I don't get the part about dolphins being so smart.
You’d have to have read Douglas Adams.
What Happens When We're No Longer the Smartest Inhabitants of Earth?
It probably depends a lot on how we define “smartest.” Humans do a lot of dumb things that are counterproductive to the species that other species do not do. I think we may be the only species ever on the plant that has the ability to render itself extinct.
Artificial Intelligence is already pretty good at doing things that can be quantified faster than humans. And as Mitch points out, augmentation of human physical capabilities, or correction of shortcomings has been around for a long time, and will possibly play a greater role in our futures.
But where humans excel is in the ability to consider, imagine, dream what cannot
be quantified/explained in any way. A.I. is exceedingly poor at imagination, love, faith, and similar emotional and illogical trains of thought.
I think our ability to consider what has never been previously considered is what makes us “smart.” And on that front A.I. / computers are exceedingly poor. Computers are very good at taking a human thought from beginning to fruition. But I think they’ll still need that initial thought, that originated with a human, for a long time to come.
But in the bigger picture of course humans will not be the smartest life form on this planet forever. Sooner or later some other species will conquer humans. (Personally the viruses get my vote.) I just don't think a human invention (e.g. computers) will be it because by definition it will have to contain the very human flaws that will allow humans to be conquered.