How did oxygen originate on Earth?

The early environment was anaerobic and abiotic. In the anaerobic environment, oxygen was combined with carbon dioxide and water, which was in the vapour form. Gradually, anaerobic environments were formed from simple inorganic compounds to complex organic compounds, and these complex organic polymers played a role in the emergence of life on Earth. Initially, in the simple life form, water was photolyzed with the aid of solar energy, releasing free molecular oxygen into the anaerobic reduction environment. Afterwards, the anaerobic environment changed to an aerobic one and the formation of organic compounds was limited to living cells.

Oxygen originated on Earth through photosynthesis, a process carried out by certain organisms such as cyanobacteria, algae, and plants. These organisms use sunlight, water, and carbon dioxide to produce oxygen as a byproduct. Over billions of years, the accumulation of oxygen in the atmosphere led to the development of aerobic organisms and the oxygen-rich atmosphere we have today.