C(4) anatomy can evolve via a single developmental change

C(4) 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(4) evolution have previously been evaluated through interspecific comparisons, which capture numerous changes besides those needed for C(4) functionality. Here, we quantify the anatomical changes accompanying the transition between non‐C(4) and C(4) 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(4) individuals is an increase in vein density, driven specifically by minor vein development that yields multiple secondary effects facilitating C(4) function. For species with the necessary anatomical preconditions, developmental proliferation of veins can therefore be sufficient to produce a functional C(4) leaf anatomy, creating an evolutionary entry point to complex C(4) syndromes that can become more specialised.