An isolated group of orangutans hiding out in a forest in Sumatra is now considered an entirely new species and the world’s most endangered great ape, researchers say. Until now, scientists had long recognized six species of living great apes: Sumatran orangutans, Bornean orangutans, eastern gorillas, western gorillas, chimpanzees and bonobos. So, to describe a new great-ape species is rare, the researchers said. Fewer than 800 individuals of the newfound species, called Pongo tapanuliensis, survive in the Batang Toru forest. “It isn’t an everyday event that we find a new species of great ape, so indeed the discovery is very exciting,” senior study author Michael Krutzen, a professor of evolutionary anthropology and genomics in the Department of Anthropology at the University of Zurich in Switzerland, said in a statement.

Scientists didn’t even know about this population, which lives south of what was thought to be the southern range of Sumatran orangutans, until 1997. After that, research had suggested this group had behavioral and genetic differences from other orangutan groups. But until now, scientists couldn’t say for certain that these differences supported a new species designation, they said. That changed in November 2013, when an adult male orangutan from Batang Toru died due to wounds sustained by local villagers. That event gave the researchers access to study material, and a careful examination revealed differences in the skull and teeth compared with other orangutans, they said.

Currently, the survival of the Batang Toru orangutans is threatened by activities such as illegal road construction, killings during human-orangutan conflicts (over crops) and illegal trade of the animals, the researchers wrote in their paper in the journal Current Biology. Perhaps an even more imminent threat, however, is the proposed development of a hydroelectric dam on Sumatra that could impact up to 8 percent of the Batang Toru population, the researchers said. “To ensure long-term survival of Pongo tapanuliensis, conservation measures need to be implemented swiftly,” they wrote.