Most of the Sumatran orangutans live on the northern part of the island. But there’s another small group that lives in Batang Toru—100 kilometers to the south, on the other side of the sizable Lake Toba. A few obscure reports from the 1930s hinted at the existence of this splinter cell, but the group was only formally described in 1997, by a team led by the conservationist Erik Meijaard. These orangutans always seemed a little unusual. They live in more mountainous forests, and they eat different kinds of food.
Their genes are also distinct. In 2013, Michael Krützen, from the University of Zurich, analyzed the DNA of 123 Sumatran orangutans, and found that, in at least one part of their genome, the Batang Toru (or Tapanuli) orangutans were distinct. If anything, they seemed more closely related to the Bornean orangutans on a different island than the Sumatran ones just a day’s walk to the north. “We didn’t expect that,” says Krützen. “It was peculiar, but we needed more data.”
He later mentioned this peculiarity while giving a talk at a conference, where both Meijaard and Nowak happened to be in the audience. The three talked, and, suspecting that these orangutans might belong to their own distinct species, they teamed up to test that idea.
Krützen’s team analyzed the entire genomes of 37 orangutans, including two from Batang Toru. This more thorough analysis confirmed that these animals are indeed genetically distinct from both the Bornean and Sumatran species—and closer to the former than the latter.
They think that the ancestors of all modern orangutans traveled from mainland Asia into Sundaland—a continuous landmass that includes what is now Sumatra, Borneo, and other islands. Around 3.4 million years ago, these ancestral apes split into two populations, one of which gave rise to the current Batang Toru lineage. The other group spread throughout Sundaland; around 670,000 years ago, they split again into two new lineages, which we now know as the Bornean and Sumatran orangutans. The Batang Toru population occasionally crossbred with their Sumatran cousins, but those interspecies shenanigans stopped almost completely 100,000 years ago, when an erupting volcano cut them off.
It’s often said that humans differ from chimps by just 1 percent of our genome, so I wondered how close the three orangutan species are. Krützen disabused me of that question; the number depends on the history of each pair of species, and there’s little to be gained by comparing between different pairs. “There’s no magic number where, beyond that level, it’s a different species,” he says. And besides, genetic differences aren’t the only line of evidence the team has.
At the time Krützen was analyzing orangutan DNA, Nowak got word of the dying male who had been attacked in Batang Toru. And when his team compared its skeleton to those of 33 other orangutans, whose remains are housed in museums, they found clear differences in the shapes of its skull, teeth, and jaw. Even from the outside, they look distinctive, with frizzier fur and a more prominent mustache, and beards on the females. They behave differently too: They eat different plants than the northern populations, and the males have a higher-pitched call.