Oxygen begins to accumulate in the atmosphere as a result of the success of cyanobacteria.
# Metadata
Title: Great Oxygenation Event Movietime: 4:24 Geographical Time: 2500 Ma
This marks one of the most transformative events in Earth's history — the rise of atmospheric oxygen. Triggered by the photosynthetic activity of cyanobacteria in shallow oceans, free oxygen began to build up, initially reacting with surface iron and forming banded iron formations.
Eventually, oxygen overflowed into the atmosphere, fundamentally changing Earth's chemistry and paving the way for aerobic life. It also led to extinction for many anaerobic organisms in what is sometimes referred to as Earth’s first mass extinction.