The forces that drove our ancestors to walk upright eons ago may seem irrelevant today, but they aren’t. For millions of years until the postindustrial era, our ancestors had to walk something like five to nine miles every day to survive. We evolved to be endurance walkers. Yet, like our ancestors, most of us retain a deep-seated drive to spend as little energy as possible by walking only when necessary. That instinct to conserve calories points to another key difference between walking today and in the past: how much we carry things like babies, food, fuel, and water.
