The ability to convert light energy into chemical energy is a tremendous evolutionary advantage. Geological evidence suggests that oxygenic photosynthesis became important approximately 2 billion years ago. Anoxygenic photosynthetic systems arose much earlier in the 3.5-billion-year history of life on Earth (Table 19.1). The photosynthetic system of the nonsulfur purple bacterium Rhodopseudomonas viridis has most features common to oxygenic photosynthetic systems and clearly predates them. Green sulfur bacteria such as Chlorobium thiosulfatophilum carry out a reaction that also seems to have appeared before oxygenic photosynthesis and is even more similar to oxygenic photosynthesis than the photosystem of R. viridis. Reduced sulfur species such as H₂S are electron donors in the overall photosynthetic reaction

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Nonetheless, photosynthesis did not evolve immediately at the origin of life. No photosynthetic organisms have been discovered in the domain of Archaea, implying that photosynthesis evolved in the domain of Bacteria after Archaea and Bacteria diverged from a common ancestor. All domains of life do have electron-transport chains in common, however. As we have seen, components such as the ubiquinone–cytochrome c oxidoreductase and cytochrome bf family are present in both respiratory and photosynthetic electron-transport chains. These components were the foundations on which light-energy-capturing systems evolved.

As we have learned, photosynthetic organisms use sunlight to oxidize H₂O, producing O₂ and protons used to power ATP synthesis and generate NADPH. Research is currently underway to try to mimic this process in order to provide clean energy. Photovoltaic cells can use light energy to oxidize water, producing O₂ as well as H₂. Hydrogen gas is a fuel that, upon reaction with oxygen, generates energy and only water as a waste product. Major difficulties in creating efficient photovoltaic cells are that the materials required are not durable and often not readily available. Recent work suggests that semiconductors composed of organic–inorganic material show great promise. One such material, perovskite (CH₃NH₃PbI₃), is remarkably efficient at capturing sunlight. It is humbling to realize that, although photosynthetic organisms have been oxidizing water for billions of years, researchers today struggle to replicate the process.