When commercial travelers brought the plague across the Black Sea to the Mediterranean region, one of the most devastating pandemics in human history began. In the eight years from 1346 to 1353 alone, this infectious disease, also known as the “Black Death”, could have killed up to 60 percent of the population in Europe and western Asia.

The starting point was probably those bacteria that still infect rodents in the foothills of the Tian Shan Mountains, report Johannes Krause from the Max Planck Institute for Evolutionary Anthropology in Leipzig and his team in the journal “Nature”.

“The origin of the medieval plague has been puzzled for many decades,” says Philipp Stockhammer from the Ludwig Maximilian University in Munich, who was not involved in this study. Conjectures often focused on China. The pathogen could have come from there to Europe via the Silk Road. At the end of the 19th century, excavations first fueled the suspicion that the plague had reached mankind near Lake Issyk-Kul in what is now Kyrgyzstan. In 1338 and 1339, an exceptionally large number of people were buried in two cemeteries near the water. Some tombstones mentioned “a devastating plague”.

The inscriptions, coins and other objects found in the graves, as well as historical records, now revealed to the group around Johannes Krause that traders who traveled to various regions of Asia lived in this region. At that time, the area belonged to one of the four khanates into which Genghis Khan’s Mongolian empire had broken up. Its northern neighbor was the Golden Horde Khanate, which stretched from present-day Kazakhstan to present-day Ukraine.

There, on the lower Volga and on the Crimean peninsula, the first Europeans died in 1345 from the plague, which had previously disappeared from Europe for almost 600 years. When the Golden Horde besieged the Crimean town of Kaffa in 1346, the plague reached the trade network of the city-state of Genoa, which then held the Black Sea port, and spread to the Mediterranean. Within eight years, the disease spread to Europe and North Africa – and large parts of the population at that time died.

Bacteria with this genome gave rise to the very similar pathogen that caused further outbreaks from 1346 to 1353 and into the early 19th century. From them emerged three more branches of the plague, which together with the branch of the Black Death represent about 80 percent of all plague lines known today. There is another, fifth original branch in the plague family tree, which includes all earlier cases of this infectious disease identified so far.