The Octopus, the Dolphin and Us: A Great Filter Tale
Some great food for thought here.
Dolphins are certainly intelligent. And they are certainly far from our line. It seems hard to find a definite answer, but it seems that the last common ancestor of humans and dolphins was a small mammal existing during the reign of the dinosaurs. Humans and dolphins have been indicated by red rectangles, and their last common ancestor with a red circle.
This red circle is well before the K-T boundary (indicated by the dotted line), hence represents a mammal living in the literal shadow of the dinosaurs.
We can apply a convergent evolution argument to this common ancestor. Thus, assuming that subsequent evolution was somewhat independent, getting from that common ancestor to dolphin level of intelligence is something that can happen relatively easily.
Can we go further? Well, what if we applied the argument twice? Let’s bring in the most alien looking of the high-intelligence animals: the octopus.
More: The Octopus, the Dolphin and Us: A Great Filter Tale - Less Wrong