1. An unsound spine

Problem: Our spines are a mess. It’s a wonder we can even walk, says Bruce Latimer, director of the Center for Human Origins at Case Western Reserve University, in Cleveland. When our ancestors walked on all fours, their spines arched, like a bow, to withstand the weight of the organs suspended below. But then we stood up. That threw the system out of whack by 90 degrees, and the spine was forced to become a column. Next, to allow for bipedalism, it curved forward at the lower back. And to keep the head in balance—so that we didn’t all walk around as if doing the limbo—the upper spine curved in the opposite direction. This change put tremendous pressure on the lower vertebrae, sticking about 80 percent of adults, according to one estimate, with lower back pain.

Fix: Go back to the arch. “Think of your dog,” Latimer says. “From the sacrum to the neck, it’s a single bow curve. That’s a great system.” Simple. Strong. Pain-free. There’s only one catch: To keep the weight of our heads from pitching us forward, we’d need to return to all fours.