13 Million years ago. We’ve known about Dryopithecus (“Oak ape”) for a while. The first specimen was found in France in 1856. They’ve since been found all over Europe, from Spain to Hungary. There are about 4 species of Dryopithecus, roughly chimp-sized.
The various Dryopithecuses are interesting because they look like they’d make good candidates for an ancestor to the great apes, both Asian and African. (They could just as easily be on a side branch though. It’s easier to tell whether something is a close or distant relative than to figure out whether it’s an ancestor or a collateral.) Dryopithecus had made the move to suspensory brachiation – hanging from branches – and had the freely-rotating shoulders, long arms, and strong hands you need for that. But it wasn’t specialized for knuckle walking like a gorilla or a chimp. This could mean it spent almost all its time in trees. Later on (10 Mya) at Rudabanya, Hungary, we find Dryopithecus living in a moist subtropical forest, among fauna including Miocene versions of pigs, horses, rhinos, and elephants. The fauna also included predators: the lynx-like Sansanosmilus, weighing about 170 lbs, and “bear-dogs” up to five feet long. So maybe up in the trees all day was the safest place to be.
Dryopithecus could be an orang ancestor, and could be an ancestor to chimps and gorillas if there was a migration from Eurasia back to Africa. A big unsettled question in human evolution is whether biped human ancestors evolved directly from a tree-dweller like Dryopithecus, or whether human ancestors were more chimp-like knuckle walkers before they started standing upright. A lot of scenarios for human evolution start with something that looked like a chimp and lived in chimp-style social groups (more on this later), but there’s a lot of guesswork in this.