C photosynthesis provided an immediate demographic advantage to populations of the grass Alloteropsis semialata.

C photosynthesis is a key innovation in land plant evolution, but its immediate effects on population demography are unclear. We explore the early impact of the C trait on the trajectories of C and non-C populations of the grass Alloteropsis semialata. We combine niche models projected into paleoclimate layers for the last 5 million years with demographic models based on genomic data. The initial split between C and non-C populations was followed by a larger expansion of the ancestral C population, and further diversification led to the unparalleled expansion of descendant C populations. Overall, C populations spread over three continents and achieved the highest population growth, in agreement with a broader climatic niche that rendered a large potential range over time. The C populations that remained in the region of origin, however, experienced lower population growth, rather consistent with local geographic constraints. Moreover, the posterior transfer of some C-related characters to non-C counterparts might have facilitated the recent expansion of non-C populations in the region of origin. Altogether, our findings support that C photosynthesis provided an immediate demographic advantage to A. semialata populations, but its effect might be masked by geographic contingencies.