Primates are the order of mammals containing monkeys, apes, humans, and other similar forms. Nearly 200 living species, grouped into more than 50 genera, are usually recognized, and almost 100 extinct genera are also known. The primates cover a wide array of adaptive modifications in size, structure, and habits. No single feature alone can be used to separate the primates from all other mammals; the unifying feature of the group is its common evolutionary descent. A number of features discussed below do, however, serve to characterize the order in a general way, although the classification of certain fossil forms remains an area of dispute.

Most of the principal features associated with the primates have arisen as a result of an arboreal, or tree-living, mode of life. Agility in trees demands good vision, and consequently one evolutionary trend has been the development of large, forward-directed eyes. The optic nerve fibers of each eye run to both halves of the brain, resulting in a superimposed, or double, image and an associated sense of depth. A bony ring around each eye socket helps support the eyes, and in the higher primates this support has become a complete bony wall behind the eye. This emphasis on vision led to the enlargement of the occipital lobe — that part of the brain which deals with vision — and to the relative reduction of structures associated with the decreasingly important sense of smell. The nose has generally become smaller, leading to a shortening of the muzzle, or face, and the olfactory portions of the brain have diminished in size.

Most primates climb by grasping with their hands. The grasp of most primates has evolutionarily been improved by the development of a big toe or thumb, which is set apart from the other digits but which can be moved toward the tips of at least two of them. This ability, one of the most important of the adaptive characteristics among primates, is called opposability; it also provides primates with capabilities such as holding objects close for examination, eating with one hand, and so on. Opposability, manipulative skills, and the muscular coordination necessary for rapid movement through the trees evolved along with enlargement of the motor, or movement, areas of the brain, thus contributing to the continuing increase in primate brain size.

Because claws were no longer needed for climbing, the primates evolved flat nails as protection and support for the tactile tips of the digits. Clawlike structures are still present among a number of primates, including the marmosets, the lorises, and lemurs (grooming claw), but argument continues as to whether these structures are clawlike nails, intermediate stages between claws and nails, or true claws.

Other features of the primates include the presence of a clavicle, or collarbone, which serves as a connecting support between the shoulder and the trunk of the body and helps reduce muscle strain in an animal often suspended by its arms. In addition, primates, along with only bears and bats, have pendant penises; that is, the penis is not permanently positioned along the abdomen. Reproductive cycles are also different. In most primates other than the lemurs and lorises, there are no definite breeding seasons. Instead, the female primate passes through a regularly recurring menstrual cycle, with sexual drive generally at its highest just prior to ovulation, or release of the egg from the ovary. Male higher primates are generally capable of breeding at any time.

Variation of Types
Living primates vary vastly in size, from the mouse lemur, Microcebus, which weighs as little as 60 g (2 oz), to the gorilla, exceeding 180 kg (400 lb) in the wild. Gestation periods vary accordingly, from the 2 months of the mouse lemur to the approximately 9 months of the gorilla, orangutan, and human. Natural longevity is difficult to estimate, but some captive chimpanzees and capuchin monkeys have lived more than 40 years. Locomotor, or movement, patterns are highly variable, from the forelimb-dominated arm-swinging (brachiation) of the gibbon to the hind-limb –dominated locomotion of galagos (clinging to and leaping between vertical supports) and humans (terrestrial bipedalism). Many types of quadrupedal, or four-legged, movement exist.

Primates feed on leaves, fruit, insects, buds, flowers, seeds, bark, roots, and other items. Some feed regularly, if infrequently, on birds and other small animals, and a few, such as baboons and chimpanzees, occasionally kill and eat larger animals. Some primates are more specialized in their diet than are others, but none is known to rely exclusively on any one component.

All primates are social, even those, such as the lorises, that lead mostly solitary lives. In general, nocturnal primates are less gregarious than are diurnal ones. Among the higher primates (monkeys, apes, and humans), only the owl monkey, Aotus, of South America is nocturnal. The lower primates show a diversity of activity rhythms. Some, such as lorises, galagos, aye-ayes, and some lemurs, are nocturnal; others, including some indris and lemurs, are diurnal; and a few of the lemurs display a 24-hour cycle of activity. Social units of the more gregarious primates vary greatly in size, composition, and organization, from the family unit consisting of an adult pair and their immature offspring to large multimale aggregations numbering in the hundreds. Neither evolutionary relationship nor ecological similarity, however, appears to have much value in predicting social structure. Indeed, even within a species, social organization may vary with environmental circumstances. This variation emphasizes the fact that among primates a great deal of behavior is learned behavior.

