In the December 1997 issue of Geology p. 1135, an article by H. Ohmoto titled "Evidence in pre-2.2 Ga paleosols for the early evolution of atmospheric oxygen and terrestrial biota", concluded: "Minimum concentration of O2 in the 3.0-2.2Ga atmosphere is calculated to be about 1.5% of the present atmospheric level [his italics]". This conclusion delivers the final blow to those wishing the Precambrian atmosphere to be reducing.
The article considers the behavior of soils under anoxic and oxic conditions, concluding the oxygen levels of the Precambrian earth were a minimum of 1.5% O2 throughout most of the Precambrian.
Miller-Urey experiments have at least 2 orders of magnitude smaller yields of amino acids in neutral and mildly reducing atmospheres, and do not produce biological products at all in an oxidizing environment. This proposal is not for a neutral or mildly reducing atmosphere. The author emphasizes repeatedly that his figure of 1.5% is a minimum, and likely there was more O2 than that. For more information on this subject click here
2010 Arthur V. Chadwick, Ph.D.