Geological history of oxygen

Photosynthetic prokaryotic organisms that produced O2 as a byproduct lived long before the first build-up of free oxygen in the atmosphere,[5] perhaps as early as 3.5 billion years ago.

[6] Early fluctuations in oxygen concentration had little direct effect on life, with mass extinctions not observed until around the start of the Cambrian period, 538.8 million years ago.

[11] The maximum of 35% was reached towards the end of the Carboniferous period (about 300 million years ago), a peak which may have contributed to the large size of various arthropods, including insects, millipedes and scorpions.

However, this long period was noticeably euxinic, meaning oxygen was scarce and the ocean and atmosphere were significantly sulfidic, and that evolution then was likely comparatively slow and quite conservative.

[9] The most celebrated link between oxygen and evolution occurred at the end of the last of the Snowball Earth glaciations, where complex multicellular life is first found in the fossil record.

[9] Models based on uniformitarian principles (i.e. extrapolating present-day ocean dynamics into deep time) suggest that such a concentration was only reached immediately before metazoa first appeared in the fossil record.

[9] Further, anoxic or otherwise chemically "inhospitable" oceanic conditions that resemble those supposed to inhibit macroscopic life re-occurred at intervals through the early Cambrian, and also in the late Cretaceous – with no apparent effect on lifeforms at these times.

[9] This might suggest that the geochemical signatures found in ocean sediments reflect the atmosphere in a different way before the Cambrian – perhaps as a result of the fundamentally different mode of nutrient cycling in the absence of planktivory.