A 5000-year-old mass grave houses the oldest bacteria of the plague ever found - Science News

A long-time Scandinavian woman gave 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 strain identified recently Yersinia pestis, the bacterium that causes the plague, the oldest ever found. The woman's bones, which date from 5,040 to 4,867 years ago, were found about 20 years ago in a mass grave in an old farm in Sweden.

The teeth of an adult male in the same grave contain traces of the same variant of the wound, affirm 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 Cell.

Compare new ones Y. pestis Tension with other ancient and modern varieties suggests 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. This ancient epidemic has helped to drastically reduce the population in Europe, which began 8,000 years ago (SN: 11/23/13, p. 12).

In particular, scientists suspect that a first form of plague has 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 inhabited centers of Trypillia, which housed 10,000 to 20,000 people, to the shepherds of Western Asia known as Yamnaya (SN: 11/25/17, p. 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, which probably occurred as a result of epidemics of sores, scientists say.

Rasmussen and other investigators had previously suspected that the Yamnaya herders would soon bring Y. pestis strains from Asia in Europe (SN: 11/28/15, p. 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 all Y. pestis variant identified so far. Based on a statistical model of how the bacterium evolved, scientists estimate that the Scandinavian strain has probably moved away from the others Y. pestis forms 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 have been abandoned and the Yamnaya breeders have walked west to Europe, Rasmussen says.

A form of ancestral plague to the current strains, found mainly in China, emerged in eastern Asia about 5,100 years ago, according to team estimates.

However, archaeologists have not found any sign of the pest bacteria in the Trypillia sites. Rasmussen and colleagues are planning to look Y. pestis DNA in human skeletons from settlements there.

The new variant of the 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 not yet discovered Y. pestis variant spread through Eurasia and Scandinavia, says Skoglund. Later, that tension could have given rise to Y. pestis strains that have infected the European farmers, the Yamnaya shepherds and the Scandinavian woman.

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