Growing evidence suggests that the unique molecular engines that drive photosynthesis arose only once during Earth’s history. All the diverse reaction centers—packets of proteins and pigments—in plants, algae, and certain bacteria appear to have evolved from a single ancestral structure. Researchers compared the most recently available structures. They developed new hypotheses that explain reaction center evolution in terms of selective pressures. These hypotheses describe a potential route of evolution for the great variety of photosynthetic reaction centers seen today.