Light from the Sun powers most life on today's Earth in some way. The core of the photosynthetic apparatus where charge separation occurs, the reaction center (RC), is thought to have originated a single time and diverged, yielding new kinds of complexes adapted to different tasks and environments. Chen et al. now present an important missing puzzle piece in our understanding of the evolution of RCs: a cryo–electron microscopy structure of the homodimeric type I RC from a green sulfur bacterium bound to a light-harvesting protein. The observed cofactor and pigment arrangement explain biochemical features of this RC and will aid in our understanding of how a single ancestral RC gave rise to the range of structures and functions seen in RCs today.