A prehistoric genome of the bacterium responsible for plague gives us clues about its ancient spread between Europe and Asia. Researchers have identified a vast expansion that occurred around 5,300 years ago. It may have linked Scandinavia, Central Europe, and northern China over several thousand kilometers.
Plague is caused by the bacterium Yersinia pestis. Its ancient history remains difficult to reconstruct, because its DNA is rarely preserved in human remains. Each new genome therefore provides a valuable landmark. It makes it possible to compare lineages, their mutations, and their closeness in time.
Yersinia pestis, direct immunofluorescence staining (DFA), magnification 40x.
Credit: CDC/Courtesy of Larry Stauffer, Oregon State Public Health Laboratory
The team examined 1,400 ancient human remains from eastern Eurasia. These individuals lived between about 8,000 and 400 years before the present. The researchers found three prehistoric genomes of Yersinia pestis, including one from East Asia.
These data were then compared with 136 previously published ancient and modern genomes. The resulting genetic tree reveals several lineages separated at almost the same time. This branching, dated to about 5,300 years ago, links bacteria found in very distant regions.
The small genetic differences indicate a separation over only a few centuries. The sites involved span more than 7,500 km. The authors calculate a median minimum dispersion speed of 32.9 km per year. This value corresponds to rapid circulation for a period without modern means of transportation.
The study does not make it possible to identify precisely the people, animals, or exchanges responsible for this spread. It nevertheless establishes that the bacterium was already circulating over long distances. Human movements, trade networks, and contacts with infected animals may have played a role, although they cannot be distinguished here.
The researchers also observed genetic changes linked to lineages that later became better adapted to fleas. Among them is the acquisition of the ymt gene. These changes do not prove that all ancient strains were already transmitted like those of historical pandemics. Rather, they specify an important step in their evolution. Future discoveries in Asia could further refine this chronology.