For the first time, scientists have successfully reconstructed the genome of the pathogen Yersinia pestis that caused the Justinian Plague - a devastating and widespread  pandemic that killed upwards of 50 million people back in the 6th century.

If that pathogen sounds familiar to you, it's because was thought to be responsible for two more history-altering plagues - the Black Death and London's Great Plague. Now that its genome is finally laid bare, we might gain a better understanding of the deadly bacterium that has devastated entire populations for well over a millennium.

"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 pestisillustrating the potential of ancient genomic reconstructions to broaden our understanding of pathogen evolution and of historical events," says team member Michal Feldman from the Max Planck Institute in Germany.

Before we get into the newly constructed genome, though, let's talk about plagues. Though there have been quite a few different ones through the ages, the three most talked-about are the Justinian Plague, the Black Death, and London's Great Plague. 

The Justinian Plague sprung up around 541 AD in the Byzantine Empire, killing roughly 50 million people.

The Black Death - probably the most well-known plague - occurred in Europe in the mid-1300s and killed around 200 million people. London's Great Plague, the most recent of the three, broke out in the mid-1600s, killing 100,000 people (a quarter of London's population).

While these three plagues all had different symptoms, were spread differently, and infected different amounts of people, there was one common factor: they were all caused by Y. pestis at different evolutionary stages.

Feldman and his team reconstructed the genome of Y. pestis by taking DNA samples from tooth pulp of five 6th-century skeletons found at an ancient burial site near Munich, Germany, known as Altenerding.

"We could clearly find preserved DNA signatures in the DNA extract we made from the pulp chamber, and from that we were able to determine that Yersinia pestis was circulating in that individual at the time of death," one of the team, Kirsten Bos, told Nicola Stanbridge at the BBC.

With these samples, the researchers sequenced the Y. pestis genome in the lab with a higher level of accuracy than a previous 'draft genome' of the bacteria, which was published in 2014 by a separate team.

Feldman's genome found 30 new mutations and corrected 19 false positive mutations found in the older draft genome. 

"We were very fortunate to find another plague victim with very good DNA preservation in a graveyard just a few kilometres from where the individual analysed in [the previous study] was found," says one of the researchers, Michaela Harbeck.

"It provided us with the great opportunity to reconstruct the first high-quality genome in addition to the previously published draft genome."

Finally having the complete sequenced genome will help to explain how Y. pestis evolved over the course of thousands of years, devastating humanity with a series of deadly plagues. With further study, the team might be able to piece together a timeline showcasing the plague's evolution with different genomes.

"We don't know why the Great Plague of London was the last major outbreak of plague in the UK, and whether there were genetic differences in the past, those strains that were circulating in Europe to those circulating today," Bos explained to the BBC

"These are all things we're trying to address by assembling more genetic information from ancient organisms."

While understanding the evolutionary history of the plague is important, the new genome could also help researchers to mitigate the effects of a plague in the future, which is something most of us don't worry about, but - historically speaking - maybe we should.

The findings were published in Molecular Biology and Evolution.