On comparing the skull and dental characteristics of the Batang Toru species to those of 33 other adult male orangutans of a similar developmental state, they found "unique" structural differences.
Unfortunately, the population of these newfound orangutans is so small, they've immediately joined the list of some of the most endangered primates in the world. To investigate the find, the researchers conducted an extensive DNA study of the orangutans, sequencing the genomes of 37 individual animals.
"If steps are not taken quickly to reduce current and future threats and to conserve every last remaining bit of forest, a great ape species may become extinct within a few decades", warns Matt Nowak, who supervises research into the Tapanuli orangutans at the Sumatran Orangutan Conservation Programme.
Sumatran orangutans have been classified as critically endangered since 2008.
A genetic examination showed that Tapanuli orangutans are closely related to Bornean orangutans, though they live in close proximity to Sumatran orangutans.
Following the discovery of the skeletal material, believed to have come from a Tapanuli orangutan killed in a human-animal conflict in 2013, an worldwide team of researchers set out to understand more about the distant ape relative of humans.
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To verify the morphological and genetic examination, the team employed a complex computer model which revealed that the Tapanuli population must have been isolated from other Sumatran populations of orangutans for least 10,000 to 20,000 years.
"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", says one of the team, evolutionary biologist Alexander Nater from the University of Zurich.
"Great apes are among the best-studied species in the world", says one of the team, Erik Meijaard from the Australian National University.
Until the Tapanuli orangutan was described, there were only six official species of nonhuman great ape: chimpanzees, two species of gorillas, bonobos, and two species of orangutan. The paper suggests that a mortality rate of even 1 percent per year could drive this newly identified orangutan species to extinction. Morphometric, behavioral, and genomic evidence for a new orangutan species.
Wiratno, director-general of natural resources conservation and ecosystem at Indonesia's Ministry of Environment and Forests, who like many Indonesians only uses his first name, said Batang Toru is not included in the protected forest area.