C anatomy can evolve via a single developmental change.

C photosynthesis is a complex trait that boosts productivity in warm environments. Paradoxically, it evolved independently in numerous plant lineages, despite requiring specialised leaf anatomy. The anatomical modifications underlying C evolution have previously been evaluated through interspecific comparisons, which capture numerous changes besides those needed for C functionality. Here, we quantify the anatomical changes accompanying the transition between non-C and C phenotypes by sampling widely across the continuum of leaf anatomical traits in the grass Alloteropsis semialata. Within this species, the only trait that is shared among and specific to C individuals is an increase in vein density, driven specifically by minor vein development that yields multiple secondary effects facilitating C function. For species with the necessary anatomical preconditions, developmental proliferation of veins can therefore be sufficient to produce a functional C leaf anatomy, creating an evolutionary entry point to complex C syndromes that can become more specialised.