To many people, viruses are simply agents for disease, but they may possess far greater power than originally thought. Some experts now believe that viruses helped give rise to the first complex cells, played a role in the diversification of life, changed how our young are born, drove evolution, and perhaps gave us emotions. One expert even suggests that an ancient virus could have entered our genes and rewired us for monogamy. National Geographic investigates a provocative new theory that suggests that all life – including humans – is descended from viruses.  (50 minutes)