Ups and downs of ancient oxygen

Abstract

The first significant increase in atmospheric oxygen levels on Earth (the 'Great Oxidation') is thought to have occurred at least 300 million years after the evolution of oxygenic photosynthesis, but the reason for this time lag is not clear. Using a new conceptual model of the global redox system, Goldblatt et al. show that atmospheric oxygen levels could have remained either at a low or a high steady state after oxygenic photosynthesis evolved. The Great Oxidation may have been a switch between these states triggered by a relatively small environmental change. The model suggests that oxygenic photosynthesis alone is insufficient to cause an oxygen-rich atmosphere. So in the absence of additional factors, Earth might have an atmosphere containing only a few parts per million of oxygen, not the 21 which we enjoy. Equally, oxygenic photosynthesis could conceivably have evolved on planets that only have low levels of atmospheric oxygen.