Ever since Darwin, the world has been struggling with the mystery of human diversity. As the historian Peter Bowler has written, an evolutionary interpretation of the history of life on the earth must inevitably extend itself to include the origins of the human race. But this has proved to be a difficult and controversial task. Understanding human origins means accounting not only for the obvious differences between people and cultures around the world, but also for the unity of "Homo sapiens" as a single biological species. As Stephen Jay Gould has said, flexibility is the hallmark of...