@Mww & @Pfhorrest : This grew out of conversations with you both. I do not aim to convince either of you, of course, but I do aim to give a rough sketch so to why I believe that a priori moral rules and moral objectivity are redundant at best, inaccurate always, and damaging at worst.

I have tried to write this in many different ways, and the OP ends up a small book on morality and natural history. If I add citations, it ends up as a normal-sized book. So this really has to be a thumbnail sketch, with padding out and citations given afterwards.

The thesis is that metaphysical and object notions of morality are wrong, unjustified, and unnecessary, that a historo-scientific approach provides a more apt foundation for morality and has as its natural consequences moral pluralism/relativism.

WARNINGS: Natural selection is taken as a given. Humans are animals throughout. I know there are many who cannot stand such ideas but I don't intend to re-fight that war. And this is going to be long.

Animals fall broadly into three categories based on their degrees of sociality: solitary, subsocial, and social. Solitary animals tend to socialise for courting and reproduction only. Subsocial ones may cohabit and raise offspring briefly as couples, but otherwise operate independently. Social animals tend to operate in their cohabitation groups: hunting, gathering, child-rearing, migrating, fighting, etc.

Humans are ultrasocial animals: we pack a lot of biological capacity specifically for operating in social groups compared with other animals. Typically, evolutionary pathways tend toward higher sociality over successive generations. We are evolved, for instance, from subsocial ancestors of rodents.

Our presocial inheritance

Brief outline only of heritable selfish traits:

• survival instinct
• genome benefit: cuckoos, parasites
• basic needs: hunger, thirst, etc.
• the costs of giving

Selfish behaviours are not reborn with every human. They can be modulated by socialisation, but we inherit instincts and drives not just from our human ancestors, but from all of our ancestors, including our subsocial ones. For subsocial animals, what is good for them personally is good, so long as they can survive the obtaining of it. There is no exile, no lynchings, no reciprocity, no basis to evolve heritable characteristics for selfless behaviour and, whatever else we get, unless it was selected against in the intervening years, we get that too.

Our social inheritance

Competing with that we have heritable social drives, many of which are likely at least partly presocial in origin. For instance, empathy is linked to selfish behaviour. Understanding the perspective of another individual allows us to assess their threat and their vulnerability. It comes under the general negotiations of subsocial and social animals.

People scoring highly on empathy tests show increased and concentrated activity in mirror neurons [Gazzola, V., Aziz-Zadeh, L., & Keysers, C. (2006). Empathy and the somatotopic auditory mirror system in humans].There are two kinds of empathy: cognitive empathy (seeing things from someone else's perspective); and emotional empathy (feeling what we'd feel in others' circumstances). The triggering of either is not a given. Some people react to empathetic stimuli more than others. People react to stimuli concerning people like them more than people unlike them. Psychopaths show no signs of emotional empathy, and only demonstrate cognitive empathy when specifically requested to imagine how a subject feels. Empathy is shown to be linked to the genes ADRA2B, 5-HTTLPR, LRRN1, and OXTR.

Such tests also demonstrate increased production of oxytocin. Oxytocin also has its roots in subsocial behaviour. It is, for instance, produced when the uterus and/or cervix expand during childbirth, when the nipples are stimulated during breastfeeding, in romantically attached parents, in physical contact (e.g. sex). Yes, I said cervix and nipples.

In ultrasocial animals (that's us), higher oxytocin is seen during grooming and social eating, in trust negotiation, and is also during empathetically-stimulating scenarios (such as those in mirror neuron tests). People given artificially high doses of oxytocin are seen to be more social altruistic: that is, they behave in a moral charitable way, and that charity tends more toward social altruism rather than a good that the subject might benefit from.

