How did oxygen get into earth early atmosphere?

Oxygen first entered Earth's early atmosphere through a process called photosynthesis by ancient cyanobacteria around 2.7 billion years ago. These photosynthetic organisms produced oxygen as a byproduct of their metabolic processes, gradually increasing oxygen levels in the atmosphere. This ultimately led to the Great Oxidation Event, which occurred around 2.4 billion years ago and significantly increased atmospheric oxygen levels.