C-4 photosynthesis is a complex trait resulting from a series of anatomical and biochemical modifications to the ancestral C-3 pathway. It is thought to evolve in a stepwise manner, creating intermediates with different combinations of C-4-like components. Determining the adaptive value of these components is key to understanding how C-4 photosynthesis can gradually assemble through natural selection. Here, we decompose the photosynthetic phenotypes of numerous individuals of the grass Alloteropsis semialata, the only species known to include both C-3 and C-4 genotypes. Analyses of C-13, physiology and leaf anatomy demonstrate for the first time the existence of physiological C-3-C-4 intermediate individuals in the species. Based on previous phylogenetic analyses, the C-3-C-4 individuals are not hybrids between the C-3 and C-4 genotypes analysed, but instead belong to a distinct genetic lineage, and might have given rise to C-4 descendants. C(3)A. semialata, present in colder climates, likely represents a reversal from a C-3-C-4 intermediate state, indicating that, unlike C-4 photosynthesis, evolution of the C-3-C-4 phenotype is not irreversible.