💀 The plague was already striking during prehistory, 5500 years ago

For centuries, the plague remained associated with medieval cities and large agricultural societies. But a recent discovery turns everything upside down: in Siberia, hunter-gatherers living in small isolated groups were already suffering deadly plague epidemics 5,500 years ago, long before the great historical pandemics.

International researchers analyzed ancient DNA preserved in the teeth of 46 skeletons from four cemeteries near Lake Baikal. By reconstructing the genomes of the Yersinia pestis bacterium, they found that 18 individuals were carriers. This rate of nearly 40% is surprisingly high, exceeding that of some Black Death burial sites.


This unusual proportion immediately intrigued archaeologists, struck by the high number of children and young adolescents buried at two of these sites. This distribution suggested a sudden event that struck the youngest. Carbon-14 dating confirmed that the deaths occurred over a short period, and DNA revealed that members of the same family, such as siblings or parents with their children, died together, which points to an epidemic.

How can the lethality of this primitive plague be explained? These ancient strains did not possess the genetic adaptations that allow the bacterium to be transmitted via fleas and rodents, the key mechanism of later great pandemics. Instead, they carried a unique superantigen, a toxin capable of triggering an excessive immune reaction, causing severe inflammation and often fatal complications.

Transmission of the disease likely occurred directly from marmots to humans. Indeed, hunter-gatherers in the region had close contact with these rodents, which are still plague reservoirs today. Thus, the bacterium likely circulated from central or eastern Asia before spreading across Eurasia through wild rodent populations.

This study challenges the idea that early forms of plague were mild. Long before the emergence of cities and agriculture, plague was already a formidable killer, adapted to societies of small nomadic groups. Scientists believe these primitive strains gradually evolved to cause the pandemics we know, but their initial virulence remains a major discovery.

Yersinia pestis: the agent of plague

Plague is caused by a bacterium named Yersinia pestis. Discovered in 1894 by Alexandre Yersin, this bacterium can infect many mammals, but its main reservoir is the rodent. In humans, it causes several forms of the disease: bubonic, pneumonic, or septicemic. The bubonic form, the most well-known, is characterized by painful swelling of the lymph nodes. Without antibiotic treatment, mortality can exceed 50%.

Historical transmission of plague occurs mainly through fleas. A flea that has bitten an infected rat regurgitates bacteria during a new bite. However, in ancient strains, this adaptation was absent. This means the bacterium used other routes to infect humans, likely through direct contact with infected animals or inhalation of droplets. This discovery shows that Yersinia pestis has great genetic plasticity.

Modern strains of plague are classified into several biovars, including Antiqua, Medievalis, and Orientalis. Each corresponds to different historical pandemics. Studying ancient strains makes it possible to trace the evolution of the bacterium and identify key genes for its virulence. The genome of Yersinia pestis contains many mobile elements that facilitate genetic exchanges, accelerating its adaptation to new hosts.