Viruses regularly invade people’s cells, make new copies of themselves, and infect their host. Sometimes they get “stuck” in the host cell’s DNA. And sometimes they get stuck in an egg cell, which replicates and survives — and that viral DNA survives for millions of years. From The Sixty-Million-Year Virus:
Scientists can identify viruses lurking in our genome (known as endogenous retroviruses) by their distinctive DNA. A fully-functioning retrovirus sequence contains three genes — one for copying DNA, one for a shell, and one for escaping and invading cells. These genes are flanked by a series of repeating DNA, which allow viruses to be inserted or snipped out of their host’s genome. The human genome carries full-fledged retroviruses, as well as viruses in various state of decay. Scientists have identified 98,000 of these viruses, along with about 150,000 fragments of defunct viruses. All told, they make up 8 percent of the human genome. In many cases, the virus genes have disappeared altogether, leaving behind flanking repeats, which have been duplicated to millions of copies that take up about 40 percent of the genome. As a point of comparison, our ‘own’ genes — in other words, those that encode proteins that make up our bodies and allow our bodies live — make up only about one percent of the genome.