I like that explanation where humans during the forest days were smarter but smaller and weaker and that we were chased out of the trees by our bigger stronger cousins. But why would we leave the safety of the trees to begin with to deal with even larger predators? We were driven out and that would also explain the ancient myth of being cast from paradise. It wasn't God, it was our big bad tree neighbors. Once on the ground, our bigger brains allowed us to fashion survival strategies and develop common language among the troupes.