There are a lot of moral theories. As a matter of logic, at most one of them could be right. On the other hand, as a matter of logic, all of them could be wrong.
The only moral theories that we should take seriously are those that are compatible with morally-observant people leaving a genetic legacy in the evolutionary world in which we live.
Here’s why. It is obvious that human beings can do wrong. Since we can do wrong, there can be natural selection for dispositions to do wrong whenever doing wrong would help us achieve a greater genetic legacy. This means that there is strong selective pressure for our behavior to conform to whatever would increase biological fitness no matter what is right or wrong. But morality sometimes requires us to do things that reduce our fitness. For instance, helping orphans involves using resources that we could otherwise use to raise our own children. At least it did throughout most of human history. Organisms with features that reduce their fitness relative to their competitors are quickly eliminated by natural selection. So, natural selection ought to have eliminated organisms that help orphans in short order. It ought to have eliminated morally-observant people.
The questions then are how morality originated, and how it can endure in the kind of biological world in which we live, which is apparently hostile to it.
There is no non-evolutionary theory that can answer either question satisfactorily. None of them is adequate. None even has the potential to be developed in such a way that it becomes adequate. So, any true moral theory has to be an evolutionary theory.
It might be objected that moral truths could be logical truths and that logical truths are unaffected by natural selection. So what? Natural selection may be unable to change such truths but it can certainly affect our ability to grasp them. It is obvious that there are many logical truths that many people cannot grasp – think of propositions in advanced mathematics – and there is no reason why there could not be some propositions that none of us could grasp. If moral truths were logical truths and if understanding them would reduce fitness, there would be selection for an inability to understand them.
Furthermore, if morality does not help us to maximize our genetic legacy, it cannot itself be adaptive or an adaptation. This is because an adaptive feature is one that helps us maximize our genetic legacy, and because an adaptation is a feature that has been naturally selected because it helps us maximize our genetic legacy.
At this point, we confront a dilemma. The dilemma is that, if morality is not evolutionary, the lineages of morally-observant individuals will be supplanted by the lineages of the amoral, but if it is adaptive or an adaptation, it will differ too much from our actual moral codes to be plausible. In other words, the dilemma is that non-evolutionary ethics is at odds with biology while adaptationist ethics is at odds with ethics.
The dilemma can be resolved. It will be resolved here by arguing that morality is not an adaptation but the inseparable by-product of an adaptation, which gives us a non-adaptationist but evolutionary morality. The adaptation confers benefits; the by-product imposes costs. The benefits of the adaptation outweigh the costs of the by-product. Thus, the combination is beneficial, albeit to a lesser degree than the unencumbered adaptation would be. Since the by-product cannot be separated from the adaptation, the combination endures.