Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the genus Pongo, orangutans were originally considered to be one species. From 1996, they were divided into two species: the Bornean orangutan (P. pygmaeus, with three subspecies) and the Sumatran orangutan (P. abelii). In 2017, a third species, the Tapanuli orangutan (P. tapanuliensis), was definitively identified. The orangutans are the only surviving species of the subfamilyPonginae, which split from the other hominids (gorillas, chimpanzees, and humans) between 19.3 to 15.7 million years ago.

According to molecular evidence, within apes (superfamily Hominoidea), the gibbons diverged during the early Miocenebetween 24.1 and 19.7 million years ago (mya), and the orangutans split from the African great ape lineage between 19.3 and 15.7 mya. Israfil and colleagues (2011) estimated based on mitochondrial, Y-linked, and X-linked loci that the Sumatran and Bornean species diverged 4.9 to 2.9 mya.[23](Fig. 4) By contrast, the 2011 genome study suggested that these two species diverged around 400,000 years ago, more recently than was previously thought. Also, the orangutan genome was found to have evolved much more slowly than chimpanzee and human DNA.[21]

A 2017 genome study found that the Bornean and Tapanuli orangutans diverged from Sumatran orangutans about 3.4 mya, and from each other around 2.4 mya. Orangutans travelled from Sumatra to Borneo as the islands were connected by land bridges as parts of Sundaland during recent glacial periods when sea levels were much lower. The present range of Tapanuli orangutans is thought to be close to where ancestral orangutans first entered what is now Indonesia from mainland Asia.[18][24]

Fossil record

The three orangutan species are the only extant members of the subfamily Ponginae. This subfamily also included the extinct genera Lufengpithecus, which lived in southern China and Thailand 8–2 mya,[17]:50 Indopithecus, which lived in India from 9.2 to 8.6 mya; and Sivapithecus, which lived in India and Pakistan from 12.5 mya until 8.5 mya.[26]These apes likely lived in drier and cooler environments than orangutans do today. Khoratpithecus piriyai, which lived in Thailand 5–7 mya, is believed to be the closest known relative of the orangutans.[17]:50 The largest known primate, Gigantopithecus, was also a member of Ponginae and lived in China, from 2 mya to 300,000 years ago.[27][17]:50

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Characteristics

Orangutans have small ears and noses; the ears are unlobed.[32] The mean endocranial volume is 397 cm3.[34] The braincase is elevated relative to the facial area, which is concave and prognathous.[32] Compared to chimpanzees and gorillas, the brow ridge of an orangutan is underdeveloped.[35] Females and juveniles have rounded skulls and narrow faces while males develop a large sagittal crest and large cheek pads or flanges,[32] which show their dominance to other males. The cheek pads are made mostly of fatty tissue and are supported by the musculature of the face.[36] Mature males also develop large throat pouches and long canines.[32][17]:14