In the Eocene, commencing around 56 million years ago, the continents began to assume their modern configuration. Australia calved from the Antarctic portion of the old supercontinent of Gondwanaland, trapping a cold current round Antarctica, which would eventually become circumpolar with the separation of South America. India’s collision with Siberia created the Himalayas, while the Laurentian supercontinent split into North America and northern Europe. Europe and Asia had yet to knit together, still girdling a shallow sea. The Eocene climate was warm and moist, with tropical rain forest covering most of the land surface, but from around 36 million years ago, a steady cooling began. Fauna had recovered from the mass extinction at the end of the Mesozoic, but most land mammals were still small: giant flightless birds and snakes were the dominant land predators, while basilosaurs (early whales) and sharks ruled the seas.
— OR —