The freak accident behind complex life. All life on earth started as single cell bacteria and stayed like that for 2 billion years. Successfully spreading across the planet. So even if we do find alien life, what are the chances of that life being complex – like us? Vanishingly rare… 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 cellular power pack: mitochondria. This almost miraculous event transforms earth into a complex interconnected food web based on a competition for food. And at the top of the pyramid sits us humans.