C4 photosynthesis evolved in warm climates but promoted migration to cooler ones.

C4 photosynthesis is considered an adaptation to warm climates, where its functional benefits are greatest and C4 plants achieve their highest diversity and dominance. However, whether inherent physiological barriers impede the persistence of C4 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 C4 origins, (2) photosynthetic types affect the rate of migration among climatic zones, and (3) C4 evolution changes the breadth of the temperature niche. Our analyses show that C4 photosynthesis in grasses originated in tropical climates, and that C3 grasses were more likely to colonise cold climates. However, migration rates among tropical and temperate climates were higher in C4 grasses. Therefore, while the origins of C4 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.