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, Dr. 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.
Ms. 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.
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.
A virus like influenza, which can spread through the air and also infect birds and pigs, may race around the planet in a matter of weeks. But hepatitis B depends on close human contact.
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.
Researchers at the Max Planck Institute for the Science of Human History in Jena, Germany, also have been gathering DNA from ancient human remains — and they, too, suspected that hepatitis B might be lurking in the collection.
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.
Dr. 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.”
In an interview, Dr. 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?
Dr. 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.
He and his colleagues have already started looking for hepatitis B in Neanderthal fossils. Other viruses may turn up in ancient human bones, Dr. Krause suspects — especially another human plague, smallpox.
“I’m quite sure we’d find it if we looked hard enough,” he said.
One of the co-authors of the Nature study has taken a novel next step: He is resurrecting extinct strains of hepatitis B in a secure laboratory.
Dieter Glebe, a molecular virologist at the National Reference Centre for Hepatitis B and D Viruses in Giessen, Germany, has manufactured DNA molecules that contain the viral genes recovered from ancient skeletons.
When he inserts that DNA into human cells, they produce viable hepatitis B viruses. It may be the first time these strains of hepatitis B have existed in several thousand years.
Dr. Glebe is bringing Bronze Age viruses back to life in order to see how they differ from the strains that infect people today. Studying ancient viruses could give researchers clues about how today’s hepatitis B might evolve in years to come.
Looking into the virus’s past, he said, “we can make predictions about the future.”
Correction: May 9, 2018
An earlier version of this article misstated the name of the city where the National Reference Centre for Hepatitis B and D Viruses is located. It is in Giessen, Germany, not Gliese.