Yersinia pestis is the bacterium responsible for the first plague pandemic (and all plague pandemics since) – the Justinian Plague of 541 to 750 CE. This public health crisis wiped out close to 100 million people in Asia, Africa, and Europe in the first five years alone.

Now, an international team of researchers writing in Proceedings of the National Academy of Sciences and led by the Max Planck Institute for the Science of Human History have analyzed the remains of 21 victims and reconstructed eight plague genomes to find out the bacteria evolved and the impact it had on Early Middle Age Europe. 

The Justinian Plague – named after Justinian I, ruler of the Eastern Roman (or, Byzantine) Empire – kicked off in 541 CE. Following an outbreak in central Asia, the disease spread to Egypt before reaching Istanbul (then Constantinople), the Empire's capital, and the rest of mainland Europe. A series of recurrent outbreaks over the next 200 or so years decimated Europe and the Mediterranean basin, obliterating up to 25 percent of the population and contributing to the breakdown of the East Roman Empire.

While recent research has named the culprit as Y. pestis, its way of spreading and the relationship between the various strains that popped up during the pandemic have remained something of a mystery. By reconstructing the genomes of eight strains sourced from 21 burials across Austria, Britain, France, Germany, and Spain, scientists were able to compare their structure to those of previously published ancient and modern strains of the bacteria.

The results show there were many more strains of Y. pestis during the Justinian Plague than previously realized, many of which were closely intertwined genetically-speaking and some of which may have co-existed in the same places at the same times.

Interestingly, the team was able to confirm the plague's presence in Anglo-Saxon England with genetic evidence for the very first time. At least four of the 149 individuals buried at Edix Hill cemetery near Barrington, Cambridgeshire, between 500 and 650 CE have tested positive for Y. pestis and almost certainly died from it. 

"As the Edix Hill cemetery served a small community or communities of perhaps 50 to 65 people, this must have been a major traumatic event, comparable to the later Black Death," Craig Cessford of the Department of Archaeology at the University of Cambridge, UK, said in a press release.

"It is unlikely that Edix Hill is unusual in being affected by the Justinianic Plague, more probably most, if not all, of Anglo-Saxon England was ravaged by it. This discovery therefore represents a major historical event that previously could only be guessed at, meaning that the story of Early Anglo-Saxon England must be rewritten."

The researchers were also able to detect signs of convergent evolution between the different strains, with those that appeared later in the pandemic showing big deletions in their genetic code that mirror similar adaptations found in plague genomes from the late stages of the second pandemic (or, Black Death). 

"[T]hese Y. pestis strains independently evolved similar characteristics. Such changes may reflect an adaptation to a distinct ecological niche in Western Eurasia where the plague was circulating during both pandemics," co-author Maria Spyrou of the Max Planck Institute said in a statement.

What's more, "the fact that all genomes belong to the same lineage is indicative of a persistence of plague in Europe or the Mediterranean basin over this time period, instead of multiple reintroductions," co-author Marcel Keller, previously of the Max Planck Institute now at the University of Tartu, added.

One thing the team was unable to do was to identify the plague's origins prior to the 541 CE outbreak in Egypt. Though the team says it is likely it emerged in Central Asia several hundred years beforehand.