The moral emotions and instincts of humans are not limited to kin selection or reciprocal altruism. Nor are they identical to those of langurs (though some of them may be homologous across primate species). We are our own species, with our own evolutionary history. That history has made us monogamous, which excludes the kind of infanticide practiced by some polygynous species. It just doesn’t fit within the logic of our societies.

We will always construct some symbolic system to explain our moral emotions and practices, but there is HUGE variability in the nature of those systems. Some involve a God or gods, others just animistic spirits or forces, and yet others impersonal forces like karma and dharma. Ethical humanists abstract even further to the ethical principles these spiritual forces are meant to underwrite, demonstrating why they remain operative across cultures regardless of which metaphysical construct you use to try to justify them.

The point is, the moral instincts came first. The metaphysical construct is just a post hoc rationalization. It isn’t real, and we don’t need it any more than Dumbo really needed his feather.