The mantle was probably oxidized early, during and shortly after accretion, and so the early atmosphere of Earth was likely dominated by CO2 and N2, not by CH4 and NH3. CO2 declined from multibar levels during the early Hadean to perhaps a few tenths of a bar by the mid-to late Archean. Published geochemical constraints on Archean CO2 concentrations from paleosols are highly uncertain, and those from banded iron formations are probably invalid. Thus, CO2 could have been suffi ciently abundant during the Archean to have provided most of the greenhouse warming needed to offset the faint young Sun. H2 might have augmented this warming prior to the origin of methanogenic bacteria. Atmospheric CH4 concentrations increased from at most tens of parts per million (ppm) on prebiotic Earth to hundreds of parts per million once methanogens evolved. CO was an important trace gas on prebiotic Earth because of its high free energy and its ability to catalyze key reactions involved in prebiotic synthesis. Large impacts could have made the atmosphere transiently CO rich, and this may have played a role in the origin of life and in fueling early biological metabolisms.