Why Compassion?

Ever since the post a few days ago about whether or not one should help a beaten man on the street, I got to thinking about compassion. In that situation, I would help the man almost every time, but I don't think it would be 'wrong' to just leave him there. So I wonder, how does evolution play into compassion? Is compassion just a psychological phenomenon? How does one feel this even when one sees himself as an animal with self-interest in mind?

My thoughts are that it is a result of a deeper understanding of the experience of life, of what it means to live and to suffer, and the understanding that all living things go through similar experiences. It's only when one becomes conscious of his own suffering that he can recognize the suffering in others. This makes me think that compassion came after the development of consciousness in man. If this is true, then compassion isn't an evolutionary instinct, so to speak, but rather something that comes after/during the experience of the conscious life. Most kids aren't compassionate until they grow up a little. Even many adults fail to feel compassion if they don't recognize the suffering of others to be like their own. We've all known people who can't seem to look beyond their own problems.

Those people aren't evil or wrong; they just lack a sense of unity with other people.

It doesn't seem that compassion therefore evolved as something necessary for our survival. It is accidental to our being conscious of our suffering, and our ability to rise beyond ourselves.

Thoughts/comments?