For decades, scientists have recognized the upright posture exhibited by chimpanzees, gorillas, and humans as a key feature separating the "great apes" from other primates, but a host of questions about the evolution of that posture -- particularly how and when it emerged -- have long gone unanswered. For more than a century, the belief was that the posture, known as the orthograde body plan, evolved only once, as part of a suite of features, including broad torsos and mobile forelimbs, in an early ancestor of modern apes. But a fossilized hipbone of an ape called Sivapithecus is challenging that...