Around 8 to 10 per cent of your DNA comes from viral ancestors. These sequences are the remains of prehistoric viruses that inserted their DNA into the genes of our ancestors, hundreds of millions of years ago. Some of them became permanent residents, and were passed down from parent to child. These endogenous retroviruses, or ERVs, are a legacy of epidemics past.

We understand how ERVs got into our DNA in the first place. But why have they been such successful invaders, to the point where they fill around a tenth of our genome? Gkikas Magiorkinis from the University of Oxford has an answer. By comparing the ERVs of 38 mammals, from humans to dolphins, he has found that the