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Oh, but we do know that there wasn't free oxygen in the atmosphere until about 2000 million years ago. There are deposits of uraninite and beds with water-laid pyrite that could not have formed under an oxidizing atmosphere; the banded iron formations in Minnesota (and many other places) formed from dissolved ferrous iron in ancient seas, also an impossibility under free oxygen.
The greatest volumes of iron formation date to about 2.2 billion years ago (or is that 2.3 bya?). As I understand it, this time marker is because this is the first abundant free oxygen in the atmosphere. There are also older iron formations - My knowledge on these is pretty limited, but as I understand it, these were a result of localized oxygenated areas.
In sediments previous to that 2.2 bya marker, detrital (sand grain) pyrite and uranenite can be found. These minerals are highly unstable in an oxygenated environment - they quickly chemically decompose. This is the most prominent evidence of an oxygen poor atmosphere, prior to 2.2 bya.
Moose
ps: The above is an example of an evolutionist side message lacking references. This is info pulled out of my general geological education. Were the above to appear in a journal, the assertations would probably be buried in multiple references.
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