This prompted the researchers to conduct 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.
Last year, the IUCN classified Bornean orangutans as critically endangered due to a precipitous population decline caused by destruction of their forest habitat for palm oil and pulp wood plantations.
A genetic examination showed that Tapanuli orangutans are closely related to Bornean orangutans, though they live in close proximity to Sumatran orangutans. Dubbed Pongo Tapanuliensis, the population was discovered 20 years ago, but it's only after comprehensive studies of the animals' genomes and skull morphology that scientists were able to confirm it as a distinct species. Sumatran and Bornean orangutans separated around 674,000 years ago, the team estimates.
Yet these apes have always been overlooked in favor of Sumatran orangutans that live in swampy forests north of the Tapanuli population.
"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.
Only Tapanuli orangutans appear to be direct descendants of the first mainland Asian orangutan ancestors to reach Sumatra, the investigators find.
The team notes that we will need to quickly implement conservation measures to ensure that illegal logging, road construction, and other human threats don't impact the long-term survival of the species.
"Great apes are among the best-studied species in the world", said Erik Meijaard of the Australian National University in a press release. The team of researchers believes that it is the most endangered of all surviving great apes, with only about 800 left.
The discovery means that now humans have seven close relatives: two species of gorilla, plus chimpanzees, bonobos and three orangutans.
Puji Rianti, a researcher from Bogor Agricultural University, told Anadolu Agency that these physical differences were one of the reasons these orangutans were declared a separate species.
The analysis revealed that the new species split away from the others around 3.3 million years ago.
P. tapanuliensis has only just been identified as a new species, but it already faces severe risk of extinction. There is also a new species that needs protection.
Professor Serge Wich, of Liverpool John Moores University, who provided ecological expertise to the study, said: "It is incredibly exciting that a new orangutan species has been described and it's a wonderful addition to Indonesia's high biodiversity".