Adam Gopnik at The New Yorker:
Why people walk is a hard question that looks easy. Upright bipedalism seems such an obvious advantage from the viewpoint of those already upright that we rarely see its difficulty. In the famous diagram, Darwinian man unfolds himself from frightened crouch to strong surveyor of the ages, and it looks like a natural ascension: you start out bending over, knuckles dragging, timidly scouring the ground for grubs, then you slowly straighten up until there you are, staring at the skies and counting the stars and thinking up gods to rule them. But the advantages of walking have actually been tricky to calculate. One guess among the evolutionary biologists has been that a significant advantage may simply be that walking on two legs frees up your hands to throw rocks at what might become your food—or to throw rocks at other bipedal creatures who are throwing rocks at what might become their food. Although walking upright seems to have preceded throwing rocks, the rock throwing, the biologists point out, is rarer than the bipedalism alone, which we share with all the birds, including awkward penguins and ostriches, and with angry bears. Meanwhile, the certainty of human back pain, like the inevitability of labor pains, is evidence of the jury-rigged, best-solution-at-hand nature of evolution.
Over time, though, things we do for a purpose, however obscure in origin, become things we do for pleasure, particularly when we no longer have to do them.