Ancient DNA from bubonic plague victims buried in cemeteries on the old Silk Road trade route in central Asia has helped solve an enduring mystery, pinpointing an area in northern Kyrgyzstan as the launching point for the Black Death that killed tens of millions of people in the mid-14th century.
Researchers said they retrieved ancient DNA traces of the Yersinia pestis plague bacterium from the teeth of three women buried in a medieval Nestorian Christian community in the Chu Valley near Lake Issyk Kul in the foothills of the Tian Shan mountains who perished in 1338-1339.
The earliest deaths documented elsewhere in the pandemic were in 1346.
Reconstructing the pathogen's genome showed that this strain not only gave rise to the one that caused the Black Death that mauled Europe, Asia, the Middle East and north Africa but also to most plague strains existing today.
"Our finding that the Black Death originated in central Asia in the 1330s puts centuries-old debates to rest," said historian Philip Slavin of the University of Stirling in Scotland, co-author of the study published in the journal Nature.
The Silk Road was an overland route for caravans carrying a panoply of goods back and forth from China through the cities of central Asia to points including the Byzantine capital Constantinople and Persia.
It also may have served as a conduit of death if the pathogen hitched a ride on the caravans.
"There have been a number of different hypotheses suggestingthat the pandemic may have originated in east Asia, specifically China, in central Asia, in India, or even close to where the first outbreaks were documented in 1346 in the Black Sea and Caspian Sea regions," said archaeogeneticist and study lead author Maria Spyrou of the University of Tübingen in Germany.
"We know that trade was likely a determining factor to the dispersal of plague into Europe during the beginning of theB lack Death. It is reasonable to hypothesise that similar processes determined the spread of the disease from central Asiato the Black Sea between 1338 and 1346," Dr Spyrou added.
Pandemic origins are hotly contested, as evidenced by the debate over the current Covid-19 pandemic's emergence.
The Black Death was the deadliest pandemic on record.
It may have killed 50% to 60% of the population in parts of western Europe and 50% in the Middle East, combining for about 50-60 million deaths, Dr Slavin said. An "unaccountable number" of people also died in the Caucasus, Iran and central Asia, he added.
"Already in medieval times we see the high mobility and fast spread of a human pathogen," said archaeogeneticist and study co-author Johannes Krause, director of the Max Planck Institute for the Science of Human History in Germany.
"We should not underestimate the potential of pathogens to spread around the world from rather remote locations, likely due to a zoonotic event" - an infectious disease jumping from animals to people.
The researchers analysed teeth, a rich source of DNA, from seven people buried in cemeteries of communities called Burana and Kara-Djigach, obtaining plague DNA from three in Kara-Djigach.
The cemeteries, excavated in the 19th century, included headstones attributing deaths to "pestilence" in the Syriac language.
Objects like pearls, coins and clothing from far-flung locales indicated that the towns were involved in international trade, perhaps offering stop-and-rest services for long-distance caravans.
Bubonic plague, untreatable at the time but now curable using antibiotics, caused swollen lymph nodes with blood and pus seeping out, with the infection spreading to the blood and lungs.
In Europe, it was transmitted mainly through bites of fleas carried on infected rats.
The pandemic originated in wild rodents, most likely marmots, a type of ground squirrel, Dr Slavin said.
Rodents tagging along in caravans may have helped spread it, but other transmission mechanisms may have included human fleas and lice.
"We found that the closest living relatives of that Y.pestis strain that gave rise to the Black Death are still found in marmots in that region today," Prof Krause said.