Not a problem at all, the codes of conduct of most human societies have evolved over time. Examples:
Human sacrifice used to be practiced routinely and was a major "sacrament" of some religions, it now is considered "wrong" by most contemporary societies.
We could start a debate about how routine this actually was. But I'll digress.
The real issue here is this implies that something is right or wrong based on how common it was. I think you will find that this is not a position you can follow through to its logical ends. Since everyone lies, should we legalize perjury? Of course not. Does the fact that everyone lies mean that it is considered morally acceptable? No. Also it sounds as if you are saying that human sacrifice wasn't wrong because it was common. (define common) And this would only lead us to conclude that it isn't objectively wrong today. It simply became non-preferred because morals evolved?
This fails in a number of areas. For one. You can't evolve an abstract concept such as morality. No more than you can evolve logic. You might evolve your understanding, but this is hardly how you are using the word evolve in this context. For logic to work there has to be a standard to evaluate when someone is being illogical. Likewise with morality. There is nothing biological to define your claims, or defend the evolving of morals. It presupposes that what you consider right or wrong today, to be more evoloved than what a primative culture considered right or wrong then. It is entrely subjective, yet relies on you evaluating the past against a backdrop of objective truth. Not to mention it has overtones of chronological snobbery. We know that while some cultures were practicing this, there were others that did not. And were abhorred by it.
Obviously I would contend this is not logical thinking. Human sacrifice is objectively wrong, and the reality is that these cultures were violating the objective nature of this moral truth. Otherwise you are left with accepting that human sacrifice is simply a prefence. As Jac would say, you like chocolate, I like vanilla. That there is no inherent wrong in sacrificing other people. In fact your position nearly justifies it.
I understand where the concept of "objective morality" comes from. My problem with OM is that way too many people claim they know what OM says is "right" & "wrong" but cannot give a clear and definitive answer as to just how they gained their knowledge of what is right & wrong according to OM.
You are making a fallacy that because one learns moral truth, that it is therefore not objective. That would be like saying there aren't objective facts in math if someone has to be taught what they are. Most people can't tell you why there are objective standards to such things. Also, OM is not to the exclusion of subjective morality. Obviously, breaking the speed limit is not an objective truth. Since the speed limit is subjective to the culture.
Do you think they still wouldn't leave their dead or seriously wounded behind if bringing them along seriously impaired the ability to complete the mission?
Yes. In fact they do. You are trying to make it sound as if the only reason they have this code is because it is convenient. I dare you to say that face to face to a Marine who risked his own life saving another. My friends son just got back from Afghanastan and was awarded the Silver star for such behavior. I can make the arrangment?
Slavery used to be practiced routinely, it now is considered "wrong" by most contemporary societies.
Again your argument fails. First you are giving a sweeping generalization to slavery. That is you are defining all slavery throughout history likely within the view of the abuses within the slavery system that led to the US civil war. Slavery is considered 'wrong' because of the atrocities we associate with it. The bible even speaks against abuses within slavery, and condemns them. So, first it is wrong to make this generalization. The fact is, abuses within slavery, whether accepted, practiced or not, were wrong then. It was objectively wrong. Because it was objectively wrong, people were able to act upon this objective truth, and stand against the subjective preferences. Otherwise you are forced to admit that slavery was not wrong. It was again, a matter of preference.
Another example is child sex slavery. It is prevelant today, and in fact has likely grown worse than it ever has in history. As has sexual deviancy of all sorts. If morality is evolving as you claim, then the evidence based on criminal activity is a hand grenade to your claim. While some practices such as human sacrifice maybe frowned on, other corruption is spreading and growing like wild fire.
Nomadic human societies abandoned sick or injured members, at least until their technology advanced such that bringing them along no longer imperiled the group.
I am curious as to how you researched this?
In the case of social animals, success of the group is a significant factor in the survival of the individuals making up the group.
This in itself presupposes that survival has inherent value. My friend, you are standing on the ground of objective truth to attempt to undermine it.