A cave girl started this. Cave girls walked around naked, and cave guys and lesbian cave girls enjoyed looking at the bare breasts. But they were always there, skyclad, enjoying the air and the sun and the bird droppings that the almighty cave god would send them daily. Everyone liked them, and would play and caress them when the mood was upon them, but nobody made much of a fuss about them for about five million years or so. And for the first few hundred thousand years of that, they all wore otter skin on their Sideboob.