One problem is a lack of fossil evidence for ancestors of the orangutans. That means we lack a lot of detail.
I found this:
Orangutan Evolution
"
Between 9 and 17 million years ago, apes from the genus
Dryopithecus were living not just in Africa, but were the first known species’ of ape to have migrated in to Europe and Asia, giving clues to the migratory patterns of our ancestors (Palmer, 2010; Begun, 2004)). Although there are variations in the five species of this genus, like
Proconsul, it resembled a monkey in many ways, but the bones in the forearm and elbow suggested it moved about in the tree tops like an orangutan or a gibbon. However, its skull formation is similar to the chimpanzee. Like all fossils, its evolutionary position is debated, but some scientists have claimed that
Dryopithecus and its close relatives were the ancestors of all the extant great apes (Palmer, 2010).
During this period, between 8.5-12.5 million years ago, three species of the genus
Sivapithecus were living in the rainforests of Asia.
Sivapithecus was around 1.5 meters in height, and many of its physical attributes resembled those of a chimpanzee. However, like an orangutan, it had a concave face with projecting incisors and large canines (Palmer, 1999). Fossils of
Sivapithecus species have been found in Turkey, China and Pakistan, and analysis of bone structure indicates they were adept at movement on both the ground and in the trees, and fed on a diet of savannah grasses and seeds (Palmer, 1999). The genus
Sivapithecus is now acknowledged as being the direct ancestor of modern day orangutans (Fleagle, 1999), and scientists believe that this line, the lineage that descended to modern day orangutans, branched off from the line that descended to modern day gorillas, chimpanzees, bonobo’s and humans at around 12 million years ago (Palmer, 2010; Fleagle, 1999)."