The most primitive of the primates appeared near the beginning of the Tertiary period, about 65 million years ago. About 50 million years ago, during the Eocene epoch, the first undisputed primates, which had characteristics similar to those of modern prosimians such as lemurs and tarsiers, began to appear. The earliest ancestors of the higher primates, primitive monkeys and apes, came into existence during the Oligocene epoch, about 38 million to 26 million years ago. Therefore, the prosimians are more closely related to the ancestral primates than are the higher primates, and the study of prosimians offers important insights into primate evolution.

In comparison to the higher primates, all prosimians tend to have smaller brains, longer snouts, and a more developed sense of smell. Many characteristics vary between species, however, because the diverse prosimians have developed adaptations based on their respective niches. All are arboreal, though the ring-tailed lemur (Lemur catta) finds much of its food on the ground. Most prosimians are nocturnal, but some lemurs are active in the daytime. The diet of many nocturnal species consists primarily of insects; other species feed mainly on fruits and leaves.

The Lemuriformes species are found only on Madagascar and the Comoros. Of the Lorisiformes, the lorises are found in southern India, Sri Lanka, and Southeast Asia, and the pottos and galagos live in Africa. Tarsiers are restricted to the islands of Southeast Asia. All prosimians are endangered, and some species are threatened with extinction.