The group of primates most closely related to humans. They include the African great apes, the gorilla and two species of chimpanzee; the Asian great ape, the orangutan; and the lesser apes from Asia, the gibbon and siamang. The apes can be distinguished from the rest of the primates by a number of anatomical and behavioral traits, which indicate their common origin; thus they are termed a monophyletic group called the Hominoidea.
Apes are distinguished from other primates through such obvious features as absence of tail and presence of an appendix. They share a number of specializations (synapomorphies) of the skeleton, which are useful as diagnostic characters, particularly when it comes to distinguishing fossil apes, because bones and teeth are the most readily preserved parts in the fossil record. The distal end of the humerus is especially useful, both because it is one of the most robust body parts, and therefore readily preserved, and because it is diagnostic of the ape condition, with a large trochlea (ulnar forearm articulation) and a well-developed trochlea ridge. The wrist is also modified for mobility of the joint. There are few synapomorphies of the skull, which in general retains the primitive primate condition except in individual species, but two shared specializations are the deep arched palate and relatively small incisive foramina. The teeth also are generally primitive, except in the broad, low-crowned incisors and enlarged molars. See also
Gibbons
Each group of ape differs from this basic ape pattern in varying degrees. The gibbons retain many of the primitive ape characteristics. They have also developed a number of unique characters that are different from any other ape: they have very elongated arms (relative to body weight) with many modifications of the musculature for a brachiating form of locomotion (swinging by the arms); their legs are also lengthened, so that they are quite efficient bipeds; they have adopted a monogamous family system, which is unusual in primates, and one of the outcomes is role sharing between males and females and lack of size variation between the sexes; and they have also developed a complex system of vocal communication related both to their close social bonds and to their thick tropical forest environment, which makes visual communication difficult. At present there are six species of gibbon (Hylobates) occupying most parts of Southeast Asia where primary forest still remains. See also Social mammals.
Orangutan
The sister group to the gibbons is the great ape and human group, which is readily distinguished by the shared presence of enlarged premolars in all its members. Within this group can be distinguished the orangutan and the African apes. The orangutan (Pongo pygmaeus) has a great many specializations that support this separation, although because of its common heritage with the chimpanzee and gorilla, and the great increase in body size of all three, the orangutan has a superficial similarity which has led in the past to all being grouped in the same family, the Pongidae. The differences separating the orangutan from other apes are in part biochemical—the structure of the blood proteins, for instance—and in part morphological. The deep face, enlarged premaxilla, narrow distance between the eyes, massive zygomatic bones, smooth floor of nose, and enlarged central incisors are all characters unique to the orangutan, and in all of these traits the African apes and humans retain the primitive ape condition. The orangutan is today confined to the tropical rain forests of Borneo and Sumatra, where it exists as a highly variable species. There are many biochemical and chromosomal differences both within and between populations, so that the possibility has been raised that, in fact, two separate species are present. This cannot be accepted on present evidence, but it would appear that the orangutan is a good example of ongoing evolution and incipient speciation. It is largely arboreal despite its great size, which ranges in body weight from 88 to 308 lb (40 to 140 kg). It leads a solitary or small-group existence, leading to the massive sexual-size variation indicated by its body weight variation. Little is known of its social structure.
African apes
The other part of the great ape group consists of the three African apes, the gorilla and two species of chimpanzee. They are distinguished from the orangutan (and other primates) by specializations of the wrist and frontal sinus and the development of massive brow ridges (all of which they also share with humans), and by a further series of unique modifications of the hand that are related to its unusual form of locomotion, called knuckle walking. Their legs are reduced in length (relative to body weight) so that their arms appear long in proportion to the rest of their body. The effects of this are further increased both by elongation of the hand and by the fact that when they walk quadrupedally on the ground they support their weight on the middle phalanges (finger bones) of the hand. This raises the body to a semiupright position even when the animals are walking on all fours.
The chimpanzee (Pan troglodytes) inhabits much of the forested region of tropical Africa, extending beyond the forest into more seasonal wooded habitats as well. The pygmy chimpanzee, or bonobo (P. paniscus), is confined to the southern loop of the Congo River where it inhabits mainly swamp forest. The gorilla is also confined to tropical Africa, but is divided into two rather distinct forms, the mountain gorilla (Gorilla gorilla beringei) and two lowland subspecies (G. g. gorilla and G. g. manyeura). The gorilla is the largest of the primates, with body weights ranging from 165 to 396 lb (75 to 180 kg), while the chimpanzee at 88–110 lb (40–50 kg) is much smaller. Gorilla social groups consist of a dominant male with several females and immature males and females, while chimpanzees live in fluctuating multimale or multifemale groups.
Relationship to humans
The orangutan is put into a separate group (or clade) from the other great apes (the chimpanzee, Pan, and the gorilla) because these have been shown to be more closely related to humans in evolutionary terms than they are to the orangutan. This signifies that the idea of the “great apes” encompassing all three is not valid, and the use of a single family, the Pongidae, to include them all is not correct. The great apes, however, have many superficial similarities to each other, mostly related to their large size, so that they may be said to be similar in grade, but in terms of clade (ancestral-descendant relationship) they are different. This could be recognized by restricting the use of the family Hominidae, but other classifications are also possible based on this set of relationships. See also Fossil humans.
