The simplest organisms are bacteria and viruses. But long ago, simple organisms like these learned to cooperate rather than compete (something many humans have yet to do!). More complicated cells (eukaryotes) appeared which contained smaller cells, called organelles, which almost certainly had been bacteria. The bigger cells gave these helpful bacteria a safe home while the bacterial cell did something useful like making energy. Mitochondria are examples of these and they occur in every cell in all our bodies. They act as energy powerhouses for the cells which contain them. About 600 million years ago, new types of bodies began to appear in which cells clumped together to cooperate in more specialised ways. Later, starting with the Cambrian explosion (about 542 million years ago) of new life forms, some cells became hard to form an outside protection (shell). Others became specialised at catching things to eat - like the suckers on the arms of an octopus. Some (simple plants) had developed a way to harness the sun's energy to make sugars which they could then store to use later. And so on. At some early stage, creatures with backbones appeared (the chordates), then clever ones with brains... and the rest you know (unless you left your brain behind).