![Photograph of Tapanuli orangutan: Frontal view of Pongo tapanuliensis Morphobank media number: M435788 See "paratypes" section of [1]](/_next/image?url=https%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F6%2F65%2FPongo_tapanuliensis.jpg%3Futm_source%3Den.wikipedia.org%26utm_campaign%3Dimageinfo%26utm_content%3Dthumbnail_unscaled&w=3840&q=75)
Suspicion began in the 1990s, when an isolated orangutan population was confirmed south of Lake Toba, well away from the rest of the Sumatran range. The formal description rested on a single adult male skeleton recovered in 2013, whose skull and tooth dimensions differed consistently from other orangutans, combined with genomic data from dozens of individuals.
The genomes carried a surprise. The Tapanuli lineage appears to be the deepest branch among living orangutans, splitting from the others before Bornean and northern Sumatran orangutans separated from each other, despite living on the same island as one of them.
Estimates of the surviving population sit below one thousand across three forest fragments. A planned hydroelectric development in the Batang Toru valley has been the focus of sustained dispute, and some researchers have questioned whether the differences justify full species rank rather than subspecies.