Fig. 5.

Rates of evolution as a function of divergence time. Grey dots represent divergence times of a tree calculated using a root constraint of 3.5 Ga (shown in Fig. 3), while orange dots represent those calculated using a root constraint of 3.8 Ga. The fastest rate of evolution occurs at the PsaA/PsaB duplication event. The plot shows that higher rates of evolution, measured as amino acid changes per site per million years, are necessary to explain the divergence of PsaA and PsaB at any point in time in the history of life.