The recent discovery of ancient DNA from 5,500-year-old Siberian graves has revealed a previously unknown strain of plague, adding to the growing evidence that the disease may have originated in Central or Northeast Asia. This strain, which contained a genetic superantigen that made infections particularly deadly for children, challenges the understanding of early plague outbreaks and the mechanisms of transmission. The findings also highlight the complex relationship between hunter-gatherer communities and zoonotic diseases, such as plague, and the potential role of marmots as the oldest reservoir species of the disease.
One of the most intriguing aspects of this discovery is the high rate of infection among children and young adolescents in the cemeteries. The study authors found that 39% of the individuals in the cemeteries showed signs of plague infection, with some graves containing siblings, parents, and children buried together, suggesting that the disease spread within families. This finding challenges the traditional epidemiological theory that infectious diseases cannot devastate entire communities, as hunter-gatherers were constantly moving around the landscape.
The study also sheds light on the kinship between individuals buried in the cemeteries, with genetic research revealing that some relatives were buried apart, presumably because they died during different waves of the disease. This suggests that the plague outbreaks occurred over a relatively short period, with multiple waves affecting the community. The discovery of marmot teeth pendants within the graves further supports the hypothesis that marmots were the primary source of infection for the hunter-gatherers, who likely hunted, skinned, and butchered them for their meat and fur.
The presence of a unique superantigen in the plague strain, which is also found in modern-day Yersinia pseudotuberculosis, raises questions about the earliest mode of transmission. The study authors suggest that humans may have contracted the infection from eating raw or undercooked contaminated food or untreated drinking water, which could have been a significant factor in the spread of the disease. The discovery of this superantigen also highlights the potential for early plague strains to have a potent combination of virulence factors, making them highly lethal even before the bacterium evolved efficient flea-borne transmission.
The implications of this discovery are far-reaching, as it challenges the understanding of the origins and spread of plague, and the role of different populations in the evolution of the disease. The study authors suggest that the plague may have originated in Central or Northeast Asia, and spread across Eurasia long before the rise of agriculture, dense populations, or crowded cities associated with later outbreaks. This highlights the importance of studying ancient plague ecology to understand the complex relationships between different life ways, such as hunter-gatherer and nomadic pastoralist communities, and their interactions with animals.
However, many mysteries about plague still remain, including how it spread across Northern Eurasia so quickly. The study authors note that the outbreak in the Baikal hunter-gatherers, who were culturally and genetically isolated from non-hunter-gatherer populations, appeared in Northern Europe only 200-300 years later. This raises questions about the role of human-to-human transmission, spillover infections into humans at either end, and the potential for rapid transmission through wild animals. Further research at different sites during the time period is needed to confirm these hypotheses and understand the complex dynamics of plague spread and evolution.
