About the oxygen content of the Earth's early atmosphere, there was an important transition at around 2.3 billion years ago, when the Earth's atmosphere became much more oxygenated.
Before that, the oxygen amount was at most 1% of the present amount, and possibly even less. There is evidence of this in uranium minerals and paleosols (fossilized soil), which are less oxidized before than after.
Some possible counterevidence is the Banded Iron Formations (BIF's), which are mostly on the "before" side. They contain Fe3+, which is insoluble, and most likely produced from soluble (and dissolved) Fe2+ by photosynthesizing microbes.
However, those microbes need not have released oxygen; they could have directly oxidized Fe2+ to Fe3+ to extract electrons for photosynthesis (oxygenic photosynthesis involves extracting electrons from water, leaving hydrogen ions and oxygen). Some present-day photosynthetic bacteria use sources of electrons other than water, so iron-oxidizing photosynthesis is completely feasible.
Furthermore, it's been seriously speculated that the early Earth atmosphere had about 10^-3 of methane (around 1000 times the present-day abundance). That, along with the early Earth's carbon dioxide, would produce enough of a greenhouse effect to keep the Earth from freezing as a result of the Sun being dimmer than it is today.
This methane would be produced by methanogens which live off of atmospheric carbon dioxide and hydrogen released from hot springs and the like.
However, for it to be stable, the oxygen content has to be at most 10^-7.
Here's a
nice little abstract on that subject.
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