Cast your mind back 40 million years and think about your ancestors. You’re probably picturing creatures that looked like a bit like today’s monkeys, but they’re only part of your family tree. To see your other ancestors, you’d have to whip out an imaginary microscope. Meet your great-great-great-etc-grandviruses.

The human genome is littered with the remains of viruses that, in ages past, integrated their genes into the DNA of our ancestors. They became a permanent fixture, passed down from parent to child. Today, these “endogenous retroviruses“, or ERVs, make up around 8% of our genome. They’re a living fossil record of prehistoric plagues.

Until recently, scientists thought that the only viruses to have left such a legacy were the retroviruses,