There is a line of philosophy, particularly popular among atheists and other people who like to call themselves rationalists, there is is some absolute morality which can be justified on the basis of the kinds of issues that you're talking about.
Just coincidentally, most people who believe in that come up with something like Western morality.
Now I grant, our desire for morality is indeed evolved. It is as much a part of the fabric of who we are as, say, the ease with which Stockholm Syndrome can set in to quickly adapt us becoming slaves. (Historically a very useful reaction to have from time to time.)
My point is that any attempt to justify current moral beliefs on evolution, game theory, and some sort of absolutes, is fundamentally flawed. We are equipped to have moral reactions. We are trained to have particular moral reactions. But we have no evidence that our current moral fashions are in any real sense fundamental.
If all successful systems shared some characteristics, and social creatures seem hardwired with them, those characteristics would be in some real sense fundamental, no?
I guess the way I see it, morality tries to answer the question how do I act. That assumes actors exist, and others exist that can be acted upon. Then assume they share enough that they can communicate their individual values, eg I want to survive, avoid pain.
There are moral systems that are going to allow them to cooperate. Those moral systems are going to have some things in common, eg, reciprocity, or don't unnecessarily hurt others.
Nature tends to hardwire those, experiments show. Those would be universal moral values.
Beyond that, societies are going to evolve more complex systems, e.g. relieve yourself in the toilet, not in the public square; drive on the right. Some are going to follow more or less directly from universal principles given the environment, some are more like local maxima that people agree on.
One could say that a moral system that allows a bigger and more varied group to cooperate more successfully in more complex ways is better. So beyond the universal principles one could distinguish moral systems in more or less objective ways, as well as subjective ways like how repugnant they seem.
So, I'd agree that a lot of things people have strong moral views about are just fashions. Also that you can't get to one ideal moral system. Also that you need some first principles about how to determine harm. But I think there is a lot of morality that flows from simple first principles and is hardwired.
One characteristic that all successful systems that we know of share is a division between "us" and "them". And our greatest revulsion is reserved for "almost us", who presumably are competing with "us". Any group that gets painted as "almost us" can receive intense bigotry - be they Jewish, black, LBTG, muslim...
One of the characteristics of current Western moral fashions is that "us" should be painted as broadly as possible. It is not in fashion just to talk of rights for ourselves, we talk about universal rights. We do not just grant those rights to people we like, we grant them to people of different religions, different races, different sexual orientations, etc.
To me this aspect of morality - that there are universal principles granted even to those we don't like - matters a lot. Yet if we go off of evidence from nature, it is exactly this that there is the least evidence that is innate.
Just coincidentally, most people who believe in that come up with something like Western morality.
Now I grant, our desire for morality is indeed evolved. It is as much a part of the fabric of who we are as, say, the ease with which Stockholm Syndrome can set in to quickly adapt us becoming slaves. (Historically a very useful reaction to have from time to time.)
My point is that any attempt to justify current moral beliefs on evolution, game theory, and some sort of absolutes, is fundamentally flawed. We are equipped to have moral reactions. We are trained to have particular moral reactions. But we have no evidence that our current moral fashions are in any real sense fundamental.