Well said.And that is a big if.So the question that naturally begs to be asked is if there is a universal code of common decency that is independent of man how come we all don't behave the same way when it comes to right and wrong? The reason man doesn't behave the same way is because of subjectivity. The difference between being objective and being subjective is bias. Bias is eliminated when there is no preference for an outcome. To eliminate a preference for an outcome one must have no thought of the consequences to one's self. If one does not practice this they will see subjective truth instead of objective truth. Subjective truth leads to moral relativism. Where consequences to self and preferences for an outcome leads to rationalizations of right and wrong.
There is absolutely no evidence of some universal code of behavior. And for there to be one then there would have to be an absolute authority that authored that moral code.
There is none
I’m always uncomfortable when the hyper-religious choose to lecture others on “morality”. We could get into the utterly chilling lack of ethics and morality that is evoked by the treatment of people under religious authoritarianism - considering that bastion of enlightenment such as the Christian Europe during the Dark Ages - but that’s a thread unto itself.
The sense of self-sacrifice, nurturing, decision making and making “moral” choices is a higher brain function and it's seen in comparably lesser degrees in lesser animals. This in and of itself is enough to suggest that "morality" is a natural phenomenon of higher brain functions. A measure of language, nurturing, survival, industry, and even environmental control, all can be attributed to animals lesser on the sentience strat than man.
Ultimately, I would propose that morality involves decision making (or making a choice), where decision making is accomplished with a cognizant awareness of consequences. The mammalian sense of nurturing the young is not morality, it’s a function of evolution that seeks to preserve the species.
I suppose what defines morality is our expectations and choices as to how we extend such gestures as compassion, empathy, fairness, etc.
Going further, I don't think all moral impulses are learned. I think left to our own devices in a world devoid of civilization, we wouldn't blindly and blatantly kill, we likely wouldn't mate upwards with our parents. Fitness for survival would likely preclude this from happening as we would likely nurture our babies and try to protect them.
If we were to posit a world where humans are stripped of their morality, they’re still going to behave in ways that favor survival, not only of self but of the species, whether that boils down to a nuclear family or a tribe.