Oxygenic photosynthesis is the principal producer of both oxygen and organic matter on earth. The primary step in this process—the conversion of sunlight into chemical energy—is driven by four multi-subunit membrane protein complexes named photosystem I, photosystem II, cytochrome b6f complex and F-ATPase. Photosystem I generates the most negative redox potential in nature and thus largely determines the global amount of enthalpy in living systems. The recent structural determination of PSI complexes from cyanobacteria and plants sheds light on the evolutionary forces that shaped oxygenic photosynthesis. The fortuitous formation of our solar system in a space plentiful of elements, our distance from the sun and the long time of uninterrupted evolution enabled the perfection of photosynthesis and the evolution of advanced organisms. The available structural information complements the knowledge gained from genomic and proteomic data to illustrate a more precise scenario for the evolution of life systems on earth. BioEssays 27:914–922, 2005. © 2005 Wiley Periodicals, Inc.