A homodimeric complex for anaerobic photosynthesis In plants, algae, and cyanobacteria, large molecular complexes—photosystems I and II—convert light energy into chemical energy, releasing oxygen as a by-product. This oxygenic photosynthesis is critical for maintaining Earth's atmospheric oxygen. At their cores, photosystems I and II contain a heterodimeric reaction center. Reaction centers evolved in an atmosphere lacking oxygen, and the ancestral complex was likely homodimeric, encoded by a single gene. Gisriel et al. describe the structure of a homodimeric reaction center from an anoxygenic photosynthetic bacterium. The structure shows perfect symmetry of the light-collecting antennae and elucidates the electron transfer chain. Science , this issue p. 1021