A 5,000-year-old mass grave harbors the oldest plague bacteria ever found

A long-dead Scandinavian woman has yielded bacterial DNA showing that she contracted the earliest known case of the plague in humans.

DNA extracted from the woman’s teeth comes from a newly identified ancient strain of Yersinia pestis, the bacterium that causes plague, the oldest ever found. The woman’s bones, which date from 5,040 to 4,867 years ago, were found nearly 20 years ago in a mass grave at an ancient farming site in Sweden.

Teeth from an adult male in the same grave contain traces of the same plague variant, say evolutionary geneticist Simon Rasmussen of the University of Copenhagen and colleagues. But plague DNA from the woman is better preserved, the team reports online December 6 in Cell.

Comparisons of the newly found Y. pestis strain with other ancient and modern strains suggest that a plague epidemic emerged more than 5,000 years ago in densely populated farming communities in southeastern Europe. Then the plague spread elsewhere, including to Scandinavia, via trade routes, Rasmussen’s team concludes. That ancient epidemic apparently contributed to sharp population declines in Europe that began as early as 8,000 years ago (SN: 11/2/13, p. 12).

In particular, the scientists suspect that an early form of plague developed among southeastern Europe’s Trypillia culture between 6,100 and 5,400 years ago. Trypillia settlements were the first to bring enough people into close contact to enable the evolution of a highly infectious version of Y. pestis, the team suggests. Trading networks then transmitted the plague from Trypillia population centers, home to as many as 10,000 to 20,000 people, to West Asian herders known as the Yamnaya (SN: 11/25/17, p. 16), the researchers argue. In this scenario, herders infected by the Trypillia people probably spread what had become a new strain of the plague both eastward to Siberia and westward to the rest of Europe, including Scandinavia. Yamnaya migrations to Europe roughly coincided with the rapid abandonment and burning of large Trypillia settlements, which probably occurred as a result of plague outbreaks, the scientists say.

Rasmussen and other investigators had previously suspected that Yamnaya herders brought early Y. pestis strains from Asia into Europe (SN: 11/28/15, p. 7). “Now we can see that the plague was in Europe before the herders came,” Rasmussen says.

DNA comparisons let the researchers calculate that the Scandinavian woman’s plague strain is the oldest of any Y. pestis variant identified so far. Based on a statistical model of how the bacterium evolved, the scientists estimate that the Scandinavian strain probably diverged from other Y. pestis forms around 5,700 years ago. A Eurasian plague variant previously dated to between 4,800 and 3,700 years ago — the oldest known until now — originated around 5,300 years ago, the team calculates. That’s around the time Trypillia towns were abandoned, and Yamnaya herders trekked westward to Europe, Rasmussen says.

A form of the plague ancestral to present-day strains, which are mostly found in China, emerged in East Asia around 5,100 years ago, the team estimates.

Still, archaeologists haven’t found any sign of the plague bacterium at the Trypillia sites. Rasmussen and colleagues plan to look for Y. pestis DNA in human skeletons from settlements there.

The newly discovered plague variant fits the scenario that the researchers propose, says evolutionary geneticist Pontus Skoglund of the Francis Crick Institute in London. But it’s also possible that centuries earlier, an as-yet-undetected Y. pestis variant spread through Eurasia and into Scandinavia, Skoglund says. Later, that strain could have given rise to Y. pestis strains that infected European farmers, Yamnaya herders and the Scandinavian woman.