My contention is that morality has been consistent from the get go. People have always had the free will to choose that consistent morality or to reject it in favour of self-interest. If you are going to go back to quoting the OT and the instances of genocide and stoning then I would agree that that can't actually be of God in spite of their protestations that they were.
I don't need to reference a book I find to made of fairy tales, GDR. We can look to actual evidence to see that different groups of people have different standards of ethics/morality. It's not like they knew they were being immoral and chose otherwise: they actually thought what they were doing was right and just. I hate to trot out Buz, but look at how he trivializes slavery and says that "life was better for them here, as slaves". This is someone now, here, in the 21st century Imagine how slaveholders viewed it back then?
Are you saying the Mayans actually knew better, that they actually knew what they were doing was immoral but continued to do it anyways?
Are you telling me that cannibal tribes actually realize they are being immoral, but continue their traditions anyways?
Are you telling me that racists realize hating someone for the color of their skin is actually immoral, but continue doing it anyways?
So your idea that people just choose to be immoral as a whole doesn't jive with what we actually see. Yes, there
are people who do
choose to be immoral, but we don't judge entire civilizations and categorize their actions based on those individuals, now do we?
"Science is interesting, and if you don't agree you can fuck off." -Dawkins