Ancient DNA analysis has uncovered evidence that the plague was infecting people in Siberia as far back as 5,500 years ago — long before the rise of cities and the infamous flea-driven epidemics of the medieval period. This discovery suggests that the disease may have been more widespread and deadly in prehistoric times than previously thought. The study focused on human remains from four hunter-gatherer burial sites in the Lake Baikal region of East Siberia. Scientists extracted genetic material from the teeth of these ancient individuals and used it to reconstruct the DNA of the bacteria that caused the plague, revealing previously unknown early forms of the disease. Among the 46 people tested, DNA from Yersinia pestis — the bacterium responsible for plague — was found in 18 of them. This means nearly 40 percent of the individuals carried evidence of the infection, a detection rate higher than that seen in some medieval plague burial sites. This finding challenges the common belief that the plague became a major human health threat only after the development of urban centers and the spread of infected fleas. What makes these ancient strains of plague particularly intriguing is the presence of a unique genetic factor known as a superantigen. Superantigens are proteins that can trigger an unusually strong immune response, leading to severe inflammation and potentially life-threatening conditions. This genetic feature is not found in the plague strains that caused the medieval Black Death, suggesting that these ancient forms of the disease may have been more virulent or deadly in their own right. The study also supports the theory that the plague may have originated in Central or North-East Asia before spreading across Eurasia through wild rodent populations. Archaeological evidence shows that the hunter-gatherers near Lake Baikal had close contact with marmots — large burrowing rodents that are still known to carry the plague today. Researchers believe that some of these early outbreaks may have occurred when the disease jumped directly from infected marmots to humans, marking the beginning of a long and complex relationship between humans and this deadly pathogen.