All life on Earth started as single-cell bacteria and stayed like that for two billion years. So even if we do find alien life out there, what are the chances of that life being complex like us? On our strange rock, it's all down to a freak event, which accidentally happened when one cell ate another to create a kind of power pack for life. This almost miraculous event transforms Earth into a complex interconnected web based on a competition for food. And at the top of the pyramid sit we humans. Astronaut host – Mae Jemison.