"Retroviruses make up about 8 percent of the human genome. When these viruses insert into the genome,…"

“Retroviruses make up about 8 percent of the human genome. When these viruses insert into the genome, the result is usually bad for the host. But not always: Some retrovirus proteins can help fight off infection with other retroviruses. And at least twice in primate evolution retrovirus insertions have added genes to the host genome that aid in making the placenta.”

– Human Genome Is Part Bornavirus | Wired Science | Wired.com