It began with a rumor, a scrap of conversation picked up by a health worker delivering antimalarial medicine in the scattered villages of southern Amazonia. In the middle of 1996, he stopped at a lumberyard in Brazil’s Guaporé River Valley, near the Bolivian border. Loggers there spoke of a wild man who roamed the surrounding rain forest, which they occasionally ventured into in search of mahogany trees. The man was a naked savage, they said, probably an Indian. But he didn’t appear to be part of any tribe. From what they could see, he lived all alone in a tiny thatched hut, with no apparent...