A primate (from Latin primat-, from primus 'prime, first rank') is a eutherian mammal constituting the taxonomic order Primates. Primates arose 85–55 million years ago first from small terrestrial mammals, which adapted to living in the trees of tropical forests: many primate characteristics represent adaptations to life in this challenging environment, including large brains, visual acuity, color vision, a shoulder girdle allowing a large degree of movement in the shoulder joint, and dextrous hands. Primates range in size from Madame Berthe's mouse lemur, which weighs 30 g (1 oz), to the eastern gorilla, weighing over 200 kg (440 lb). There are 376–522 species of living primates, depending on which classification is used. New primate species continue to be discovered: over 25 species were described in the 2000s, 36 in the 2010s, and three in the 2020s.
Primates are classified as the strepsirrhines (lit. 'twisted-nostriled') and the haplorhines (lit. 'simple-noses'). Strepsirrhines include the lemurs, galagos, and lorisids, while haplorhines include the tarsiers and the simians (apes and monkeys). Simians (lit. 'snub-noses') can be further reduced to the platyrrhines (lit. 'flat-noses'), or New World monkeys, and the catarrhines (lit. 'narrow-noses'), which are Old World monkeys and apes (including humans). Forty million years ago, simians from Africa migrated to South America presumably by drifting on debris, which gave rise to the five families of New World monkeys. The remaining simians diverged into apes (Hominoidea) and Old World monkeys (Cercopithecoidea) approximately twenty-five million years ago. Common species that are simians include the (Old World) baboons, macaques, gibbons, and great apes; and the (New World) capuchins, howlers and squirrel monkeys.
Primates have large brains (relative to body size) compared to other mammals, as well as an increased reliance on visual acuity at the expense of the sense of smell, which is the dominant sensory system in most mammals. These features are more developed in monkeys and apes, and noticeably less so in lorises and lemurs. Some primates are trichromats, with three independent channels for conveying color information. Except for apes including humans, primates such as prosimians and monkeys have tails. Most primates also have opposable thumbs. Many species are sexually dimorphic; differences may include muscle mass, fat distribution, pelvic width, canine tooth size, hair distribution, and coloration. Primates have slower rates of development than other similarly sized mammals, reach maturity later, and have longer lifespans. Depending on the species, adults may live in solitude, in mated pairs, or in groups of up to hundreds of members. Some primates, including gorillas, humans, and baboons, are primarily terrestrial rather than arboreal, but all species have adaptations for climbing trees (in humans this can be seen, for example, in sports like climbing and parkour). Arboreal locomotion techniques used include leaping from tree to tree and swinging between branches of trees (brachiation); terrestrial locomotion techniques include walking on two limbs (bipedalism) and modified walking on four limbs (knuckle-walking).
Primates are among the most social of animals, forming pairs or family groups, uni-male harems, and multi-male/multi-female groups. Non-human primates have at least four types of social systems, many defined by the amount of movement by adolescent females between groups. Most primate species remain at least partly arboreal: the exceptions are humans, some other great apes, and baboons, all of which left the trees for the ground and now inhabit every continent.
Close interactions between humans and non-human primates (NHPs) can create opportunities for the transmission of zoonotic diseases, especially virus diseases, including herpes, measles, ebola, rabies, and hepatitis. Thousands of non-human primates are used in research around the world because of their psychological and physiological similarity to humans. About 60% of primate species are threatened with extinction. Common threats include deforestation, forest fragmentation, monkey drives, and primate hunting for use in medicines, as pets, and for food. Large-scale tropical forest clearing for agriculture most threatens primates.