It is believed that the RC arose only once during Earth’s history, and that all the RCs around today are distantly related to that original ancestor. Over time, these RCs have changed to perform different chemistries. While the amino acid sequences have changed a great deal, astonishingly, the overall structure of RCs has remained similar. The team believes that the ancestral reaction center (ARC) was simpler than the versions that exist today. This ARC was probably homodimeric and interacted with molecules in the membrane, like the modern Type II RCs (and the heliobacterial RC), instead of with soluble proteins.
They instead compared the positions of protein structural elements and chlorophylls within the RCs. In essence, they focused more on the structure and function of the RCs to reconstruct the evolutionary history, starting by making predictions about the ARC’s structure and function.
Once the ARC had acquired the metal cluster, thus becoming the ancestor to all modern Type I RCs, more changes occurred to further direct electrons to a soluble acceptor, which resulted in extracting more energy for the cell’s metabolism. These included changes in the positions and identities of the chlorophyll cofactors. Much of the later changes in the Type I RCs were driven by the need to deal with the presence of oxygen, as the unstable intermediates within RCs can react with oxygen to generate very damaging molecules. In the opinion of the ASU team, the heliobacterial RC retains clear vestiges of the changes leading to the early Type I RC and that understanding the fine details in how modern RCs work allows for informed hypotheses about how they evolved.