The bacterium that is responsible for the plague may have started infecting humans much earlier than researchers had previously believed.
In the study – published October 22 in the journal Cell – the researchers analysed the DNA sequences in tooth samples from the remnants of 101 individuals who lived during the Bronze Age and Iron Age in Europe and in Asia.
The researchers found that the Yersinia pestis bacterium was present in samples taken from seven human teeth. The earliest plague sample dated back to 2794 B.C. (from Bronze Age Siberia) and the latest dated back to 951 B.C. (from Iron Age Armenia).
Simon Rasmussen, and a bioinformatician at the Technical University of Denmark, said that based on the DNA samples, Yersinia pestis may have infected people 3,000 years earlier than scientists had thought.
This means that the plague may have been responsible for many other epidemics throughout human history. According to the researchers, mysterious epidemics such as the one that weakened the Imperial Roman army may have been triggered by the Yersinia pestis bacterium.
The findings also show that the plague has changed over time. About 3000 B.C., during the Bronze Age, the plague genomes lacked the ymt gene, which protects the Yersinia pestis bacterium once it gets inside the fleas’ guts (fleas used to spread the disease to humans).
Samples taken from the Iron Age human remnants (1200 B.C.), suggest that – about 2000 year later – Yersinia pestis had developed the ymt gene. Fleas were thus able to transmit the disease sometime after the Bronze Age.
Plague strains are also known to have a mutation that does not allows them to produce flagellin – a protein which the immune system of mammals can detect and destroy – however, the mutation was not found in the two oldest Bronze Age samples, only in the youngest Bronze Age sample.
The plague may have triggered large migrations. Morten Allentoft, an evolutionary biologist at the University of Copenhagen, said that:
“Perhaps people were migrating to get away from epidemics or recolonizing new areas where epidemics had decimated the local populations.”
Researchers plan on conducting further studies to uncover more details about the history of the plague.
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