Apes and humans, commonly referred to as hominoids, are a closely related group of primates classified together in their own superfamily, the Hominoidea. The living hominoids are subdivided into two families, the Hylobatidae and the Hominidae (see table). The hylobatids or lesser apes (belonging to a single genus, Hylobates) are represented by 11 or so species found throughout Southeast Asia. Humans and the great apes—the orangutan (Pongo pygmaeus), the gorilla (Gorilla gorilla), the common chimpanzee (Pan troglodytes), and the pygmy chimpanzee or bonobo (Pan paniscus)—are grouped together in the Hominidae. In the past, the great apes were included in a separate family, the Pongidae, but recent anatomical and molecular studies have shown that the African apes ( Gorilla and Pan) are more closely related to humans than they are to the Asian orangutan. The revised taxonomy better reflects, therefore, the phylogenetic relationships among members of the Hominoidea.
The evolutionary history of the extant hominoids is poorly known, with the notable exception of humans, which have a relatively complete fossil record extending back to more than 4 million years ago. The earliest fossil apes that can be definitively linked to the living hylobatids are known from sites in China dated to less than 1.5 million years ago, while the fossil record for the African apes is restricted to a few possible fragmentary finds reported from the late Miocene and Pleistocene of East Africa, dating back to 6 million years ago. The evolution of the orangutan is, by comparison, much better documented. Fossil teeth from cave sites in Asia dating back to more than 1 million years ago show that orangutans in the past were considerably larger than they are today, and that unlike their living relatives, which are found only on Sumatra and Borneo, they once had a wider distribution in Southeast Asia that extended as far north as southern China. In contrast to the paucity of fossils available to trace the evolutionary history of hominoids over the past 5 million years, there is a wealth of evidence from the Miocene period (23-5 million years ago) to show that apes were once much more common and more diverse in the past than they are today.
The remains of the earliest apelike fossil primates, commonly known as proconsulids, have been recovered from sites in Kenya, Uganda, and Saudi Arabia dating to the late Oligocene and early Miocene (28-16 million years ago). There are a dozen species of proconsulids, ranging in size from the small Nyanzapithecus harrisoni (8 kg), which was about the size of a modern-day black-and-white colobus monkey, to Proconsul major (60-90 kg), which was the size of a female gorilla. Comparisons of their teeth, jaws, and skeletons indicate that proconsulids exhibited a wide diversity of dietary and locomotor behaviors, but they were typically arboreal quadrupeds that ate various combinations of soft, ripe fruits and young leaves. The best known species are Proconsul heseloni, Proconsul nyanzae, Turkanapithecus kalakolensis, and Afropithecus turkanensis. Recent studies have shown that proconsulids represent either the earliest known hominoids or primitive stem catarrhines (the group that gave rise to both Old World monkeys and apes). They are certainly more primitive than any of the living apes, retaining generalized skulls and teeth, and monkeylike postcranial skeletons. However, during the early Miocene, there was at least one species of hominoid living in East Africa, Morotopithecus bishopi, which had already acquired some of the unique post-cranial features of modern apes. This species, from Moroto in Uganda, dated to more than 20 million years ago, had specializations of the lumbar vertebrae and scapula, not found in the contemporary proconsulids, that indicate that it had developed the stiff-backed, partially upright posture and suspensory forelimbs that are characteristic of modern apes.
During the middle Miocene (16-10 million years ago), conditions in East Africa became drier, cooler, and more seasonal, and open woodland habitats replaced the humid tropical forests typical of the early Miocene. These ecological changes coincided with the appearance in East Africa of more advanced types of apes, including the earliest true hominoids, as well as stem hominids. The best known fossil apes from this time period are Equatorius, Kenyapithecus and Nacholapithecus, which are classified together as kenyapithecines. Kenyapithecus is known only from a handful of specimens from the locality of Fort Ternan in Kenya, whereas Equatorius and Nacholapithecus are represented by large samples of jaw fragments and isolated teeth, as well as partial skeletons, from several localities in Kenya. These apes are more derived than the proconsulids in having thickened enamel on their cheek teeth, more robust jaws, a simian shelf (a distinctive bony buttress on the internal inferior surface of the front of the mandible), and relatively larger upper premolars. Their limb bones indicate that they were probably more terrestrially adapted than proconsulids, and they exhibit a number of specialized features for forelimb-dominated climbing that link them more closely to modern hominoids. Nacholapithecus, at least, lacked a tail as in living apes. Another intriguing fossil ape from this time period is Otavipithecus from Namibia, dated to approximately 13 million years ago, and the only Miocene hominoid species recorded from southern Africa. Until recently, it was known only by a single lower jaw fragment, so its relationship to other fossil and extant hominoids has been difficult to establish. However, additional cranial and postcranial specimens of Otavipithecus have now been discovered, and these suggest that it was more closely related to earlier proconsulids than to contemporary apes from East Africa.
