The Archean ends
Two billion years of a microbial, oxygen-free world close — and the air is about to change.
- When
- Known to
- ± 10 million years
- Era
- Paleoproterozoic, Proterozoic
The Archean–Proterozoic boundary at 2.5 billion years ago is a round number rather than a marker horizon, but it sits close to a genuine turning point. Continents are largely assembled, plate tectonics is running in its modern form, and cyanobacteria have been making oxygen for hundreds of millions of years while the oceans and volcanic gases absorbed all of it. Within 100 million years that balance breaks, and the atmosphere fills with a gas that is poison to almost everything then alive.