Lasting from about 56 to 34 million years ago, the Eocene is the second of three epochs in the Paleogene period. Known for its "hothouse" climate and the rise of modern mammal groups, it ended with major global cooling.
The Eocene started with the warmest temperatures of the Cenozoic. Tropical forests stretched nearly from pole to pole. Early horses, such as the small, dog-sized Eohippus, appeared in the forests. Early relatives of bats took to the air. Whale ancestors transitioned from small, four-legged land dwellers into massive marine mammals. Later in the epoch, global temperatures cooled, and Antarctica began to move toward an ice-covered state. The name Eocene comes from the Greek eos, "dawn," and kainos, "new."