@Tuna,
Tuna, as I'm sure you know, a lot of great thinkers have argued for and against the concept of "natural law" (as opposed to positive law) which can be, and has been, argued by replacing the word "law" with morality
Nietzsche's thought, particularly as expressed in his book "The Genealogy of Morals," has some analysis of the "basis" for morality in it that is (or was) somewhat unique. And, of course, virtually every conceivable basis for morality from deontology to utilitarianism to egoism to divine ordination to cultural relativism to "might makes right," ad nauseum have been suggested.
I know you're not looking for a history course here. Can we start with your views on the topic, if you have any?
Or, if you prefer, let me ask you this. The authors of our declaration of independence (Jefferson, mainly) refers to our "inherent and inalienable rights."
Do such things exist, ya figure? Jefferson had them as having been bestowed upon men by their "Creator." I know you claim to be an atheist, so I'm guessing you wouldn't accept that as a source, if you accept the concept at all.