By comparing viral "insertions" that occurred in closely related animal species, the authors determined that some of these traces of past infections were left behind as long as 40 million years ago.
The research drew praise from W. Ian Lipkin, director of the Center for Infection and Immunity at Columbia University's Mailman School of Public Health.
"It has profound implications for understanding how we've evolved with viruses and how viruses have molded us to become what we are today," said Lipkin, who was not involved with the study. "It's like doing genomic archaeology."
Genetic sequences from the filovirus family - the one that includes Ebola and its deadly cousin, Marburg - were found in six vertebrate species. These included the wallaby, the opossum, and a kind of bat called the microbat, but not humans. Ebola and Marburg both cause hemorrhagic fevers.
Sequences from Bornaviruses, which cause neurological disease, were found in the genomes of 13 animal species, including humans, squirrels, and mice.
The researchers found that sequences from the ancient viruses were about 30 percent or 40 percent similar to their modern counterparts, a surprisingly high rate, according to Cedric Feschotte, a University of Texas scientist who was not involved with the paper.
"You would never expect to see anything that close," said Feschotte, an associate professor of biology at UT's Arlington campus.
The high rate of similarity might indicate that some of these ancient viral insertions provide a measure of protection against disease, said Anna Marie Skalka, one of the paper's authors and director emerita of the Institute for Cancer Research at Fox Chase.
