Photosynthetic reaction centers (RCs) harvest light energy and convert it into chemical energy, a process that ultimately sustains most life on Earth. RCs are classified as type I (Fe-S type) or type II (quinone type) on the basis of their terminal electron acceptors. Most extant RCs are heterodimers represented by photosystems I and II (PSI and PSII), and they are evolved from homodimeric RCs similar to those seen in green sulfur bacteria (GSB) and heliobacteria. GSB grow in the absence of oxygen, capturing sunlight energy with a light-harvesting structure known as the chlorosome. Fenna-Matthews-Olson proteins (FMO) transfer this energy to a type I RC (GsbRC) to initiate charge-separation and electron-transfer reactions.