It is estimated that there are about 55 000 Orangutans left in the wild.
In Borneo and Sumatra.
Several million years ago, orangutans moved from the South Asian mainland onto what is now Sumatra and occupied an area south of the Toba caldera. Around 3.3 million years ago, a group of them moved north to colonize the area north of Toba. While the two groups did interbreed from time to time, they would remain largely distinct. (Read why orangutans are more like us than you think.)
Then, some 600,000 years ago, a second split occurred—this time between the original population south of Toba and the orangutans that went on to settle in Borneo. As ice ages progressed and sea levels changed, orangutans moved effortlessly between landmasses—which explains how the Batang Toru orangutans could be more closely related to those from Borneo.
Around 75,000 years ago, Toba erupted. Perhaps not coincidentally, the genomic data indicates a population crash around 75,000 years ago as well. Because the lava destroyed the surrounding rain forest, the orangutans living on either side of the volcano were permanently separated.