Evolution is important in understanding ourselves. In our inner evolution, we developed a frontal cortex and gained a higher capacity for reasoning. At this point, the moral law began to emerge. And as this moral law developed, we began to develop conscience.
Since humans sprang from animals, our animal nature is still with us. This animal nature is the part that predisposes us to self-preservation and self-concern. It is the enemy of benevolence—not in every case, but in some, especially when the good of others is equal to our own.
This is why reason is so important in the development of morality. When reason reaches the point where it can discern right and wrong in a wider context, we become moral agents. We know that we should treat others as we want to be treated. We know that others are equal to ourselves, and that we should treat one another fairly.
We have an ongoing struggle with this animal nature, which in some traditions is called the lower nature or the carnal nature. We must put this to death, shut it off, and shut it out. We must use our reason to override our animal nature.