About 1/12 of our genome consists of stretches of DNA from viruses that infected our ancestors millions of years ago.

Image courtesy of the NIH

These viruses at some point inserted themselves into our genome via DNA parasitism, and have been evolving with us for millions of years. Just like other mutations, the mutations that are caused by the insertion of viruses into our genome can produce genetic diseases, as well as potentially beneficial adaptations.

Even more amazing, in our chimp cousins, we can see portions of their genome which have the exact same virus DNA inserted at the exact same point in their genome. This is unequivocal evidence that both ourselves and our chimp cousins had a common ancestor at some point which means that evolution has indeed taken place!