The Miocene is the fourth epoch of the Cenozoic. It started approximately 23 million years before the present and lasted eighteen million years. It is during this epoch that we see the first true apes and Old World monkeys. The Miocene was a time of great mammalian diversity, and many species originated and became extinct in response to environmental change.

The Miocene started out warmer than the preceding epoch. The warmth melted the polar ice cap that had formed over Antarctica. Fluctuating climate, which was characteristic of this epoch, caused a majority of the woodland environments to be replaced by savanna grasslands. These grasslands were home to a diverse fauna. In North America, species of rhino roamed the countryside alongside as many as a dozen horse species. Volcanic activity increased during the Miocene. Africa became more arid during this epoch, and India's collision with the Asian mainland continued to form the Himalayas. As South America moved north, the passageway between the continent and Antarctica opened up. The resulting circulation of cold waters around Antarctica led to the formation of deep, cold bottom waters in Earth's oceans.

Primates of the Miocene were diverse, not only in species but also in size, locomotor patterns, and social structures. Primates moved into most of the locomotor and dietary niches they now occupy. In the early Miocene, ape species evolved and diversified in Africa, and by about 16 million years ago, apes spread to Eurasia and began to diversify there as well. The variety of species creates a very complex picture of ape evolution. These apes exhibit a greater range of sizes than is found amongst modern apes, in fact, the largest primate known evolved during this epoch. Although most of the Miocene apes were fruit-eaters (frugivorous), at least two species are thought to have been leaf-eaters (folivorous). Analysis of the fossil remains shows that many of the species had a combination of primitive and derived features, which makes it hard to tell which ones were ancestors of modern apes, orangutans, gibbons and humans. While the apes of Africa and Eurasia had their greatest diversity during the early Miocene, relatively few species of Old World monkeys are known from this time span. Monkeys underwent a major radiation in the late Miocene and throughout the Pliocene. During the late Miocene, the diversity of large apes began to decline as tropical and subtropical habitats of Europe and Asia began to contract and become concentrated closer to the equator.

At the end of this epoch we see yet another cooling event, related to the expansion of the ice sheet that covered Antarctica. Ocean levels dropped in response to the formation of ice on land, which resulted repeatedly in the drying and catastrophic refilling of the Mediterranean Sea.

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