You rely on your wrists for hundreds of motions, from a quick flick to cut carrots or hands on hips to make a point, to something more exaggerated, even strenuous. Your wrists flex to knead dough, push a lawnmower uphill, type a report. You don't even think about it—until something goes wrong, until the numbness, tingling, and pain are so bad that you wake up in the middle of the night, your hands so weak that uncapping the pill bottle is a struggle.

A zebra has its stripes and a giraffe its neck, birds are known by their wings, and a fish by its gills. For humans, it's two feet and a unique locomotion—bipedalism.

A lot of theories try to explain why humans evolved to walk two-legged. Some say that bipedalism let early hominids rise up to see over the savannah grass and check for predators. Others say that having free hands to tend offspring and gather food was the advantage. Herman Pontzer, a biological anthropology major, supports a third idea: Bipedalism may have been the best way to travel.