C photosynthesis evolved in warm climates but promoted migration to cooler ones.

C photosynthesis is considered an adaptation to warm climates, where its functional benefits are greatest and C plants achieve their highest diversity and dominance. However, whether inherent physiological barriers impede the persistence of C species in cool environments remains debated. Here, we use large grass phylogenetic and geographical distribution data sets to test whether (1) temperature influences the rate of C origins, (2) photosynthetic types affect the rate of migration among climatic zones, and (3) C evolution changes the breadth of the temperature niche. Our analyses show that C photosynthesis in grasses originated in tropical climates, and that C grasses were more likely to colonise cold climates. However, migration rates among tropical and temperate climates were higher in C grasses. Therefore, while the origins of C photosynthesis were concentrated in tropical climates, its physiological benefits across a broad temperature range expanded the niche into warmer climates and enabled diversification into cooler environments.