The orangutans, or Pongo Tapanuliesensis, were found in the isolated highlands in the Batang Toru forests in Tapanuli, where there are less than 800 individual orangutans estimated by researchers living in the high altitude of over 850 meters above sea level.
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.
The new species lived south of what was the known range for Sumatran orangutans and remained a mystery to scientists until two decades ago. "We identified three very old evolutionary lineages among all orangutans, despite only having two species now described", says Maja Mattle-Greminger, a postdoctoral researcher at University of Zurich.
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.
Scientists weren't sure the unique orangutan population existed until 1997. The results confirmed their observations, with another surprising revelation: not only did they uncover three very old evolutionary lineages, but the newly discovered population was the oldest.
She also said she hoped it would spark new scientific debate on whether the three subspecies of the Bornean orangutan should themselves be elevated to full species of great ape, in particular the orangutan of eastern Borneo. The team of researchers believes that it is the most endangered of all surviving great apes, with only about 800 left. If after 200 years of serious biological research we can still find new species in this group, what does it tell us about all the other stuff that we are overlooking: hidden species, unknown ecological relationships, critical thresholds we shouldn't cross?
"Great apes are among the best-studied species in the world", said Erik Meijaard of the Australian National University in a press release.
Tragically, the new addition to the hominid family is already on its way out.
"When we realized that Batang Toru orangutans are morphologically different from all other orangutans, the pieces of the puzzle fell into place", said Michael Krützen, a professor at the University of Zurich and a member of the research team. There is also a new species that needs protection.
"It is crucial that we work to conserve the forest, because if we do not take the steps needed to protect the Tapanuli orang-utans, we could see their discovery and extinction within our lifetime", Dr Benoit said.