by David Reich
The high differentiation of human populations in the Near East ten thousand years ago was a specific instance of a broader pattern across the vast region of West Eurasia, documented by Iosif Lazaridis, who led the analysis. Analyzing our data, he found that about ten thousand years ago there were at least four major populations in West Eurasia—the farmers of the Fertile Crescent, the farmers of Iran, the hunter-gatherers of central and western Europe, and the hunter-gatherers of eastern Europe. All these populations differed from one another as much as Europeans differ from East Asians today. Scholars interested in trying to create ancestry-based racial classifications, had they lived ten thousand years ago, would have categorized these groups as “races,” even though none of these groups survives in unmixed form today.
Spurred by the revolutionary technology of plant and animal domestication, which could support much higher population densities than hunting and gathering, the farmers of the Near East began migrating and mixing with their neighbors. But instead of one group displacing all the others and pushing them to extinction, as had occurred in some of the previous spreads of hunter-gatherers in Europe, in the Near East all the expanding groups contributed to later populations. The farmers in present-day Turkey expanded into Europe. The farmers in present-day Israel and Jordan expanded into East Africa, and their genetic legacy is greatest in present-day Ethiopia. Farmers related to those in present-day Iran expanded into India as well as the steppe north of the Black and Caspian seas. They mixed with local populations there and established new economies based on herding that allowed the agricultural revolution to spread into parts of the world inhospitable to domesticated crops. The different food-producing populations also mixed with one another, a process that was accelerated by technological developments in the Bronze Age after around five thousand years ago. This meant that the high genetic substructure that had previously characterized West Eurasia collapsed into the present-day very low level of genetic differentiation by the Bronze Age. It is an extraordinary example of how technology—in this case, domestication—contributed to homogenization, not just culturally but genetically. It shows that what is happening with the Industrial Revolution and the information revolution in our own time is not unique in the history of our species.
The fusion of these highly different populations into today’s West Eurasians is vividly evident in what might be considered the classic northern European look: blue eyes, light skin, and blond hair. 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.
5
The Making of Modern Europe
Strange Sardinia
In 2009, geneticists led by Joachim Burger sequenced stretches of mitochondrial DNA from ancient European hunter-gatherers and some of the earliest farmers of Europe.1 Although mitochondrial DNA is hundreds of thousands of times shorter than the rest of the genome, it has enough variation to allow categorization of the peoples of the world into distinct types. 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.
Mitochondrial DNA is only a small portion of the genome, however, and the whole-genome studies that followed delivered strange results. In 2012, a team of geneticists sequenced the genome of the “Iceman,” a natural mummy dating to approximately fifty-three hundred years ago that was discovered in 1991 on a melting glacier in the Alps.2 The cold had preserved his body and equipment, providing a vivid snapshot of what obviously had been an extraordinarily complex culture dating to thousands of years before the arrival of writing. His skin was covered with dozens of tattoos. He wore a woven grass cloak and finely sewn shoes. He carried a copper-bladed axe and a kit for lighting fires. An arrowhead in his shoulder and a torn artery showed that he had been shot and had stumbled to the top of a mountain pass before collapsing. Based on the isotopes of the elements strontium, lead, and oxygen in the enamel capping his teeth, it seemed likely he had grown up in a nearby valley where isotopes (contained in groundwater and plants, and derived from the local rocks) had similar ratios.3 But the ancient DNA data showed that his closest genetic relatives are not present-day Alpine people. Instead, his closest relatives today are the people of Sardinia, an island in the Mediterranean Sea.
Figure 14a. Archaeology and linguistics provide evidence of profound transformations in human culture. Archaeological evidence shows that farming expanded from the Near East to the far northwest of Europe between about 11,500 years ago and about 5,500 years ago, transforming economies across this region.
This strange link to present-day Sardinians kept turning up. In the same year that the Iceman’s genome was published, Pontus Skoglund, Mattias Jakobsson, and colleagues at the University of Uppsala published four genome sequences from individuals who lived about five thousand years ago in Sweden.4 A leading theory up until their study was that the Swedish hunter-gatherers who lived at that time descended from farmers who had adapted a hunter-gatherer lifestyle to exploit the rich fisheries of the Baltic Sea, and were not directly descended from the hunter-gatherers who had lived in northern Europe (including Sweden) several thousand years earlier. But ancient DNA disproved this theory. Instead of being genetically close to each other, the farmers and hunter-gatherers were almost as different from each other as Europeans are from East Asians today. And the farmers once again had that strange link to Sardinians.
