The evolution of the human ape's head.

New York Times - Born, and Evolved, to Run

Here's two paragraphs from Born, and Evolved, to Run:

Our heads are what make our species interesting. If you were to meet a Neanderthal or a Homo erectus, you’d see that they are the same as us — except from the neck up. We’re different in our noses, ears, teeth, how we swallow and chew. When you think about what makes us human, it’s our big brains, complex thought and language. We speak with our heads, breathe and smell with our heads. So understanding how we got these heads is vital for knowing who we are and what we are doing on this planet.

This was my “eureka!” moment. I’d observed pigs on treadmills for hundreds of hours and had never thought about this. So Dennis and I started talking about how, when these pigs ran, their heads bobbed every which way and how running humans are really adept at stabilizing their heads. We realized that there were special features in the human neck that enable us to keep our heads still. That gives us an evolutionary advantage because it helps us avoid falls and injuries. And this seemed like evidence of natural selection in our ability to run, an important factor in how we became hunters rather than just foragers and got access to richer foods, which fueled the evolution of our big brains.