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There is evidence of free oxygen in rocks, like the banded iron
formation. The red is caused when iron reacts and forms oxides.
Weathering became an oxygen consumer, taking free oxygen and combining
it with elements. This must have been the start of an abundant supply.
Without molecular oxygen life could not have evolved to its present
form. It probably originated with cyanobacteria (like present day
stromatolites) creating free oxygen as a by-product of
photosynthesis.
During the Proterozoic the amount of free O2 in the
atmosphere rose from 1 - 10%. Most of this was released by cyanobacteria,
which increase in abundance in the fossil record 2.3 billion years ago.
Present levels of O2 were probably not achieved until about
400 million years.
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