 They begin coming out of the trees, again, due to climate change, and it just makes more sense for them to walk on two feet so they can see further over the horizon. And there's a lot of theories on why, but we became bipedal. We started walking on two feet. And that's an awful lot easier to do with a narrower pelvis. So over time, the females that had narrower pelvises were more likely to survive. Their genes were passed on in a Darwinian sense.