Hypothetical phylogeny for photosynthetic reaction centres. Prior to the last common ancestor of all extant life the primitive reaction centre, a homodimer with two bound quinones, each donating electrons to a primitive cytochrome cc complex, evolved into the heterodimeric type found in green non-sulphur bacteria (Chlorobacteria). This was duplicated prior to divergence of cyanobacteria and gracilicutes to generate a modified homodimeric type of cytochrome bc1 complex with iron-sulphur clusters (FF); for a mechanistic explanation of this duplication see . Cyanobacteria converted the two versions into photosystems I and II. Proteobacteria replaced chlorosomes in the original heterodimeric type by an H subunit with purple carotenoid, but did not retain the new duplicate with FeS clusters. By contrast, this was the only version retained by green sulphur bacteria (Sphingobacteria) and Heliobacteria, both losing the earlier heterodimeric type. This scenario is simplified from ref. 1 and congruent with the cladistic tree in Fig. 7 and the concatenated rRNA tree  and is compatible with photosynthetic protein trees, if properly rooted (see text).