Scientists have identified the oldest strain ever found of the Yersinia pestis bacteria that causes the plague.
The strain, discovered in the DNA of a 20-year-old woman who lived in Sweden more than 5000 years ago, is the closest to the plague microbe’s genetic origin and pre-dates all modern and other ancient known strains.
This suggests, say researchers led by Simon Rasmussen from the Technical University of Denmark, that there were multiple strains of plague in existence at the end of the Neolithic period, and that the disease may have contributed to population decline at the dawn of the Bronze Age.
“These analyses revealed that multiple and independent lineages of Y. pestis branched and expanded across Eurasia during the Neolithic decline, spreading most likely through early trade networks rather than massive human migrations,” they write in a paper published in the journal Cell.
“Our results are consistent with the existence of a prehistoric plague pandemic that likely contributed to the decay of Neolithic populations in Europe.”
The discovery was made as the researchers trawled through publicly available genetic data from ancient humans, screening for sequences similar to more modern plague strains.
The new strain found in the remains of the Swedish woman had the same genes that make pneumonic plague deadly today. There were also traces in another individual at the same grave site, suggesting, Rasmussen and colleagues say, that the woman did likely die of the disease.
As well as the oldest strain of plague every found it is also the most basal, making it the closest to the original form of Y. pestis.
“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 Rasmussen.
He believes the finding offers a new theory about how plague spreads.
Massive human migrations from the Eurasian region into Europe are known to have occurred around 5000 years ago, and researchers – including Rasmussen – previously have suggested the invaders brought the plague with them, wiping out the large settlements of Stone Age farmers when they arrived.
However, the newly discovered strain appears to have diverged from the rest of Y. pestis before these migrations began – when the Neolithic European settlements were already starting to collapse. At that time, the mega-settlements of up to 20,000 inhabitants becoming common in Europe were prime breeding grounds for the disease.
“We think our data fit,” Rasmussen explains. “If plague evolved in the mega-settlements, then when people started dying from it, the settlements would have been abandoned and destroyed. This is exactly what was observed in these settlements after 5500 years ago.
“Plague would also have started migrating along all the trade routes made possible by wheeled transport, which had rapidly expanded throughout Europe in this period.”
Eventually, he suggests, it would have arrived through these trade interactions at the small settlement in Sweden where the woman lived. She wasn’t genetically related to the people who invaded Europe from the Eurasian steppe, supporting the idea that the bacterium arrived before the mass migrations.
Rasmussen acknowledges there are limitations to what the study data can determine, and that his team hasn’t yet identified the plague in individuals from the mega-settlements where it may have evolved. But that’s in part because they haven’t looked yet.