http://plus.maths.org/content/does-it-pay-be-nice-maths-altruism-pa...

Mathematical evidence that cooperation evolves naturally as a successful strategy.

This is one of the most interesting things I've read for a long time.  Don't worry, it's not too mathematical.  I recommend printing it out if you want to read it. 

I believe we need to know why these things evolved, in order to justify them today; but it's not necessary to know how they evolved.