What influenced the abundance of oxygen in Earth’s atmosphere?
The concentration of oxygen in the atmosphere has changed a lot throughout Earth's history. Theories based on geological evidence suggest the processes listed below are responsible for these changes.
Photosynthesis
Photosynthetic organisms like cyanobacteria were the first oxygen producers on Earth. Fossil evidence for stromatolites, reef-like structures built up by cyanobacteria, dates back as far as 3.5 byabillion years ago. Cyanobacteria are also called blue-green algae because they look like green algal growth in water. They evolved sometime between 3.5 to 2.5 byabillion years ago, but the concentration of O2 in the atmosphere did not increase immediately.
Absorption by oceans
The Earth’s early atmosphere did not contain any free O2 until about 2.4 byabillion years ago. It is theorized that the O2 produced by cyanobacteria was directly captured in the oceans and rocks on the seafloor. Various evidence in the geological record indicates that the oceans acted as one of the first big oxygen sinks. For example, O2 oxidized dissolved iron in the oceans, forming Banded Iron Formations.
The Great Oxidation Event
Free oxygen in the atmosphere is recorded from around 2.4 byabillion years ago. The first significant increase in atmospheric O2 is referred to as the “Great Oxidation Event”. There is evidence that populations of cyanobacteria increased and oxygen sinks were used up. Thus, O2 could overflow into the atmosphere. After 2.4 byabillion years ago, pyrite was not found in rocks on land, since it is unstable and breaks down in an oxygenated environment.
The Cambrian Explosion
The following period of relatively stable, low atmospheric O2 levels is called the “boring billion”, between 1.8 - 0.8 byabillion years ago. O2 in the atmosphere was mainly absorbed by rocks on land, and the evolution of life stagnated. However, from around 800 myamillion years ago O2 levels in the atmosphere increased because oxygen sinks were filled up again. During the Cambrian Explosion, around 542 myamillion years ago, many new species evolved in the oceans and on land. Plants evolved and eventually, the atmosphere reached today’s O2 levels of around 21%. 
Figure 1: Atmospheric oxygen abundance over Earth’s history: The increase of O2 abundance in Earth's atmosphere (solid line) can be linked to two major events, the Great Oxidation Event around 2.4 bya billion years agoand the Cambrian Explosion around 542 myamillion years ago, after which the atmospheric O2 abundance fluctuated (shaded area shows uncertainty of different theories and data) and finally reached today’s levels of 21 %.