Hundreds of millions of years ago, at a time when back-boned animals were just starting to crawl onto land, one such creature became infected by a virus. It was a retrovirus, capable of smuggling its genes into the DNA of its host. And as sometimes happens, those genes stayed put. They were passed on to the animal’s children and grandchildren. And as these viral genes cascaded through the generations, they changed, transforming from mere stowaways into important parts of their host’s biology.