The rule could be anything, as funny or as serious as you want. The universe will progress in a similar way that it has up until this point, unless your changed rule prevented it from doing so.
Some examples might be:
- The invention of currency is not allowed.
- Iron is slightly less stable.
- The Ancient Greeks are able to cultivate Silphium, which does not go extinct now.
I think what we have is something much better. A flexible morality which is taught from generation to generation and adapted as needed. An absolut moral code wouldn’t work.
Funny enough, that’s exactly the way it actually is. And I believe this system still works; not that we could change it anyway.
Still, this is how it is. It discourages immoral actions even when nobody sees them happening, because the person doing them still knows and feels bad/shame/worries.