In the film Alienwe were presented with the horrifying prospect of another being living inside a human body. But this isn't science fiction. Our cells all contain the remnants of simple organisms that inserted themselves into our distant ancestors - a union that may have kick-started complex life. These are the powerhouses of our bodies. Without them we couldn't survive. But how did the fusing of two life forms happen? Quentin Coopertalks to the experts. Producer Jonathan Rides E-MAIL: material.world@bbc.co.uk