Scientists may have found the oldest-ever evidence of the bubonic plague, locked away in an ancient flea trapped in amber.

In a study published this month in the Journal of Medical Entomology, researchers report that the 20-million-year-old flea found in what's now the Dominican Republic — a new species dubbed Atopopsyllus cionus — contains structures thought to be an ancient strain of the bacteria that once devastated Europe and Asia.

The human plagues (bubonic, pneumonic and septicemic) are caused by the bacterium Yersinia pestis. Studies have found that Y. pestis developed its ability to kill humans quite recently — maybe not long before the first major outbreak about 1,500 years ago. It may have existed as a minor human infection for a few thousand years before that, but it's probably only been infecting humans at all for 10,000 years or so.

But it's possible this flea contains an earlier version of Y. pestis -- or at least a microbe that shares a common, ancient ancestor with the modern strain -- that preyed upon other animal hosts.

The fossilized bacteria found in the flea show physical similarities to modern Y. pestis. They were also found in the right spots: The bacteria were found on the proboscis, which the flea uses to drink blood, as well as compacted in the flea's rectum — a standard hangout spot for plague bacteria.

"Since the dried droplet with bacteria is still attached to the tip of the proboscis, the flea may have become entrapped in resin shortly after it had fed on an infected animal," lead author George Poinar Jr. of Oregon State University said in a statement.

That drop of blood is a clue, too. When fleas consume blood from a plague victim, the bacteria gunk up the route to the flea's stomach, keeping them from feeling satiated. That drives them to feed again, which pushes some of the bacteria back up the proboscis and infects a new victim. 

But it's hard to say for sure what the bacteria are, and they may not even be related to Yersinia. Tara C. Smith, an associate professor at Kent State University who specializes in microbiology, was skeptical.

"At that resolution, all they have are the shapes and sizes of what they assume are bacteria in the epipharynx of the flea," Smith told The Post. "But without testing them using some kind of staining technique, more advanced microscopy, or being able to examine the DNA of the purported bacteria, it’s really impossible to tell what they are—if they’re even bacteria to begin with, which is only an assumption made by the author based on size and shape," she explained.

With a sample this ancient, it's unlikely anyone will be able to prove the connection. But if the study's assumptions are correct, the findings suggest a long, influential history for plague bacteria's lineage.

"If this is an ancient strain of Yersinia, it would be extraordinary," Poinar said. "It would show that plague is actually an ancient disease that no doubt was infecting and possibly causing some extinction of animals long before any humans existed. Plague may have played a larger role in the past than we imagined."

Read More: