Researchers have uncovered new information about the dark matter of our genomes, which could reveal clues about viruses which infected our ancestors as far back as the age of the dinosaurs.

The scientists set out to uncover why 90 per cent of a mammal’s functionless genome harbours bits of DNA from ancient viruses. They searched the genomes of 38 mammals covering a large range of genetic species – including rat, mouse, bat, human, elephant and dolphin – using in silico screening.

Researchers assembled from the University of Oxford, the Aaron Diamond AIDS Research Centre in New York and Rega Institute in Belgium, using mathematical models to compare the genetic material from all the residing viruses they had collected.

They found that one particular group of viruses had become unable to infect new cells. The genetic material could still amplify itself in this case, but the rest of the lifecycle was contained within a single cell. There was then a dramatic proliferation of the virus’ genetic material within the genomes.

By comparing it with all the other viruses in the genomes, they found that this was a universal phenomenon and that loss of cell infectivity is associated with a 30-fold increase in the abundance of the virus.

Robert Belshaw from Oxford’s Zoology department said: “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 the spread massively within it.”

This is a pattern that is comparable to that seen in epidemic outbreaks, where a small proportion of so-called superspreaders infect the rest of the population. Dr Gkikas Magiorkinis, lead author of the study published in Proceedings of the National Academy of Sciences explained: “We know that much of the ‘dark matter’ in our genome plays by its own rules, in the same way as an epidemic in an infectious disease, but operating over millions of years.”