Biologists have a pretty good idea of both how flies become resistant to DDT and how humans and primates have diverged over time.

We do not understand how cultures evolve nearly so well. The majority of human evolution does not involve changes in our DNA, but rather alterations in the gigantic library of nongenetic information, the culture, that our species possesses. This library is orders of magnitude larger than that of our genetic information, and the elements on its diverse shelves usually have meaning only in connection with other elements. Indeed, there has been a long, bitter debate about whether it is sensible even to use the term evolution to describe changes in culture.

Despite the great difficulties of building a comprehensive theory of cultural change deserving of the label of "evolution," progress in that direction has begun. We are finally starting to understand the patterns of culture change and the role of natural selection in shaping them. And since everything from weapons of mass destruction to global heating are the results of changes in human culture over time, acquiring a fundamental understanding of cultural evolution just might be the key to saving civilization from itself.