The origin of life could have involved autotrophy, but this is most probably chemolithotrophic rather than photolithotrophic. There is evidence, from the natural abundance of carbon isotopes, of autotrophy involving Rubisco and the Benson–Calvin–Bassham cycle from about 4 Ga. However, other autotrophic CO2 fixation pathways could also have occurred. Evidence on the evolution of photosynthetic reactions suggests an early origin of the photochemical reaction centre, with the possibility of the occurrence of two photosystems in series (photosystem II plus photosystem I) and the possibility of oxygenic photosynthesis, before the origin of the single photosystem (reaction centre I or reaction centre II) photosynthesis in the multiple clades of anoxygenic photosynthetic bacteria. The origin of photosystem II and photosystem I preceded the origin of cyanobacteria and the subsequent Great Oxidation Event at about 2.4–2.3 Ga. The occurrence of oxygenic photolithotrophy is a necessary, but not sufficient, condition for the occurrence of the Great Oxidation Event and the Neoproterozoic Oxidation Event. There is no consensus on what other factors are involved in initiating the Great Oxidation Event and the Neoproterozoic Oxidation Event.