Scientists reported on Wednesday that they have recovered DNA from the oldest viruses known to have infected humans - and have succeeded in resurrecting some of them in the laboratory.
The viruses were all strains of hepatitis B. Two teams of researchers independently discovered its DNA in 15 ancient skeletons, the oldest a farmer who lived 7,000 years ago in what is now Germany.
Until now, the oldest viral DNA ever recovered from human remains was just 450 years old.
The research may provide clues to the continuing evolution of hepatitis B, a plague that infects an estimated 257 million people worldwide and contributes to an epidemic of liver cancer.
"It's a hugely important moment in our understanding of one the most important pathogens of humans," said Edward C. Holmes, a virologist at the University of Sydney.
The reconstructing of the DNA
One of the teams was led by Eske Willerslev, a geneticist at the University of Copenhagen who has helped revolutionize our understanding of human prehistory by collecting DNA from age-old skeletons.
In some instances, by grinding teeth and bits of bone to powder, and then pulling out fragments of genetic material, he and his colleagues have succeeded in reconstructing all of the DNA of individuals. But along the way, the scientists discovered that human genes aren't the only ones hidden in bones and teeth.
In 2015, Willerslev and his colleagues discovered DNA of the bacteria Yersinia pestis, which causes bubonic plague, in seven Bronze Age skeletons unearthed in Europe and Asia.
His team turned over raw genetic data they had gathered from hundreds of ancient skeletons to the Centre for Pathogen Evolution at the University of Cambridge for further evaluation.
"He handed us a gold mine," said Barbara Mühlemann, a graduate student at the university and co-author of the new study.
Mühlemann and her colleague Terry Jones led an inspection of 114 billion fragments of DNA retrieved from the skeletons of 304 people who lived 200 to 7,100 years ago. In most of the fragments, the researchers found nothing of interest.
But in the remains of 12 skeletons, they discovered that a tiny fraction of the recovered DNA came from viruses. A closer look revealed that the bones carried the same infection: hepatitis B.
Today, the virus represents a massive burden on human health. Present in blood and saliva, hepatitis B can be transmitted by pregnant mothers to their unborn children, and also can be spread through sex or by sharing needles.
Chronic infections can lead to liver cancer. Each year, the World Health Organization estimates, hepatitis B kills 887,000 people. Researchers have long wondered how it became a worldwide menace.
In 2012, researchers studying a mummified body in Korea from the late 1600s discovered DNA from the hepatitis B virus, specifically a strain common today in Asia.
In January, another team recovered the virus's DNA from a 450-year-old mummy from Italy. That virus belonged to a strain still found around the Mediterranean today.
But the skeletons in which the Cambridge geneticists found hepatitis range from 820 to 4,500 years old. The research, published in the journal Nature, demonstrates that hepatitis B existed across Europe and Asia as early as the Bronze Age.
"It gives a whole new light on understanding human suffering in the past," said Hendrik Poinar, an expert in ancient DNA at McMaster University.
Johannes Krause and his colleagues examined DNA extracted from the teeth of 53 ancient people in what is now Germany. Three of them were infected with hepatitis B, it turned out: one who lived about 1,000 years ago, a second person who lived 5,300 years ago and a third who lived 7,000 years ago.
The last individual belonged to a population of Europe's first farmers, who had spread across the continent from Turkey.
Krause was struck by the abundance of viral DNA found in the teeth, which suggests that the individuals had massive infections of the virus. "It must be really late stage," he said. "It could have contributed to the death of these people."
Krause and his colleagues posted a copy of their paper online on Sunday; the final version is to appear in the journal eLife on Thursday.
In an interview, Krause said it was striking that both teams of researchers found several skeletons with hepatitis B, including a number of extinct strains. "It seems to have been quite prevalent in the past," he said.
The two studies together raise a host of new questions about the history of a killer. For example, how did the virus first evolve?
Krause and his colleagues found that their Stone Age viruses were most closely related to strains of hepatitis B found today only in chimpanzees and gorillas.
He speculated that the virus jumped from apes to humans early in the history of our species in Africa. "It's more likely this is really an old pathogen in humans for the last hundred thousand years or more," he said.
