"Bubonic plague" calls to mind medieval images of the havoc that was wreaked by one of the most devastating calamities in our history.
Caused by the bacterium Yersinia pestis, the so-called Black Death refers to the plague that hit Europe in the mid-1300s, killing millions of people.
The same strain is responsible for the two other main plague pandemics that hit humankind: the Justinian plague, which started in 541 AD, and the Modern Plague, which started in China in the late 19th century.
The plague is still "active" today. In fact, the World Health Organization (WHO) report that between 2010 and 2015, 3,248 people had the infectious disease, 584 of whom died from it.
Despite its current prevalence and historical significance, little is known about how and when the disease originated.
For instance, just this year, researchers started to question the initial assumption that the disease spread because of rats and shifted the blame to human bodily parasites, such as fleas and lice.
Now, a team of scientists led by researchers at the Max Planck Institute for the Science of Human History in Jena, Germany, suggests that the origins of the bacterium go further back in time than previously believed.
The researchers analyzed the genomes of two people who died of the plague 3,800 years ago and were buried together in a tomb site in Russia. The scientists' findings were published in the journal Nature Communications.
Plague may have spread 4,000 years ago
Previous studies of Y. pestis had traced its earliest genetic variants back to the Late Neolithic/Early Bronze Age.
However, none of these genetic variants showed the signs that would've made the disease spread as fast as it did — that is, they didn't display the genetic mutations that enabled the virus to survive in fleas, for instance, which recent studies have shown were the main vector for the virus.
But, the DNA of the two bodies analyzed by the researchers in the new study did show such signs. Also, after combining the newly acquired genetic data with already existing information, the team recalculated the date at which the plague must have started, pushing it back by 1,000 years.
"Our Y. pestis isolates from around 4,000 years ago possessed all the genetic characteristics required for efficient flea transmission of plague to rodents, humans, and other mammals," notes Maria Spyrou, of the Max Planck Institute, who is also the first author of the study.
Study co-author Kirsten Bos, also of the Max Planck Institute, continues, "Both individuals appear to have the same strain of Y. pestis. [...] And this strain has all the genetic components we know of that are needed for the bubonic form of the disease."
"So plague, with the transmission potential that we know today, has been around for much longer than we thought."
Senior author Johannes Krause, also at the Max Planck Institute, comments on potential directions for future research.
He says, "Additional Bronze Age and Iron Age plague genomes could help pinpoint key events that contributed to the high virulence and spread of one of humankind's most notorious pathogens."