The ozone layer forms
Oxygen high in the atmosphere becomes a shield, and the land surface stops being lethal.
- When
- Known to
- ± 100 million years
- Era
- Paleoproterozoic, Proterozoic
As oxygen accumulates, ultraviolet light converts some of it to ozone, building a stratospheric layer that absorbs the shortest-wavelength radiation. Before this, unshielded UV made exposed land surfaces hostile to anything without shelter. The shield is a precondition for life eventually leaving the water for good, though that will not happen for another 1.9 billion years.