In the birds the viral fragments were located in "the exact same genomic position," a finding that would be "extremely unlikely" to occur at different times across different species more recently, the researchers noted in their paper. Thus, the hepadnaviruses instead worked their way into common ancestors tens of millions of years ago and has been passed down in the genetic code to new species that evolved in the intervening millennia.
As the field of paleovirology has taken off in recent years—thanks in large part to quicker genetic sequencing technology—more and more surprises are emerging from the dark, heretofore little-explored reaches of vertebrate genomes. And Feschotte predicts an increasing number of viral findings in the near future.
"Any kind of viruses can potentially do this as long as they can infect the germ cells," Feschotte says. "This is not going to stop. We're picking the low-hanging fruit right now," he says of viral fossil study. And new research profiling more endogenous viruses is slated to be published soon.
Genetic scans can also provide a new, more even-keeled perspective on viral history, Feschotte says. He notes that the study of virus evolution has likely been subjected to a general selection bias in what strains undergo examination. Most viral subjects have been the cause of modern illness—whether in humans or other organisms. But looking into the genetic past "opens an interesting window, a much less biased window, I would argue, to study viruses," he says.
Feschotte proposes that most viruses that have come under study show such a fast mutation rate because they have recently—in the past thousands of years—crossed over into a new species and "are in their adaptation phase, so it kind of makes sense that we see them evolving much faster." He points to the presence of the Ebola virus, which is relatively new to—and so deadly in—humans, that seems to have existed for far longer in bats and hardly makes them sick. But taking a broader viral history survey can offer a more representative picture of how viruses evolved in and with their hosts.
Malik is not entirely convinced by this interpretation, noting that the unexpectedly slow average mutation rate might have more to do with host genetics than those of the viruses. But many of these uncertainties might be answered with further study in the lab, assessing and even reassembling these extinct viruses.
"These are DNA fossils, and we can always put back together and resurrect extinct species," Feschotte says. "I know it sounds very scary, like science fiction," he adds, "but that can really answer questions in terms of the biology of the virus."
Feschotte explains that other researchers might have to puzzle over fossilized bones and argue indefinitely about whether a hominid walked upright. But with the genetic material of an extinct virus in front of them, researchers can recreate it in the lab. The process (which Malik hastens to note is done "in a highly protected setting") could eventually "reveal to us why these hosts were able to survive and thrive with these insertions," Malik says.