During the Black Death in the mid-1300s, most people first believed that rats spread the bubonic plague. Scientists would later understand that it was the fleas on the rats. It wasn’t until the 1890s when European scientists, studying an outbreak in colonial Hong Kong, found that it was really a bacterium in the fleas on the rats.
This culprit is called Yersinia pestis.
The Black Death wasn’t Y. pestis’s only appearance in history. It’s caused several major outbreaks in Europe and Asia over the last 2000 years. A paper published today in the journal Nature Communications found the earliest historical evidence for this species in Britain dates back to about 4000 years ago.
An old modern microbe
Ancient genome researchers in the United Kingdom, Germany, Spain, and France found evidence that the bacterium responsible for the bubonic plague may have spread to Britain about 4000 years ago. While we’ve known that Y. pestis had come to continental Europe during the Bronze Age, this finding demonstrates that it had moved west as well.
For reference, humans contain bacteria that predate use by millions of years. This pesky microbe is considered a modern pathology, so 4000 years makes it quite young by comparison. In fact, Yersinia pestis only diverged from Yersinia pseudotuberculosis, the tuberculosis bacterium, as recently as 6000 years ago in Eurasia, according to a study that analyzed the dental plaque of prehistoric humans.
When the plague came to Britain
This discovery marks the earliest evidence thus far of the plague in Britain, though we’ve seen evidence of this bacterium from as far back as 5000 years ago in other parts of Eurasia. One extinct strain of Y. pestis, known as the Late Neolithic and Bronze Age (LNBA) lineage, was believed to spread through Europe as humans traversed the Eurasian steppe between the two continents. However, when it arrived in Britain has remained a mystery.
According to the findings, the bacterium could have been prevalent from Eastern Asia all the way to Britain as long as 4700 years ago.
What about the virulence gene?
However, that doesn’t mean the bubonic plague was rampant at all these times — just that the bacterial species that would later cause it was present.
The researchers analyzed samples from 34 bodies buried at two British Early Bronze Age sites. Three victims contained traces of Yersinia pestis, so then the team sequenced the three bacterial genomes. These remains and the bacteria genomes dated back to about 4000 years old. Two of the plague victims were children buried in a mass grave in Somerset. The third victim, a woman between 35 and 45 years old, came from a grave in Cumbria and was one of four tested bodies from that burial ground.
But these Y. pestis were missing a gene crucial to the plague, which made it clear something had changed in this bacterium between then and the time of the Black Death. This gene, known as the ymt virulence gene, is highly infectious and best known for transforming infection with the bacterium into a gruesome death sentence, in part because it lets the bacterium survive within the flea. This survival tactic helped further increase transmission. The lineage containing this gene was found in Central Asia about 2800 years ago.
While scientists have evidence that Y. pestis was cruising through Britain 4000 years ago, right now there’s no telling the severity of the plague caused by the LNBA lineage. What’s important, though, is just how wide the little bacterium spread over the course of just a few centuries. The finding, in addition to telling us when these microbes spread to Britain, demonstrates its transmissibility even before the Plague decimated the continent.
Future research could tell us more about what the plague looked like in this strain, as well as its prevalence within Britain alone.