Carl Zimmer in his excellent blog, The Loom:
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 our cells much as they read their own genes. In many cases, infected cells die as they manufacture hundreds of new viruses that burst out of them. But in some cases the viruses get stuck. They sit in the cell’s genome, and the cell goes on living. When the cell duplicates, it duplicates the virus DNA as well. Just because the virus spares the cell is not necessarily a good thing. The virus may still be able to pop out of dormancy and wreak havoc. It may also trigger its host cell to duplicate like mad–giving rise to cancer. One in five cancers is associated with these viruses.
Now imagine what might happen if one of these viruses happened to infect an egg. The egg might well die. Or not. And if it started to divide (as a fertilized embryo), the virus would be passed down to all the daughter cells. In other words, a baby would be born with the virus throughout its body.