Viruses can be thought of as hyperspeed shape-shifters, organisms that can adapt quickly to overcome barriers to infection. But recent research has been finding ancient traces of many viruses in animal genomes, DNA insertions that have likely been there for much longer than the viruses were previously thought to have existed at all.
A new study describes evidence of a hepadnavirus (a virus group that includes hepatitis B, which infects humans as well as other mammals and ducks) hiding in the genomes of modern songbirds. By tracing back to these bird species' common ancestors, the researchers behind the new work estimate that this family of viruses has been around for at least 19 million years—and possibly as long as 40 million years—rather than the several thousand years researchers had estimated.
Such a primordial start is difficult to square with how similar these ancient snippets' DNA looks to currently circulating versions of the virus and what we know about viruses' ability to change so rapidly. "It's just something we don't quite grasp in the evolution of viruses," says Cédric Feschotte, an associate professor at the University of Texas Arlington's Department of Biology and co-author of the new study. "I think that's pretty exciting." By complicating the understanding of viral evolution, the new findings also promise to help inform transmission dynamics and the ways in which viruses move among different host species.
The new estimate would slow the average rate of hepadnavirus mutation some 1,000-fold, wrote the researchers of the new the study, published online September 28 in PLoS Biology.
Given the bold new picture of virus evolution that many of this and other recent genomic insertion discoveries imply, however, Feschotte is braced for blowback from the field. But the study's numbers look solid, says Harmit Malik, an evolutionary geneticist at the Fred Hutchinson Cancer Research Center who was not involved in the new research. "There was a fairly high burden of proof" in the study, he says; "I think the authors have done a really good job."