Although mutant B4 of Chlamydomonas reinhardtii lacks photosystem I (PS I), it is capable of photoautotrophic assimilation of atmospheric carbon dioxide and sustained simultaneous photo-evolution of molecular oxygen and hydrogen. Here we report that at saturating light intensities, carbon dioxide reduction is stable under anaerobiosis but unstable in air. At lower light intensities, carbon dioxide reduction is stable in both atmospheres. The data indicate that OS I is not necessary for autotrophic photosynthesis. One interpretation of these results is that oxygenic photosynthesis developed as a single-light-reaction process, presumably from a bacterium with a phaeophytin-quinone reaction centre, but became unstable as oxygen in the Earth’s atmosphere accumulated. PS I was the seconde light reaction, added to confer stability in oxygen-containing atmospheres. Viewed from this perspective, the well-known Z scheme of modern photosynthesis is seen as a specialized adaptation for performing low-potential reductive photochemistry in oxygen-containing atmospheres, but is not an irreducible necessity for satisfying the thermodynamic and mechanistic requirements of carbon dioxide photoreduction using water as the source of reductant.