Fifty thousand years ago, a tired hunter spots a cave opening blocked by a boulder: a safe place to rest if he can move the heavy obstruction. Unlike his apish forebears, the hunter stands upright. In that straight-up posture, he pushes mightily against the boulder, but nothing happens. To get a better angle, the hunter places his feet at a greater distance from the rock. When his body is almost horizontal, the applied force has a much larger component in the right direction. The boulder moves.
