Washington (AFP)

He is the first known victim of the plague: a hunter-gatherer who lived 5,000 years ago in present-day Latvia was a carrier of the bacteria causing the disease, researchers revealed on Tuesday.

The plague ravaged mankind for thousands of years, possibly wiping out up to half the population of Europe in the 14th century, during the Black Death.

But scientists have long been puzzled about its origin and evolution.

The man in question, called "RV 2039", was in his twenties.

Its skeleton was found at the end of the 19th century, but then disappeared before being rediscovered in 2011. Its remains show the bacterium Yersinia pestis, according to this work published in the journal Cell Reports.

"Analyzes of the strain we have identified show that Y. pestis evolved earlier than we thought," Ben Krause-Kyora of the University of Kiel in Germany told AFP.

Scientists say this strain is part of a lineage that emerged around 7,000 years ago, 2,000 years older than previously established.

Finding this bacteria was "really a surprise," said Ben Krause-Kyora.

The team of scientists were initially looking to establish a possible family link between this man and three other people found in the same place, when they made this discovery.

Yersinia pestis likely killed the individual, although researchers believe the disease was slow.

At the time of his death, he had high levels of this bacteria in his blood, which has been associated with less aggressive infections in rodents.

The people around him had not caught the disease, which seems to show that he was not infected with pulmonary plague, a highly contagious form of plague.

"RV 2039" may have been infected from a rodent bite, scientists say.

The found bacteria are missing key genes, such as the one that allows fleas to transmit the disease.

This old version was therefore less contagious and deadly than the medieval version.

The most recent strain of plague that can be transmitted by fleas dates back to around 3,800 years ago, when towns of more than 10,000 people began to form.

The increasing population density probably caused the evolution of the bacteria.

Tracing the history of Yersinia pestis could help to understand how man himself evolved to defend himself, according to Ben Krause-Kyora: "We are very interested in future studies on how these ancient infectious diseases influenced our current immune system ".

© 2021 AFP