You write “[W]hy do we assume that our most primitive and universal moral laws, the ones written in our conscience and to which we are beholden (or at the very least to which we all feel and act as though we’re beholden), would exist without a law giver?” We assume this because evidence, drawn from looking at the “mechanics” of evolution, tells us why we are moral creatures and that there is no evidence that any “extra-natural” force is acting upon us or inspiring us to be good (“extra-natural” is my suggested new term because “super-natural” is so overused and drained of meaning).

You really ought to read The Moral Animal by Robert Wright. Sure, Wright’s word isn’t the last word, but he does offer an explanation that accounts for all your questions. As I think I mentioned to you or maybe it was someone else, Patricia Churchland has just published a new book, Conscience: The Origins of Moral Intuition. I haven’t read it (but plan to), but I can tell you this much: Churchland will convincingly argue that our moral intuitions have nothing to do with any “commandments” drawn from a supposed “holy” text. (This just occurred to me: Another book I intend to read that discusses moral issues is Justice for Hedgehogs by Ronald Dworkin.)

Never mind your remote island scenario (it’s not bad) when we can deal with a comparable scenario in real life: In Saudi Arabia thieves will get punished by getting a hand chopped off. We in the United States and in Europe recoil at such barbarism. How come? Well, for starters it seems extreme to permanently maim someone because of a theft when a severe fine or jail time seems more commensurate. You may respond with “Okay, but where did we get that idea? Where do we get the feeling that chopping off a hand is an extreme reaction that is way too disproportionate to the crime in question?” Good question! And I can tell you that the answer is not rooted in any “extra-natural” source influencing us or telling us that chopping off a hand is a barbaric response to stealing food or clothing or whatever.

Asking questions is always fine, but I’d like to make a recommendation. Ask yourself something like “Is there truly a Celestial Being who has endowed me with certain moral principles and drives, and if so what is the evidence for this claim?” Not to be snarky, but theists always come up empty-handed. In the history of the world, they’ve never been able to demonstrate that our moral impulses can be traced back to an “outside” extra-natural force that is acting upon us, and the reasons that theists have failed consistently at this endeavor is because there are Earth-based explanations — see Wright; see Churchland — that account for why we act and live as we do in the world.