It was only after identifying key differences in the teeth, skull, DNA, diet and calls of the Tapanuli orangutan (Pongo tapanuliensis) that the global team concluded they had found a unique species.
An global team of researchers from 34 institutions, led by anthropologist Alexander Nater of the University of Zurich in Switzerland, pursued two lines of evidence to determine if the ape colony was different enough from the two already acknowledged orangutan species - known as the Bornean and Sumatran - to be defined as a third.
Galdikas, president of Orangutan Foundation International, said she hoped media attention over the announcement will further efforts to protect remaining orangutan populations in Borneo and Sumatra.
Rianti said in the Pleistocene period - 2.6 million years ago when the last of the five documented ice ages occurred - these giant apes experienced extreme climate change, and were separated from other orangutan species. 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.
Only Tapanuli orangutans appear to be direct descendants of the first mainland Asian orangutan ancestors to reach Sumatra, the investigators find. "Humans are conducting a vast global experiment, but we have near-zero understanding of what impacts this really has, and how it could ultimately undermine our own survival".
There are only believed to be around 800 Tapanuli orangutans left.
All three species of orangutan face dire threats to their populations at the hands of humans.
Tragically, the new addition to the hominid family is already on its way out. Upon closer inspection, the primate's skull exhibited skull and teeth features dissimilar from other orangutan species.
Wiratno, the director general of conservation of natural resources and ecosystems at Indonesia's Forestry and Environment Ministry, told a news conference in Jakarta that most of Batang Toru forest was designated as protected in December 2015.
"When we realized that Batang Toru orangutans are morphologically different from all other orangutans, the pieces of the puzzle fell into place", said Dr Michael Krützen, a professor at the University of Zurich and a member of the research team.