The mantle was probably oxidized early, during and shortly after accretion, and so the early atmosphere of Earth was likely dominated by CO 2 and N 2 , not by CH 4 and NH 3 . CO 2 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 CO 2 concentrations from paleosols are highly uncertain, and those from banded iron formations are probably invalid. Thus, CO 2 could have been sufficiently abundant during the Archean to have provided most of the greenhouse warming needed to offset the faint young Sun. H 2 might have augmented this warming prior to the origin of methanogenic bacteria. Atmospheric CH 4 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.