Chris,
To be honest, I'm no longer sure what point you're trying to make. I think it has something to do with the relativism and determinism (or non-determinism?) of the universe according to the atheistic worldview.
I'll try to lay out the principles as I see them in hopes that somewhere along the way I might answer some of your questions.
The universe is finite in all dimensions. The Big Bang gve rise to the universe and any questions about what happened BEFORE make no sense as it makes no sense to ask what lies north of the North Pole. The dimension of time did not exist prior to the Big Bang and thus questions of "before" are meaningless. Thus I preoccupy myself with the question of the origin of Big Bang as much as I preoccupy myself with the "North Pole" question.
According to my worldview (backed by the evidence of Quantum Mechanics) the universe is not inherently deterministic. It is purely material (ie everything that exists is at least in principle measurable) and indeterministic at the atomic level (aka god plays dice with the universe) as outlined through Heisenberg's Uncertainty Principle. And no, I don't know what "causes" the wavefunction to collapse but I don't believe it is a "soul" or a disembodied mind.
I believe (based on much research summarized in many books on biogenesis) that life on earth arose billions of years ago through environmental factors acting upon the primoridal soup of chemicals. Yes, it took a lot of time and we don't know all the steps but we also don't know all the steps of a "desert rose" formation yet it's not "Intelligently Designed" but rather weathered by interesting wind/temperature patterns. We can't produce a living cell in a petri dish yet but science has not closed the book on that. Give it time and I am confident that such feat will be accomplished eventurally.
I am positive that once early simple life forms sprouted on earth the early diversification through random genetic mutations gave rise to inter spiecies competition for resources. this competition for resources pushed those microbes to evolve to more and more sophisticated forms until today when we see the enormous diversity of life on this planet.
The human brain evolved after the mammalian dominance took hold and as homonoids (which were quite inferior physically) developed larger and larger neocortex which allowed them to better model their world and helped them pursue increasingly more complex analyses and predictions. At the same time the evolutionary pressure pushed the human beings toward cooperative behaviour as this was the best tactic that humanoid spiecies had against other, stronger predators. This long process led to the evolution of the clever and cooperative (though still capable of ruthless competition and selfish desire) Homo Sapiens that we are today.
The sense of "ethical" vs "unethical" is wired into the Human brain because this is what has helped us survive on earth for the past several million years. While there is no ultimate cosmic "right" or "wrong" there is one in the context of the human society. The definition of "ethical" in this scheme is all that advances the human society and helps us humans lead easier, longer, healthier and safer lives. What is "unethical" in this scheme is all that which destroys human life, incites violence between human tribes (aka nations) and destroys the principle of cooperative collaboration (theft etc).
What I THINK you posit is that such an ethics code is merely an illusion of a fallible human brain if it's not backed by a supernatural Creator. I can't agree with such a position at all. We have more than just "opinion" to go on here. For a start, we have much empirical evidence. We can observe that societies which permit murder, theft, rape or torture to go unpunished either disintegrate rapidly or at best keep barely functioning (usually with substantial foreign aid only) until the spiral towards collapse plunges them into chaos eventually. Thus we do have a wealth of experience (both positive and negative) of what it takes to build a successful human society. Based on that we can draw a lot of inspiration as to what is "right" or "wrong", what is to be labeled ethical and what is unethical. And no, there are not always cut and dried answers within this worldview. A lot of it is tentative and will evolve as we learn more about ourselves but we already have a wealth of past experience to start with. I don't know why this is so hard for you and others here to understand (or perhaps rather accept and acknowledge). I see this meme "There is no ethics without God" appear here all the time but it is completely without basis to anyone but the most rabid fundamentalist.
I'll state my conclusion again. A divine being is NOT required for a succesful code of ethics to exist among humans. We have the "generous" gene coupled with the societal experience of countless previous generations of humans upon which we can build a pretty decent set of secular ethics. We already do this. We call it the Law. The Constitution The judicial governance. And it works most of the time.
<comic_relief>This might come as a shock to you but I do not consider eating shellfish unethical under this secular code of ethics of mine
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