A study shows how extensively viruses from as far back as the dinosaur era still thrive in our genetic material.
It sheds light on the origins of a big proportion of our genetic material, much of which is still not understood.
The scientists investigated the genomes of 38 mammals including humans, mice, rats, elephants and dolphins.
The research was carried out at Oxford University, the Aaron Diamond AIDS Research Centre in New York and the Rega Institute in Belgium.
It is reported in the journal Proceedings of the National Academy of Sciences.
One of the viruses was found to have invaded the genome of a common ancestor around 100 million years ago with its remnants discovered in almost every mammal in the study.
Another infected an early primate with the result that it was found in apes, humans and other primates as well.
The work established that many of these viruses lost the ability to transfer from one cell to another.