[File:Oxygenation-atm-2.svg|thumb|right|420px|O
2 build-up in the
Earth's atmosphere. Red and green lines represent the range of the estimates while time is measured in billions of years ago (Ga).
Stage 1 (3.85–2.45 Ga): Practically no O
2 in the atmosphere.
Stage 2 (2.45–1.85 Ga): O
2 produced, but absorbed in oceans & seabed rock.
Stage 3 (1.85–0.85 Ga): O
2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer.
Stages 4 & 5 (0.85 Ga–present): O
2 sinks filled, the gas accumulates.]]