by David Reich
The Tide from the East
The grasslands of the steppe stretch about eight thousand kilometers from central Europe to China. Prior to five thousand years ago, the archaeological evidence indicates that almost no one lived far from the steppe river valleys, because in between these areas there was too little rain to support agriculture, and too few watering holes to support livestock. The European third of the steppe was a hodgepodge of local cultures, each with its own pottery style, spread thinly over the landscape in places where water could be found.18
All this changed with the emergence of the Yamnaya culture around five thousand years ago, whose economy was based on sheep and cattle herding. The Yamnaya emerged from previous cultures of the steppe and its periphery and exploited the steppe resources far more effectively than their predecessors. They spread over a vast region, from Hungary in Europe to the foothills of the Altai Mountains in central Asia, and in many places replaced the disparate cultures that had preceded them with a more homogeneous way of life.
One of the inventions that drove the spread of the Yamnaya was the wheel, whose geographic origin is not known because once it appeared—at least a few hundred years before the rise of the Yamnaya—it spread across Eurasia like wildfire. Wagons using wheels may have been adopted by the Yamnaya from their neighbors to the south: the Maikop culture in the Caucasus region between the Black and Caspian seas. For the Maikop, as for many cultures across Eurasia, the wheel was profoundly important. But for the people of the steppe, it was if anything even more important, as it made possible an economy and culture that were entirely new. By hitching their animals to wagons, the Yamnaya could take water and supplies with them into the open steppe and exploit the vast lands that had previously been inaccessible. By taking advantage of another innovation—the horse, which had recently been domesticated in a more eastern part of the steppe, and which made cattle herding more efficient as a single rider could herd many times the number of animals than could be herded by a person on foot—the Yamnaya also became vastly more productive.19
The profound transformation in culture that began with the Yamnaya is obvious to many archaeologists of the steppe. The increase in the intensity of the human use of the steppe lands coincided with a nearly complete disappearance of permanent settlements—almost all the structures that the Yamnaya left behind were graves, huge mounds of earth called kurgans. Sometimes people were buried in kurgans with wagons and horses, highlighting the importance of horses to their lifestyle. The wheel and horse so profoundly altered the economy that they led to the abandonment of village life. People lived on the move, in ancient versions of mobile homes.
Prior to the explosion of ancient DNA data in 2015, most archaeologists found it inconceivable that the genetic changes associated with the spread of the Yamnaya culture could be as dramatic as the archaeological changes. Even the archaeologist David Anthony, a leading proponent of the idea that the spread of Yamnaya culture was transformative in the history of Eurasia, could not bring himself to suggest that its spread was driven by mass migration. Instead, he proposed that most aspects of Yamnaya culture spread through imitation and proselytization.20
But the genetics showed otherwise. Our analysis of DNA from the Yamnaya—led by Iosif Lazaridis in my laboratory—showed that they harbored a combination of ancestries that did not previously exist in central Europe. The Yamnaya were the missing ingredient, carrying exactly the type of ancestry that needed to be added to early European farmers and hunter-gatherers to produce populations with the mixture of ancestries observed in Europe today.21 Our ancient DNA data also allowed us to learn how the Yamnaya themselves had formed from earlier populations. From seven thousand until five thousand years ago, we observed a steady influx into the steppe of a population whose ancestors traced their origin to the south—as it bore genetic affinity to ancient and present-day people of Armenia and Iran—eventually crystallizing in the Yamnaya, who were about a one-to-one ratio of ancestry from these two sources.22 A good guess is that the migration proceeded via the Caucasus isthmus between the Black and Caspian seas. Ancient DNA data produced by Wolfgang Haak, Johannes Krause, and their colleagues have shown that the populations of the northern Caucasus had ancestry of this type continuing up until the time of the Maikop culture, which just preceded the Yamnaya.
The evidence that people of the Maikop culture or the people who proceeded them in the Caucasus made a genetic contribution to the Yamnaya is not surprising in light of the cultural influence the Maikop had on the Yamnaya. Not only did the Maikop pass on to the Yamnaya their technology of carts, but they were also the first to build the kurgans that characterized the steppe cultures for thousands of years afterward. The penetration of Maikop lands by Iranian- and Armenian-related ancestry from the south is also plausible in light of studies showing that Maikop goods were heavily influenced by elements of the Uruk civilization of Mesopotamia to the south, which was poor in metal resources and engaged in trade and exchange with the north as reflected in Uruk goods found in settlements of the northern Caucasus.23 Whatever cultural process allowed the people from the south to have such a demographic impact, once the Yamnaya formed, their descendants expanded in all directions.24
How the Steppe Came to Central Europe
On the eve of the arrival of steppe ancestry in central Europe around five thousand years ago, the genetic ancestry of the people who lived there was largely derived from the first farmers who had come into Europe from Anatolia beginning after nine thousand years ago, with a minority contribution from the indigenous European hunter-gatherers who mixed with them. In far eastern Europe also around five thousand years ago, the genetic structure of the Yamnaya reflected a different mixture of ancestries: an Iranian-related population along with an eastern European hunter-gatherer population, in approximately equal proportions. Populations that were mixes of European farmers and steppe groups related to the Yamnaya had not yet formed.
