I'm sure some here know about what happens (or what used to happen) to the elderly and/or very young in some of the more traditional societies - the ones that have survived despite the encroachment of 'civilisation', and which no doubt existed for long periods in our past - most of such in fact. That is, that if any individuals weren't seen as essential to the group survival and they might have caused the group not to survive, then they were 'jettisoned' - a more benign word perhaps for often what did happen to them. This often applied to any elderly who were not really capable of looking after themselves alone, or where the very young could not be taken care of successfully given the resources available - birth of infants too close together perhaps. And any who know much about animal behaviour know this is true of many non-human species too - whether it is those that will kill the offspring of a future partner (so their genes lived on) or where those deemed sickly at birth were simply abandoned or killed - being a waste of resources.
So, why do we expect any objective morality to occur in human life, especially when we have cases such as hatred towards homosexuality, trans people, or those deemed worthy of death for certain crimes, and where appropriate morality might equally be seen as 'expedient to our purposes' - as to which some tend to accept these and others not. With the religiously minded often determining as to such things. Unless one does have the view that we just are not evolved animals - like all other life.
And isn't it the case that we might deem other such things necessary (or expedient) - as to human survival in the future - so as to throw objective morality into the bin?
PS I did already know about much of the above, but the thread was initiated by some of the things in the Jared Diamond book I'm currently reading - The World Until Yesterday: What Can We Learn From Traditional Societies?