even emails discussing biological differences across populations should be banned,

Pääbo took seriously his moral responsibility as head of an ambitious German institute of anthropology, and wondered whether the truth about human population structure could be more like the anthropologist Frank Livingston’s suggestion that “there are no races, there are only clines”—a view in which human genetic variation is characterized by gradual geographic gradients that reflect interbreeding among neighbors.

Pääbo showed that such nonrandom sampling could account for some of the effects Feldman and colleagues observed. However, later work proved that nonrandom sampling could not account for most of the structure, as substantial clustering of human populations is observed even when repeating analyses on geographically more evenly distributed sets of samples.

Reference here is earlier than Paabos work?

In 2005, the U.S. Food and Drug Administration lent support to this way of thinking when it approved BiDil, a combination of two medications approved to treat heart failure in African Americans because data suggested it was more effective in African Americans than in European Americans.

except tht it didnt

But on the other side of the argument, David Goldstein suggested that U.S. racial categories are so weakly predictive of most biological outcomes that they do not have long-term value.14 He and his colleagues showed that the frequencies of genetic variants that determine dangerous reactions to drugs are poorly predicted by U.S. census categories. He acknowledged that the reliance on racial and ethnic categories is useful given our poor present knowledge, but predicted that the future will involve testing individuals directly for what mutations they have, and doing away altogether with racial classification as a basis for making individualized decisions about care.

Hmmm

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.

Compared to most academics, the politics of genome bloggers tend to the right—Razib Khan17 and Dienekes Pontikos18 post on findings of average differences across populations in traits including physical appearance and athletic ability. The Eurogenes blog spills over with sometimes as many as one thousand comments in response to postings on the charged topic of which ancient peoples spread Indo-European languages,

What real differences do we know about? We cannot deny the existence of substantial average genetic differences across populations, not just in traits such as skin color, but also in bodily dimensions, the ability to efficiently digest starch or milk sugar, the ability to breathe easily at high altitudes, and susceptibility to particular diseases. These differences are just the beginning. I expect that the reason we don’t know about a much larger number of differences among human populations is that studies with adequate statistical power to detect them have not yet been carried

The indefensibility of the orthodoxy is obvious at almost every turn. In 2016, I attended a lecture on race and genetics by the biologist Joseph L. Graves Jr. at the Peabody Museum of Archaeology and Ethnography at Harvard. At one point, Graves compared the approximately five mutations known to have large effects on skin pigmentation and that are obviously different in frequency across populations to the more than ten thousand genes known to be active in human brains. He argued that in contrast to pigmentation genes, the patterns at genes particularly active in the brain would surely average out over so many locations, with some mutations nudging cognitive and behavioral traits in one direction and some pushing in the other direction. But this argument doesn’t work, because in fact, if natural selection has exerted different pressures on two populations since they separated, traits influenced by many mutations are just as capable of achieving large average differences across populations as traits influenced by few mutations. And indeed, it is already known that traits shaped by many mutations (as is probably the case for behavior and cognition) are at least as important targets of natural selection as traits like skin color that are driven by a small number of mutations.

Naturl selection is nit the iaaue

By compiling the information on the number of years of education for over four hundred thousand people of European ancestry whose genomes have been surveyed in the course of various disease studies, Daniel Benjamin and colleagues identified seventy-four genetic variations each of which has overwhelming evidence of being more common in people with more years of education than in people with fewer years even after controlling for such possibly confounding factors as heterogeneity in the study population.25 Benjamin and colleagues also showed that the power of genetics to predict number of years of education is far from trivial, even though social influences surely have a greater average influence on this behavior than genetics.

Correlation vs causation

How do these genetic variations influence educational attainment? The obvious guess is that they have a direct effect on academic abilities, but that is probably wrong. A study of more than one hundred thousand Icelanders showed that the variations also increase the age at which a woman has her first child, and that this is a more powerful effect than the one on the number of years of education. It is possible that these variations exert their effect indirectly, by nudging people to defer having children, which makes it easier for them to complete their education.

