"It's the first declaration of a new great ape species in about 100 years," Ian Singleton, co-author of the study and director of the Sumatran Orangutan Conservation Programme, told AFP.
The species, called 'Tapanuli orangutan', lives in the Batang Toru forest on Sumatra island, and numbers only about 800 in total, making it the most endangered great ape in the world, Singleton added.
Until recently, scientists thought there were only two genetically distinct types of orangutan, Bornean and Sumatran.
Researchers studied the DNA, skulls and teeth of 33 orangutans killed in human-animal conflict before concluding that they had indeed discovered a new species, giving it the scientific name Pongo tapanuliensis.
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Outwardly the Tapanuli orangutan bears a closer resemblance to its Bornean counterpart, with cinnamon-coloured fur that is frizzier than its Sumatran relative. It also has a "prominent moustache", according to the findings published in the journal Current Biology.
Scientists believe the three types of orangutans share a common ancestor but began to diverge into different species about 3.4 million years ago.
"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 co-author Alexander Nater of the University of Zurich.
The Tapanuli orangutan species became isolated from its Sumatran relatives about 10-20,000 years ago, Nater added, eventually settling in the Batang Toru forest.
The authors of the study said conservation measures need to be urgently implemented.
"Orangutans reproduce extremely slowly, and if more than one percent of the population is lost annually this will spiral them to extinction," co-author Serge Wich, professor at Liverpool John Moores University, said.
Both Sumatran and Bornean orangutans are listed as critically endangered by the International Union for Conservation of Nature (IUCN).
The Sumatran orangutan population is estimated to be just under 15,000, while about 54,000 orangutans are thought to live in Borneo, according to the IUCN.