Almost all plants use a classical photosynthetic pathway called 'C3,' fixing carbon dioxide directly into a three-carbon molecule. About 30 million years ago, however, a biochemical innovation arose independently several times — the C4 pathway — offering a far more efficient way to capture carbon under heat and aridity. With a specialised arrangement of leaf cells, CO₂ is first bound to a four-carbon intermediate and then delivered at high concentration to the cells where photosynthesis takes place. Where atmospheric CO₂ is scarce, where heat accelerates photorespiration, and where water is limited, this strategy gives C4 plants a clear advantage.

The C4 pathway appeared in some grass genera in the early Oligocene–Miocene, but its global expansion happened around 8–6 million years ago. Carbon-isotope records read this expansion clearly worldwide: δ¹³C values in calcareous concretions, paleosol carbonates, and animal tooth enamel show C4 plants becoming dominant across the tropics and subtropics during the late Miocene. At the same time, atmospheric CO₂ continued to fall through the Cenozoic, the Antarctic ice sheet expanded, and the African and Asian monsoon systems strengthened — all conditions that worked in C4 grasses' favour.

The ecological consequences were transformative. As forest patches contracted and savannas spread, a new food source opened for the ancestors of ruminants, equids, and elephants, animals able to handle the high-silica leaves of grasses; dental adaptations such as hypsodonty (high-crowned teeth) became widespread in this interval. In East Africa, this savanna expansion is also the ecological context in which apes — and ultimately the lineage to hominins — moved from the trees toward the ground and toward bipedal walking. Today's vast savanna belts, and the fact that the world's major cereal grains (maize, sorghum, sugarcane) are C4 plants, all trace back to this quiet biochemical revolution of the late Miocene.