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. Scientists discovered layers in South Africa’s Pongola Basin which bear witness to oxygen production by bacteria as early as 2.97 billion years ago. That makes the Basin the earliest known home to oxygen-producing organisms — known as an oxygen oasis. The study has been published in the latest Nature Geoscience.
Conditions on Earth some three billion years ago were inhospitible to say the least. The atmosphere contained only one-one hundred thousandth of the oxygen it has today. The primeval oceans contained hardly any sulfate; but they did contain large amounts of ferrous iron. When bacteria started producing oxygen, it
— source Universitaet Tübingen | Jan 18, 2018