Viruses have affected the development of several mammalian genomes, says Carl Zimmer at The Loom. In 2000, researchers found that the human gene syncytin — which is necessary for the gestation of offspring — may have come from a virus. "Viruses have insinuated themselves into the genome of our ancestors for hundreds of millions of years," Zimmer says. "They typically have gotten there by infecting eggs or sperm, inserting their own DNA into ours.