We may never find dinosaur DNA in an amber-encased mosquito, but that doesn't mean we can't learn anything from blood-sucking insects trapped in fossilized resin.

In fact, a flea that succumbed to amber 20 million years ago may now offer clues about one of the most nefarious — and influential — diseases in human history.

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Discovered in what's now the Dominican Republic, this flea contains bacteria that resemble Yersinia pestis, the notorious pathogen behind the plague, aka Black Death. Half of Europe's population was killed by Y. pestis in the 14th century, and if these ancient microbes are its ancestors, that would mean the plague is much older and more worldly than previously thought, pre-dating even our own species.

"If this is an ancient strain of Yersinia, it would be extraordinary," says George Poinar, an entomologist at Oregon State University, in a statement about his new study on the fossil. "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."

The bacteria were found in a dried droplet on the flea's proboscis, the mouthpart used for drinking blood, and in its rectum. Poinar can't say for sure that they're related to Y. pestis, but he cites several reasons to suspect they are. The microbes are coccobacillus bacteria with nearly spherical and rod shapes, for example, which are unique to Yersinia among modern flea-borne bacterial pathogens.

"Aside from physical characteristics of the fossil bacteria that are similar to plague bacteria, their location in the rectum of the flea is known to occur in modern plague bacteria," Poinar adds. "And in this fossil, the presence of similar bacteria in a dried droplet on the proboscis of the flea is consistent with the method of transmission of plague bacteria by modern fleas."

ancient bacteria on flea proboscisThe arrow above points to bacteria still on the 20 million-year-old flea's proboscis. (Photo: George Poinar Jr./OSU)

The idea of pre-human plague bacteria clashes with recent genomic research, which suggests the flea-plague-vertebrate cycle evolved within the last 20,000 years. But several strains of Y. pestis exist today, Poinar points out, and ancient plagues may have been caused by varieties that are now extinct. So even if human-infecting strains are only 20,000 years old, others could have evolved to target rodents long before the rise of Homo sapiens about 200,000 years ago.

It's rare to discover any kind of flea trapped in amber, Poinar says, and this is the first time one has been found with associated microorganisms. The flea itself also seems to be an extinct species, but it's the bacteria that are stealing the spotlight.

Plague is still endemic in many countries, including the U.S., where it sometimes jumps to people via prairie dogs. But it was much worse in the Middle Ages, when periodic outbreaks killed up to 200 million people across Europe and Asia, triggering cultural changes that have shaped human history for centuries. (Plague is classified into three types — bubonic, septicemic and pneumonic — depending on how it infects the body.) It's now treatable with antibiotics, but insights about the disease are always welcome, whether for modern health or historical sleuthing.

Poinar has previously suggested that insect-borne diseases played a role in the demise of dinosaurs — along with other factors, like that asteroid. If these new microbes really are ancestors of modern plague bacteria, is it possible that even older ancestors were killing T. rexes 66 million years ago? We can only wonder for now, although Poinar says this flea may still reveal secrets about other ancient wildlife.

"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," Poinar says. "This might have been one of the rodents that occurred in the Dominican amber forest. Rodent hair has been recovered from that amber source."

It's no Jurassic World, but does that mean we can at least resurrect some weird rats from the Miocene Epoch? Unfortunately not, since DNA has a half-life of 521 years, meaning it only lasts about 6.8 million years at best. But on the bright side, we shouldn't need to worry about catching an extinct Caribbean plague, either.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

Flea fossil may hold ancestor of bubonic plague
It's no Jurassic Park, but a flea stuck in amber for 20 million years may still shed light on the Black Death.