When the plague came to Europe in the 14th century, the “Black Death” triggered one of the worst plagues in human history. But where the pathogen, the bacterium Yersinis pestis, came from, was previously a matter of dispute. Now scientists may have found the origin of the medieval pandemic. This was achieved through DNA analyzes of some dead people who died in 1338 in northern Kyrgyzstan from a “new pestilence” according to the grave inscriptions. Gene comparisons revealed that these dead people carried an early, still very primordial strain of the plague pathogen. The Black Death could therefore have reached Europe from Central Asia – probably via old trade routes.
The year 1347 marks the outbreak of the “Black Death” in Europe. The plague pathogen first reached the Mediterranean region via merchant ships from the Black Sea and spread there, transmitted by fleas, at breakneck speed. The first wave of this plague wiped out up to 60 percent of the population of western Eurasia, establishing a pandemic that produced repeated local outbreaks well into the 19th century. Gene comparisons of different strains of the bacterium Yersinia pestis have long suggested that various forms of plague have been repeatedly transmitted to humans from rodents and other animal reservoir hosts in this region for thousands of years. Because even in late antiquity there was a major outbreak of plague, the so-called Justinian plague. The medieval Black Death, however, seems to have its roots further east, according to most hypotheses.
Searching for clues in medieval graves
But where exactly the origin of the medieval “Second Plague Pandemic” lay has so far been disputed. “Hypotheses based on historical records and genetic data have considered a number of possible places of origin, ranging from western Eurasia to East Asia,” explain Maria Spyrou from the University of Tübingen and her colleagues. She and her team have now followed another lead, which led them near Lake Issyk-Kul in Kyrgyzstan. A good 140 years ago, archaeologists had discovered medieval graves in two cemeteries there, the gravestones of which indicate the death of some of these people in an unknown epidemic in the years 1338 and 1339 – almost a decade before the arrival of the Black Death in Europe. Since then, the tombstones, inscribed in Syriac-Aramaic, have fueled speculation as to whether these dead in the cemeteries of Kara-Jigach and Burana might have died of the plague.
In order to clarify this question, Spyrou and her colleagues have now taken samples from seven dead people from Kara-Djigach and Burana and analyzed them for the genome of the plague pathogen Yersinia pestis. They found what they were looking for in three of these medieval plague victims: the tissues of the dead contained DNA from Yersinia pestis. All three people had also died in the epidemic year 1338/1339, as their grave inscriptions revealed. “We were finally able to prove that the epidemic mentioned on the tombstones was actually caused by the plague,” says co-author Phil Slavin of the University of Sterling. The question now remained how the plague pathogens from the Kyrgyz foothills of the Tienshan Mountains were connected to the medieval Black Death in Europe. To do this, the research team compared the DNA sequences of the plague genomes from Kyrgyzstan with the sequences of 203 modern and 47 historical representatives of Yersinia pestis.
At the origin of the plague pandemic
The comparative analyzes revealed that the genome from Kyrgyzstan is more original and older than all other known gene sequences of the plague pathogen from the 14th century. In addition, a genetic tree analysis positioned the plague variant from near Lake Issyk-Kul at a crucial point in plague evolution: “We found that the ancient tribes from Kyrgyzstan are right at the nexus of a massive diversification event,” reports Spyrou. During this event, which previously could only be roughly dated to between the 10th and 14th centuries, the plague pathogen split into four strains that still exist today. This massive diversification of Yersinia pestis was almost unique in the millennia-old history of this pathogen and is considered the “big bang of plague diversity”. It is also closely linked to the spread of the Black Death – the medieval pandemic is believed to have triggered this split.
By classifying the Kyrgyz pathogens at the base of these four lines, the researchers were now able to date this “big bang” more precisely for the first time: it must have taken place after 1338 and was therefore actually linked to the Black Death. At the same time, the genetic analyzes also prove that the ancestor of the plague that raged in Europe in the 14th century must have come from Kyrgyzstan. “So we actually succeeded in determining the origin of the Black Death and the exact time when it broke out – the year 1338,” says Spyrou. The team theorizes that the bacteria spread from wild rodent populations to humans in the Issiköl Lake area. “Today, we find modern tribes most closely related to the ancient tribe in plague reservoirs around the Tian Shan Mountains, very close to where this ancient tribe was found. So the ancestor of the Black Death appears to have originated in Central Asia,” explains senior author Johannes Krause from the Max Planck Institute for Evolutionary Anthropology in Leipzig.
How the ancient plague pathogen then got from the foothills of the Tien Shan Mountains to western Eurasia has not yet been clearly clarified. The scientists suspect, however, that it was transported onwards via trade connections between this region in Central Asia and the ports and trading centers on the Black Sea. “The beginning of the Black Death is commonly associated with the first eruptions that occurred in the Black Sea region in 1346,” Spyrou and her colleagues explain. From there the Black Death came to Europe by ship.
Source: Maria Spyrou (Eberhards-Karls-Universität Tübingen) et al., Nature, doi: 10.1038/s41586-022-04800-3