Perhaps they built the civilizations, realized the pointlessness, went back to frolicking in the water and ended up with flippers from lack of use.

My roundabout point is, why do they have to build civilizations? Because we humans think that's the pinnacle of advancement? Perhaps the dolphins have a difference in opinion. Perhaps they're debating this very idea over some delicious herring: "do you think humans are intelligent?"

"Mmm, could be. But they keep scurrying around building pointless crap, so I don't know."

"Yeah, good point. More herring, darling?"

Looking forward, these cross-coastal cities and villages, and even underwater towns, may come. We each have our part to play, humans with tech, dolphins with point.