A TEAM of researchers from France, Sweden, and Denmark have identified a new strain of Yersinia pestis, the bacteria that causes plague, in DNA extracted from 5,000-year-old human remains.
Their analyses, publishing December 6 in the journal Cell, suggest that this strain is the closest ever identified to the genetic origin of plague. Their work also suggests that plague may have been spread among Neolithic European settlements by traders, contributing to the settlements’ decline at the dawn of the Bronze Age.
“Plague is maybe one of the deadliest bacteria that has ever existed for humans. And if you think of the word ‘plague,’ it can mean this infection by Y. pestis, but because of the trauma plague has caused in our history, it’s also come to refer more generally to any epidemic.
The kind of analyses we do here let us go back through time and look at how this pathogen that’s had such a huge effect on us evolved,” says senior author Simon Rasmussen (@simonrasmu), a metagenomics researcher at the Technical University of Denmark and the University of Copenhagen.
To better understand the evolutionary history of the plague, Rasmussen and his colleagues trawled through publicly available genetic data from ancient humans, screening for sequences similar to more modern plague strains.
They found a strain they had never seen before in the genetic material of a 20-year-old woman who died approximately 5,000 years ago in Sweden.
The strain had the same genes that make the pneumonic plague deadly today and traces of it were also found in another individual at the same grave site–suggesting that the young woman did likely die of the disease.
This strain of the plague is the oldest that’s ever been discovered. But what makes it particularly interesting is that, by comparing it to other strains, the researchers were able to determine that it’s also the most basal–meaning that it’s the closest strain we have to the genetic origin of Y. pestis.
It likely diverged from other strains around 5,700 years ago, while the plague that was common in the Bronze Age and the plague that is the ancestor of the strains in existence today diverged 5,300 and 5,100 years ago, respectively. This suggests that there were multiple strains of plague in existence at the end of the Neolithic period.
Rasmussen also believes that this finding offers a new theory about how plague spreads.
Massive human migrations from the Eurasian steppe down into Europe are known to have occurred around 5,000 years ago, but how these cultures were able to displace the Neolithic farming culture that was present in Europe at the time is still debated.