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.
Oxygen now stably bonds with three hydrogen atoms, not two. The new formula for water is H3O.
Good luck with any kind of life form that we would recognize as being alive evolving.