The DNA of a strain of bacteria responsible for the infamous Black Death plague has been found in the teeth of three individuals found buried in the UK thousands of years before the deadly pandemics raged across Europe.

Two of those individuals, determined to be young adolescents, were buried in a mass grave in Charterhouse Warren in Somerset; the third was a middle-aged woman aged around 35 to 45 buried in a ring cairn monument in Levens in Cumbria. All three lived at around the same time, although it's unclear whether plague was the cause of their deaths.

The strain of the Yersinia pestis bacterium identified was different from the strain that caused the Black Death, an outbreak that established a plague pandemic that lasted 500 years and killed millions across Europe and the Middle East.

Nevertheless, these are the oldest cases of Y. pestis infection identified in Britain, suggesting the plague spread easily, even before it developed the mutations that would contribute to recurrent bouts of deadly virulence from the 14th century.

"The ability to detect ancient pathogens from degraded samples, from thousands of years ago, is incredible," explains genomicist Pooja Swali of The Francis Crick Institute in the UK.

"These genomes can inform us of the spread and evolutionary changes of pathogens in the past, and hopefully help us understand which genes may be important in the spread of infectious diseases. We see that this Yersinia pestis lineage, including genomes from this study, loses genes over time, a pattern that has emerged with later epidemics caused by the same pathogen."

The Levens ring cairn, under which just one person was buried. (Ian Hodkinson)

The second plague pandemic was a pretty major series of events for human history, having altered not just economics, politics, and culture but potentially – even disputably — our own genome.

Although we know a lot about the history of the disease, how it originated and spread is something genetic detectives are still working to figure out. Traces of it have been found from Eurasia dating back to around 5,000 years ago, to the Late Neolithic and the Bronze Age (LNBA).

There are two forms of the disease: bubonic or septicemic plague, transmitted by flea bites; and pneumonic plague, spread through human-to-human aerosol transmission. The most common LNBA strains of the bacterium lack the adaptations for flea transmission, suggesting that earlier forms of plague were not as horrendously deadly.

But how transmissible was pneumonic plague? Swali and her colleagues investigated the early spread of Y. pestis in Britain by extracting DNA from the teeth of 34 individuals. Our choppers have a reputation for preserving identifiable chunks of microbial DNA in detectable levels, providing researchers with valuable resource for tracking the history of many diseases.

Charterhouse Warren, photographed in 1972. (Tony Audsley)

Swali's team detected Y. pestis in three of the individuals' teeth, all radiocarbon dated to around 4,000 years ago.

Interestingly, the mass grave at Charterhouse Warren containing the two children isn't typical of the period. What's more, many of the bones of the 28 individuals therein showed signs of fatal trauma, suggesting that the site was used for the disposal of bodies after a violent event.

There's a strong possibility that the children were infected with plague at the time of their death, but it's unlikely that the infection was the primary cause of their death.

How the older woman buried in the ring cairn died, by contrast, is impossible to tell.

In all three, the bacterium DNA lacked the genetic mutation for flea transmission. The earliest identification of this mutation remains at around 3,800 years ago.

Charterhouse Warren. (Tony Audsley)

What the new discovery tells us, though, is Y. pestis had spread from mainland Europe into the British Isles earlier than we knew. This early, wide geographic spread seems to suggest that pneumonic plague spread not just far, and therefore rather easily. Further investigation could help reveal the impact this had on humanity at that time.

"We understand the huge impact of many historical plague outbreaks, such as the Black Death, on human societies and health, but ancient DNA can document infectious disease much further into the past," says geneticist Pontus Skoglund of The Francis Crick Institute.

"Future research will do more to understand how our genomes responded to such diseases in the past, and the evolutionary arms race with the pathogens themselves, which can help us to understand the impact of diseases in the present or in the future."

The research has been published in Nature Communications.