Interesting article in New Scientist: Charity begins at Homo sapiens.
And a bit off-topic quote from there:
To make his point he gives the example of sexual desire, which most biologists agree evolved to spur the conception of offspring. Today, however, individuals experience sexual desire in many situations in which procreation is clearly impossible, "even when they know the object of their desire is imaginary, or a piece of paper", as Tooby says.
Sexual desire for a piece of paper! :-D