Scientists have uncovered clues as to how our genomes became riddled with viruses, some dating back 100 million years.
More than 90 per cent of every mammal's genome has no known function, with much of this genetic 'dark matter' harbouring pieces of DNA from ancient viruses that infected our ancestors.
The new investigation, by researchers at Oxford University, the Aaron Diamond AIDS Research Center in New York and the Rega Institute in Belgium examines how these ancient viruses got into their hosts' genomes in such abundance.
The team searched the genomes of 38 mammals covering a large range of species, from mouse, rat and bat to human, elephant and dolphin, collecting genetic material from all of the residing viruses and comparing it using mathematical models.
Some were found to have been infecting our ancestors hundreds of millions of years ago - and to remain today.
One particular group of viruses, the team discovered, has lost the ability to infect new cells. Their genetic material can still amplify itself, but the whole life cycle of the virus takes place within a single cell.
And this change, they found, was followed by a dramatic proliferation of the virus' genetic material within the genomes. All of the other viruses in the genomes revealed the same phenomenon, with the loss of cell infectivity associated with a 30-fold increase in the abundance of the virus.
"We suspect that these viruses are forced to make a choice: either to keep their 'viral' essence and spread between animals and species, or to commit to one genome and then spread massively within it," says Robert Belshaw from the University of Oxford.
"This is the story of the epidemic within every animal's genome, a story which has been going on for 100 million years and which continues today."