Civilization changes human nature. The same way our civilization produces dogs, caws, sheep, etc. it also changed us into a less aggressive, more cooperative being with way way way higher self control.
EDIT: I can give you countless examples. Compare the human reactions in developing countries with those of say a London native. You'll see that just a few centuries of life in big cities have given Englishmen incredible manners and self control.
A good point. The question then is "does civilization change human nature itself or just the contract between members of said society?" Extreme circumstances seem to break the social contract of a civilization and reveal that human nature didn't change much if at all.
EDIT: I can give you countless examples. Compare the human reactions in developing countries with those of say a London native. You'll see that just a few centuries of life in big cities have given Englishmen incredible manners and self control.