If you think about fossils, you probably picture a piece of bone or shell, turned to stone and buried in the ground. You visit them in museums; some of you may even have found some. But your closest fossils are inside you, scattered throughout your genome. They are the remains of ancient viruses, which shoved their genes among those of our ancestors. There they remained, turning into genetic fossils that still lurk in our genomes to this day.

We’ve known about our viral ancestors for 40 years, but a new study shows that their genetic infiltration was far more extensive than anyone had realised. The viral roots of our family tree have just become a lot bigger.

When viral genes were first