The amount of oxygen in the Earth's atmosphere increased primarily due to the process of photosynthesis carried out by early life forms, especially cyanobacteria. These microorganisms used sunlight, water, and carbon dioxide to produce glucose and oxygen as a by-product. Over billions of years, the oxygen produced by these organisms accumulated in the atmosphere
. This buildup of oxygen was not immediate. For a long time, oxygen was absorbed by oceans and land surfaces, and reacted with elements like iron, leading to the formation of banded iron formations. Around 2.4 billion years ago, during the Great Oxidation Event (GOE), there was a dramatic rise in atmospheric oxygen levels as oxygen began to accumulate faster than it was consumed by chemical reactions. This event transformed Earth's atmosphere from one that was almost devoid of oxygen to one containing significant free oxygen
. The rise in oxygen levels continued over hundreds of millions of years, reaching about 20% of the atmosphere by around 350 million years ago, and even peaking at about 35% during the Carboniferous period. This oxygen increase allowed for the evolution of more complex aerobic life forms and changed the planet’s climate and chemistry significantly
. In summary, the increase in atmospheric oxygen was driven by photosynthesis in cyanobacteria and other early plants, coupled with geological and chemical processes that allowed oxygen to accumulate rather than being consumed, culminating in major events like the Great Oxidation Event that reshaped Earth's atmosphere
