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… Uh… Hm… 🤔
What does this do?
maybe it would look like a reverse snowfall underwater if the ice is buoyant? I imagine if the ice stayed at the bottom it would be interesting evolution wise since most things survive the cold by being at the bottom.