Figure 1. Distribution of orang-utan subspecies on Borneo, in relation to Tanjung Puting National Park.

Three subspecies diverged from a common ancestor during climatic fluctuations around 176,000 years ago³⁸, with marked population differentiation evolving over the last 80,000 years³⁹. Pongo pygmaeus pygmaeus is restricted to northern West Kalimantan and Sarawak, and is primarily isolated by the Kapuas River from P. p. wurmbii, in southern West and Central Kalimantan. P. p. morio is found in East and North Kalimantan and in Sabah, and is isolated by rivers, mountains and geographic distance from the other two subspecies. Figure (including base map) drawn in Adobe Illustrator CC 2015 (http://www.adobe.com/illustrator); orang-utan distribution follows that previously described⁷⁷.