From what I've gathered, it's pretty much similar to today. Holier-than-thou monkeys in trees thought they were better than others and wouldn't allow the underclass up into the tree. So those on the ground had move on and over time they evolved, began walking, probably forced to stand to see over grass for predators. Thoudands of years go by and now we have the same Holier-than-thou class living in the hills above the underclass.