The simians (infraorder Simiiformes) are the "higher primates" very common to most people: the monkeys and the apes, including humans. Simians tend to be larger than the "lower primates" or prosimians.
The mammalian order to which humans belong. Primates are generally arboreal mammals with a geographic distribution largely restricted to the Tropics. Unlike most other mammalian orders, the primates cannot be defined by a diagnostic suite of specializations, but are characterized by a combination of primitive features and progressive trends. These include:
Increased dominance of vision over olfaction, with eyes more frontally directed, development of stereoscopic vision, and reduction in the length of the snout.
Eye sockets of the skull completely encircled by bone.
Loss of an incisor and premolar from each half of the upper and lower jaws with respect to primitive placental mammals.
Increased size and complexity of the brain, especially those centers involving vision, memory, and learning.
Development of grasping hands and feet, with a tendency to use the hands rather than the snout as the primary exploratory and manipulative organ.
Progressive elaboration of the placenta in conjunction with longer gestation period, small litter size (only one or two infants), and precocial young.
Increased period of infant dependency and more intensive parenting.
A tendency to live in complex, long-lasting social groups.
It has been recognized for a long time that many of these features are adaptations for living in trees. However, it has been proposed more recently that primates may have developed their specializations as a consequence of being visually directed predators, living among the smaller branches of the forest canopy or undergrowth, that captured insects with their hands.
Classification of the primates is as follows:
Family: Lorisidae (lorises)
Family: Cheirogaleidae (dwarf lemurs)
Lepilemuridae (sportive lemur)
Lemuridae (true lemurs)
Indriidae (sifakas, indri, woolly lemur)
Family Tarsiidae (tarsiers)
Family: Callitrichidae (marmosets, tamarins)
Cebidae (capuchins, squirrel monkeys, douroucoulis, titis)
Atelidae (sakis, uakaris, howler monkeys, spider monkeys, woolly monkeys)
Family Cercopithecidae (Old World monkeys)
Family: Hylobatidae (gibbons, siamang)
Hominidae (orangutan, gorilla, chimpanzees, humans)
There are two major groups of primates: the strepsirhines or “lower” primates, and the haplorhines or “higher” primates. Strepsirhines have elongated and forwardly projecting lower front teeth that form a toothcomb, used for grooming the fur and for obtaining resins and gums from trees as a source of food. The digits of the hands and feet bear flattened nails, rather than claws, except for the second toe, which retains a sharp toilet claw for grooming. They also have a moist, naked rhinarium and cleft upper lip (similar to the wet noses of dogs). Most strepsirhines are nocturnal, with large eyes and a special reflective layer (the tapetum lucidum) behind the retina that intensifies images in low light. Compared with haplorhines, the brain size is relatively small and the snout tends to be longer.
The strepsirhines are subdivided into two major groups: the lorisoids, which are found throughout tropical Africa and Asia, and the lemuroids, which are restricted to Madagascar.
The lorisoids include the galagids or bushbabies (Galago, Otolemur, Euoticus, and Galagoides) and the lorisids or lorises (Loris, Nycticebus, Perodicticus, Pseudopotto, and Arctocebus). They are small nocturnal primates, in which the largest species, the greater bushbaby, weighs only about 1 kg (2 lb). Their diet consists mainly of a combination of insects, fruits, and gums. Lorisoids are semisolitary, living in small, dispersed social groups.
The greatest diversity of strepsirhines is found on Madagascar, where more than 30 species are represented, belonging to five different families.
Tarsiers, tiny primates (weighing only about 120 g) from the islands of Southeast Asia, all belong to a single genus, Tarsius. They are nocturnal with the largest eyes of any primate, and other adaptations for a specialized lifestyle as vertical clingers and leapers. In the past, tarsiers have been grouped together with the strepsirhines as prosimians, because they retain many primitive features lost in higher primates. However, tarsiers share a number of distinctive specializations with anthropoids that suggest that they are more closely related to each other than either is to the strepsirhines. For this reason, tarsiers and anthropoids are classified together as haplorhines.
