A long-time Scandinavian woman has given off bacterial DNA by demonstrating that she contracted the first known case of plague in humans.
The DNA extracted from the woman's teeth comes from an ancient identified strain of Yersinia pestis the bacterium that causes the plague, the oldest ever found. The bones of women, dating from 5,040 to 4,867 years ago, were found about 20 years ago in a mass grave in an old farm in Sweden.
Teeth of an adult male in the same grave contain traces of the same variant of the plague, say the evolutionary geneticist Simon Rasmussen of the University of Copenhagen and colleagues. But the DNA of the woman's scourge is better preserved, the team reports online on December 6th at Cell .
Comparisons between the rediscovered Y. pestis strain with other ancient and modern strains suggest that a plague epidemic has emerged more than 5,000 years ago in densely populated farming communities in southeastern Europe. Then the plague spread elsewhere, even in Scandinavia, through the trade routes, concludes the Rasmussen team. That ancient epidemic apparently contributed to a sharp decline in the population in Europe that began as early as 8,000 years ago ( SN: 1
In particular, scientists suspect that the first form of plague developed between the Trypillia culture of south-eastern Europe between 6,100 and 5,400 years ago. The Trypillia settlements were the first to bring enough people to close contact to allow the evolution of a highly infectious version of Y. pestis suggests the team. The commercial networks transmitted the scourge from the centers of the population of Trypillia, which housed 10,000 to 20,000 people, to the shepherds of Western Asia known as Yamnaya ( SN: 25/11/17, 16 ), the researchers claim. In this scenario, the shepherds infected by the people of Trypillia probably spread what had become a new strain of the plague both to the east and to Siberia and to the west to the rest of Europe, including Scandinavia. Yamnaya's migrations to Europe coincided approximately with the rapid abandonment and destruction of large Trypillia settlements, probably caused by plague epidemics, scientists say.
Rasmussen and other investigators had previously suspected that the Yamnaya herders would soon bring Y. pestis strains from Asia to Europe ( SN: 28/11/15, page 7 ). "Now we can see that the plague was in Europe before the shepherds arrived," says Rasmussen.
DNA comparisons allow researchers to calculate that the scourge of the Scandinavian woman is the oldest of any Y. pestis variant identified so far. Based on a statistical model of how the bacterium evolved, scientists estimate that the Scandinavian strain probably diverged from others Y. pestis formed about 5,700 years ago. A variant of Eurasian plague previously dated between 4,800 and 3,700 years ago – the oldest known so far – originated about 5,300 years ago, calculates the team. This is the period when the cities of Trypillia were abandoned and the Yamnaya breeders walked west to Europe, says Rasmussen.
A form of ancestral plague to the current strains, found mainly in China, emerged in Eastern Asia about 5,100 years ago, the team estimates
However, archaeologists have not found any sign of the pest bacteria in the Trypillia sites. Rasmussen and colleagues plan to look for Y. pestis DNA in human skeletons from settlements there.
The variant of the newly discovered plague fits the scenario proposed by researchers, says the evolutionary geneticist Pontus Skoglund of the Francis Crick Institute in London. But it is also possible that centuries before, a still unknown Y . The pestis variant spread through Eurasia and in Scandinavia, says Skoglund. Later, that tension could have given rise to Y. pestis strains that infected the European peasants, the shepherds Yamnaya and the Scandinavian woman.