New research published in Cell reveals the hitherto unknown past of the bacterium responsible for plague, Yersinia pestis (Y. pestis). The findings show that people would commonly get infected by the pathogen hundreds of years earlier than scientists previously believed. While Y. pestis existed during the Bronze Age, it only experienced the genetic changes that account for the disease 1,000 years later; it took centuries to propagate via fleas and outwit the immune system of its host.
When researchers from the Center for GeoGenetics at the University of Copenhagen in Denmark analysed 89 billion raw DNA sequences obtained from tooth samples extracted from remains of 101 Bronze Age people from Europe and Asia, they discovered that plague infections would occur around 4,800 years ago.
Scientists from University of Gothenburg in Sweden think this implies that plague might have existed within populations much earlier than it was previously suspected.
“We found that the Y. pestis lineage originated and was widespread much earlier than previously thought, and we narrowed the time window as to when it developed. This study changes our view of when and how plaque influenced human populations and opens new avenues for studying the evolution of diseases,” says senior study author Eske Willerslev.
The researchers wanted to figure out whether people from these populations migrated because of the occurrence of epidemics; it was unclear as to whether they escaped places affected by some plague or they shifted to places emptied following the large-scale deaths because of the disease.
They spotted the DNA of Y. pestis in 7 individuals that existed between 2794 BC and 951 BC (early Iron Age). As per evolutionary analysis, the most recent common ancestor of Y. pestis strains was found to be 5,783 years of age – much older than scientists’ theories would suggest. However, the plague was not yet a widely infectious disease during the Bronze Age (c. 3000 – 1500 BC).
It appears that the ancestors of the current Y. pestis strains living during the Bronze Age were vulnerable enough to be subdued by the immune system; the flagella was detected in earlier forms of the bacteria and this feature allows the immune system to detect it and combat it. By the Iron Age, the flagella had been lost such that the Y. pestis could not be identified by the immune system. Furthermore, by that time, genes coding for Yersinia murine toxin (ymt) that would safeguard the pathogen in the gut of the fleas developed such that propagation from insects to humans was possible. It seems that Y. pestis had adapted fully around 1,000 BC, when records of plagues were made.