Oxytocin production is heritable via the gene OXT and oxytocin reception via the gene OXTR. We also have a genetic amenability toward socialisation, mediated by oxytocin, dopamine, vasopressin and seratonin. Socialisation is important because most of the above are capacities rather than drives we are born with. To that extent, an immediate empathetic response to an individual in distress is not fully natural but learned via natural capacities for empathy, altruism, and socialisation together.

Generally, people lie on a selfish-selfless spectrum from psychopaths and narcissists, through arseholes, normal people, and good people, to annoyingly aggressive and obsessive altruists.

The underlying neural circuitry differs between psychopaths and altruists with emotional processing being profoundly muted in psychopaths and significantly enhanced in altruists. But both groups are characterized by the reward system of the brain shaping behavior. Instead of rigid assignment of human nature as being “universally selfish” or “universally good,” both characterizations are partial truths based on the segments of the selfish–selfless spectrum being examined. In addition, individuals and populations can shift in the behavioral spectrum in response to cognitive therapy and social and cultural experience, and approaches such as mindfulness training for introspection and reward-activating compassion are entering the mainstream of clinical care for managing pain, depression, and stress. — Sonne & Gash

-- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5917043/

A Brief Natural History of Morality

The above covers the how and the what: we are ultrasocial animals with heritable altruistic and empathetic capacities that compete with other, selfish heritable characteristics that, together with a heritable amenability to socialisation, allows us to make moral decisions concerning other individuals. That on its own isn't much of a foundational morality though.

Why need the why and the who to finish off part one (PART ONE!!! IT'S SO LONG ALREADY AND THIS IS THE WHISTLESTOP VERSION!!!).

Moving from solitary and social-but-solitary behaviours to higher subsocial behaviours increased out survival benefit by ensuring that more young lived to reproductive age. This was fairly trivial: oxytocin is structurally very similar to vasopressin, so likely evolved as a mutation. Evolution of the strictly social behaviours were instrumental in allowing social groups to cohere: they cannot be stable if everyone is out just to benefit themselves, their spouse, and their kids. The survival benefits of social groups are well known and include lower risk of starvation, of being killed in struggles for survival, etc.

A key behaviour to social coherence is reciprocal altruism, and nature also had a selection basis for that. Our ancestors lived in small social groups where whoever they met on a given day was overwhelmingly likely to be either a relative or a neighbour they would see again and often. There existed then the potential for reciprocal altruism to benefit the group as a whole, as one kind act begets many more. Mirror neurons and cognitive empathy were likely originally survival mechanisms for gauging threat and opportunity, but they also provided nature with an operating empathy system to hijack for social altruism. Simply understanding how another might respond to kindness by knowing how you would respond to kindness is a great start, but we have, at some point, also evolved the ability to feel how we would feel in another's circumstances. (This is an area of on-going study. It might be that cognitive empathy triggers oxytocin production which trigger emotional empathy for instance, which would fit the aforementioned feedback loop.) Add oxytocin production -- already a thing for subsocial animals -- into the mix to build a general drive toward altruistic behaviour along with feelings of attachment and protectionism toward the specific individual, and you have a part of basis for morality:

In one aspect, natural morality is a biological drive toward altruism triggered by empathy on the survival basis of reciprocity.

As I said, this would compete with other, selfish drives we also inherit, but those drives are now accompanied by a health hazard. If you can get away with benefiting yourself at the expense of another, you are more likely to behave selfishly (viz. the rioters and looters comprised of yesterday's law-abiding citizens who accompany today's peaceful protests). But... A) You have to live with yourself. You have a capacity for emotional empathy that will literally make you hurt if you hurt others, unless you can train yourself or be trained by others or have a natural inclination to suppress it (socialisation and pseudo-socialisation, genetic impairment).

In another aspect, natural morality is an incentive away from antisocial behaviour.

And B) if you don't get away with it, you know damn well you are surrounded by other empathetic individuals who you have also hurt by hurting one. In social animals and ultrasocial ones, this can manifest itself in attacks, exiles, even lynchings. I'm hazy on the mechanisms, but I think its sufficient to say that a) there is a survival benefit in a social group that punishes antisocial behaviour among its ranks, and b) that social groups do punish antisocial behaviour among its ranks.

