Dolphins evolved from land animals that returned to the ocean due to environment conditions.
Dolphins are animals perfectly adapted to the aquatic life with adaptations product of million of years of evolution but still preserving a set of features of their life on land.
Cetaceans and dolphins, in consequence, descend from a terrestrial ancestor which after a slow evolution process returned to live in the ocean.
It is believed that dolphins follow similar types of humans patterns in their communication efforts.
There are many studies out there that compare the size of brains from one living thing to the next.