How do we know that we are kin to chimpanzees and howler monkeys and the other primates? For one thing, it’s by far the best explanation for the fossil record. For another, our DNA shows signs of kinship to other primates, much like the genetic markers that are shared by people from a particular ethnic group. There’s a third line of evidence that I find particularly fascinating: the viruses carried by humans and other apes.

Every day, viruses traffic in and out of human bodies. They invade people’s cells, make new copies of themselves, and then, if they’re lucky, infect a new host. Some viruses do this by stapling themselves into our DNA, so that their own genes are read by