Scientists have used genetic data extracted from 700-year-old DNA to solve one of the great mysteries of medical history: the origins of the Black Death that killed up to half of Europe’s population in the 14th century.
An international team of researchers now believes that the continent’s worst recorded pandemic began in Kyrgyzstan, after analysing the ancient DNA taken from human remains in two cemeteries where the surge of burials in 1338 and 1339 indicated a deadly epidemic. Some tombs near Lake Issyk-Kul are inscribed with the word “pestilence” in the Turkic language.
As well as human genes, the scientists found DNA from the bacteria that cause plague. They believe the Yersinia pestis infection had been transmitted by fleas from marmots, a type of ground squirrel, to the local trading communities.
Genomic analysis showed these bacteria were the ancestors of the plague that travelled along trade routes to reach the Black Sea in about 1346. The infection — named after the black patches of dying tissue that appeared on victims’ skin — spread rapidly across the Mediterranean and through western Europe, reaching peak mortality from 1349 to 1351.
“We found that the ancient strains from Kyrgyzstan are positioned exactly at the node of this massive diversification event,” said Maria Spyrou of the University of Tübingen, lead author of the study published in Nature. “In other words, we found the Black Death’s source strain and we even know its exact date: 1338.”
The researchers believe their findings disproved a popular theory that the Black Death strain originated in China.
Further evidence emerged when the scientists discovered that the wild marmots still living in the region today carried a similar strain of Y pestis bacteria. These may have been the animal reservoir from which the Black Death emerged.
“We found that modern strains most closely related to the ancient strain are today found in plague reservoirs around the Tian Shan mountains [in central Asia], so very close to where the ancient strain was found,” said Johannes Krause, senior author of the paper from the Max Planck Institute for Evolutionary Anthropology in Leipzig.
The infection would then move from marmots into rats, which are the main animal responsible for spreading Y pestis to humans — most often by flea bites, though direct transmission from rodents and between people is also known.
The high mortality rate of the Black Death had profound social and economic consequences for the later medieval period, weakening the feudal system as labour shortages enabled surviving peasants to demand better working conditions.
The plague waves that followed the Black Death, which lasted until the early 19th century, are known as the second plague pandemic. The first pandemic, sometimes called the Justinian plague, ran for about 200 years from 541AD before mysteriously dying out.
The third (and last) plague pandemic began in China in the mid-19th century and largely petered out in the mid-20th century, though cases do still occur occasionally around the world.
Krause said there were a number of reasons why plague was not viewed as such a serious threat to global health today, but the main one was hygiene. “People do not live in such proximity to rodents as they used to and we have fewer fleas.”
As a bacterial disease, the infection can be controlled with antibiotics. A final factor might be the displacement of black rats in parts of the world by brown rats, which seem to be less susceptible to the Y pestis infection.
“Just like Covid, the Black Death was an emerging disease and the start of a huge pandemic that went on for some 500 years,” Krause said. “It is very important to understand in what circumstances it emerged.”