Where, then, does the fall of evolutionary psychology leave the idea of human nature? Behavioral ecology replaces it with "it depends"—that is, the core of human nature is variability and flexibility, the capacity to mold behavior to the social and physical demands of the environment. As Buller says, human variation is not noise in the system; it is the system. To be sure, traits such as symbolic language, culture, tool use, emotions and emotional expression do indeed seem to be human universals. It's the behaviors that capture the public imagination—promiscuous men and monogamous women, stepchild-killing men and the like—that turn out not to be. And for a final nail in the coffin, geneticists have discovered that human genes evolve much more quickly than anyone imagined when evolutionary psychology was invented, when everyone assumed that "modern" humans had DNA almost identical to that of people 50,000 years ago. Some genes seem to be only 10,000 years old, and some may be

younger