Our DNA is embedded with a strange genetic code that protects us against viruses and infections.
It's an evolutionary irony. The genetic code is comprised of remnants of ancient viruses called endogenous retroviruses (ERVs) that apparently inserted themselves in the genomes of our prehistoric ancestors. The ERVs then evolved through time and now has the ability to fight against modern viruses.
But scientists are interested to know how and when ERVs began to spread, which places they had targeted, and which animals they attacked millions of years ago.
Efficient Colonizers And Invaders
Good thing there's an answer to that. About 15 to 30 million years ago, the global spread of a specific group of ERVs affected 28 out of 50 ancestors of modern mammals, including species as diverse as rodents, primates and carnivores.
A research team from Boston College found that a group of viruses called ERV-Fc had spread among prehistoric mammals found in almost all continents, except Australia and Antarctica.
This meant that the ERV-Fc were efficient invaders, jumping from one species to the next more than 20 times.
When did the ancient group of virus first came up in evolution? According to the study, ERV-Fc emerged from at least as far as the start of the Oligocene epoch, which was a period of drastic global change marked by climate cooling that eventually led to the Ice Ages.
At this period, vast expanses of grasslands began to grow and large mammals became the world's predominate fauna.
Boston College Professor Welkin Johnson said viruses exist everywhere life is found and have significant impacts on the evolution and ecology of organisms, from bacteria to humans.
While viruses do not leave fossils behind, he said viral genetic sequences in the DNA of organisms serve as molecular "fossils" that will help scientists explore the natural history of ERVs and their hosts.
Unraveling The Mystery
Johnson and his team performed an exhaustive search of mammalian genome sequences for ERV-Fc loci, and then compared the sequences.
For each genome with enough ERV-Fc sequence, the team reconstructed protein sequences that represented the viral invaders that colonized hosts. The sequences were used to infer the natural history and the relationships of ERV-Fc-related viruses.
The team found that the genome of mammals indeed contained hundreds of thousands of molecular fossils similar to ERV-Fc.
What's most interesting is the fact that these viruses exchanged genes with one another and with other viruses. This suggests that genetic recombination is vital for their evolutionary success.
"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," said geneticist William Diehl, the report's lead author.
Diehl said they could possibly assess the impact of HIV on human health 30 million years into the future. It will allow them to discern better why and when new viruses emerge, as well as how long-term contact with viruses impacts the evolution of hosts.
The findings of the study are featured in the journal eLife.