Humans are mammals of the Primate order. The earliest primates evolved about 65 million years ago in the geological period known as the Paleocene epoch. They were small-brained, arboreal fruit eaters, similar to modern tree shrews . Primates of the Eocene epoch (55 to 38 million years ago) were similar and ancestral to contemporary tarsiers , lemurs , and tree shrews, and are classified as lower primates or prosimians.
During the late Eocene, the higher primates, or anthropoids, developed from prosimian ancestors and, aided by continental drift , diverged into New World (or platyrrhine) and Old World (or catarrhine) monkeys . The branching of Old World monkeys and hominoids apparently occurred in the late Oligocene (38 to 25 million years ago) or early Miocene (25 to 8 million years ago), a time period poorly represented in the fossil record.
The lesser apes (gibbons and siamangs) and other hominoid lines diverged about 20 million years ago, while the Asian great apes (the orangutan being the only surviving form) diverged from the African hominoids about 15 to 10 million years ago. Genetic evidence suggests that the ancestral lines of gorillas diverged about 8 million years ago and that chimpanzees and hominids diverged about 5 million years ago.