In the Earth’s early history, several billion years ago, only traces of oxygen existed in the atmosphere and the oceans. Today’s air-breathing organisms could not have existed under those conditions. The change was caused by photosynthesizing bacteria, which created oxygen as a by-product – in vast amounts. 2.5-billion-year-old rock layers on several continents have yielded indications that the first big increase in the proportion of oxygen in the atmosphere took place then.