We can understand the prehistoric past only by interpreting the things people left behind. Finds don't come with words to explain how an object arrived at a site or why people decorated a pot a certain way. So there’s a lot of detail about prehistoric people’s lives, cultures, and interactions that these objects can only hint at. In recent years, however, the DNA of ancient people has added depth and detail to the information gleaned from artifacts. Genomic studies, it turns out, can tell us who the people using those artifacts were and where they came from.

Most of the genomic work so far has been relatively small-scale due to the massive effort involved in sampling and processing ancient DNA, but two new studies add several hundred prehistoric genomes to the existing data.

“The two studies published this week approximately double the size of the entire ancient DNA literature and are similar in their sample sizes to population genetic studies of people living today,” Harvard Medical School geneticist David Reich, who coordinated the studies, told Ars. “We can pick out subtleties in ancient demographic process that were more difficult to appreciate using the small sample size studies available before.”

And those studies reveal some surprising things about how prehistoric Europeans moved around, interacted, and shared ideas.

How genes can trace the flow of ideas

We have no way of knowing what prehistoric people called themselves, or even what their neighbors called them, so in most cases we can only refer to them based on the things they left behind. The Bell Beaker culture, for example, left behind stylized bell-shaped pots, along with copper daggers, perforated buttons, stone wrist guards, and arrowheads. These bell-shaped pots first turned up in Spain around 4,750 years ago, and they had spread throughout Central and Western Europe and Northwest Africa by about 4,500 years ago, largely replacing local styles of pottery. By 4,300 years ago, Bell Beaker artifacts began appearing in Britain, replacing artifacts associated with the culture that built Stonehenge.

But when you can only see that one style of pot replaces another, it’s hard to tell if that happened because the people who made the Bell Beaker artifacts swept in and wiped out the earlier culture or if it happened because people saw Bell Beaker artifacts and decided to copy them. Both processes look pretty much the same in the archaeological record that’s currently available, but they’re very different versions of human history.

Genomic studies of ancient remains can offer some answers by telling archaeologists whether the people associated with Bell Beaker artifacts in one place were related to the people who lived there before the change in pottery style. And the answer, according to the genomes of 400 prehistoric people from Britain, Iberia, and Central Europe, is that both versions of the story happened in different parts of Europe.

The genetic profile of people buried with Bell Beaker goods in Iberia during the Bell Beaker heyday looks very different from those who were using the same kinds of objects in Central Europe. In fact, there seems to have been very little interbreeding at all. That implies that people weren’t migrating en masse and displacing others, but their ideas were spreading through social and trade networks.

“It is clear from this that the Beaker phenomenon spread between the two regions largely by copying of ideas by people in contact with each other,” said Reich. “These people traveled long distances and traded up and down the rivers and seaways and encountered each other.”

But Britain is a different story. People buried with Bell Beaker artifacts in Copper Age Britain are genetically similar to people buried with Bell Beaker artifacts from Central Europe, while the people associated with Britain’s earlier culture are from a completely different population. Sometime around 4,300 years ago, people from Central Europe migrated to Britain, bringing their bell-shaped pots with them, and, within a century or two, they had completely replaced the older Britons who built Stonehenge.

Glimpses of prehistoric social networks

An even larger mass migration brought agriculture into Europe about 8,000 years ago, when farmers from Anatolia moved north and west into Europe, mostly following the Danube River and the Mediterranean coastline. That’s not a new idea, but genomic data adds some surprising detail to the story, described in a second study of 225 sets of ancient European remains, dating from 14,000 to 2,500 years ago.

The incoming population from Anatolia almost completely replaced the previous residents of Southeastern and Central Europe within centuries of their arrival. This wasn’t a mere cultural shift; it was a nearly complete population turnover.

“It’s not clear how these movements happened—whether by disease, or by violence, or by some kind of biased intermarriage process—but what the genetics shows unambiguously is that these changes did happen and much more dramatically than any archaeologists expected,” said Reich. But the genomic data offers some hints about how that population turnover may have played out, and it may actually have happened a little differently everywhere two groups of people interacted.

For instance, the northward spread of farming stopped for several thousand years at the boundary of the latitudes where most crops grow well; archaeologists knew that based on earlier finds. What the genomic study reveals is that along that boundary, in Northern Europe, genes associated with earlier hunter-gatherer groups tended to show up more on Y-chromosomes than X-chromosomes, which implies that males were passing on hunter-gatherer ancestry more than females. That’s a hint that women from farming communities tended to marry into hunter-gatherer communities, but not necessarily the other way around.

The genomic data can’t tell us why that may have happened, but it raises some fascinating questions and adds a layer of detail that we wouldn’t have had otherwise.

Reich and his colleagues hope to carry out similar studies in other parts of the world, where they might unravel more nuances of past human movements. “These two studies are exciting in part because they show what it will be possible to do in other regions of the world where similarly large-scale studies are deployed,” said Reich.

Nature, 2017. DOI: 10.1038/nature25738 (About DOIs).