Scientists have found that an isolated population of orangutans living on Indonesia's Sumatra island is a unique species, a researcher involved in the study said on Thursday.
Until now, the orangutan has been classified as belonging to two distinct species reflecting their geographical distribution: the Bornean orangutan (Pongo pygmaeus) and the Sumatran orangutan (Pongo abelii).
The new species, called the Tapanuli orangutan (Pongo tapanuliensis), live in the Batang Toru area in North Sumatra and has an estimated 800 members. 
"This new species is as old as the earliest of our own direct ancestors," said Erik Meijaard, a conservation scientist at the Australian National University who was involved in the research.
"It has evolved over some 3 million years to quietly adapt to the specific conditions in this part of the Asian tropics," he told DPA.
Behavioural and genetic differences suggested in earlier studies were not clear enough to support its designation as a new species, the scientists wrote in a paper published in the journal Current Biology. But careful studies dating back to 2013 of a skeleton belonging to a Batang Toru orangutan killed in a human-animal conflict revealed consistent differences in its skull and teeth, the paper said.
The finding means that there are fewer than 7,000 Sumatran orangutans in the wild. 
Like the other orangutan species, the Tapanuli group are also threatened by poaching and the destruction of their habitat. 
Plans are afoot to build a hydroelectric dam that would flood large parts of their best habitat.  
"It is super important that everything is done to protect all the remaining individuals, and ideally allowing the population to grow," Meijaard said.