Abstract The origin of life on Earth seems to demand a highly reduced early atmosphere, rich in CH₄, H₂, and NH₃, but geological evidence suggests that Earth's mantle has always been relatively oxidized and its emissions dominated by CO₂, H₂O, and N₂. The paradox can be resolved by exploiting the reducing power inherent in the “late veneer,” i.e., material accreted by Earth after the Moon-forming impact. Isotopic evidence indicates that the late veneer consisted of extremely dry, highly reduced inner solar system materials, suggesting that Earth's oceans were already present when the late veneer came. The major primary product of reaction between the late veneer's iron and Earth's water was H₂. Ocean-vaporizing impacts generate high pressures and long cooling times that favor CH₄ and NH₃. Impacts too small to vaporize the oceans are much less productive of CH₄ and NH₃, unless (i) catalysts were available to speed their formation, or (ii) additional reducing power was extracted from pre-existing crustal or mantle materials. The transient H₂–CH₄ atmospheres evolve photochemically to generate nitrogenated hydrocarbons at rates determined by solar radiation and hydrogen escape, on timescales ranging up to tens of millions of years and with cumulative organic production ranging up to half a kilometer. Roughly one ocean of hydrogen escapes. After the methane is gone, the atmosphere is typically H₂- and CO-rich, with eventual oxidation to CO₂ rate-limited by water photolysis and hydrogen escape.