Abstract

Six extant species of non-human great apes are currently recognized: Sumatran and Bornean orangutans, eastern and western gorillas, and chimpanzees and bonobos. However, large gaps remain in our knowledge of the fine-scale variation in hominoid morphology, behavior, and genetics, and aspects of great ape taxonomy remain in flux. This is particularly true for orangutans (genus: Pongo ), the only Asian great apes, and phylogenetically our most distant relatives among extant hominids. Definitive designation of Bornean and Sumatran orangutans, P. pygmaeus (Linnaeus 1760) and P. abelii (Lesson 1827), as distinct species took place as recently as 2001. Based on morphological, behavioral and environmental data, and corroborated by population genetic analyses of 37 orangutan genomes, we show that an isolated population of the Sumatran orangutan south of Lake Toba, Batang Toru, is highly distinct from the northern Sumatran and Bornean populations. The deepest split in the evolutionary history of extant orangutans occurred ~3.38 Ma between this remnant population south of Lake Toba and those to the north, while both currently recognized species separated much later about 674 ka. A morphometric analysis based on cranio-mandibular and dental characters as well as behavioral data revealed consistent differences between individuals from Batang Toru and other extant Ponginae. Our combined analyses support a new classification of orangutans into three extant species. One of them, P. tapanuliensis, encompasses the Batang Toru population, of which fewer than 800 individuals survive.