The Great Oxidation Event

What happened

Oxygen produced by photosynthesising cyanobacteria built up in the atmosphere around 2.4 billion years ago, rising by orders of magnitude over roughly 200 million years and turning an oxygen-poor world into an oxygen-rich one.

Why it matters

The rise of atmospheric oxygen was a turning point in Earth's history — deadly to many early microbes, but the essential foundation for the later evolution of large, complex organisms.

Sources

• NASA Astrobiology. Revisiting Earth's Oxygenation 2.4 Billion Years Ago · Reputable source