The variety in moral conclusions that different societies draw from their circumstances may be less varied than you think. In fact, the root commonality of all societies formed the basis of C S Lewis's conversion and his first chapter in Mere Christianity. As he reasoned, there may have been no societies that have extolled lying, cheating, or cowardice as moral.
Check it out, Right and Wrong as Clues to the Meaning of the Universe.
Of course, Lewis is a far sight from concluding the existence of a personal god, let alone the God of Christianity. But this is his first step