In another aspect, natural morality is an incentive toward punishment of antisocial behaviour.

These effectively take the place of a priori knowledge in metaphysical constructions of morality. They are biological in origin, honed for purpose by natural selection but completely divorced from that purpose, i.e. there is no so that at root (e.g "punish antisocial behaviour so that...") Formulated without reference to their biological bases, they might look something like:

1. Do not harm others to benefit yourself: Antisocial behaviour is counter to what mankind is. Antisocial people are, to that extent, regressive... less than human... governed more by presocial capacities than social ones. Given the capacity for empathy, you are given the understanding of the harm you cause. If you cause that harm anyway, you are not making a rational error, you are making a social error: you are a hypocrite. From this we can derive a qualified version of the golden rule.
2. Help others, for they may help you later: There is a selfish and a selfless aspect to this which is more in keeping with real human ethics than the ethics of metaphysics. We are not just social animals, we are ultrasocial ones. It is in our being to seek reciprocal relationships with relatives and neighbours.
3. Do not allow antisocial behaviour: As said before, social groups cannot be sustained if there is sufficient antisocial behaviour to make reciprocal altruism unfeasible. It is not only a direct danger to you, your friends and your neighbours, it is a waste of your resources to invest in people who will not reciprocate but who will exploit.

That we are moral beings, i.e. moral in our being, segues nicely into...

Moral existentialism

We are a long way from living in small social groups acting out a few simple moral rules derived from biological capacities honed by natural selection. It would be interesting here to think about the ways our capacities for socialisation were perverted by the powerful over the last several thousand years, including the current situation where morality is so starkly polarised: antisocial behaviour itself is lionised and yet altruism and empathy is being extended more and more thoroughly. Such a description would take us into the field of memetics, a generalisation of natural selection. But I have to wrap this up, so...

There were two major effects pertinent here of the advent of transport and breakdown of the small social group structure:
1. the people you generally see are strangers
2. the people you live near underwent different socialisations to yourself.
We have no genetic capability to deal with either of these problems, because those problems did not exist when we evolved. In the past, strangers were less likely objects of empathy, since they were more likely to be competitors and less likely to reciprocate altruism. (It is also seen in tests that in-group members' empathy responses to out-group members is less than to other in-group members, suggesting that counter-empathetic responses are not contingent.)

The social group can no longer be regarded as family and neighbours. Families are generally distributed, and neighbours often unknown to us. Social groups are virtual and malleable: we work with one set of people, live with another set, socialise with another set, etc. These days, people are likely to have friends and relatives who live or come from a different country. There is no even vaguely-defined boundary you can draw around yourself and say: this is my social group. Your virtual social group encompasses the globe and is overwhelmingly diluted by strangers.

How does one 'do morality' authentically in such a world? Individualism is the accepted political answer. Care for yourself, your spouse, your children, and otherwise do what you can to prosper. Harm, within lawful bounds (and lawful bounds are also malleable) is okay in the name of self-fulfilment. This is not a solution: we are hard-wired toward empathy and altruism and to suppress that is to make ourselves less than human. If we engage in behaviour toward others knowing or being able to know, via empathy, that it would harm us, we are hypocrites, and hypocrisy is the fundamental immoral act. That we accept, through our natural capacity for socialisation, antisocial behaviour as our social conditioning, it is only insofar as we have been morally perverted.

The polar opposite is to accept the world as our social group, and this drives the trend to extend (and codify in law) altruism and empathy to more and more kinds of stranger, who are now less strange because they enter into our virtual social lives. But this is hardly a perfect fit for our natural morality either. The biological drive to do no harm is as grounded as ever but, when your social group is the world:
1. harm of others is continuous and cannot be consistently opposed (empathy fatigue)
2. reciprocal altruism is no longer useful, and yet we are surrounded by those we can help.
Our biological tendency to oppose harm and help others is no longer feasible, which raises a potentially infinite number of moral conundrums and from which we derive a (philosophically porous) distinction between morally obligatory and morally praiseworthy acts.

