Cyanobacteria evolved photosynthesis and began releasing oxygen as waste. At first, Earth absorbed it into oceans, rocks, iron, and volcanic gases. But over time, those buffers filled up. Oxygen began to accumulate, transforming the planet and setting off one of life’s greatest crises—and opportunities.

Photosynthetic cyanobacteria began releasing oxygen into the oceans and atmosphere, transforming Earth’s chemistry. For many early anaerobic organisms, oxygen was toxic. But for the future of complex life, this was a turning point. Oxygen opened the path toward more energy-hungry cells, larger bodies, and eventually animals.

Oxygen spilled into the atmosphere paving the way for profound environmental and biological changes. The creation of an ozone layer, the oxygenation of the oceans, and the consequent rise of aerobic life forms set the stage for the later explosion of complex life on Earth, fundamentally altering the course of our planet’s history.

Breathable air to animals, including humans, started about 540 million years ago.