Indeed, highly developed capacities for learning and for problem solving are primate characteristics. Even the lower primates, usually considered less "intelligent" than the higher forms, have recently been shown to perform at least as well as New World monkeys in a variety of laboratory tests. Most primate learning is social; young primates learn those things they must know to survive within the context of their social group and its specific environment. Systems of social communication among primates are complex and varied and include scent, vocal, and visual signals; even "solitary" primates constantly employ scent marking to communicate with neighbors with whom they rarely come into physical contact.

Because they were present in fairly wide variety in the Paleocene Epoch, primates most likely originated from insectivore ancestors sometime during the latter half of the preceding Cretaceous Period, possibly between 100 million and 65 million years ago. The first great increase in primates occurred during the Paleocene, between 65 million and 53.5 million years ago. Until 1990, Plesiadapis was believed to be the oldest known primate, in part because its relatively low-cusped molars resembled certain later primates, reflecting the dietary shift away from insects and toward plant products thought to have characterized the origin of primates. However, detailed examination of skulls and fingers from that animal has shown features distinct to the colugo, a small, tree-dwelling mammal from the rain forests of Borneo and the Philippines. This discovery eliminates Plesiadapis as the link between early mammals and primates. Fossilized teeth from a new form, dubbed Altiatlasius for its discovery in the High Atlas Mountains of Morocco, date back 60 million years and show primatelike characteristics. Altiatlasius may have resembled the mouse lemur Microcebus of Madagascar, one of the smallest primates alive. Researchers are still looking for a transition between early mammals such as Plesiadapis and primates.

Primates of the Eocene Epoch, from 53.5 million to 37 million years ago, were considerably more "modern," possessing grasping extremities and resembling some of the Madagascan lemurs. Some paleontologists classify lemurs and other lower primates as prosimians. Bony eye rings and shortened faces appeared among these fossil animals, reflecting an emphasis on vision over smell. Such primates spread over the tropical and subtropical worlds, which included the now-temperate regions of Europe and North America.

Old World higher primates first appear in the fossil record during the Oligocene Epoch, from 37 million to 23.5 million years ago, and are known at this time only from the Fayum region of Egypt. Opinions vary as to whether those Fayum fossils classified in the extinct family Parapithecidae are related to the Old World monkeys, but the late Oligocene Aegyptopithecus unquestionably represents an early ape. This affinity is seen more clearly in its teeth than in its skull, which remains rather primitive. In its locomotion Aegyptopithecus seems to have been a fairly generalized arboreal climber.

During the Miocene Epoch, from 23.5 million to 5 million years ago, a variety of apes were present in Africa, Asia, and Europe, although whether any of these were directly ancestral to any living ape has yet to be demonstrated. The earliest potential human relative, Ramapithecus, is known from the Miocene by a number of jaw fragments (and possibly some partial limb bones) from Eurasia and Africa between about 14 million and 8 million years ago. The dental similarity between Ramapithecus and later members of the human family, Hominidae, indicates not only a probable evolutionary relationship, but also that this form had shifted toward the consumption of tough food morsels, probably gathered by hand at ground level. Old World monkeys are known from Miocene deposits in Africa and Eurasia; during this epoch the two modern subfamilies (Cercopithecinae and Colobinae) became differentiated. A marked increase in the types of Old World monkeys occurred during the Pliocene Epoch, about 5 million to 1.6 million years ago, and the Pleistocene Epoch, about 1.6 million to 10,000 years ago.

Paleontologists have for many years assumed that the South American New World monkeys are probably descended from the earlier forms discovered in North America. South American monkeys are unknown before the early Oligocene, however, and then only known by an enigmatic jaw fragment; of the several later fossil genera that have been found, none appears to be ancestral to any living form. In the mid-1980s a new wave of conjecture was induced by the discovery of a fossil primate in Wyoming, because the fossil dated from about 53 million years ago and appeared to be ancestral to both lemurs and tarsiers. The primate, named Cantius torresi, was more primitive than other fossil species from either North America or Europe, raising the possibility that primitive primates spread from Asia into North America across a land bridge earlier than they reached Europe. Fossil fragments of another, very primitive primate, some 45 million years old, were found in China in the 1990s. Paleontologists named this animal Eosimias, or "dawn monkey," because they considered it the earliest of the animals that evolved into the higher primates after they branched off from simpler forms. Eosimias would have been only about the size of an adult human's thumb.

The classification of living primates is currently a matter of some dispute. The primates are sometimes divided into two suborders, the Prosimii, which includes the lower primates such as lemurs, lorises, and aye-ayes, as well as tarsiers, and the suborder Anthropoidea, which contains monkeys, apes, and humans. Most authorities, however, would provisionally accept the following simplified arrangement in the table that accompanies this article.

Ian Tattersall

Reviewed by Kenneth Mowbray

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