The renowned biologist E. O. Wilson explores the necessary and delicate balancing act of the human conscience and its evolutionary role. Vice / Virtue or selfishness vs. selflessness: we embody both extremes - often simultaneously and they are biologically rooted..
When an individual is cooperative and altruistic, this reduces his advantage in competition with other members but increases the survival and reproduction rate of the group as a whole. In a nutshell, individual selection favors what we call sin and group selection favors virtue. The result is the internal conflict of conscience that afflicts all but psychopaths, estimated fortunately to make up only 1 to 4 percent of the population.
...The internal conflict in conscience caused by competing levels of natural selection is more than just an arcane subject for theoretical biologists to ponder. It is not the presence of good and evil tearing at one another in our breasts. It is a biological trait fundamental to the human condition, and necessary for survival of the species. The opposed selection pressures during human evolution produced an unstable mix of innate emotional responses. They created a mind that is continuously and kaleidoscopically shifting in mood -- variously proud, aggressive, competitive, angry, vengeful, venal, treacherous, curious, adventurous, tribal, brave, humble, patriotic, empathetic and loving. All normal humans are both ignoble and noble, often in close alternation, sometimes simultaneously.
The instability of the emotions is a quality we should wish to keep. It is the essence of the human character, and the source of our creativity. We need to understand ourselves in both evolutionary and psychological terms in order to plan a more rational, catastrophe-proof future. We must learn to behave, but let us never even think of domesticating human nature.
Read more here.
(Hat tip to Andrew Sullivan)