C4 photosynthesis is a CO2-concentrating mechanism that has evolved from conventional C3 photosynthesis in various species, such as tropical plants. Heckmann et al. carried out theoretical modelling to characterize the fitness consequences of the various possible evolutionary paths to C4 photosynthesis. They found that each step is typically accompanied by an incremental increase in fitness, thus showing how selective pressure can drive the stepwise assembly of complex biochemical systems.