The oldest strain of the bacteria behind the plague that caused the Black Death has been discovered in the remains of a 5000-year-old hunter-gatherer.
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The pandemic may have killed up to half of Europe's population in the 1300s.
But genetic analysis from the University of Kiel in Germany reveals this ancient strain of the bacteria Yersinia pestis (Y pestis) was likely to have been less contagious and not as deadly as its medieval version.
The plague-carrying hunter-gatherer was a 20- to 30-year-old man who has been named RV 2039.
He was one of two people whose skeletons were excavated in the late 1800s in a region called Rinnukalns in present-day Latvia.
Soon afterwards, the remains of both vanished until 2011, when they reappeared as part of German anthropologist Rudolph Virchow's collection.
Following this rediscovery, two more burials were uncovered from the site.
Researchers used teeth and bone of all four hunter-gatherers to sequence their genomes and then tested them for pathogens.
They were surprised to find evidence of Y pestis in RV 2039, and determined it was the oldest strain ever discovered, according to the journal Cell Reports.
It was likely to be part of a lineage that emerged about 7000 years ago, only a few hundred years after Y pestis split from its predecessor, Yersinia pseudotuberculosis, according to researchers.
This ancient strain lacked one crucial thing - the gene that first allowed for fleas to spread the plague.
This gene was responsible for efficient transmission to human hosts, which resulted in the growth of the pus-filled buboes in those infected, and was associated with the medieval bubonic plague.
Flea-based transmission also required the death of the host, meaning the appearance of the gene could have driven the evolution of a deadlier disease.
Researchers say it is likely it took more than a thousand years from RV 2039 for the bacteria to acquire all the mutations needed for flea-based transmission.
RV 2039 probably died from the infection, which was likely to have been transmitted directly via a bite from an infected rodent.
However, researchers think the course of the disease might have been fairly slow, and the people he was buried with were not infected, suggesting a highly contagious respiratory version of the plague was less likely.
Australian Associated Press
