Man is the Only Animal that blushes. Or needs to.......................Mark Twain, Following the Equator.
Mark Twain's witticism poses a fundamental question for evolutionary biology: What was the circumstance that impelled and produced this unique adaptation? Darwin himself thought deeply about it, and wrote letters to naturalists around the world to ask if people everywhere did in fact blush - they do.
Humans are also unique in their extra-familial generosity, or altruism. Christopher Boehm draws a straight line between these behaviors, and believes that they connect directly with our ability to cooperate in large groups. The evolutionary pressure against altruistic behavior, whether it involves giving money to support some child in a different country or going to war to defend your country, are huge. Free riders: cheaters, draft dodgers, etc.,(the Dick Cheneys, George W. Bushes, et. al.) get a big advantage out their free ride. Without active suppression of free riding, large scale cooperation does not seem possible.
Boehm speculates that active suppression of free riders, who at first were probably just bullies, arose with hunting of large game, something a lone human with crude weapons can hardly accomplish. Such suppression was probably violent, just as it has been in recent hunter-gatherers, and favored those who could internalize society's rules. That internalization software and hardware became the "conscience" - the basis of our sense of shame.
Of course some people lack that sense of shame - the Nietzschean and Randian "Supermen" - perhaps better described as Superchimps, since they are really just slightly upgraded chimpanzees, lacking in some of the critical faculties that made us human. Usually, though, we just call them sociopaths.