Plague In Dental DNA From Much Earlier Than Thought, Says Study

Like a lurker, the plague has been around for millennia--nearly twice as long as previously thought, it turns out. A new study shows that the disease was endemic to humans as late as the early Bronze Age, then changed to its better-known, flea-carried form sometime between the second and first millennium B.C.

The study looked at ancient DNA, finding that in early days the plague would have been largely spread by humans to other humans. Later, genetic mutations made Yersinia pestis (Y. pestis) able to live in fleas' guts. The later form of the bacteria killed half Europe's population in the 1300s, of course. The researchers are from the universities of Copenhagen (Denmark) and Cambridge (U.K.), and their paper was recently published in the journal Cell.

The team says that these new learnings suggest that plague might have caused large population declines that are thought to have taken place in the late 4th and early 3rd millennium B.C., according to a release.

"The underlying mechanisms that facilitated the evolution of Y. pestis are present even today. Learning from the past may help us understand how future pathogens may arise and evolve," said Professor Eske Willerslev, who recently joined Cambridge University's Department of Zoology from the University of Copenhagen, in the release.

In their work, the scientists studied genomes from 101 adults' teeth that date from the Bronze Age, drawn from across Eurasia. In the DNA of seven of those adults, the researchers found Y. pestis bacteria. The oldest of those died 5,783 years ago, noted a release.

This makes those teeth the earliest evidence of plague, because previous to this the earliest direct molecular evidence for the disease from a skeleton was not older than 1,500 years.

As it happens, six of those seven samples missed two genetic components currently found in plague: ymt, a "virulence gene" and pla, an "activator gene." The first of those helps prevent flea-gut toxins from detroying the bacteria-it then increases and fills the flea's digestive tract. The flea, starving, bites anything it can and spreads the plague. Pla, alternatively, helps the bacteria to spread across various tissues, contaminating blood and lymph nodes rather than just lungs, the release noted.

The scientists concluded that without those two genes, plague could not have been carried by fleas or cause bubonic plague, which automatically goes into the lymphatic immune system, said the release.

Their conclusion, then, is that in the Bronze Age people affected by plague likely had the disease in their respiratory systems and experienced hacking cough fits before dying. The disease was also spread by inhaling bacteria when near infected people, according to the release.

The findings are not just medical in nature, said Professor Robert Foley, from Cambridge, in the release. "The Bronze Age is the edge of history, and ancient DNA is making what happened at this critical time more visible."

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