A study in the journal Cell reported the recovery of plague DNA from a 4,000-year-old sheep found at the Bronze Age settlement of Arkaim in Russia. Conducted by an international team of researchers, the work offers new clues to how an early form of Yersinia pestis spread across Eurasia.
Investigators examined bones and teeth from cattle, sheep, and goats buried at the Sintashta-Petrovka settlement. They identified Yersinia pestis, the bacteria responsible for the plague, in a sheep molar, assembling a near-complete genome that matched the Late Neolithic Bronze Age (LNBA) lineage known to have infected people at the same time. “If we didn’t know it was from a sheep, everyone would have assumed it was just another human infection—it’s almost indistinguishable,” said Christina Warinner of Harvard University, according to Phys.org.
The LNBA strain appeared about 5,000 years ago, circulated for nearly three millennia, and vanished roughly 2,000 years ago. Because it lacked the genes that enable flea-borne spread, its transmission route has remained a puzzle. The new evidence suggested that domesticated livestock served as intermediary hosts, bringing humans into contact with the pathogen as herders moved across the steppe.
Genetic comparisons indicated that an unknown wild reservoir repeatedly spilled the bacterium into both sheep and humans. “We can show that the ancient lineage evolved under elevated pressure, which is in contrast to the Y. pestis still found today. Moreover, the ancient sheep as well as human infections are likely isolated spillovers from the unknown reservoir, which remains at large. Finding that reservoir would be the next step,” said Felix Key, head of the Evolutionary Pathogenomics Lab at the Max Planck Institute of Infection Biology, according to EurekAlert.
Nearly identical LNBA genomes have been recovered up to 6,000 kilometers apart, a range that is hard to explain by the slow movement of people or herds alone. “Sheep and humans are unlikely to have been the main agents spreading the disease,” said Ian Light-Maka of the University of Arkansas, according to EurekAlert. The team also observed repeated mutations in the same genes, pointing to unusual evolutionary pressures.
The work highlighted the value of screening animal remains. “I think there will be more and more interest in analyzing these collections—they give us insights that no human sample can,” said Key, according to Phys.org. Warinner noted that the Sintashta-Petrovka culture’s extensive herding “provided plenty of opportunity for their livestock to come into contact with wild animals infected by Y. pestis. From then on it is just one more short hop into humans”.
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