Earth's early atmosphere.
http://seis.natsci.csulb.edu/rbehl/300i-L06.htm
Evidence for the lack of free oxygen
Urananite and pyrite minerals are found unoxidized in Precambrian sediments
Both readily oxidized today
No early Precambrian iron oxides
Fe-rich minerals in sediments deposited on land tend to rust (oxidize)
E.g., in alluvial fans, rivers, desert sand dunes
Typically referred to as "red beds"
Banded Iron Formations present in early Precambrian
Mostly disappear by 1.8 - 2.5 Ga (billions of years ago)
Chemical building blocks of life can't survive in the presence of O2
Amino acids
DNA
The simplest living organisms have an anaerobic metabolism
E.g., some bacteria (such as botulism), archaebacteria or archaea which inhabit unusual
environments
They are killed by oxygen
http://www.csuchico.edu/~jpushnik/6Abiol/abiotic.htm
Formation of the Second Atmosphere
Gases escaped from the molten core through volcanic activity.
This atmosphere contained H2, H2O, CO, CO2, N2, H2S, HCN, H2CO.
Noticeable absent is O2, making this second atmosphere a "Reducing atmosphere"
Water was present as vapor providing conditions for the development of weather.
Rains would have formed surface water deposits ("Primordial soups").
So my view is that Art's statement that the non-reducing nature of the early earth is well established is something that might be more wishing than reality. I believe that there is still quite a bit of disagreement here.