Lurid novels of Cave Bear Clans aside, the beetle-browed and bandy-legged Neanderthal died out after a few millennia for one simple reason: he seldom lived past the age of 30. What that meant, say the authors of this theory of human ancestry, was that each generation of Neanderthal youth had to relearn the hard lessons of daily life—that fire burns, sabre-tooth tigers bite and that certain mushrooms are poisonous.