Two sixth-century skeletons are giving some huge insights into the murky and brutal history of the bacterium behind the Black Death.

A team of bioarchaeologists analyzed samples taken from the skeletons discovered in Altenerding, an ancient southern German burial site near Munich, who died of the Plague of Justinian. Using “molecular clues” from the victims and the first high-coverage genome of the bacteria, they showed that the Justinian plague was caused by the same bacterium, Yersinia pestis, as the bubonic plague, or Black Death, that rampaged across Europe in the fourteenth century.

However, they found many genetic differences between the two strains, despite being the same species. The sequencing showed 30 newly identified mutations and structural rearrangements unique to the Justinianic strain. It's believed these mutations could be linked to the virulence of Y. pestis. This discovery is also revealing new information about the molecular evolution of Y. pestis and its effect on human history.

^{Yersinia pestis bacterium under a 200x magnification. CDC/ Larry Stauffer/Oregon State Public Health Laboratory/Public Domain}

The Plague of Justinian swept through the Byzantine Empire in 541 CE, taking with it the lives of as many as 50 million people – around 15 percent of the world’s population. Some 800 years later, the Black Death swept through Europe and took out around 60 percent of the population. The two plagues have been linked before, but it was never clear just how similar or different the strains were to each other.

"Our research confirms that the Justinianic plague reached far beyond the historically documented affected region and provides new insights into the evolutionary history of Yersinia pestis, illustrating the potential of ancient genomic reconstructions to broaden our understanding of pathogen evolution and of historical events," research colleague Michal Feldman said in a statement. "Our reanalysis of previous datasets stresses the importance of following strict criteria to avoid errors in the reconstruction of ancient pathogen genomes."

The full study can be found in the journal Molecular Biology and Evolution