See Kasting figure: The rise of atmospheric oxygen. How do we know?

There are rocks which are 2.5 billion years old which contain the banded iron formation.

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPaleo1.html

These are sedimentary rocks and the redness of the iron layers indicates that there was oxygen in the water. Oxygen (O2) was starting to appear in the atmosphere of the Earth.

After about 2 billion years ago, we no longer see banded iron formations in the rocks; instead we see redbeds.

http://earth.usc.edu/~geol150/evolution/precambrian2.html

Redbeds are sandstones which contain iron which has rusted! This implies (for various reasons) that the amount of free oxygen -- that is, not in any other molecule -- had further increased.

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