The genetic impact of steppe ancestry on central Europe came in the form of peoples who were part of the ancient culture known to archaeologists as the Corded Ware, so named after its pots decorated by the impressing of twine into soft clay. Beginning around forty-nine hundred years ago, artifacts characteristic of the Corded Ware culture started spreading over a vast region, from Switzerland to European Russia. The ancient DNA data showed that beginning with the Corded Ware culture, individuals with ancestry similar to present-day Europeans first appeared in Europe.25 Nick Patterson, Iosif Lazaridis, and I developed new statistical methods that allowed us to estimate that in Germany, people buried with Corded Ware pots derive about three-quarters of their ancestry from groups related to the Yamnaya and the rest from people related to the farmers who had been the previous inhabitants of that region. Steppe ancestry has endured, as we also found it in all subsequent archaeological cultures of northern Europe as well as in all present-day northern Europeans.
The genetic data thus settled a long-standing debate in archaeology about linkages between the Corded Ware and the Yamnaya cultures. The two had many striking parallels, such as the construction of large burial mounds, the intensive exploitation of horses and herding, and a strikingly male-centered culture that celebrated violence, as reflected in the great maces (or hammer-axes) buried in some graves. At the same time, there were profound differences between the two cultures, notably the entirely different types of pottery that they made, with important elements of the Corded Ware style adapted from previous central European pottery styles. But the genetics showed that the connection between the Corded Ware culture and the Yamnaya culture reflected major movements of people. The makers of the Corded Ware culture were, at least in a genetic sense, a westward extension of the Yamnaya.
The discovery that the Corded Ware culture reflected a mass migration of people into central Europe from the steppe was not just a sterile academic finding. It had political and historical resonance. At the beginning of the twentieth century, the German archaeologist Gustaf Kossinna was among the f
irst to articulate the idea that cultures of the past that were spread across large geographic regions could be recognized through similarities in style of the artifacts they left behind. He also went further in viewing archaeologically identified cultures as synonymous with peoples, and he originated the idea that the spread of material culture could be used to trace ancient migrations, an approach he called the siedlungsarchäologische Methode, or “Settlement Archaeology.” Based on the overlap of the geographic distribution of the Corded Ware culture with the places where German is spoken, Kossinna suggested that the cultural roots of the Germans and of Germanic languages today lay in the Corded Ware culture. In his essay “The Borderland of Eastern Germany: Home Territory of the Germans,” he argued that because the Corded Ware culture included the territories of Poland, Czechoslovakia, and western Russia of his day, it gave Germans the moral birthright to claim those regions as their own.26
Kossinna’s ideas were embraced by the Nazis, and although he died in 1931, before they came to power, his scholarship was used as a basis for their propaganda and a justification for their claims to territories to the east.27 Kossinna’s suggestion that migration was the primary explanation for changes in the archaeological record was also attractive to the Nazis because it played into their racist worldview, as it was easy to imagine that migrations had been propelled by innate biological superiority of some peoples over others. Following the Second World War, European archaeologists reacting to the politicization of their field began picking apart the arguments of Kossinna and his colleagues, documenting cases in which changes in material culture were brought about through local invention or imitation and not the spread of people. They urged extreme caution about invoking migration to explain changes in the archaeological record. Today, a common view among archaeologists is that migrations are only one of many explanations for past cultural change. Many archaeologists still argue that when there is evidence for major cultural change at a site, the working assumption should be that the changes reflect communication of ideas or local invention, not necessarily movements of people.28
Discussions of the Corded Ware culture and migration in the same breath ring particularly loud alarm bells because of Kossinna’s and the Nazis’ attempt to use the Corded Ware culture to construct a basis for national German identity.29 While we were in the final stages of preparing a paper for submission in 2015, one of the German archaeologists who contributed skeletal samples wrote a letter to all coauthors: “We must(!) avoid…being compared with the so called ‘siedlungsarchäologische Methode’ from Gustaf Kossinna!” He and several contributors then resigned as authors before we modified our paper to highlight differences between Kossinna’s thesis and our findings, namely that the Corded Ware culture came from the east and that the people associated with it had not been previously established in central Europe.
The correct theory that the Corded Ware culture spread through a migration from the east had already been proposed in the 1920s by Kossinna’s contemporary, the archaeologist V. Gordon Childe,30 although this idea too fell out of favor in the wake of the Second World War and the reaction to the abuse of archaeology by the Nazis, a reaction that took the form of extreme skepticism about any claims of migration.31 Our finding about the genetic link between the Yamnaya and the Corded Ware culture demonstrates the disruptive power of ancient DNA. It can prove past movements of people, and in this case has documented a magnitude of population replacement that no modern archaeologist, even the most ardent supporter of migrations, had dared to propose. The association between steppe genetic ancestry and people assigned to the Corded Ware archaeological culture through graves and artifacts is not simply a hypothesis. It is now a proven fact.
