Scientists had thought that there were two orangutan species: the Sumatran orangutan (Pongo abelii) and the Bornean orangutan (Pongo pygmaeus).
However, the physical and genetic data now show that an isolated population of orangutans in Batang Toru, a region within the three Tapanuli districts in the province of North Sumatra, represents a distinct species.
Tapanuli orangutans were morphologically different from all other orangutans.
The researchers also reconstructed the history of the Tapanuli population.
Their calculations show that the population appears to have been isolated from all other Sumatran populations for at least 10,000 to 20,000 years.
The oldest evolutionary line in the genus Pongo is actually found in Tapanuli orangutans, which appear to be direct descendants of the first Sumatran population in the Sunda archipelago.