The Earth formed about 4600 million years ago and, 400–700 million years later, chemical evolution began, leading to living entities with metabolic pathways, catalysts, information storage molecules and, finally, membrane-enclosed ‘organisms’. Evolution continued, with high levels of mutation and lateral gene transfer, to produce microbial anaerobic heterotrophs, autotrophs, lithotrophs, phototrophs (anoxygenic and later oxygenic) and, by 2200 million years ago, aerobes.