ABSTRACT
Earth’s early atmosphere was probably a weakly reduced mixture of CO2 and N2, along with smaller amounts of H2 and other reduced gases. Surface pressure during the Hadean may have been significantly higher than today, but by Archean times, it was closer to today’s value. Hadean climate may have been warm, despite the faint young Sun, and Archean climate was more like that of the Phanerozoic, with glacial intervals interspersed with warm, ice-free periods. Life, which arose either in the Hadean or the early Archean, needed a source of thermodynamic free energy both for its origin and for its later maintenance and proliferation. Several different reactions involving species derived from atmospheric photochemistry could have provided this energy source. These reactions are analyzed using the concepts of biological energy quantum and maintenance energy. Based partly on this analysis, the so-called “water-gas shift reaction,” in which CO reacts with H2O to form CO2 and H2, is the most likely metabolic driver for the very earliest organisms. Methanogenesis is less primitive but should have led to a more robust biosphere once it was invented. Reactions of nitrogen oxides and/or O2 with available reductants are less likely to have been biologically important.
