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Transmission Electron Micrograph of smallpox viruses. For the first time, scientists have found ancient smallpox genomes from the Americas, hiding in skeletons in Chile that date back about 500 years. Photo / Phanie via AFP
Scientists have found smallpox viruses in the remains of people in Chile from around 500 years ago, revealing how the virus may have evolved into a global killer.
Perhaps no disease has inspired more terror than smallpox. Edward Jenner, who invented the smallpox vaccine in 1796, called it “the
most dreadful scourge of the human species”. In the 20th century alone, smallpox is estimated to have killed 300 million people. The eradication of the virus in the 1970s stands as one of the great triumphs of medicine. And yet, five decades later, smallpox still haunts us, as the world worries about its resurrection as a biological weapon.
For all the anxiety that smallpox inspires, its history remains remarkably mysterious. Researchers don’t know how smallpox first evolved or how long it has plagued humanity. But over the past decade geneticists have offered a new line of evidence, as they find smallpox DNA still lurking in the bodies of people who died centuries ago.
Last month, researchers revealed the latest genetic trace in a paper published in Science. For the first time, they have found ancient smallpox genomes from the Americas, hiding in skeletons in Chile that date back about 500 years.
These DNA fragments have prompted scientists to think about smallpox in a new light. It may not have begun as the dreaded killer we think of it as today. It’s possible that it was a relatively mild infection for centuries, taking many generations to evolve into Jenner’s dreadful scourge.
Hendrik Poinar, an expert on ancient smallpox at McMaster University in Ontario, Canada, who was not involved in the new study, said he was not aware of any other disease that had taken such a course. “This is very strange,” he said.
Scientists generally agree that the smallpox virus jumped into humans from another species. Other animals carry a variety of related viruses, but scientists have yet to find a virus that could have been the forerunner.
“We’re not any closer to knowing the actual reservoir species from which smallpox came, which to me is kind of amazing,” Poinar said.
To understand when smallpox first started killing humans, researchers have looked at mummified remains, searching for the sores and scars the virus creates. The mummy of Pharaoh Ramses V, which dates back 3000 years, has pockmarks on its cheeks; many experts have suggested it’s the oldest trace of smallpox in human history.
But Poinar no longer shares that view. “I don’t buy Ramses,” he said.
Poinar changed his opinion after he discovered how well other viruses can mimic smallpox. He and his colleagues examined the body of a child who had died in Italy nearly 500 years ago, considered the oldest physical evidence of smallpox there.
But Poinar and his colleagues found no smallpox DNA. Instead, they found genes from hepatitis B, which can cause a facial rash that resembles smallpox sores.
In 2016, Poinar and his colleagues offered up far more compelling proof of ancient smallpox: its DNA. They found genetic fragments of the virus in the body of a child who lived in Lithuania in the 1600s.
