In the early 2000s, scientists identified two distinct species of orangutans: the Bornean and Sumatran, which live amidst the tropical forests of Malaysia and Indonesia, respectively. But in a remarkable new study published in Current Biology, researchers claim to have uncovered an elusive third species of orangutan, bumping the total number of (non-human) great apes up to seven, as Ben Guarino and Sarah Kaplan of the Washington Post report.
The newly identified Pongo tapanuliensis, or Tapanuli orangutan, is an isolated orangutan group that live in a high-elevation Sumatran forest called Batang Toru, located south of Lake Toba. Researchers have known about the species, informally, for decades. In the 1930s, Dutch travelers described an orangutan population that dwelled in the Sumatran highlands. And Erik Meijaard, a conservation scientist at the Australian National University, observed the Tapanuli group in 1997, according to Joe Cochrane of the New York Times.
“[I]t has taken us 20 years to get the genetic and morphological data together that shows how distinct the species is,” Meijaard, who joined an international team of scientists in authoring the study, tells Cochrane.
The breakthrough came in 2013, when researchers found parts of a skeleton belonging to a male orangutan that had been killed by locals in Batang Toru. They noticed significant physical differences between this specimen and other Sumatran orangutans, including the size of its skull, jaw and teeth. Researchers then set about conducting what they call the “largest genomic study of wild orangutans to date,” according to Chochrane.
After sequencing the genomes of 37 orangutans living across Sumatra and Borneo, the team found that the Sumatran, Bornean and Tapanuli groups “comprise three distinct evolutionary lineages,” writes Jason Goldman of National Geographic. Surprisingly, the Tapanuli group boasted the oldest lineage. Just as surprisingly, the species appeared to be more closely related to orangutans from Borneo than to other orangutans living close by on Sumatra.
The results of the study suggest that several million years ago, orangutans migrated from South Asia to an area near what is now Lake Toba in Sumatra. Approximately 3.3 million years ago, the population split, with one group moving north of Lake Toba and the other one staying south.
A second split occurred some 600,000 years back—“this time between the original population south of Toba and the orangutans that went on to settle in Borneo,” Goldman writes, “which explains how the [Tapanuli] orangutans could be more closely related to those from Borneo.”
Tapanuli orangutans became isolated from Sumatran creatures up to 20,000 years ago—no longer sharing genes between the populations. “They've been evolving totally independently for at least 15,000 years,” Michael Krützen, an evolutionary geneticist at the University of Zurich in Switzerland and one of the authors of the study, tells the Washington Post.
The authors of the study acknowledge that there are some limitations to their work, most notably that it relies on just one Tapanuli skeleton “[d]ue to the challenges involved in collecting suitable specimens for morphological and genomic analyses from critically endangered great apes,” they write in the paper. But they also note that other species have been defined based on the remains of one individual.
If the classification sticks, it will mean that the world is home to seven great apes—a family that already includes Bornean and Sumatran orangutans, eastern and western African gorillas, chimpanzees and bonobos. The Tapanuli is the rarest of all the species; according to the study authors, fewer than 800 individuals are living today. Krützen tells the Washington Post that he expects the new species to be classified as critically endangered “really soon.”