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Plague Bacteria Entered Europe During Neolithic Age, Genetic Analysis Finds

NEW YORK (GenomeWeb) – Yersinia pestis, the bacterium that causes the plague, likely traveled to Europe as people from the Central Eurasian steppes expanded there, according to a new genetic analysis.

Y. pestis has been linked to a number of historical outbreaks such as the Plague of Justinian and the Black Death, but recent work has uncovered evidence that the bacterium caused disease even earlier.

A team led by researchers from the Max Planck Institute for the Science of Human History sifted through human teeth and bones uncovered at different European sites that date back to the Late Neolithic to Bronze Ages — about 4,800 years to 3,700 years ago — for signs of the plague. As they reported today in Current Biology, the researchers recovered and reconstructed Y. pestis genomes from six individuals. By analyzing those genomes in conjunction with human archeological and genetic evidence, they pieced together how the plague came to Europe.

"In our view, the human genetic ancestry and admixture, in combination with the temporal series within the Late Neolithic-Bronze Age Y. pestis lineage, support the view that Y. pestis was possibly introduced to Europe from the steppe around 4,800 years ago, where it established a local reservoir before moving back towards Central Eurasia," Max Planck's Alexander Herbig said in a statement.

The researchers screened 563 bone and tooth samples dating from the Late Neolithic to the Bronze Age, from Russia, Germany, Hungary, and other sites in Eastern Europe for Y. pestis by mapping sequencing reads from those samples against a reference set of more than 12 Yersinia genomes. Based on the number of reads from the samples that corresponded to the Yersinia references, the researchers uncovered half a dozen samples that appeared to harbor the plague.

Those six samples then underwent deeper sequencing, and Herbig and his colleagues reconstructed their genomes with a mean three- to 12-fold coverage. Using three different approaches, the researchers added their Y. pestis samples to the plague phylogenetic tree. Their six samples along with two previously reported ancient strains formed their own clade.

All the plague genomes dating back from the Late Neolithic to the Bronze Age were highly similar, even those uncovered in different European regions, the researchers noted. Based on this, they said two scenarios could explain how Y. pestis spread into Europe. In the first possible chain of events, the plague was introduced into Europe a number of times from a common reservoir in Central Eurasia. Conversely, in the second scenario, the plague could have entered Europe once and established a reservoir there from which it circulated, before later returning to Central Eurasia.

To gauge which of those scenarios was the more likely, Herbig and his colleagues decided to examine their bacterial genomic data in the context of human genetic and archaeological data from the Late Neolithic to Bronze Ages.

Some 4,800 years ago, people from the Central Eurasian steppes began to migrate into Europe, bringing distinctive genetic markers with them. The earliest indication of plague in Europe coincides with this influx of migrants — the Lithuanian and Estonian plague samples in this study were isolated from individuals with this distinctive genetic complex — and suggests that the plague spread with the migration of the steppe nomads, the researchers said. The plague then likely persisted in Europe until the middle of the Bronze Age when it moved again with human populations back to Central Eurasia.

The researchers also noted that the plague bacteria dating back to this era lacked plasmid virulence and other infection-related genetic factors now found in Y. pestis. This suggested that the ancient form of the disease might have differed in severity and disease dynamics.