(CN) — An international team of scientists has found smallpox strains in the teeth of 1,400-year-old Viking skeletons, providing not only the earliest record of the deadly virus by far but also a rare glimpse into the extinct virus’ mutations and strains.

“Smallpox is the only virus humans have managed to basically erase. That’s an achievement because it was the biggest killer of all known pathogens; it has killed more people in history than all other epidemics together,” said Eske Willerslev, the University of Cambridge evolutionary geneticist who led the study. “The challenge is that, because it has been erased, we know very little about how it developed. … With these ancient sequences from the Vikings, this is where we start getting a glimpse into that.”

Willerslev and more than two dozen scientists recovered the variola virus, which causes smallpox, from the skeletons of more than a dozen northern Europeans, 11 of whom died during the Viking age — between 763 and 1066 A.D.

The scientists, whose research was published Thursday in the journal Science, extracted samples from the skeletons’ teeth, where bloodborne pathogens such as viruses and bacteria are deposited and preserved, to sequence the DNA and screen it for all known pathogens.

“In this case, we found some evidence of smallpox, so we needed to get more sequencing data out,” Willerslev said in an interview.

The material was meticulously dated so the team could rule out the possibility that modern bacterial or viral material contaminated the samples.

Of course, it’s unlikely that modern smallpox could have contaminated the material. Globally administered vaccinations led scientists to declare that smallpox was eradicated in December 1979, but not before the disease killed at least 300 million people in the 20th century alone.

Smallpox’s defeat was cause for celebration, but its rarity also poses a problem for current research on the globe’s deadliest plague.

“Because we erased it, which is obviously a good thing, it means there is extremely little information about it. We can’t just go out and do a lot of experiments with it — and also because it’s so dangerous,” Willerslev said. “But by looking at ancient materials, where the virus was once viral but is stone-dead now, we can play with it and reconstruct its genome.”

A 1,200-year-old, smallpox-infected Viking skeleton was found in Öland, Sweden. (Swedish National Heritage Board)
A 1,200-year-old, smallpox-infected Viking skeleton was found in Öland, Sweden. (Swedish National Heritage Board)

Notably, the smallpox that the scientists reconstructed from the Viking-era remains is not identical to the modern strains of smallpox that ravaged the globe before its eradication. By looking to the past, scientists can catalogue potential future pathogens before mutations naturally develop.

That’s an important step for research in virology and vaccination, since the surviving poxes that persist among animals — cowpox, monkeypox and so on — could, in principle, jump to humans, “just as we have seen with the coronavirus,” Willerslev warned.

“When I talk to my biologist friends, they think, ‘Well, mutation has been successful in the past; it’s only a matter of time before it reappears,’” he said. “So basically, all this ancient data gives us the possibility to see what mutations may reappear, and you can also start investigating current vaccines to counter these pathogens and mutations, if they turn up again.”

The samples from Viking skeletons, which predate the earliest confirmed cases of smallpox by about 1,000 years, also help date the plague’s global spread.

“Of course, we can never find the first smallpox in an individual,” said Willerslev, “but we don’t see it before [the Viking period], so this is the first time it becomes common. Vikings were travelling all across Europe,” acting as smallpox super-spreaders.

Similar to influenza and Covid-19, smallpox was spread between humans through infected fluids, including respiratory droplets, or contaminated objects such as clothing.

The researchers’ findings grew from a larger project that has sequenced more than 1,000 ancient human genomes to study pathogens preserved in their remains.

“It’s really revising our understanding of the history of pathogens. We start realizing how little we actually knew about the mutation and spread of these pathogens, and that is of course super exciting, because it is a whole new area,” Willerslev said.