All organisms are made up of cells. An adult human has around 300 trillion body cells - a three with 14 zeros. The vast majority of organisms on Earth are however made up of only a single cell. Life on Earth began 3.5 billion years ago with the single-cell organisms. Only around 700 million years ago did the first multicellular organisms being to emerge, from which all other organisms, including humans, evolved. This film looks at a number of micro-organisms to explain the first steps in that evolution.