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. There are 100,000 known fragments of viruses in the human genome, making up over 8% of our DNA." In 2005, French virologist Thierry Heidmann and his colleagues found that syncytins weren't just present in primates, but that a variant was present in mice as well, Zimmer adds. In 2009, Heidmann found another variant in rabbits.

In an new PNAS article, Heidmann reports his discovery of a variant of syncytin in dogs and cats, and all the other mammals that belong to the carnivoran branch, as well, Zimmer says. "The big picture that's now emerging is quite amazing. Viruses have rained down on mammals, and on at least six occasions, they've gotten snagged in their hosts and started carrying out the same function: building placentas," he adds. "The complete story will have to wait until scientists have searched every placental mammal for syncytins from viruses."