chimpanzee-like ancestor. Bonobo males had to become less aggressive, females more adept at forming coalitions powerful enough for their hierarchy to control that of the males. Although the point cannot yet be proved, it seems more likely that bonobos are descended from chimpanzees, rather than the other way around. Still, both are descended from the joint ancestor of chimps and humans, and the joint ancestor presumably included both chimplike and bonobolike features in its behavioral template. That makes it easier to understand how humans came by their contradictory impulses of aggression and conciliation. "Being both systematically more brutal than chimps and more empathic than bonobos, we are by far the most bipolar ape," writes the primatologist Frans de Waal. "Our societies are never completely peaceful, never completely competitive, never
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