A study of ancient DNA from hunter-gatherer cemeteries near Lake Baikal in East Siberia has identified plague infections dating to about 5,500 years ago. The findings indicate that Yersinia pestis was already capable of causing deadly outbreaks long before the rise of farming communities and cities commonly associated with later plague epidemics.
An international research team analyzed genetic material preserved in ancient teeth from human remains recovered at four cemeteries. Plague DNA was detected in 18 of the 46 individuals examined, or nearly 40 percent of the sample. The researchers reconstructed bacterial genomes and identified early strains that had not previously been known.
The study, published in Nature, combined the genetic results with archaeological evidence and radiocarbon dating. At two of the largest cemeteries, many of the deceased were children and young teenagers. Radiocarbon results showed that numerous burials occurred within relatively short periods, while some graves contained people who appear to have been siblings or parents and children who died at around the same time.
According to the researchers, these patterns are consistent with rapid, family-based outbreaks. The findings may also explain a concentration of young individuals that archaeologists had struggled to interpret at the cemeteries.
Earlier research had suggested that ancient Yersinia pestis strains lacked genetic features that later enabled efficient transmission through fleas and rodent hosts. That led some scientists to think early forms of plague may have been less capable of producing large or deadly outbreaks. The new analysis instead suggests that the strains identified at Lake Baikal were highly lethal, particularly for children, even without those later transmission mechanisms.
The team also reported a distinctive superantigen in the ancient strains. This toxin-producing genetic factor has not been found in later historic plague strains. Superantigens can provoke powerful immune reactions and are associated with severe inflammation, which the researchers say may have increased the danger posed by these infections.
The study adds support to the possibility that plague emerged in Central or North-East Asia before spreading through wild rodent populations. The hunter-gatherers examined had close contact with marmots, which still carry plague. Researchers propose that infected marmots may have transmitted the disease directly to people, triggering outbreaks within the prehistoric communities.
