A French activist I know, a person with a flair for assailing the idiocies of modern life, someone who pines, though not romantically, for a trapdoor to a better world, was telling me about a hunter-gatherer tribe that has lived in isolation, some believe, for sixty-thousand years. Called the Sentinelese, they are the lone occupants of North Sentinel Island, in the Andaman archipelago of the Bay of Bengal. For all recorded history they have repelled visitors. Once, to make researchers go away, scores of them, the French activist told me, held an orgy on the sand as anthropologists watched…