The great ape family just acquired a new member, but its numbers are frighteningly low. Maxime Aliaga

Humanity has just recognized a new living cousin, but it is even more endangered than other members of the great ape family. The new species, dubbed Pongo tapanuliensis, probably represents the oldest existing orangutan species, but it's restricted to a small patch of Sumatran rainforest and is estimated to number just 800.

The hominid family was once quite diverse, but most of its members are gone. Today it's just us, two species of gorilla, two chimpanzees, and the orangutans. Two decades ago it became accepted that the orangutans of Borneo and Sumatra are actually separate species, P. pygmaeus and P. abelii, an unsurprising finding given that hundreds of kilometers of sea divide them.

Now, in Current Biology, scientists from four continents have made the case that the orangutans that live south of Lake Toba, Sumatra, are a different species from those north of the lake. More remarkably, the north-Sumatran apes are much more closely related to those in Borneo than they are to their relatives a short distance away in the Sumatran forests of Batang Toru.

Although local residents have known about these orangutans for a long time, the outside world only recently became aware the population existed in the region's rugged terrain. Zoologists set out to explore whether there were any differences between this population and the better-known ones, but the task was hampered by reluctance to trap members of such an endangered species. Instead, the researchers relied on genetic analysis of fecal samples, compared with the genomes of 37 orangutans from other areas, and the skull of a single male that had been killed some time before.

The genetics indicated that the newly named Tapanuli orangutans are quite distinct from others. So distinct, the paper's authors argue, they deserve to be regarded as their own species, rather than a Sumatran subspecies. Indeed, the genetics are so different the authors conclude P. tapunuliensis and other orangutans separated 3.4 million years ago. The Bornean and north-Sumatran populations only split into different species 670,000 years ago.

The one Pongo tapanuliensis skull, smaller than other adult males of the same age, but with larger teeth. Nater et al./Current Biology


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