How was it that the low-population-density shepherds from the steppe were able to displace the densely settled farmers of central and western Europe? The archaeologist Peter Bellwood has argued that once densely settled farming populations were established in Europe, it would have been practically impossible for other groups coming in to make a demographic dent, as their numbers would, he thought, have been dwarfed by the already established population.32 As an analogy, consider the effect of the British or Mughal occupations of India. Both powers controlled the subcontinent for hundreds of years, but left little trace in the people there today. But ancient DNA shows definitively that major population replacement happened in Europe after around forty-five hundred years ago.
How were people with steppe ancestry able to have such an impact on an already settled region? A possible answer is that the farmers who preceded them may not have occupied every available economic niche in central Europe, giving the steppe peoples an opportunity to expand. Although it is difficult to estimate population sizes from archaeological evidence, the number of people in northern Europe before two thousand years ago has been estimated to be around one hundred times less than today or even smaller, reflecting less efficient farming methods, lack of access to pesticides and fertilizers, the absence of high-yielding plant varieties, and higher infant mortality.33 When the Corded Ware culture arrived, many tilled fields in central Europe were surrounded by virgin forests. But studies of pollen records in Denmark and elsewhere show that around this time, large parts of northern Europe were transformed from partial forest to grasslands, suggesting that the Corded Ware newcomers may have cut down forests, reengineered parts of the landscape to be more like the steppe, and carved out a niche for themselves that previous peoples of the region had never fully claimed.34
There is also a second possible explanation for why the steppe peoples were able to become established in Europe—one that no one would have thought plausible without ancient DNA. Eske Willerslev and Simon Rasmussen, working with the archaeologist Kristian Kristiansen, had the idea of testing 101 ancient DNA samples from Europe and the steppe for evidence of pathogens.35 In seven samples, they found DNA from Yersinia pestis, the bacterium responsible for the Black Death, estimated to have wiped out around one-third of the populations of Europe, India, and China around seven hundred years ago. Traces of plague in a person’s teeth are almost a sure sign that he or she died of it. The earliest bacterial genomes that they sequenced lacked a few key genes necessary for the disease to spread via fleas, which is necessary to cause bubonic plague. But the bacterial genomes did carry the genes necessary to cause pneumonic plague, which is spread by sneezing and coughing just like the flu. That a substantial fraction of random graves analyzed carried Y. pestis shows that this disease was endemic on the steppe.
Is it possible that the steppe people had picked up the plague and built up an immunity to it, and then transmitted it to the immunologically susceptible central European farmers, causing their numbers to collapse and thereby clearing the way for the Corded Ware culture expansion? This would be a great irony. One of the most important reasons for the collapse of Native American populations after 1492 was infectious diseases spread by Europeans who plausibly had built up some immunity to these diseases after thousands of years of exposure as a result of living in close proximity to their farm animals. But Native Americans, who by and large lacked domesticated animals, likely had much less resistance to them. Was it possible that, in a similar way, northern European farmers after five thousand years ago were decimated by plagues brought from the east, paving the way for the spread of steppe ancestry through Europe?
How Britain Succumbed
After the wave of steppe ancestry crashed over central Europe, it kept rolling. Beginning around forty-seven hundred years ago, a couple of centuries after the Corded Ware culture swept into central Europe, there was an equally dramatic expansion of the Bell Beaker culture, probably from the region of present-day Iberia. The Bell Beaker culture is named for its bell-shaped drinking vessels that rapidly spread over a vast expanse of western Europe alongside other artifacts including decorative buttons and archers’ wristguards. It is possible to learn about the movement of people and objects by studying the ratios of isotopes of elements like strontium, lead, and ox
ygen that are characteristic of materials in different parts of the world. By studying the isotopic composition of teeth, archaeologists have shown that some people of the Bell Beaker culture moved hundreds of kilometers from their places of birth.36 Bell Beaker culture spread to Britain after forty-five hundred years ago.
A major open question for understanding the spread of the Bell Beaker culture has always been whether it was propelled by the movement of people or the spread of ideas. At the beginning of the twentieth century, the recognition of the massive impact of the Bell Beaker culture led to the romantic notion of a “Beaker Folk,” a people who disseminated a new culture and perhaps Celtic languages—a nod to the nationalistic fervor of the time. But, like the claim made for the Corded Ware culture, this position fell out of favor after the Second World War.
In 2017, my laboratory succeeded in assembling whole-genome ancient DNA data from more than two hundred skeletons associated with the Beaker culture from across Europe.37 Iñigo Olalde, a postdoctoral scientist, analyzed the data to show that individuals in Iberia were genetically indistinguishable from the people who had preceded them and who were not buried in a Bell Beaker culture style. But Bell Beaker–associated individuals in central Europe were extremely different, with most of their ancestry of steppe origin, and little if any ancestry in common with individuals from Iberia associated with the Bell Beaker culture. So, in contrast to what happened with the spread of the Corded Ware culture from the east, the initial spread of the Bell Beaker culture across Europe was mediated by the movement of ideas, not by migration.