If you look at avg skill level, yes, there has been evolution. The biggest evolution is in the striking. Multiple strike combos are much more common. Having said that, at the championship level, I don't think there's been some big evolution as some try to suggest, wrestling & BJJ hasn't evolved nearly as much. It isn't like wrestlers of today are pulling off some advanced wrestling technique that GSP couldn't. For all the talk about evolution, there aren't any techniques that the current champs do that couldn't be done in yesteryear
Francis - Francis hits hard, its not like he's introducing never before seen striking techniques. His BJJ is definitely suspect, his defensive wrestling looks improved, though its just based off 1 takedown attempt by old man Stipe
Jan - solid all around, nothing revolutionary
Adesanya = Anderson (I would say Anderson had better BJJ, bigger threat off his back)
Usman - I'm not saying Usman would lose to GSP, but technically GSP is better IMO. Usman still could win, he hits hard, is hard to take down etc, but I think GSP is more skilled
Oliveira - there are plenty of LWs who would be a bad style matchup for him
Weight classes below 155 weren't present in the UFC until more recent times.