Starting around 20 million years ago during the Miocene epoch, global cooling and drying trends led to a massive expansion of open grassland and savanna ecosystems at the expense of dense forests. Grasses (Poaceae) evolved C4 photosynthesis, allowing them to thrive in low-CO2, arid, and fire-prone environments. Because grass grows from the base rather than the tips, it could withstand heavy grazing. This triggered an evolutionary arms race among mammals, driving the rise of large herds of hoofed herbivores (ungulates like horses and antelopes) with specialized high-crowned teeth (hypsodonty) to chew abrasive silica-rich grass. The opening of the landscape also forced animals to adapt to open terrain, giving rise to long-legged runners and, in Africa, prompting hominin ancestors to descend from the trees and evolve bipedalism to navigate the savannas.