Extinct members of the order of mammals to which humans belong. All current classifications divide the living primates into two major groups (suborders), but zoologists differ as to whether the tarsier (Tarsius) should be classified with the lower primates (lemurs, lorises, and bushbabies) or the higher primates (New and Old World monkeys, greater and lesser apes, and humans).
All primates have a common origin which, however, is not reflected in the universal possession of a suite of diagnostic features. The order as a whole has been characterized in terms of showing a group of progressive evolutionary trends, notably toward the predominance of the visual sense, the reduction of the sense of smell and associated structures, improved grasping and manipulative capacities, and enlargement of the higher centers of the brain. Among the extant primates, the lower primates more closely resemble forms that evolved relatively early in the history of the order, while the higher primates represent a group more recently evolved (see illustration).
Representatives of living primate families.
Early primates
The earliest primates are placed in their own suborder, Plesiadapiformes, because they have no direct evolutionary links with, and bear no adaptive resemblances to, any group of living primates. However, the chewing teeth and the locomotor anatomy of these fossil forms sufficiently resemble those of later primates to demonstrate the common origin of the two groups. Best known from the Paleocene Epoch, around 66–54 million years ago (Ma), and found in both the Old World and the New World, the plesiadapiforms retained clawed hands and feet, had rather small brains compared to their body size, possessed large specialized front teeth, and were probably arboreal in habit.
Eocene primates
Often termed euprimates, they are divided broadly into lemurlike forms, usually grouped into the superfamily Adapoidea, and tarsierlike forms (Omomyoidea). Eocene primates of both the Old and New Worlds already display the trends that mark modern primates as a whole: These arboreal animals possessed grasping hands and feet in which sharp claws were replaced by flat nails backing sensitive pads; the snout was reduced, suggesting a deemphasis of smell, while the bone-ringed eyes faced more forward, producing stereoscopic vision and suggesting primary reliance on the sense of sight; and the brain was somewhat enlarged relative to body size when compared to those of other mammals of the period.
Modern lower primates
The extant lower primates are allocated to the suborder Prosimii if Tarsius is included, and to the suborder Strepsirhini if this strange primate is excluded, as is provisionally done here (see illustration). There is no ancient primate fossil record in Madagascar, home of the most diverse group of modern lower primates, but extinct species little more than a thousand years old document a much wider adaptive radiation before the arrival of humans on the island.
Tarsiers
The tiny Tarsius, which lives today in Southeast Asia, represents a link between the strepsirhines and the anthropoids (monkeys, apes, and humans). In many respects it is related to the anthropoids, but dentally it is usually thought primitive, although some authors have found similarities to some strepsirhines. The extinct Eocene omomyoids are close to tarsiers skeletally; they are often considered broadly ancestral to the anthropoids for that reason and also because some have monkeylike front teeth.
Higher primates
The anthropoids include three main groups (infraorders) of living animals and their extinct relatives paracatarrhini (archaic anthropoids), platyrrhini (New World anthropoids), and catarrhini (old world anthrapoids). Their divergence from a possibly omomyoid stock probably took place some 50 Ma. The monkeys of the New World and those of the Old are of similar evolutionary grade, but the latter share a much more recent common ancestry with apes and humans, with which they are grouped in the infraorder Catarrhini.
Three main groups of early higher primates are the oligo-pithecids, the parapithecids, and the propliopithecids. The former two appear to be only distantly related to any of the living monkeys or apes. The propliopithecids may be close to the common ancestry of later catarrhines (Old World anthropoids). These arboreal animals were the size of small cats, with apelike teeth, small brain, and limbs similar to those of the acrobatic South American atelines. Representatives of modern lineages begin to occur in the fossil record by about 28–20 Ma in both hemispheres.
New World primates
The platyrrhine or ateloid monkeys of South and Central America are divided into two families. All living (and well-known extinct) forms are arboreal and occupy forested areas between Argentina and southern Mexico. A distinctive evolutionary pattern observed in this group is the antiquity of the extant lineages as reflected by the close relationships of most of the few known fossils to modern genera. The small marmosets and the common squirrel and capuchin monkeys are grouped into the family Cebidae, while the generally large-bodied spider-howler and saki-uakari groups are linked to the smaller titis and (probably) owl monkeys in the Atelidae. These two families differ in the relative robustness of their jaws and reduction of last molars. See also Monkey.
The earliest fossil platyrrhine, 26-million-year-old Branisella from Bolivia, is as yet known only by teeth and jaw fragments. The largest number of fossil platyrrhines comes from the La Venta beds of Colombia, dated about 13 Ma. The last 100,000 years saw another flowering of extinct platyrrhine lineages. A cave site in eastern Brazil has yielded partial skeletons of “giant” relatives of the howler and spider monkeys, while several localities in the Caribbean produced controversial fossils perhaps related to howlers, saki-uakaris, and squirrel monkeys (or possibly representing a distinct lineage whose members came to resemble those other groups).
