A man who died more than 5000 years ago in Latvia because of the plague that caused the Black Death could be "patient zero", new evidence has found.
Scientists have identified the man who was infected with the earliest-known strain of the disease, according to research published in the journal Cell Reports.
The plague spread rampantly through Europe in the 1300s, wiping out about half of the population, causing millions of deaths.
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"Up to now, this is the oldest-identified plague victim we have," Dr Ben Krause-Kyora, from the University of Kiel in Germany told BBC.
The 5300-year-old human remains were found buried with three other people at a Neolithic burial site in Latvia beside the River Salac.
DNA was extracted from the bones and teeth of the four people, whose remains were then tested for bacteria and viruses.
One individual, a hunter-gathered believed to be a man in his 20s, was infected with an ancient strain of plague, caused by the bacterium Yersinia pestis.
"He most likely was bitten by a rodent, got the primary infection of Yersinia pestis and died a couple of days [later] - maybe a week later - from the septic shock," Dr Krause-Kyora said.
The ancient strain is thought to have emerged about 7000 years ago when agriculture first appeared in central Europe.
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Researchers believe the bacterium may have been first present in animals, before leaping across to humans without initially causing large outbreaks.
However, over time, the bacterium evolved into the form known as bubonic plague, which infected humans, spread by fleas.
Humans can contract plague after being bitten by a rodent flea that is carrying the bacterium, or by touching an animal infected with the disease.
The disease still exists today but can be treated with antibiotics following early diagnosis.
The new evidence has challenged many theories about the development of human civilisation in Europe and Asia, and that the disease caused population declines in Western Europe at the end of the Neolithic Age.
