About 2.4 billion years ago, Earth’s air had almost no oxygen—just oceans of microbes and a toxic atmosphere.

Then, tiny blue-green organisms called cyanobacteria began doing something revolutionary: photosynthesis. As they made energy from sunlight, they released oxygen—a waste product that slowly transformed the planet.

This moment—called the Great Oxygenation Event—killed off many early microbes, but made complex life possible. It was Earth’s first major extinction and the beginning of breathable air.

How do we know?

• Rusty rocks (called banded iron formations) show when oxygen started building up.

• Chemical shifts in sulfur confirm it happened.

• Fossils of oxygen-producing microbes tell us who did it.

The air we breathe today?
Started with a bacterial bloom.