Nof primarily cognitive

The Benjamin study has already been joined by others finding genetic predictors of behavioral traits,29 including one of more than seventy thousand people that found mutations in more than twenty genes that were significantly predictive of performance on intelligence tests.

The average time separation between pairs of human populations since they diverged from common ancestral populations, which is up to around fifty thousand years for some pairs of non-African populations, and up to two hundred thousand years or more for some pairs of sub-Saharan African populations, is far from negligible on the time scale of human evolution.

Writing now, I shudder to think of Watson, or of Wade, or their forebears, behind my shoulder. The history of science has revealed, again and again, the danger of trusting one’s instincts or of being led astray by one’s biases—of being too convinced that one knows the truth.

From the errors of thinking that the sun revolves around the earth, that the human lineage separated from the great ape lineage tens of millions of years ago, and that the present-day human population structure is fifty thousand years old whereas in fact we know that it was forged through population mixtures largely over the last five thousand years—from

We truly have no idea right now what the nature or direction of genetically encoded differences among populations will be. An example is the extreme overrepresentation of people of West African ancestry among elite sprinters. All the male finalists in the Olympic hundred-meter race since 1980, even those from Europe and the Americas, had recent West African ancestry.46 The genetic hypothesis most often invoked to explain this is that there has been an upward shift in the average sprinting ability of people of West African ancestry due to natural selection. A small increase in the average might not sound like much, but it can make a big difference at the extremes of high ability—for example, a 0.8-standard-deviation increase in the average sprinting ability in West Africans would be expected to lead to a hundredfold enrichment in the proportion of people above the 99.9999999th percentile point in Europeans. But an alternative explanation that would predict the same magnitude of effect is that there is simply more variation in sprinting ability in people of West African ancestry—with more people of both very high and very low abilities.47 A wider spread of abilities around the same mean and a hundredfold enrichment in West Africans in the proportion of people above the 99.9999999th percentile point seen in Europeans is in fact exactly what is expected given the approximately 33 percent higher genetic diversity in West Africans than in Europeans.48 Whether or not this explain...

Males and females differ by huge tracts of genetic material—a Y chromosome that males have and that females don’t, and a second X chromosome that females have and males don’t. Most people accept that the biological differences between males and females are profound, and that they contribute to average differences in size and physical strength as well as in temperament and behavior, even if there are questions about the extent to which particular differences are also influenced by social expectations and upbringing (for example, many of the jobs in industry and the professions that women fill in great numbers today had few women in them a century ago). Today we aspire both to recognize that biological differences exist and to accord everyone the same freedoms and opportunities regardless of them. It is clear from the abiding average inequities that persist between women and men that fulfilling these aspirations is a challenge, and yet it is important to accommodate and even embrace the real differences that exist, while at the same time struggling to get to a better place.

Suppose you are the coach of a track-and-field team, and a young person walks on and asks to try out for the hundred-meter race, in which people of West African ancestry are statistically highly overrepresented, suggesting the possibility that genetics may play a role. For a good coach, race is irrelevant. Testing the young person’s sprinting speed is simple—take him or her out to the track to run against the stopwatch. Most situations are like this.

Examples of such medical conditions include poor eyesight, which can now be fully corrected with spectacles, or infertility, which can now be corrected by medical interventions, or cognitive challenges, which can now be controlled by medication and psychotherapy. It is possible that this change in natural selection is leading to a buildup of mutations contributing to altering these traits in the population.

Travel to exotic places and a gold rush to obtain key bones are central to this way of doing science. Some in the second generation of ancient DNA research have adopted this model. But others, including myself, travel far less, and instead spend most of our time developing expertise in improved laboratory techniques or statistical analysis, obtaining the samples we study through increasingly equal partnerships with archaeologists and anthropologists.

The future for ancient DNA laboratories that I find appealing is based on a model that has emerged among radiocarbon dating laboratories.