Figure 14b. European languages are nearly all part of the Indo-European language family that descends from a common ancestral language as recently as about 6,500 years ago. (The map labels show the pre-Roman distribution of Indo-European languages.)
Skoglund and Jakobsson proposed a new model to explain these findings—that migrating farmers whose ancestors originated in the Near East spread over Europe with little mixture with the hunter-gatherers they encountered along the way, a sharp contrast to Luca Cavalli-Sforza’s model for the farming expansion into Europe that had been popular until this time and that emphasized extensive mixture and interaction with the local hunter-gatherers during the expansion.5 The new model would not only explain the striking genetic contrast between hunter-gatherers and farmers in Sweden around five thousand years ago. It would also explain why the ancient farmers were genetically similar to present-day Sardinians, who plausibly descend from a migration of farmers to that island around eight thousand years ago that largely displaced the previous hunter-gatherers. Isolated on Sardinia, the
descendants of these farmers were minimally affected by demographic events that later transformed the populations of mainland Europe. So far, so good— this new model explained the genetic composition of most Europeans up until around five thousand years ago. But Skoglund and Jakobsson also went further and proposed that these two sources—hunter-gatherers and farmers—might have contributed almost all the ancestry of Europeans living today. Here they missed something extraordinarily important.
A Cloud on the Horizon
In 2012, it seemed that the big question of the ancestral sources of present-day European populations might be solved. But there was an observation that didn’t fit.
In that year, Nick Patterson published a perplexing result from his Three Population Test. As described in the previous chapter, he showed that the frequencies of mutations in northern Europeans today tend to be intermediate between those of southern Europeans and Native Americans. He hypothesized that these findings could be explained by the existence of a “ghost population”—the Ancient North Eurasians—who were distributed across northern Eurasia more than fifteen thousand years ago and who contributed both to the population that migrated across the Bering land bridge to people the Americas and to northern Europeans.6 A year later, Eske Willerslev and colleagues obtained a sample of ancient DNA from Siberia that matched the predicted Ancient North Eurasians—the Mal’ta individual whose skeleton dated to around twenty-four thousand years ago.7
How could the finding of an Ancient North Eurasian contribution to present-day northern Europeans be reconciled with the two-way mixture of indigenous European hunter-gatherers and incoming farmers from Anatolia that had been directly demonstrated through ancient DNA studies? The plot became even thicker as we and others obtained additional ancient DNA data from hunter-gatherers and farmers between eight thousand and five thousand years ago and found that they fit the two-way mixture model without any evidence of Ancient North Eurasian ancestry.8 Something profound must have happened later—a new stream of migrants must have arrived, introducing Ancient North Eurasian ancestry and transforming Europe.
In 2014–15, the ancient DNA community and especially my own laboratory published data from more than two hundred ancient Europeans from Germany, Spain, Hungary, the steppe of far eastern Europe, and the first farmers from Anatolia.9 By comparing the ancient individuals to West Eurasian people living today, Iosif Lazaridis in my laboratory was able to figure out how it was that the Ancient North Eurasian ancestry entered Europe within the last five thousand years.
Our initial approach was to carry out a principal component analysis, which can identify combinations of mutation frequencies that are most efficient at finding differences among samples. In doing this, we benefited from our extraordinarily high resolution data from around six hundred thousand variable locations on the genome, around ten thousand times more locations than Cavalli-Sforza had been able to analyze in his 1994 book.10 While Cavalli-Sforza had tried to make sense of the principal component summaries of genetic variation by plotting their values onto a map of the world, we could do far more. We plotted a single dot for each individual depending on where he or she fell relative to the two principal components. On the scatterplot we obtained for close to eight hundred present-day West Eurasians, two parallel lines appeared: the left containing almost all Europeans, and the right containing almost all Near Easterners, with a striking gap in between. By placing all the ancient samples onto the same plot, we could watch their positions shift over time, and the last eight thousand years of European history unfurled before our eyes, offering a time-lapse video showing how present-day Europeans formed from populations that had little resemblance in their ancestry to most Europeans living today.11
First came the hunter-gatherers, who themselves were the product of a series of population transformations over the previous thirty-five thousand years as described in the last chapter, the most recent of which was a massive expansion of people out of southeastern Europe by around fourteen thousand years ago that displaced much of the previously established population.12 In principal component analysis, the hunter-gatherers who lived in Europe at this time fell beyond present-day Europeans along an axis measuring the difference between Europe and the Near East. This was consistent with their having contributed ancestry to present-day Europeans but not to present-day Near Easterners.
