The ancient organisms that made most of the oxygen on Earth were cyanobacteria, also known as blue-green algae. These single-celled organisms, which appeared around 3.5 billion years ago, developed the process of photosynthesis, which converts sunlight, water, and carbon dioxide into energy and oxygen.

Over millions of years, the oxygen produced by cyanobacteria gradually accumulated in the atmosphere, transforming Earth from an oxygen-poor to an oxygen-rich environment. This dramatic shift in atmospheric composition had profound consequences, paving the way for the evolution of more complex life forms that rely on oxygen for respiration.