“I discovered the population south of Lake Toba in 1997, but it has taken us 20 years to get the genetic and morphological data together that shows how distinct the species is,” said Dr. Erik Meijaard, a conservation scientist affiliated with Australian National University and an author of the paper.

Image Researchers used measurements and the overall shape of a specimen’s skull, jaw and teeth, as well as genetic analysis, to distinguish the Tapanuli orangutans from two other species. Credit... Nater et al.

Orangutans are an endangered species, with populations in some areas critically endangered. The new species, Pongo tapanuliensis, or the Tapanuli orangutan, was described as living only in an area of forest about 425 square miles in size. The team researching the Tapanuli orangutan contends that it’s the most endangered of all surviving great apes, with only about 800 left.

In 2013, researchers involved in conservation efforts in an area of North Sumatra province known as the Batang Toru ecosystem recovered parts of a skeleton from an adult male orangutan killed by local residents. They were surprised to find singular characteristics that consistently differed from other Sumatran orangutans, including in the measurements and overall shape of its skull, jaw and teeth, said Matthew G. Nowak, a conservation biologist with the Sumatran Orangutan Conservation Program, an organization involved in the research.

“When we realized that Batang Toru orangutans are morphologically different from all other orangutans, the pieces of the puzzle fell into place,” said Dr. Michael Krützen, a professor at the University of Zurich and a member of the research team.

Researchers then conducted what they called the “largest genomic study of wild orangutans to date,” comparing the genes from the recovered orangutan with data collected in the past from other field sites on Sumatra. They found that the Tapanuli population had become isolated from other Sumatran orangutan populations sometime in the last 10,000 to 20,000 years.