It’s a lot to conclude from just a few ancient teeth.

The team had set out looking for evidence of plague in publicly available genetic data sets. Getting DNA from ancient bones and teeth is still a fairly new procedure, and it has largely focused on the stuff from humans. But modern sequencing techniques pick out all the genetic material in a sample, which frequently includes a lot of contaminating bacteria. For this reason, “normally 95 percent, 99 percent of the data was just thrown into the Dumpster,” Simon Rasmussen, who studies pathogen evolution at the Technical University of Denmark and co-authored the study, told me. The idea was to look through all this genetic detritus for signs of Yersinia pestis. When people die of the plague, their blood has high levels of bacteria, which leave a distinct genetic signature in the dental pulp inside teeth.

Rascovan ultimately sorted through the genomes of about 100 ancient individuals found throughout Europe. When he found Yersinia pestis in the teeth of the 4,900-year-old woman in Sweden, he looked at other people buried in the same tomb. Up to 78 people were buried around the same time, suggesting a surge in deaths that could have become an epidemic. Indeed, a young man in the tomb also had fragments of plague bacteria in his teeth. The strain of Yersinia pestis in the grave site was distinct from all others ever sequenced. The team thinks that it diverged from other known strains 5,700 years ago.

Rasmussen and others had previously found plague bacteria in ancient human remains all over Eurasia. Prior to this discovery, the oldest and most genetically distinct strain had been found in Central Asia. This fit a neat narrative: Steppe pastoralists from Eurasia began migrating into western Europe around 4,900 years ago, and they could have brought the plague with them. The disease seemed to explain how they were able to supplant the farmers—like the Swedish woman—already living in Europe. In the span of a few centuries, the steppe pastoralists would replace 70 percent of the population in Central Europe and 90 percent in Great Britain. In fact, modern Europeans derive most of their ancestry from steppe pastoralists who came during this period.

But now, the dates don’t match up. If the Swedish plague strain diverged from Central Asian ones 5,700 years ago, but human migration to the region didn’t start until 4,900 years ago, how did plague get to Sweden? Rascovan and Rasmussen turned to their archaeologist colleagues, who saw that this development fit into another pattern: Populations in Europe were falling even before people from the steppes migrated there.

Around 6,000 years ago, mega-settlements as big as 10,000 to 20,000 people sprang up in what is now Moldova, Romania, and Ukraine. The settlements were regularly abandoned and then burned. Perhaps, the new study argues, plague spread through these sites. At the same time, innovations such as wheeled transport and metallurgy allowed distant trade routes to emerge, possibly connecting mega-settlements with far-flung villages like those in Sweden. Thus, some populations were dense enough for a new pathogen to emerge, and trade routes helped spread it across the continent. “This might actually be one of the first plague epidemics,” Rascovan says.