Evolution and systematics

The great apes have traditionally been grouped in the family Pongidae, clearly distinguishing them from the prosimians, monkeys, lesser apes, and humans. Three genera and four species were usually recognized within this family, which included the orangutans (Pongo pygmaeus), gorillas (Gorilla gorilla), chimpanzees (Pan troglodytes), and bonobos (Pan paniscus). In the past, bonobos have been referred to as pygmy chimpanzees, a misnomer attributed to their gracile appearance. They were not recognized as a distinct species until 1929.

Like any branch of science, taxonomic classification involves the search for greater comprehension. Incremental progress relies on debate among colleagues, constructive criticism, and innovative methods for measuring the relationship between species, such as DNA and chromosomal analyses. Based on these factors, Pongidae has been replaced by the family Ho-minidae, which more accurately describes the evolutionary relationship between all of the great apes and humans.

The family Hominidae consists of four genera and seven species. Orangutans, the only Asian great apes, are divided into two species based on their geographically distinct ranges. Those from the island of Borneo are Pongo pygmaeus, while those from the island of Sumatra are Pongo abelii. The remaining members of the family Hominidae are all African in origin. Chimpanzees, Pan troglodytes, are divided into four subspecies, P. t. troglodytes, P. t. verus, P. t. vellerosus, and P. t. schweinfurthii. Bonobos, Pan paniscus, are the remaining species in the genus. There are two species of gorilla, western gorillas (Gorilla gorilla), and eastern gorillas (Gorilla beringei). Two subspecies of eastern gorillas exist, mountain gorillas (G. b. beringei) and eastern lowland gorillas (G. b. graueri). The final species in the family Hominidae is Homo sapiens sapiens, otherwise known as modern humans. This species is discussed in a separate chapter.

Within the family Hominidae, two subfamilies further clarify the evolutionary and associated geographical origins of the species. The Asian orangutans are members of the subfamily Ponginae, while all of the African species are members of the subfamily Homininae. Based on comparisons of DNA from each species in both subfamilies, it is clear that orangutans diverged from the members of Homininae approximately 14 million years ago (mya). Within Homininae, gorillas split from Pan and Homo approximately 7 mya. Humans were the next to diverge approximately 6 mya. The final group to emerge was Pan, with bonobos and chimpanzees diverging approximately 3 mya.

Although commonly misunderstood, humans are not the "end-product" of great ape evolution. Humans did not evolve from chimpanzees (or any other living species of great ape), and the other species of great ape are not evolving into humans. Each species within Hominidae has evolved on its own distinct pathway, although all share a common ancestry.

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