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.
It was only after identifying key differences in the teeth, skull, DNA, diet and calls of the Tapanuli orangutan (Pongo tapanuliensis) that the worldwide team concluded they had found a unique species.
Until recently, scientists thought there were only two genetically distinct types of orang utan - Bornean and Sumatran. Previously, scientists have recognized six great ape species: Sumatran and Bornean orangutans, eastern and western gorillas, chimpanzees and bonobos.
The Tapanuli orangutan bears a close resemblance to its Bornean and Sumatran cousins, but close observers may notice that it has a smaller head, slightly frizzier cinnamon-colored fur, and a "prominent moustache", the scientists wrote.
"We have proposed that the Tapanuli orangutans be categorized as critically endangered species by the International Union for Conservation of Nature [IUCN]", Puji Rianti, a researcher from the Bogor Agricultural Institute (IPB), said.
"We identified three very old evolutionary lineages among all orangutans, despite only having two species now described", added Maja Mattle-Greminger, a postdoctoral researcher at UZH.
"The Batang Toru orangutans appear to be direct descendants of the initial orangutans that had migrated from mainland Asia, and thus constitute the oldest evolutionary line within the genus Pongo", said Alexander Nater, also from the Unversity of Zurich.
While new species are discovered often, especially as scientists use modern techniques like genome sequencing, it is highly unusual for a new great ape species to be identified in the 21st century, since they are among the most extensively studied animals on the planet.
Computer models of their history suggests that the Tapanuli population has been completely isolated from other groups for at least 10,000 to 20,000 years.
This is a photograph of Pongo tapanuliensis, a newly identified distinct species of orangutans, bringing the total number of non-human great apes to seven.
"It isn't an everyday event that we find a new species of great ape, so indeed the discovery is very exciting", Michael Krutzen, an anthropologist at the University of Zurich in Switzerland, said in a news release.
"If steps are not taken quickly to reduce current and future threats to conserve every last remaining bit of forest we may see the discovery and extinction of a great ape species within our lifetime", they said.
A new species of frizzy-haired orangutans has been discovered in the remote jungles of Indonesia.
Wiratno, the director general of conservation of natural resources and ecosystems at Indonesia's Forestry and Environment Ministry, told a news conference in Jakarta that most of Batang Toru forest was designated as protected in December 2015.