Scientists have uncovered the history of ancient viruses that stretches back as far as 30 million years ago. The findings could help researchers better understand the evolution of viruses and learn more about the viruses that exist today.

Retroviruses are abundant in nature and include human immunodeficiency viruses (HIV-1 and -2) and human T-cell leukemia viruses. In this latest study, the researchers looked at a specific group of these viruses called ERV-Fc that affected a wide range of hosts.

"Viruses have been with us for billions of years, and exist everywhere that life is found," said Welkin Johnson of Boston College, co-author of the new study. "They therefore have a significant impact on the ecology and evolution of all organisms, from bacteria to humans. Unfortunately, viruses do not leave fossils behind, meaning we know very little about how they originate and evolve. Over the course of millions of years, however, viral genetic sequences accumulate in the DNA genomes of living organisms, including humans, and can serve as molecular 'fossils' for exploring the natural history of viruses and their hosts."

Using these "fossil" remains, the researchers focused on uncovering the natural history of ERV-Fc. First, the scientists performed a search of mammalian genome sequence databases for ERV-Fc loci and then compared the recovered sequences. For each genome with sufficient ERV-Fc sequence they then reconstructed the sequences of proteins representing the virus that colonized the ancestors of that particular species.

The researchers were able to pinpoint patterns of evolutionary change in the genes of these viruses, reflecting their adaptation to different kinds of mammalian hosts. Interestingly, the researchers found that these viruses often exchanged genes with each other and with other viruses, suggesting that genetic recombination played a significant role in their evolutionary success.

"Mammalian genomes contain hundreds of thousands of ancient viral fossils similar to ERV-Fc," said William E. Diehl from the University of Massachusetts, lead author of the study. "The challenge will now be to use ancient viral sequences for looking back in time, which may prove insightful for predicting the long-term consequences of newly emerging viral infections. For example, we could potentially assess the impact of HIV on human health 30 million years from now. This method will allow us to better understand when and why new viruses emerge and how long-term contact with them impacts the evolution of host organisms."