To start I would like to say I have my own reservations about all of this. Anyway, for the sake of gaining better understanding, I will submit to other's views for this particular argument.
Stealing a car from your neighbor with 100% assurance of you not getting caught would only be beneficial, but only beneficial to you and not the group (group-selection theory). Merely saying that the existence or moral behavior automatically excludes any naturalistic explanation is bunk due to "survival of the fittest" meaning, the most fit to adapt to environmental changes. You are left with the origins of such. The only logical predecessor of morals would be instincts. So I am inclined to inquire what are the origins of instincts? For if we can accurately postulate the origins of such we can formulate a sound hypothesis.