primate, member of the mammalian order Primates, which includes humans, apes, monkeys, and prosimians, or lower primates. The group can be traced to the late Cretaceous period, where members were forest dwellers. As a whole, they are arboreal, although a few species are terrestrial; nearly all inhabit warm climates. All higher primates and some prosimians display some degree of social organization.

Primates are very unspecialized anatomically, and the order is more easily described by the evolutionary trends within it, tending generally toward increased dexterity and intelligence, than by specific traits characteristic of all its members. Significant trends have been the enlargement of the braincase, elaboration of the brain and of the sensory pathways to it, flattening of the face and shifting of the eyes to a forward position, development of stereoscopic vision, and increased flexibility of the hands and feet. Nearly all primates have flat fingernails and opposable thumbs and big toes.

The prosimians ("premonkeys") are small, arboreal, mostly nocturnal animals. The most primitive, the tree-shrews, strongly resemble the insectivores, a primitive, unspecialized group of mammals from which primates branched at an early stage of mammalian evolution. The prosimians also include the lemurs and the aye-aye of Madagascar, the lorises of Africa and Asia, and the tarsiers of SE Asia.

Monkeys are diurnal animals, generally with flatter, more expressive faces and better developed brains than the prosimians. Like prosimians, they retain the skeletal structure of quadripedal animals and usually walk or run on four feet. The New World monkeys are anatomically distinct from Old World monkeys; most have prehensile tails, and all are arboreal. The Old World monkeys, which lack prehensile tails and include some terrestrial species, are more closely related to the hominids (apes and humans).

The apes (gibbons, orangutans, gorillas, and chimpanzees) are characterized by modification of the upper skeleton for brachiation (arm swinging) and by high intelligence. Tool use and limited toolmaking are found among apes. Humans, of which Homo sapiens is the only living species, have a pelvic structure adapted to upright posture and is characterized by the use of language and by a highly developed ability to manipulate the environment (see human evolution).

Any of more than 300 species of the order Primates, including monkeys, apes, humans, and others. Primates are distinguished from other mammals by one or more of the following traits: unspecialized structure, specialized behaviour, a short muzzle, comparatively poor sense of smell, prehensile five-digit hands and feet possessing flat nails instead of claws, acute vision with depth perception due to forward-facing eyes, a large brain, and prolonged pre- and postnatal development. Most species bear a single young and live in troops headed by a male. The primates are one of the most diverse orders of mammals on Earth. They include the lemurs (more than 70 species in six families), the lorises (three or more species in one subfamily), the tarsiers (six or more species in one family), the New World monkeys (almost 100 species in five families), the Old World monkeys (more than 100 species in one family), and the apes and humans (about 20 species in two families). The oldest known fossil remains of primates are about 60 million years old.

Learn more about primate with a free trial on Britannica.com.