Three billion years ago single-celled underwater bacteria used sunlight to convert carbon dioxide and water into tiny oxygen bubbles. Soon plants were turning an atmosphere full of volcanic carbon dioxide into oxygen. As we learn in this video segment from Interactive NOVA: "Earth," photosynthesis created a good home for animals and humans, though not for some primitive organisms. They had to retreat to where oxygen couldn't reach them. Join researchers as they search for these organisms, now considered tiny time capsules from a time before there was oxygen on Earth.