Why is there morality?

I'm admittedly a skeptic and have a habit of asking that most annoying question in the arsenal of skeptics: Why?

I think this is the most important question that needs to be settled in the field of ethics before any useful conversation can happen. My reasoning for what I believe is the most reasonable basis of morality is bellow.

Based on our current system of popular belief there is no god, there is no alpha and omega that all existence stands upon, or, if there is, it's existence isn't particularly relevant to the life of man. So assuming there is no entity for morality to flow from then we are left with a social or natural basis for morality. Either morality is naturally a part of us or it exists as a convention.

I lean towards morality being a social convention mixed with certain elements that are naturally innate to every human. There would have been no way for evolution to imprint in us an entire morality, and the differing systems of morality between different times and cultures attests to a constantly shifting morality. However it would make sense that evolution imparted a mechanism in us to prevent us from killing another member of the species. This internal prerogative could be ignored however by a cultures morality, as witnessed by human sacrifice in many early human civilizations.