The anthropoids include the platyrrhines or New World monkeys and the catarrhines or Old World monkeys, apes, and humans. Anthropoids are distinguished from strepsirhines and tarsiers in having a larger brain, relatively small eyes (all anthropoids are diurnal, active by day, except for the nocturnal douroucouli from South America), eye sockets almost completely enclosed by a bony septum, the two halves of the lower jaw fused in the midline rather than separated by a cartilage, small and immobile ears, the hands and feet bearing nails with no toilet claws (except for the callitrichids that have secondarily evolved claws on all fingers and toes), a single-chambered uterus rather than two-horned, and a more advanced placenta.
The platyrrhines from South and Central America are a diverse group of primates comprising more than 50 species and 16 genera. Primatologists have had a difficult time establishing a classification of platyrrhines that reflects their evolutionary interrelationships, and no consensus has been reached. There is agreement, however, that three distinct clusters can be defined: the callitrichids, the pitheciines, and the atelines. The last two groups appear to be closely related and are commonly included together in the family Atelidae. The relationships of the remaining platyrrhines are uncertain, and they are often placed together for convenience in the Cebidae.
All platyrrhines are arboreal, and they are widely distributed throughout tropical forests extending from Mexico to northern Argentina. They are small to medium-sized primates ranging from 100 g to 15 kg (0.2 to 33 lb). Platyrrhines exhibit a variety of quadrupedal locomotor types ranging from squirrellike scrambling, to leaping and forelimb suspension. Atelines and capuchin monkeys are unique among primates in having a specialized prehensile tail that can grasp around branches for extra support.
The catarrhines include all anthropoid primates from Africa, Asia, and Europe. There are two main groups: the cercopithecids or Old World monkeys, and the hominoids or apes and humans. Catarrhines are distinguished from platyrrhines by a reduction in the number of premolars from three to two in each half of the upper and lower jaw, and the development of a tubelike (rather than ringlike) tympanic bone to supports the eardrum.
Old World monkeys are widely distributed throughout sub-Saharan Africa and tropical Asia, and also occur in the extreme southwestern tip of the Arabian Peninsula, northwest Africa, Gibraltar (their only European record), and East Asia. They are a highly successful group comprising more than 80 species. They are distinguished from other anthropoids in having bilophodont molar teeth that bear a pair of transverse crests. They also have naked, roughened sitting pads on their rumps, called ischial callosities—a feature that they share with hylobatids. In addition, most Old World monkeys are highly sexually dimorphic, with males considerably larger than females.
Hominoidea is the superfamily to which apes and humans belong. Hominoids are distinguished from cercopithecoids in having primitive nonbilophodont molars, larger brains, longer arms than legs (except in humans), a broader chest, a shorter and less flexible lower back, and no tail. Many of these specializations relate to a more upright posture in apes, associated with a greater emphasis on vertical climbing and forelimb suspension.
Hominoids can be classified into two families: the Hylobatidae, which includes the gibbons and siamang, and the Hominidae, which includes the great apes and humans. The gibbons and siamang (Hylobates) are the smallest of the hominoids (4–11 kg or 9–24 lb), and for this reason they are sometimes referred to as the lesser apes. The nine or so species are common throughout the tropical forests of Asia. They are remarkable in having the longest arms of any primates, which are 30–50% longer than their legs. This is related to their highly specialized mode of locomotion, called brachiation, in which they swing below branches using only their forelimbs. Gibbons are fruit eaters, while the larger siamang incorporates a higher proportion of leaves in its diet. Hylobatids live in mo
^ Milner, Richard (1990). The Encyclopedia of Evolution: Humanity's Search for Its Origins. Facts on File, New York, s.v. "Primates".
^ British Union for the Abolition of Vivisection - Primates