In the early 2000s, biologists recognized that orangutans were two species, Bornean and Sumatran orangutans. An analysis of ape DNA in 2011 indicated these orangutan species split apart about 400,000 years ago. Now a team of evolutionary biologists and anthropologists, publishing their work Thursday in the journal Current Biology, says there is a third orangutan offshoot: Pongo tapanuliensis, or the Tapanuli orangutan.

The great ape family, then, has grown to eight species: bonobos, chimpanzees, eastern and western gorillas, humans and the three types of orangutans. The Tapanuli orangutan is the rarest of them all.

“The number of great apes worldwide have plummeted massively,” said Michael Krützen, an evolutionary geneticist at the University of Zurich in Switzerland and an author of the report. “I really hope that our paper will put these orangutans more on the radar.”

This isolated group of orangutans began to evolve into a separate species once their genetic exchange with other Indonesian apes slowed to a trickle 100,000 years back, the authors say. Tapanuli orangutans have subtle differences in their facial features as well as smaller skulls than other orangutans. This species has a “prominent mustache,” the authors wrote, and the female Tapanuli orangutans have beards, unlike Bornean orangutans. The males emit longer roars, too, than other orangutans.

There are 54,000 Bornean orangutans and fewer than 7,000 Sumatran orangutans. These two species are listed as critically endangered by the International Union for the Conservation of Nature. The Tapanuli population consists of fewer than 800 individuals, according to the new report. They are found only in forests patches on the Indonesian island of Sumatra, at high altitudes south of the swamp forests where the Sumatran orangutans live.

A Pongo tapanuliensis skull. (Nater et al.)

Sumatran orangutans are hunted by jungle cats, but otherwise the apes have few natural predators. Humans pose the biggest threat. Agriculture, particularly palm oil plantations, encroaches on orangutan habitat.

As early as the 1930s, records from Dutch travelers hinted that an isolated orangutan group lived in the Sumatran highlands. A conservationist at the Australian National University and an author of the new work, Erik Meijaard, first encountered these orangutans decades ago, Krützen said. Yet until recently, there was not enough evidence to pin down their identity as a separate species.

In 2013, villagers killed an orangutan near Tapanuli forest. Its body provided the single skeleton studied in the new report. Analysis of the orangutan bones, plus a detailed genetic analysis of blood samples, convinced the authors that the Tapanuli orangutan was distinct from other apes.

DNA revealed that these orangutans are descended from their cousins who live across a few hundred miles of sea, in Borneo. They ceased interbreeding and no longer share any gene flow. “They've been evolving totally independently for at least 15,000 years,” Krützen said.

They are distributed over about 1,000 square kilometers, an area about the size of Dallas. This orangutan species is too new to science to have an official conservation status, but “they will probably be named critically endangered really soon,” Krützen said.

Correction: A previous version of this article incorrectly stated that orangutans live only in Indonesia. Bornean orangutans are also found in the Malaysian states of Sabah and Sarawak on Borneo.