Is the concentration of Oxygen in the Earth's atmosphere constant since the planet's formation?

The concentration of oxygen in the Earth's atmosphere has not remained constant since the planet's formation. Initially, the Earth's atmosphere was composed primarily of gases released from volcanic activity, such as carbon dioxide, nitrogen, and water vapor, with virtually no free oxygen. It wasn't until the advent of photosynthetic organisms, particularly cyanobacteria, around 2.4 billion years ago, that significant amounts of oxygen began to accumulate in the atmosphere. This event is often referred to as the Great Oxidation Event, marking a dramatic increase in atmospheric oxygen levels.

Since that time, the concentration of oxygen has fluctuated due to various natural processes, including photosynthesis and respiration, geological activity, and the impacts of human activity in more recent times. Presently, oxygen makes up about 21% of the atmosphere, but that percentage has varied considerably through Earth's geological history, making the assertion that oxygen concentration has been constant inaccurate.