Harry Belafonte, at age eighty-nine, has been a star for more than sixty years, as well as an activist, he says, since birth. But he feels that the civil-rights movement is in decline, and he’s doing what he can to turn things around. Jon Lee Anderson journeys up the Amazon to witness a once-in-a-lifetime encounter: the emergence of an isolated tribe from the forest to make contact with other civilizations. And one of our writers tries out the new voice-recognition device from Amazon—living with Alexa, she says, is like “2001: A Space Odyssey” crossed with “The Golden Girls.”