What event 2.5 billion years ago led into earths atmosphere getting oxygen?

Around 2.5 billion years ago, a significant event known as the Great Oxidation Event occurred, largely driven by photosynthetic microorganisms, particularly cyanobacteria. These organisms began to produce oxygen as a byproduct of photosynthesis, gradually increasing the levels of oxygen in Earth's atmosphere. This dramatic rise in atmospheric oxygen transformed the planet's environment, enabling the evolution of aerobic life forms and significantly altering the course of biological and geological history.