The first organisms to release oxygen into the atmosphere were cyanobacteria, also known as blue-green algae. These single-celled organisms evolved around 3.5 billion years ago and were the first to develop photosynthesis, the process of using sunlight to convert carbon dioxide and water into energy, releasing oxygen as a byproduct.

Before cyanobacteria, the Earth's atmosphere was essentially oxygen-free. The release of oxygen by cyanobacteria was a major event in Earth's history, leading to the development of complex life forms, the formation of the ozone layer, and the eventual oxygenation of the atmosphere we breathe today.