Human Evolution: The Biology of Man

Bipedalism: A Strange Step Forward

The next few human traits I will discuss involve our efficiency of locomotion. Let us start with bipedal walking:

Walking on Two Feet

If you think about the  physics of walking on two feet, the process is very efficient. When you place one foot in front of the other, you plant that foot onto the ground for the first half of the motion. You bring your body over that foot, with the center of mass rotating around the ankle. At the peak of this step, your entire body weight is suspended over this foot. In short, by lifting yourself up, you have stored a lot of potential energy.

When you pass this peak, the potential energy becomes kinetic energy, moving you forward. As you come down, your other leg swings in front of you, using the hip as the point of rotation, in preparation of absorbing the weight of the body, starting the cycle again. These two points of rotation work as a “inverse pendulum,” constantly converting potential energy to kinetic energy and vice versa. This can be compared to a Newton’s cradle in terms of energy, making walking a low energy endeavor for humans.

Adaptations to Walking: Arches, Hips, and Heels

On top of this evolutionary mechanism, humans have developed many more adaptations to make walking more efficient. For instance, the arches in our feet allow us to use them as springs, stretching the foot ligaments when our feet are planted on the ground and launching us forward as our short toes push off, converting elastic energy into kinetic energy.

Unlike other primates, our hips are flared sideways, allowing us to balance our weight above two legs without losing balance. Our uniquely flat heels enable us to absorb the shock of walking more efficiently as well. Just like the heels, our leg bones are thicker and our knee joints are larger than our ancestors to cope with the chronic mechanical stress of walking.