Apes and humans are closely related primates in the superfamily Hominoidea. The living hominoids are subdivided into the families Hylobatidae and Hominidae. The hylobatids or lesser apes (genus Hylobates) are represented by approximately nine 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 in the Hominidae. In the past, the great apes were included in a separate family, the Pongidae, but subsequent anatomical and molecular studies showed that the African apes (Gorilla and Pan) are more closely related to humans than they are to the Asian orangutan.
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 more than 4 million years. The earliest fossil apes that can be definitively linked to the modern hylobatids are known from sites in China dated to less than 1.5 million years ago (Ma), while the fossil record for the African apes is entirely unknown. The evolution of the orangutan is, by comparison, much better documented. 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 Ma). This evidence shows that apes were once much more common and more diverse than they are today.
Proconsulids
The remains of apelike fossil primates, commonly known as proconsulids, have been recovered from sites in Kenya, Uganda, and Saudi Arabia dating to the early Miocene (23–16 Ma). Studies have shown that proconsulids represent either the earliest known hominoids or primitive stem catarrhines (the group which 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 features of modern apes.
During the middle Miocene (16–10 Ma), conditions in East Africa became somewhat drier, cooler, and more seasonal, and open woodland habitats replaced the humid tropical forests that were typical of the early Miocene. These ecological changes coincided with the appearance in East Africa of a more advanced type of hominoid, Kenyapithecus. The limb bones indicate that Kenyapithecus was more terrestrially adapted than proconsulids and exhibited a number of specialized features that link it more closely to extant hominoids.
Eurasian hominoids
Until the middle Miocene, hominoids were restricted to Africa, but during this period they migrated into Eurasia. Once in Eurasia, hominoids became established over a wide geographical region, extending from Spain in western Europe to eastern China, and they became increasingly diversified during the middle and late Miocene (16–5 Ma). The best-known fossil Eurasian hominoids are Dryopithecus (western and central Europe), Oreopithecus (Italy), Graecopithecus or Ouranopithecus (Greece), Ankarapithecus (Turkey), Sivapithecus (Indo-Pakistan), and Lufengpithecus (China). Of these forms, Sivapithecus is evidently closely related to the living orangutan, but the relationships of the other Eurasian Miocene hominoids remain contentious (see illustration).
Cladogram of the evolutionary relationships of fossil and living apes. Broken lines indicate uncertain relationships.
An ecological shift from moist temperate woodlands to drier, more seasonal habitats during the later Miocene coincided with a sharp decline in the diversity of hominoids in Eurasia. The only survivor in Europe toward the end of the Miocene was Oreopithecus, a highly specialized relative of Dryopithecus. Lufengpithecus and Sivapithecus, along with Gigantopithecus, are found in the late Miocene of Asia. All of these Eurasian hominoids became extinct by the close of the Miocene, except for Gigantopithecus, whose remains have been recovered from Pleistocene cave sites in China dated to less than 1 Ma.
Hominoids also became extremely rare in Africa during the late Miocene. A large hominoid, Samburupithecus, known only by a single maxilla from Kenya (dated to 10–8 Ma), may represent a close relative of the African apes and humans. The earliest definitive record of fossil hominoids that are more closely related to humans than they are to the African great apes is known from the Pliocene (5.2–1.6 Ma) with the appearance of Ardipithecus ramidus from Ethiopia (4.4 Ma), Australopithecus anamensis from Kenya (4.2–3.9 Ma), and Australopithecus afarensis from Ethiopia and Tanzania (4–3 Ma). See also Apes; Fossil humans; Mammalia; Monkey.
