Moving to the Light: The Evolution of Photosynthesis
Photosynthesis enabled early life to severe its ancestral dependence on geochemistry. The paleogeochemical record suggests that photosynthetic life colonized the planet photic zone as early as 3.4 GYA. Photosynthesis evolved in the Bacteria domain, and initially utilized compounds of geochemical origin such as ferrous iron or hydrogen as sources of electrons, without producing oxygen. Several variants of anoxygenic photosynthesis are present in extant bacteria. The cyanobacteria evolved oxygenic photosynthesis, a pathway that deploys two types of photosystem working in series to sum the energy of two photons for each electron transported from water to carbon dioxide. Multiple sources of evidence suggest that the cyanobacteria and oxygenic photosynthesis appeared at least 2.7 GYA, viz. 300 MY before the stable oxygenation of the planet. Endosymbiosis horizontally transferred oxygenic photosynthesis to the eukaryotes. Major similarities in the molecular architecture of photosystems in extant bacterial lineages point to a monophyletic origin of the core photosynthetic machine, followed by horizontal transfer among distantly related taxa, duplication and neo-functionalization. The Archaea lack photosynthesis but independently evolved a phototrophic pathway based on rhodopsins. Among a diversity of metabolic pathways for inorganic carbon fixation, the RubisCO-based Calvin, Benson and Bassham cycle is by far predominant.
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