Figure 15. This plot shows a statistical analysis of the primary gradients of genetic variation in present-day people (gray dots) and ancient West Eurasians (black and open dots). Ten thousand years ago, West Eurasia was home to four populations as differentiated from one another as Europeans and East Asians are today. The farmers of Europe and western Anatolia from nine thousand to five thousand years ago were a mixture of western European hunter-gatherers (A), Levantine farmers (C) and Iranian farmers (D). Meanwhile, the pastoralists of the steppe north of the Black and Caspian seas around five thousand years ago were a mixture of eastern European hunter-gatherers (B) and Iranian farmers (D). In the Bronze Age, these mixed populations mixed further to form populations with ancestry similar to people today.
Second came the first farmers, who lived between about eighty-eight hundred and forty-five hundred years ago in Germany, Spain, Hungary, and Anatolia. Ancient farmers from all these places were genetically similar to present-day Sardinians, showing that a pioneer farmer population had landed in Greece probably from Anatolia, and then spread to Iberia in the west and Germany in the north, retaining at least 90 percent of their DNA from that immigrant source, which meant that they mixed minimally with the hunter-gatherers they encountered along the way. Further investigation, though, showed that it was not quite so simple. We also found that farmers from the Peloponnese in southern Greece who lived around six thousand years ago may have derived part of their ancestry from a different source population in Anatolia—a population that descended more from Iranian-related populations than was the case in the northwestern Anatolian farmers who were a likely source population for the rest of Europe’s farmers.13 The first farming in Europe was practiced in the Peloponnese and the nearby island of Crete by people who did not use pottery. This has led some archaeologists to wonder if they were from a different migration.14 Our ancient DNA is consistent with this idea, and suggests the possibility that this population held on for thousands of years.
Third, we identified a new development in farmers living between six thousand and forty-five hundred years ago. In many of these later farmers, we observed a shift toward approximately 20 percent extra hunter-gatherer ancestry, not present in the early farmers, implying that genetic mixing between the previously established people and new arrivals had begun, albeit after a couple of thousand years’ delay.15
How did the farming and hunter-gatherer cultures coexist? Hints come from the Funnel Beaker culture, which is named for decorated clay vessels in graves dated after about sixty-three hundred years ago. The Funnel Beaker culture arose in a belt of land a few hundred kilometers from the Baltic Sea, which was not reached by the first wave of farmers, probably because their methods were not optimized for the heavy soils of northern Europe. Protected by the stronghold of their difficult-to-farm environment, and sustained by the fish and game resources of Baltic Europe, the northern hunter-gatherers had more than a thousand years to adapt to the challenge of farming. They adopted domesticated animals, and later crops, from their southern neighbors, but kept many elements of their hunter-gathering ways. The people of the Funnel Beaker culture were among those who built megaliths, the collective burial tombs made of stones so large it would have taken dozens of people to move them. The archaeologist Colin Renfrew suggested that megalith building might be a direct reflection of this boundary between southern farmers and hunter-gatherers turned farmers—a way of laying claim to territory, of distinguishing one people and culture from others.16 The genetic data may bear witness to this interaction, as there was clearly a stream of new migrants into the mixed population. Between six thousand and five thousand years ago, most of
the northern gene pool was overtaken by farmer ancestry, and it was this mixture of a modest amount of hunter-gatherer-related ancestry and a large amount of Anatolian farmer–related ancestry—in a population that retained key elements of hunter-gatherer culture—that characterized the Funnel Beaker potters and many other contemporary Europeans.
Europe had reached a new equilibrium. The unmixed hunter-gatherers were disappearing, persisting only in isolated pockets like the islands off southern Sweden. In southeastern Europe, a settled farmer population had developed the most socially stratified societies known up until that time, and rituals that as the archaeologist Marija Gimbutas showed featured women in a central way—a far cry from the male-centered rituals that followed.17 In remote Britain, the megalith builders were hard at work on what developed into the greatest man-made monument the world had seen: the standing stones of Stonehenge, which became a national place of pilgrimage as reflected by goods brought from the far corners of Britain. People like those at Stonehenge were building great temples to their gods, and tombs for their dead, and could not have known that within a few hundred years their descendants would be gone and their lands overrun. 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.
