Until 65 million years ago, dinosaurs ruled the land and huge marine reptiles ruled the oceans. Mammals were small creatures, no larger than badgers, who spent their lives dodging the jaws of hungry dinosaurs. Then one or more catastrophic events wiped out the dinosaurs and large marine reptiles. However many small mammals managed to survive this extinction event.
5 million years later, at the start of the Eocene, vast stores of methane were released from the ocean. Methane is a greenhouse gas, twenty times more powerful than carbon dioxide. Rapid and extreme global warming followed and the Eocene soon became the warmest time in Earth's recent history.
The mammals flourished in the warm climate, and a period of rapid evolution followed. Animals ranging from the first recognisable forms of horses, rodents, bats and primates appeared, along with a bizarre menagerie of evolutionary dead ends.
In these warm humid conditions, forests spread from pole to pole, and crocodiles bathed in the warm waters off Greenland. The continents moved closer to the positions they're in today and India began its collision with Asia, marking the start of the creation of the Himalayas.