Which process do scientists think provided earth with an oxygen-rich atmosphere?

Scientists believe that the process of photosynthesis played a crucial role in providing Earth with an oxygen-rich atmosphere. This process, primarily carried out by cyanobacteria and later by plants, began around 2.4 billion years ago during the Great Oxidation Event. As these organisms converted carbon dioxide and water into glucose and oxygen using sunlight, the accumulation of oxygen in the atmosphere significantly transformed Earth's environment, enabling the evolution of aerobic life forms.