Some authorities think bipedalism is at least as important a defining characteristic of what it is to be human as our high-functioning brain. Many theories have been proposed as to why our distant ancestors dropped out of trees and adopted an upright posture—to free their hands to carry babies and other objects; to gain a better line of sight across open ground; to be better able to throw projectiles—but the one certainty is that walking on two legs came at a price. Moving about in the open made
