Photosynthesis evolved in the Archean. Before the Great Oxidation Event, the dominant form of photosynthesis was anoxygenic bacterial photosynthesis, although there is molecular phylogenetic evidence of the occurrence of oxygenic cyanobacteria (or their ancestors) in the Archean, explaining the occurrence of ‘whiffs of oxygen’ in the Archean. Recent molecular genetic evidence shows that phototrophy is a synapomorphy of only one of the six clades of anoxygenic phototrophs (the Chlorobi) and of the oxygenic cyanobacteria, so the occurrence of phototrophy in the other five clades of anoxygenic phototrophy involves horizontal gene transfer. Photolithotrophy only occurs in three clades of anoxygenic bacteria, that is, the Chlorobi with reaction centre I for photochemistry and the reverse tricarboxylic acid cycle for CO2 fixation, the Chloroflexi with reaction centre II for photochemistry and the 3-hydroxypropionate bi-cycle or the Benson–Calvin–Bassham cycle for CO2 fixation, and the γ-proteobacteria with reaction centre 2 for photochemistry and the Benson–Calvin–Bassham cycle for CO2 fixation. The oxygenic cyanobacteria have photosystem I (homologue of reaction centre I) and photosystem II (homologue of reaction centre II) in linear electron flow, and photosystem I in cyclic electron flow, and the Benson–Calvin–Bassham cycle for CO2 fixation.