New research is reshaping the timeline of one of humanity's most studied pathogens. Scientists analyzing ancient DNA from burial sites near Lake Baikal in Siberia have identified evidence of plague dating back about 5,500 years, making it the oldest known outbreak of its kind.
The study examined remains from 42 hunter-gatherers buried across four cemeteries. In nearly two-fifths of the individuals, researchers found traces of Yersinia pestis, the bacterium responsible for plague. The findings suggest that severe outbreaks were already affecting small mobile communities long before farming societies and large cities became widespread.
What the genomes show
By combining ancient bacterial genomes, radiocarbon dating, and kinship analysis, the team reconstructed two early phases of infection that likely began around 5,600 to 5,400 years ago. The genetic data place these strains on an early branch of the Y. pestis family tree, close to the point where it split from Y. pseudotuberculosis.
One notable detail is the absence of the ymt gene, which later helped plague spread through fleas. That means this early form of the disease likely moved in a different way, possibly through close human contact. The researchers also point to wild marmots as a possible animal reservoir in the region.
A window into early disease evolution
The burial evidence suggests the outbreaks were fast-moving and may have affected children especially hard. Several infected individuals were between 8 and 11 years old, offering a rare glimpse into how ancient communities experienced infectious disease.
Beyond the historical significance, the study highlights how zoonotic diseases can emerge in very different societies and environments. It also adds a new chapter to the story of plague's evolution, showing that major pathogen shifts were already underway thousands of years ago. This discovery may help scientists better understand how animal-to-human spillovers shaped the future of infectious disease.
