The continents continued to move. The large island that was to become India met the continental mass of Asia, and created what would become the Himalayan mountain range. North America separated from Europe and Australia, and South America broke away from Antarctica and migrated northward. The Eocene is regarded as the warmest epoch of the Tertiary. Subtropical conditions could be found up to the Arctic Circle at the start of the epoch. Toward the end of this epoch the climate began to deteriorate, and temperatures began dropping significantly.

Increasingly warm conditions at the start of the Eocene caused the extinction of some prominent species of the prior epoch. But, overall, land mammals flourished as new species diversified and adapted. In particular, mammals with a keen sense of smell thrived in the dense forests and warm conditions, even around the Arctic Circle, which enabled these species to migrate between North America and Europe via Iceland and Greenland.

Primates from this epoch had evolved all the characteristic features of the order. And so it appears that these animals lived a different sort of life from that of their Paleocene ancestors. These primates had grasping hands and feet that had nails rather than claws. One of the most influential developments is the primate reliance on sight rather than smell that evolved around fifty-five million years ago. These primates were abundant on several continents, but were evidently absent from South America and Antarctica. Both of the modern suborders of primates originated in the Eocene, or possibly in the late Paleocene. One suborder includes lemurs and lorises (strepsirhines). The descendants of these species still thrive in the tropical forests of Africa, Madagascar, and Asia. The other suborder (haplorhines) includes the higher primates, such as monkeys, apes, and humans, which are often referred to as "anthropoid primates". These primates come from Asia, Arabia and Africa and still inhabit these areas.

Monkeys, apes and humans are the living descendants of the first anthropoids, which evolved in the Eocene. Anthropoids diversified greatly during the late Eocene and the Oligocene. The boundary between the two epochs is marked by a 10-million-year-long fluctuation in climate and environment. One of the environmental results was the shrinking of the subtropical and tropical forests. These forests, which had fostered the adaptive radiation of species, started their retreat to the modern tropical zones. As a result of these changes, grasses evolved, which greatly influenced the evolutionary history of land mammals. At the end of the Eocene the primates of the Northern Hemisphere nearly disappeared; many primate species took refuge in Africa and Arabia. It was in this refuge area where the ancestors of Old World monkeys and apes evolved in the Oligocene.

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