The millennia-old hepatitis B viruses were all very strange. They looked less like modern-day human viruses than the hepatitis B viruses that today infect chimpanzees and gorillas in Africa. This was true for not just one or two but five hepatitis B viruses across the two studies—found in people living from Germany to Poland, from 7,000 to 3,800 years ago. Whatever version of hepatitis B infected western Europe thousands of years ago seems to have since gone extinct in humans—but lived on in African chimps and gorillas. But was it humans who infected primates with that virus? Or vice versa? And where in the world did it cross over?

Meanwhile, similarly ancient hepatitis B viruses from central Asia resemble viruses found in humans today everywhere from western Europe to the Caribbean to western Africa, according to Willerslev’s study. What makes these so different from the viruses infecting people at the same time in western Europe?

“The papers are really interesting,” says Margaret Littlejohn, a virologist at the Doherty Institute who has studied the origins and evolution of the hepatitis B virus. “But we felt they’re giving us more questions than answers.”

Ancient human DNA also caused a similar state of confusion, initially. The evidence confounded a tidy out-of-Africa hypothesis, in which humans left Africa and sequentially moved through the rest of the continents. In fact, ancient human DNA has revealed, different populations may have left Africa at different times and perhaps even returned. And in most places on Earth, the people who live there have been serially replaced by other groups, over and over. Other populations met dead ends and went extinct. The same appears to have happened with hepatitis B.

The truly surprising takeaway, says Krause-Kyora, an author on one of the studies, is how diverse old hepatitis B lineages once were. The virus has only a fraction of that diversity today.

Willerslev, the lead author on the other study, suggested that this past diversity may be a clue for scientists trying to predict what could happen with hepatitis B in the future. “It provides a catalog of possible mutations this virus has had that have been viable in the past, and obviously some of those mutations could come back,” he said at a press briefing. The prevalence of hepatitis varies widely across the world (ranging from 0.01 percent in the United Kingdom to 22.38 percent in South Sudan), but in total, complications arising from it kill nearly 900,000 people a year.

It might even be possible eventually, says Hendrik Poinar, an ancient DNA researcher who was not involved in the studies, to resurrect these ancient hepatitis B viruses and test them in mice to see exactly how they affected the body. Were the ancient viruses more virulent? Or milder? Did they have a different course of disease entirely?