To one nineteenth-century scholar, their fierce, ridged brows were evidence of a "moral darkness" that set them irrevocably apart from human beings. Some commentators accused them of cannibalism. Yet by the 1970s the Neandertals were being hailed as "the first flower people" and praised for their apparent compassion and religious piety. The story of how scientists could come to such divergent conclusions about a set of bones unearthed in Germany in...