It’s one of the worst diseases in human history, responsible for multiple pandemics and millions of deaths worldwide, and it has literally been plaguing us for thousands of years. That’s the conclusion of a new study, published in Cell, which found the plague bacterium was infecting people in Europe and Asia about 5,000 years ago. This is a significant find, because it predates any historical records of plague by about 3,300 years.
This wasn’t all, though: plague may have been around for three millennia earlier than prior evidence suggested, but it took at least 1,000 years to evolve into the extremely deadly bubonic strain capable of spreading by fleas, acquiring two key genetic changes.
This research actually comes off the back of a significant study by members of the same group, published just a few months ago. Sampling the genomes of Bronze Age Eurasian individuals, dated between 3,000 to 1,500 B.C.E., they found evidence for large-scale migrations in which groups left southern Russia and moved into Europe, replacing a significant proportion of the population and altering the genetic make-up of the region.
This led the team to question the motives behind these mass migrations, and their idea was that disease could have played a big part. “Today, if there is an epidemic, people try and leave from it,” co-lead author Simon Rasmussen from the Technical University of Denmark told IFLScience. “So why not back then?”
Onto the case, Rasmussen, alongside colleagues at the University of Copenhagen, scoured through billions of DNA reads obtained from the teeth of 101 Bronze Age individuals from both Europe and Asia. Seven of these, dating between 2800 B.C.E and 950 B.C.E., were found to possess DNA sequences from the plague bacteria, Yersinia pestis. That’s significantly earlier than the first documented plague pandemic, which began with the Plague of Justinian around 540 C.E. There have been earlier suggestions of outbreaks, though, such as the Plague of Athens which took place around 430 B.C.E., but evidence wasn’t strong enough to confidently say Y. pestis was to blame.
The revelation didn’t end there, though. While the early Y. pestis genomes resembled those we see today, there were a couple of key differences. First off, the Bronze Age plague appears to be missing a gene that allows transmission by fleas; a feature that defines its lifecycle today.
The gene, responsible for a protein called Yersinia murine toxin, helps the bacterium survive in fleas, enabling it to proliferate and block up the gut of its host, preventing blood meals from actually entering the stomach. “The flea gets so hungry it bites and bites, and every time it bites it transmits bacteria into the host,” Rasmussen explained to IFLScience. “So this is a really important part of the lifecycle, and we can show it basically wasn’t there.” An Iron Age individual, however, did show this gene, indicating this ability was acquired between 3,700 and 3,000 years ago.
So how was it transmitting before that? “We don’t know, but back then people were in more contact with nature, catching animals for food, so maybe they got it directly from animals that are natural hosts,” said Rasmussen.
They also found that the oldest samples lacked a single mutation, responsible for the evasion of immune responses, that endows the ability to cause “classic” plague – bubonic – meaning the bacteria could only cause the other two types: pneumonic and septicemic.
The researchers still can’t definitively say the plague was to blame for the population migrations, but their work hasn’t ended yet. They want to go back and see if they can find evidence for other diseases that could have affected these populations.
Imageintext: Bronze Age human skull. Rasmussen et al., Cell 2015.
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