Old World monkeys
The living Cercopithecidae (see illustration) are divided into two subfamilies, Colobinae and Cercopithecinae. The oldest cercopithecids are found in Africa, with a few fossil forms such as Victoriapithecus of 20–15 Ma probably predating the divergence of the modern subfamilies. The cercopithecines include a wide variety of forms, all of which share cheek pouches for temporary food storage and usually large incisors reflecting a fruit diet; colobines, by contrast, are more restricted in morphology, range, and behavior pattern, and all are leaf eaters with a complicated digestive tract to facilitate the low-nutrition diet.
The earliest members of the two living subfamilies also are mainly African. One colobine jaw is known by 9 Ma, and from 7 to 5 Ma species of both cercopithecines and colobines become more abundant. Large collections of Old World monkey fossils have been recovered from East and South African sites (often in association with early human remains) in the 4–1.5 Ma interval.
Cercopithecids entered Eurasia from Africa. Mesopithecus pentelicus, an 11 to 8-million-year-old colobine known in a geographical range from Germany through Afghanistan, is the best-represented Eurasian fossil monkey, with dozens of individuals recovered from sites in Greece. The living macaques (Macaca) are widespread across eastern Asia and in North Africa, and their fossil record adds to that large range. Scattered specimens are known from North Africa after 7 Ma, and populations have been recovered across Europe from 5.5 Ma to about 100,000 years ago.
Hominoids
The most humanlike of all primates are the apes, which form a group distinguished by generally large body size, relatively large brain, lack of an external tail, and advanced placentation pattern. Living forms include the lesser apes, or gibbons (Hylobates), placed in their own family Hylobatidae, and the several great apes: orangutan (Pongo), chimpanzee (Pan), and gorilla (Gorilla). The great apes and humans, along with some extinct relatives, are grouped as the Hominidae by some authors, while others place only humans in the Hominidae and class all great apes in the Pongidae.
One of the earliest probable members of Hominoidea is Proconsul, of the East African Miocene, 23–14 million years old; a few teeth of a similar form date to 26 Ma. Proconsul is well known by most of its skeleton. Several species ranged in size from small chimpanzee to small gorilla, with a somewhat chimplike skull, large projecting canine teeth, and limb bones seemingly adapted to quadrupedal running. However, Proconsul has few of the defining features of the ape group. For the present, Proconsul is retained as a hominoid belonging to a distinct archaic family of its own. Two other groups of roughly contemporaneous species (the Eurasian Pliopithecidae and the African “Dendropithecus-group”) are clearly more “primitive” than Proconsul, although they have at times wrongly been included in Hominoidea, usually as purported relatives of the gibbons. The oldest fossil gibbons date only to about 1 Ma.
Although interpretations vary, there appear to be three groups of Eurasian hominids between 13 and 7 Ma. Dryopithecus is characteristic of the Dryopithecinae, which may include the common ancestors of all later great apes (and humans).
Spread of modern ape ancestors
Most scientists today agree that, of the great apes, the orangutan is evolutionarily farthest from humans. As a result, orangutans and their extinct relatives are here placed in the subfamily Ponginae, while African apes, humans, and their relatives are included in Homininae. The orangutan lineage is, however, the oldest well-documented one among all catarrhines. See also Apes.
Two species which probably belong to the Ponginae are placed in the genus Gigantopithecus: One dates to about 9–6 Ma from India and Pakistan; the other lived about 1.5–0.5 Ma in China and perhaps Vietnam. Hundreds of specimens, mostly isolated teeth, are known from China, and these document a species which was probably the largest primate that ever lived (perhaps weighing 200–400 kg or 440–880 lb).
The pongines probably evolved in Asia from an arboreal dryopithecine (or even kenyapithecine) ancestry which expanded into less forested environments. Such a habitat would have provided an abundance of gritty and tough food objects, to which this group's dentition appears adapted. They share with orangutans and some early humans a complex of dental-related features.
The origin of the Homininae is more problematic. The fossil ape Graecopithecus (also termed Ouranopithecus) is known from several Greek localities estimated to date between 10 and 8 Ma. Well-preserved facial material of this animal and of Dryopithecus recovered or reanalyzed in the 1990s has led different workers to suggest that one or both forms may lie near the split between Ponginae and Homininae or already on the hominine lineage, effectively close to the common ancestor of African apes and humans. At present Graecopithecus appears more derived in the direction of later hominines. At about the same time in Africa, the only known ape fossil is a single upper jaw which was named Samburupithecus in 1997 and which may also represent an early, gorillalike member of Homininae. These fossils imply that the hominines may have evolved in Eurasia and then returned to Africa after about 10 Ma. Previous workers often thought that the hominine lineage could be traced purely within Africa, but that hypothesis now appears less likely.
The ancestry of the African apes is still a mystery, as no fossils have yet been found which clearly represent their lineage before or after separation from humans. See also Fossil humans; Mammalia.
