We propose the hypothesis that natural selec- tion, acting on the specificity or preference for CO2 over O2 of the enzyme rubisco (ribulose-1,5-bisphosphate car- boxylase/oxygenase), has controlled the CO2:O2 ratio of the atmosphere since the evolution of photosynthesis and has also sustained the Earth’s greenhouse-set surface tempera- ture. Rubisco works in partnership with the nitrogen-fixing enzyme nitrogenase to control atmospheric pressure. To- gether, these two enzymes control global surface temperature and indirectly the pH and oxygenation of the ocean. Thus, the co-evolution of these two enzymes may have produced clement conditions on the Earth’s surface, allowing life to be sustained.