Fig. 2.

Percentage of sequence identity of PsaA and PsaB as a function of time. The data points are listed in Table 1. Dots in orange represent the level of sequence identity of PsaA from one species compared to the same subunit in a different species. Dots in grey represent PsaB for similar pairs of species. The blue bar represents the GOE. The oldest pair of dots are those from G. kilaueensis and are placed arbitrarily at about the start of the GOE. The second oldest node was set at 1.45 Ga and taken as the average of the estimated divergence time for the clade containing Cyanidioschizon merolae as calculated in Ref. [22] and [24], dated in the range of 1.1–1.8 Ga respectively. The red bars represent the level of uncertainty on the estimated divergence times of the clades that contain these species. The blue dashed line marks the average level of seq_id between PsaA and PsaB in all known oxygenic phototrophs, at about 42.5%.