Let's make the further assumption that our common ancestor with dolphins was dumber than the modern octopus. This doesn't seem a stretch seeing how intelligent the modern octopus can be, how minor in terms of ecological role the common dolphin-human ancestor must have been, and seeing the stupidity of many of the descendants of that common ancestor.

Could you expand on the second point, as to why it must have a minor ecological role, and why this means it would be dumber? I know little evolutionary theory, and would appreciate the explanation. Cheers.