To possibly shortcut some of the semantic arguments here, I’d like to offer the definition provided by sociobiology for consideration:
Weak altruism: behavior that provides more benefit to another individual than it does to the individual performing the behavior
Strong altruism: behavior that provides benefit to another individual, but at a net cost to the individual performing the behavior
The distinction between strong and weak is an important one. Sharing of food, for example, although altruistic cannot be truly considered as a cost to the sharer if the individual otherwise has sufficient food (or has finished eating). On the other hand, jumping into a river to save a stranger or risking one’s life to save a little girl from a falling piano are assuredly examples of strong altruism.
The question then, for evolutionists, boils down to whether or not there can be a natural explanation for the existence and persistence of strong altruistic behavior among humans. I am purposely singling out
evolutionists, because theists will simply use their catch-all non-explanation goddidit;
strong altruism because weak altruism is both observable in non-human species and relatively easy to explain via kin selection, in-group cooperation, and reciprocal altruism theory; and
humans because, afaik, they are the only species to exhibit strong altruistic behavior.
Anyone care to take a stab at it? (A hint: is there a difference between genetically-programmed altruism such as in kin selection and culturally-modified behaviors?)