plague pit
© Archeodunum SAS/Gourvennec MichaëlMass grave, Toulouse
In the 14th century, the Black Death wiped out as much as 60% of the population of Europe, spreading rapidly from the shores of the Black Sea to central Europe. Although historical records first document its appearance in 1346 C.E. in the lower Volga region of Russia, researchers didn't know whether the highly virulent strain of Yersinia pestis bacterium that caused the deadly pandemic came from a single source or was introduced to Europe more than once by travelers carrying diverse strains of plague from different parts of the ancient world.

Now, by analyzing 34 ancient genomes of Y. pestis from the teeth of people buried at 10 sites across Europe from the 14th to 17th centuries (including a mass grave in Toulouse, France — above), researchers at the Max Planck Institute for the Science of Human History in Jena, Germany, have found the earliest known evidence of this pandemic comes from Laishevo, in Russia's Volga region. There, researchers found a strain of Y. pestis that was ancestral to all other genomes they studied, differing by only one mutation from those that caused the Black Death in Europe, they report today in Nature Communications.

That doesn't mean the Volga region was ground zero for the Black Death — it could have come from elsewhere in western Asia, where scientists have yet to sample ancient DNA of Y. pestis. The researchers found that once the plague made it to Europe, a single strain was responsible for the Black Death, from Italy to the United Kingdom. This strain also gave rise to other variants of Y. pestis that caused deadly plague outbreaks from the late 14th through the 18th centuries. This suggests the bacterium persisted locally in Europe, perhaps in rodent hosts, where it evolved into diverse strains that caused later epidemics.