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, the bush babies of Africa, 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 hominoids (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
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