Driving along the other day listening to the radio, I found I was listening to a scientist who was also a Christian talking about why he believed in God (the oddness and joy of public radio). He claimed that God existed because people seemed to have an inherant desire to do/be good, and that goodness had a certain similarity across cultures. He argued that there was no evolutionary reason why people would help a drowning person out of a river, or that they would widely agree that that was the action of a good person. Hense there must be a moral or spiritual power which had created us to be moral beings.
I think that you can explain altruism in evolutionary terms (tho I'm by no means an evolitionary psychologist). Firstly, there is an evolutionary benefit from altruism and empathy if the group in question is a small related group (a family or tribe). All other things being equal, you'd expect people who develop the ability to empathise and act altruistically to live longer and procreate more often. Aside from the obvious benefits of taking care of people who are temporarily sick, it is helpful if you can predict what will upset or piss off the people around you. Also you are more likely to get laid if you are not a selfish and self interested person (well, I like to think so).
So there are good reasons for humans to be naturally disposed to empathy and altruism. However, by their nature, these qualities are easy to exploit. Once you start to empathise, you are on a slippery slope to empathising with all people or even animals (e.g. kittens and dolphins), and getting ripped off. So there needed to be a counterveilling force which could control the otherwise rampant desire to help other people.
So humans needed to develop a way of fine-tuning their altruistic urges so they benefitted the right people (and the right genes).
In general, it seems to be very easy to create racism and hatred of the other in groups of otherwise nice people. The classic experiment was with school kids. The teachers singled out the blue eyed children as different, and pretty soon the blue eyed kids were being abused by the majority. The conclusion I once saw drawn from this was that human beings seemed to have a reasonably hair trigger xenophobic reaction. Very easy to turn on, and difficult to retract.
If this is true, it would appear that our evolution has left us with a general moral sense, and a xenophobic filter to control it.
The only nice thought out of this lot, is that we do at least have the moral sense, and we are also developing the ability to see that the xenophobia is unnecessary.