If you step in to stop a stranger being harmed, it is morally praiseworthy because we are naturally inclined to do this under one set of circumstances but cannot do it at all times in our current set circumstances. It must be permissible to not act end harm. Likewise if we help someone, it is morally praiseworthy because we are morally driven to help others who may help us but the person we help will likely never (have the opportunity to) reciprocate. It must be morally permissible to withhold kindness.

The true moral condition of the global virtual social group is then:
1. do not harm others to benefit yourself (unless you can absolutely get away with it)
2. help others as you see fit
3. oppose harm as you see fit

i.e. 2-3 are now optional capacities. The existential problem is that, if I only have a capacity to do good/oppose harm and no directive or reason for doing so, and I cannot do so in every circumstance, how do I choose when to do good/oppose harm? There are many factors, but the only unaltered fundamental rule we have is: do not be a hypocrite. If you bemoan someone standing by while another is harmed, you oblige yourself to step in when someone is harmed. The moral rule is contingent ("if you...") on your socialisation -- which is not that of your neighbour -- and self-regulating ("you oblige yourself").

It is perfectly acceptable therefore to do what you will that harms no other (hedonism), to never step in to help others or resist others who harm, so long as you are liberal (allow others their hedonism) and never expect others to step in to help you or oppose harm toward you. That is a perfectly valid moral frame of reference, and how one "ought" to behave is assessed only within that frame of reference. It is perfectly feasible that a particular socialisation could and has produced this sort of moral frame. It is equally feasible to for someone to be socialised such that they dedicate their lives to helping others and opposing harm, with no expectation of reciprocity, and moral decisions taken by such a conditioned person would be with respect to that socialisation, i.e. that moral frame of reference.

There cannot be, then, a meaningful objective moral universe. Morality, viewed (correctly imo) in this bottom-up way, cannot have top-down rules because that is not what morality really is. Even the nearest to a fundamental rule -- do not be a hypocrite -- is not objective but statistical: there exist many for whom this is a practical impossibility because they lack empathy. They simply cannot equate the harm they do with the harm they'd feel if roles were reversed. Such people must be allowed their own moral frames of reference, because if you were in their shoes, that's what you should expect.

Moral philosophy is overwhelmingly concerned with questions around how one "ought" to act in a given hypothetical situation without reference to the moral agent's socialisation, state, capacity, or any other details that a real person can use to decide whether and how to act. It is pretend-morality precisely because it denies relativism, pluralism, and existentialism.

But these do not constitute a nihilism. Antisocial behaviour still exists, is still hypocritical, and is still sub-human. Those who pool wealth and resources at others' expense, who in small social groups would have been attacked, exiled or lynched, are still moral (i.e. social, not rational) failures. One is not obliged to oppose the harm they do, because one is not obliged to oppose every harm committed in one's lifetime, but if one is inclined to oppose harm, it is morally meaningful to oppose that harm. Likewise racists, misogynists, rapists, drug dealers, paedophiles, political opportunists, looters, etc., etc. You are justified in ignoring or opposing them, so long as you yourself are not a hypocrite. To oppose racism and be racist would be antisocial on all grounds.

I believe that a more thorough description along those lines is sufficient to describe the why, when, how and who of morality, and that top-down or bottom-up a priori approaches are fundamentally in error precisely by not factoring in evidence of the why, when, how and who and, erroneous or not, are unjustifiable on the grounds that it can add nothing, only subtract.

I appreciate the above is too long for this sort of platform, but I promised the opportunity to give me a thorough philosophical kicking so here it is. This is not a complete picture even of my thinking, but could be fleshed out ad hoc.