Phylogenetic analyses reveal the shady history of C4 grasses

Grasslands cover more than 20% of the Earth's terrestrial surface, and their rise to dominance is one of the most dramatic events of biome evolution in Earth history. Grasses possess two main photosynthetic pathways: the C-3 pathway that is typical of most plants and a specialized C-4 pathway that minimizes photorespiration and thus increases photosynthetic performance in high-temperature and/or low-CO2 environments. C-4 grasses dominate tropical and subtropical grasslands and savannas, and C-3 grasses dominate the world's cooler temperate grassland regions. This striking pattern has been attributed to C-4 physiology, with the implication that the evolution of the pathway enabled C-4 grasses to persist in warmer climates than their C-3 relatives. We combined geospatial and molecular sequence data from two public archives to produce a 1,230-taxon phylogeny of the grasses with accompanying climate data for all species, extracted from more than 1.1 million herbarium specimens. Here we show that grasses are ancestrally a warm-adapted clade and that C-4 evolution was not correlated with shifts between temperate and tropical biomes. Instead, 18 of 20 inferred C-4 origins were correlated with marked reductions in mean annual precipitation. These changes are consistent with a shift out of tropical forest environments and into tropical woodland/savanna systems. We conclude that C-4 evolution in grasses coincided largely with migration out of the understory and into open-canopy environments. Furthermore, we argue that the evolution of cold tolerance in certain C-3 lineages is an overlooked innovation that has profoundly influenced the patterning of grassland communities across the globe.