Olivia Judson has a great article on evolutionary origins of altruism: The Selfless Gene.
How does a propensity for self-sacrifice evolve? And what about the myriad lesser acts of daily kindness—helping a little old lady across the street, giving up a seat on the subway, returning a wallet that’s been lost? Are these impulses as primal as ferocity, lust, and greed? Or are they just a thin veneer over a savage nature?