“It seems to have started impacting human populations over large geographical scales way earlier than we thought”, said evolutionary geneticist Eske Willerslev of the University of Copenhagen and University of Cambridge.
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The study results demonstrate that the plague was an endemic in the human populaces of Eurasia no less than 3,000 years before any chronicled recordings of pandemics.
Experts also said that was unlikely that the plague was going to pose a threat which was as great as it did in the past since it’s quite treatable now. That divergence, and therefore the bacteria’s possibility of infecting humans, is much earlier than scholars previously estimated. The mode of transmission of the bacteria back then may be through inhalation thus, warranting human-to-human transfer. Instead, they believe that the plague that could be spread through fleas is a plague that evolved from what was found in the Bronze Age remains. Morten Allentoft, an evolutionary biologist at the University of Copenhagen, quipped that: “Perhaps people were migrating to get away from epidemics or recolonizing new areas where epidemics had decimated the local populations”. However, before this occurred, the plague was occurring in humans in Eurasia 3,000 years before the first recorded bubonic plagued epidemic.
To further probe this mystery, an worldwide team led by Danish researchers sequenced the DNA pulled from the teeth of 101 Bronze Age humans excavated from around Europe and Asia. After sequencing the genome, they found seven adults to have Y. pestis bacteria in their DNA, the oldest of whom died during the earliest report of plague, which is 5,783 years ago.
“However, based on the absence of crucial virulence genes, unlike the later Y. pestis strains that were responsible for the first to third pandemics, these ancient ancestral Y. pestis strains likely did not have the ability to cause bubonic plague, only pneumonic and septicemic plague”, they wrote.
Samples taken from the Iron Age human remnants (1200 B.C.), suggest that – about 2000 year later – Yersinia pestis had developed the ymt gene. In mammals, the immune system has evolved to recognize and mount protective responses against a protein called flagellin, which is the principal component of the flagella-the whip-like appendage that helps bacteria move around.
Genetic analysis of the recovered ancient bacteria revealed a pair of key differences: it lacked a “virulence” gene called ymt, and an “activator gene” called pla.
However, this mutation was not present in the two oldest Bronze Age individuals, and the flagella system was still in the process of devolving in the youngest Bronze Age individual.
Before this study, the earliest signs of the plague was in 540 AD. But now Simon Rasmussen and his colleagues from the University of Denmark have found it in 2800 BC as well. And while it’s possible that humans’ experience with Y. pestis goes back much longer than the historical record, the oldest evidence pulled from human skeletons goes back only about 1,500 years.
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In future studies, the researchers will look for evidence of plague in other geographic regions and time periods to get a better grasp of the history of this disease.
