The fateful arrival of plague bacteria in the Mediterranean during the mid 14th century sparked one of the deadliest pandemics of all of human history, dubbed the Black Death. The pandemic killed up to 50 percent of the European population as it rapidly spread. In the following four centuries, plague outbreaks continued to flare up in pockets across the continent. And in the late 19th century, the plague took hold in the East, sparking the next historic pandemic in China.

For decades, researchers have tried to retrace the plague’s steps. Some have speculated that multiple strains of the bacteria creeped onto the continent—most likely from Asia—igniting new bouts of disease with sometimes different sets of symptoms. The hypothesis follows with the current state of the plague in China, where there are multiple lineages floating around. But a new study casts doubt on the idea of multiple initial strains.

Fresh genetic sequencing data of plague bacteria from victims in Spain, Germany, and Russia suggest that a single wave of the deadly microbes sparked the Black Death as well as the subsequent outbreaks that flared for centuries in Europe and in the 19th century pandemic in China. This single wave also gave rise to plague strains behind some modern outbreaks. The study is the first to make a genetic link between the Black Death and modern plague, the authors report in Cell Host & Microbe.

For the study, researchers collected bacterial DNA from the teeth of 178 individuals found in a mass-grave site in Barcelona, Spain, a single grave in Bolgar City in Russia, and a mass-grave site in Ellwangen, Germany.

Based on radiocarbon dating of the teeth and bone fragments, the Spanish remains are likely the oldest, with a date range of 1300 to 1420. The plague is estimated to have first ignited in the Mediterranean in 1346 or 1347 and quickly ravaged the whole continent by 1353.

The Bolgar City remains were likely the next oldest, with radiocarbon dating giving an age range of 1298 to 1388, while coin artifacts found at the site were known to have been minted after 1362. That leaves the remains at the German site to be the youngest, with radiocarbon dating giving an age range of 1486–1627.

From the teeth, the researchers found DNA of plague bacteria, Yersinia pestis, on 32 individuals from the three sites. From there, the researchers did full genome sequences of three Y. pestis isolates, one from each location. Then, the researchers fit the genomes into a bacterial family tree, which included 148 previously sequenced genomes of ancient and modern isolates.

The oldest, Spanish isolate was identical to those found in London during the same time frame, which suggests that a single strain was behind the Black Death.

Based on the family tree, this Black Death strain evolved into the isolate found in Ellwangen, Germany, which also matched an 18th century strain sequenced from the Great Plague of Marseille in France. (That outbreak occurred about 200 years after the Ellwangen outbreak.)

But the key finding, the authors wrote, came from the Russian strain, which was found to be a close relative of the Black Death strain as well as a relative to modern strains that link to the Chinese pandemic—the missing link between old and new. This suggests that the single wave of plague that first crashed through Europe and lingered to cause recurring outbreaks also went on to cause the late 19th century pandemic in China.

The finding “provides solid evidence of plague’s eastward travel subsequent to the Black Death,” the authors conclude. Strains of the plague that circulate today in Sub-Saharan Africa are only slightly different from the isolate found in Bolgar City, the authors note.

Of course, the study isn’t the last word on the topic. The authors acknowledge that it’s possible that multiple but very similar strains entered Europe at different times to create the tight family tree revealed in the new study. “We regard the likelihood of such similar strains leaving Asia in a short time frame to be low, but acknowledge it would be possible,” the authors write. More samples and genetic analysis are necessary to put the nail in the coffin.