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How did savannas develop during the Cenozoic Era?

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Flat savanna plain with scattered acacia trees under a partly cloudy sky in Serengeti National Park, Tanzania. — Savanna A savanna marked with acacia trees in Serengeti National Park, Tanzania. Africa's Serengeti Plain is commonly used as an example of the savanna biome—a hot, seasonally dry ecological region characterized by an open tree canopy (i.e., scattered trees) above an understory of continuous grasses.

Savannas emerged during the Cenozoic Era as a response to long-term global cooling and a progressive reduction in rainfall at the edges of the tropics, particularly over the past 25 million years. As ocean surface temperatures dropped, the global hydrologic cycle slowed, leading to decreased precipitation and the expansion of seasonally dry habitats.1

While grasses appeared approximately 50 million years ago, well-developed, grass-rich vegetation equivalent to modern savanna was established by the early Miocene Epoch, about 20 million years ago. During the late Eocene, global cooling had already begun converting rainforests into more open forests and grasslands. By the late Miocene, grassland communities analogous to modern savannas were established on most continents. These environments further expanded at the expense of forests during the cool, dry glacial intervals of the Pleistocene Epoch. Because continents were already separated by ocean barriers, distinct plant and animal species evolved independently in each region to occupy these new habitats.12