Question about Conquest of the Planet of the Apes

Did they ever explain how the apes became anthropomorphized? Caesar makes sense because he is the offspring of the two sentient and evolved apes from the future but what about the others? As soon as the movie starts we already have the more advanced and more human-like simians seen in the other films, which took place well into the future. I know that apes became pets and subsequently slaves, but that doesn't explain how they mutated into the more human-like form seen in this movie. So is this ever explained?