The Black Death was only the beginning. Countless millions perished in this terrible early wave – an estimated 60 percent of Europe was wiped out – but the virulent bacterium responsible was never actually contained.
When the Black Death of the mid–14th century was over, Yersinia pestis was far from done, laying waste to human life for another 500 years. This grim, recurring saga of outbreaks – called the second plague pandemic – lasted until the 19th century. But where did its deadly antagonist originate?
In a new study, an international team of scientists reconstructed 34 Y. pestis genomes sourced from the teeth of 34 individuals who died in 10 different countries – tracing a kind of genetic family tree of shadowy pestilence spanning the 14th to 17th centuries.
The family tree, encompassing the remains of people who were infected by the bacterium in England, France, Germany and elsewhere, reveals a diversification of the Y. pestis lineage over time into multiple genetically distinct clades. Nonetheless, these clades appear to have one common starting point.
"These findings indicate a single entry of Y. pestis into Europe through the east", says archaeogeneticist Maria Spyrou from the Max Planck Institute for the Science of Human History, on the basis that one strain in particular looks to be the ancestor of all the second plague pandemic strains that came after it.
The precursor, the researchers say, came from Russia, specifically a town called Laishevo in the historical Volga region, based on the evidence of a sample known as LAI009.
"Our phylogenetic reconstruction shows that the LAI009 isolate from Laishevo is ancestral to the Black Death isolates from southern, central, western and northern Europe, as well as to the previously published late 14th-century isolates from London and Bolgar City," the researchers explain in their paper.
"We interpret LAI009 as the most ancestral form of the strain that entered Europe during the initial wave of the second pandemic that has been identified to date."
Of course, in reconstructions like this, conclusions are necessarily limited by the scope of skeletal remains you get to dig up and study. In other words, the researchers acknowledge it's entirely possible that the pestilence – in this era of history, at least – may have had earlier forms in other places that have not yet been sufficiently tested.
"It is possible that additional interpretations may be revealed with future discoveries of unsampled diversity in western Eurasia," Spyrou says.
In any case, once the Russian strain took hold in the early stages of the second plague pandemic, it branched off into multiple variant forms over the following centuries. Modern descendants of these variants have never been found, the team says, suggesting they have become extinct.
While the new findings cannot be truly definitive about the ancestor of the Black Death outbreak, they do nonetheless illustrate the earliest known genetic origins of what became a 500-year plague – telling us more about an ancient pathogen that existed long before the Black Death's medieval shadow, and still darkens our days even now.
For such a persistent and powerful companion to humanity, there's probably no such thing as too much information. If we must share the planet with Y. pestis, we need all the intel we can get.
"The second plague pandemic has arguably caused the highest levels of mortality of the three recorded plague pandemics," the researchers write.
"It serves as a classic historical example of rapid infectious disease emergence, long-term local persistence and eventual extinction for reasons that are currently not understood."
The findings are reported in Nature Communications.