Biological role of oxygen - Build-up in the atmosphere

The presence of large amounts of dissolved and free oxygen in the oceans and atmosphere may have driven most of the anaerobic organisms then living to extinction during the oxygen catastrophe about 2.4 billion years ago. However, cellular respiration using O₂ enables aerobic organisms to produce much more ATP than anaerobic organisms, helping the former to dominate Earth's biosphere. (Freeman 2005, 214, 586) Photosynthesis and cellular respiration of O₂ allowed for the evolution of eukaryotic cells and ultimately complex multicellular organisms such as plants and animals.

Since the beginning of the Cambrian era 540 million years ago, O₂ levels have fluctuated between 15% and 30% per volume. (Berner 1999, 10955–57) Towards the end of the Carboniferous era (about 300 million years ago) atmospheric O³4_{5³ levels reached a maximum of 35% by volume, allowing insects and amphibians to grow much larger than today's species. Human activities, including the burning of 7 billion tonnes of fossil fuels each year have had very little effect on the amount of free oxygen in the atmosphere. At the current rate of photosynthesis it would take about 2,000 years to regenerate the entire O³67³ in the present atmosphere. (Dole 1965, 5–27)}

See also:

• oxygen

• Biological role of oxygen
• oxygen