Up until the middle Miocene, hominoids were restricted to Africa, but during this period they migrated for the first time into Eurasia. The earliest Eurasian hominoid, dated to 16-14 million years ago, is Griphopithecus, which has been found in Turkey and central Europe. The teeth and jaws are similar to those of Kenyapithecus from East Africa, to which it is probably closely related. Once in Eurasia, hominoids became established over a wide geographical region, extending from Spain and France in Western Europe to eastern China, and they became increasingly diversified during the middle and late Miocene (16-5 million years ago). The best known fossil Eurasian hominoids are Dryopithecus (Western and Central Europe), Pierolap-ithecus (Spain), Oreopithecus (Italy), Graecopithecus or Ouranopithecus (Greece), Ankarapithecus (Turkey), Sivapithecus (Indo-Pakistan), and Lufengpithecus (China). Of these forms, Sivapithecus is clearly closely related to the living orangutan because it shares many unique specializations of its teeth and cranium with Pongo, including thick-enameled cheek teeth, reduced upper lateral incisors and large central incisors, orbits (eye sockets) oval in shape and taller than they are broad, narrow interorbital distance, lack of development of a distinctive bony bar above the orbits, a dished face in profile, and a tiny incisive foramen penetrating the palate. In contrast, the relationships of the other Eurasian Miocene hominoids to each other and to living apes remain contentious. A number of alternative hypotheses about the interrelationships of Eurasian Miocene hominoids has been proposed: (1) that they form a closely related group with the living orangutan, all being derived from a common ancestor that migrated into Europe from Africa sometime during the middle Miocene; (2) that some of the Eurasian hominoids, such as Dryopithecus and Graecopithecus, are more closely related to the African apes and humans than they are to Sivapithecus and the orangutan; and (3) that they represent a diverse group containing primitive hominoids, as well as forms belonging to the orangutan and African great ape lineages (see figure). The later Miocene apes from Asia, such as Ankarapithecus, Gigantopithecus, and Lufengpithecus, are possibly related to Pongo, but if they are, they are certainly more distantly related than is Sivapithecus (see figure). The relationships of Lufengpithecus from the late Miocene (7-10 million years ago) from southern China are especially problematic, because although its teeth are remarkably similar to those of living orangutans, even when compared with Sivapithecus it lacks the specialized features of the cranium shared by Sivapithecus and Pongo. The recently described fossil ape from the late Miocene of Thailand, Khoratpithecus, may provide an important link between Sivapithecus and Pongo. The absence of a scar on the chin region of the mandible for the attachment of the anterior digastric muscle in Khoratpithecus is a peculiarity among hominoids that is found only in orangutans.
The skeletons of Dryopithecus, Pierolapithecus, and Oreopithecus are relatively well known, and they show that these Western European apes were specialized for stiff-backed, forelimb-dominated arboreal climbing, clambering, and suspension, quite similar to modern great apes, especially the orangutan. Although few postcranial remains of Lufengpithecus are known, they indicate a similar locomotor pattern. Sivapithecus, by contrast, living in the subtropical woodlands of northern India and Pakistan, was primarily adapted for arboreal quadrupedal running and walking along larger branches, but it also probably spent time feeding on the ground. This is quite different from the uniquely specialized quadrumanous climbing and clambering locomotor pattern characteristic of modern orangutans. Later Miocene Eurasian homi-noids were specialized for a variety of different diets, ranging from leafy and fibrous foods in Oreopithecus, to hard seeds and nuts in Graecopithecus, to soft fruits and young leaves in Dryopithecus and Sivapithecus.
An ecological shift from moist temperate and sub-tropical woodlands to drier, more seasonal habitats during the later Miocene coincided with a sharp decline in the diversity of hominoids in Eurasia. This event, which is dated to 9.6 million years ago in Western Europe, has been dubbed the “Vallesian Crisis” (the Vallesian is the European Land Mammal Age for this time period). The only survivor in Europe toward the end of the Miocene was Oreopithecus, a highly specialized relative of Dryopithecus, which was isolated on a group of islands in the northern Mediterranean that today form part of Italy. It survived in isolation until about 7 million years ago when the islands became connected to the European mainland and allowed an influx of more competitive mammals, including monkeys. Lufengpithecus and Sivapithecus, along with the aptly named Gigantopithecus, continue in the late Miocene of Asia. The latter ape was the largest known hominoid, with massive jaws and teeth specialized for eating tough, fibrous vegetation, such as bamboo, and an estimated body weight that may have exceeded 200 kg (living male gorillas, by comparison, average only 170 kg). All of these Eurasian hominoids became extinct by the close of the Miocene (5 million years ago) except for Gigantopithecus, whose remains have been recovered from Pleistocene cave sites in southern China dated to less than 1 million years ago.
Hominoids also became extremely rare in Africa during the late Miocene. A large fossil ape, Samburupithecus, known only by a single maxilla from Kenya (dated to 10-8 million years ago), may represent a close relative of the African apes and humans. Until recently, a few isolated teeth of fossil hominoids from the late Miocene sites of Lukeino and Lothagam (dating to 7-5 million years ago) in northern Kenya were all that were available to document the earliest known occurrence of the human lineage prior to 5 million years ago, but the remains were too scrappy to be confident about their affinities or tell us much about their anatomy. Then, beginning in the mid-1990s, paleontologists working in Ethiopia, Kenya, and Chad made some remarkable discoveries that have helped to fill this critical gap in the fossil record. The recently described Ardipithecus, Orrorin, and Sahelanthropus, dating from about 7 million years ago to 4.4 million years ago, are argued to be the earliest representatives of the human lineage. They each show a combination of features that indicate reduced canine size and bipedal locomotion, both of which are unique features of later human ancestors. Most researchers agree that these fossil hominoids are close to the ancestry of humans, although there has been some debate about the evidence presented in favor of this viewpoint. The earliest definitive record of fossil hominoids that are closely related to humans is known from the Pliocene with the appearance of Australopithecus anamensis from Kenya (4.2-3.9 million years ago) and Australopithecus afarensis from Ethiopia and Tanzania (4.0-3.0 million years ago).
References:
- Begun, D. R., Ward, C. V., and Rose, M. D. (1997). Function, phylogeny, and fossils: Miocene hominoid evolution and adaptations. New York: Plenum Press.
- Fleagle, J. G. (1999). Primate adaptation and evolution. San Diego, CA: Academic Press.
- Hartwig, W. C. (Ed.). (2002). The primate fossil record.Cambridge, UK: Cambridge University Press.
