Apes, Fossil

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, [Page 217][Page 218]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.

...