Discovery and naming An isolated
population of orangutans in the
Batang Toru area of South Tapanuli was reported in 1939. The population was rediscovered by an expedition to the area in 1997, but it was not recognized as a distinct species then.
Pongo tapanuliensis was identified as a distinct species, following a detailed
phylogenetic study in 2017. The study analyzed the
genetic samples of 37 wild orangutans from populations across Sumatra and Borneo and conducted a
morphological analysis of the
skeletons of 34 adult males. The holotype is stored in the
Zoological Museum of Bogor. Comparisons of the
genomes of all 37 orangutans using
principal component analysis and
population genetic models also indicated that the Batang Toru population is a separate species.
Phylogeny Genetic comparisons show that Tapanuli orangutans diverged from Sumatran orangutans about 3.4 million years ago, and became more isolated after the
Lake Toba eruption that occurred about 75,000 years ago. They had continued sporadic contact that stopped between 10,000 and 20,000 years ago. Tapanuli orangutans diverged from Bornean orangutans about 674,000 years ago. Orangutans were able to travel from Sumatra to Borneo because the islands were connected by
land bridges as parts of
Sundaland during
recent glacial periods when sea levels were much lower. The present range of Tapanuli orangutans is thought to be close to the area where
ancestral orangutans first entered what is now
Indonesia from mainland Asia. ==Description==