Here’s a startling thought: the human genome contains ancient viruses. Researchers recently detected a 100-million-year-old virus called a human endogenous retrovirus (HERV)—that would have infected our ancestors when dinosaurs roamed the Earth—in the blood of pregnant women. They are still puzzling over how retroviruses might affect us in the long term. Eight percent of the human genome is made up of ancient viruses and scientists are still trying to puzzle out their function.