This graph represents the concentration of oxygen in the atmosphere over the last 1000 million years. As a reference, the dashed red line shows the present concentration of 21%.

It should be noted that the O₂ concentration variation which occurred in past periods can often be estimated only relatively coarsely. Therefore, the chart makes no claim to accuracy, but can only give a broad overview of change at the level of geological periods. This chart is based on the averaged data of several publications. ^[1][2]

Explanation of the curve:

After having already begun in the Precambrian, the outgassing of oxygen from the oceans caused atmospheric oxygen to jump from 3% to 12% around the beginning of the Cambrian, because by then all oxygen sinks were finally saturated. Around the same time, the Cambrian explosion took place. During the Silurian and Devonian plants spread over the land, while the animal kingdom was still almost exclusively confined to the water. This caused a further rapid and continuous increase of the oxygen concentration. In the following period carbon burial brought the oxygen level to 35%,^[3] thus favoring overgrowth especially in insects. Amphibians and early reptiles were now present on land.

Massive volcanic activity at the Permian-Triassic transition caused not only a decline of the oxygen content to 15%, but also the largest mass extinction in geological history. The oxygen concentration then recovered over a long period and reached 26% again by the middle of the Jurassic, and probably even rose to 30% in the Cretaceous. During this time the largest dinosaurs evolved.

1. ↑ Gunnar Ries: Dicke Luft bei den Sauriern
2. ↑ Geologiefznzzrz-Forum @ Geoversum: Massensterben an der Perm-Trias Grenze: Durch Sauerstoffmangel erstickt?
3. ↑ wissenschaft.de: Gigantismus, Fliegen und Antiaging: Sauerstoffreiche Luft löste vor 300 Millionen Jahren einen Innovationsschub aus