Third orangutan species developed near Mount Toba

Until very recently everybody thought that there were only two species of orangutan: the Bornean orangutan (Pongo pygmaeus) and the Sumatran orangutan (Pongo abelii). The Bornean species is divided into three subspecies.

But now, scientists have discovered that a group of orangutans, living in a high-elevation forest called Batang Toru in the mountainous region of Tapanuli on Sumatra, is in fact a seperate species[1].
Their DNA showed that Bornean orangutans, Sumatran orangutans, and the new species, Tapanula orangutan (Pongo tapanuliensis), comprise three distinct evolutionary lineages. Further analysis revealed that the oldest lineage belongs to the newest species. The Tapanula orangutan is more closely related to its counterparts from Borneo, across the sea, than to other orangutans living on the same island.

The genetic work suggests that several million years ago, orangutans moved from the South Asian mainland onto what is now Sumatra and occupied an area south of the Toba caldera. Around 3.3 million years ago, a group of them moved north to colonize the area north of Toba. While the two groups did interbreed from time to time, they would remain largely distinct.
Then, some 600,000 years ago, a second split occurred—this time between the original population south of Toba and the orangutans that went on to settle in Borneo. As ice ages progressed and sea levels changed, orangutans moved effortlessly between landmasses—which explains how the Batang Toru orangutans could be more closely related to those from Borneo.

Around 75,000 years ago, Mount Toba erupted, causing widespread destruction Perhaps not coincidentally, the genomic data indicates a population crash of orangutans around 75,000 years ago as well. Because the lava destroyed the surrounding rain forest, the orangutans living on either side of the volcano were permanently separated.

[1] Nater et al: Morphometric, Behavioral, and Genomic Evidence for a New Orangutan Species in Current Biology - 2017

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