"Their work produced the first description and statistical tests of the pattern of change in brain size relative to body size in cetaceans over 47 million years. They found that encephalization level increased significantly in two critical phases in the evolution of odontocetes.

The first increase occurred with the origin of odontocetes from the ancestral group Archaeoceti nearly 39 million years ago, and was accompanied by both an increase in brain size and a decrease in body size. This change in encephalization occurred with the emergence of the first cetaceans to possess echolocation - the processing of high frequency acoustic information within a perceptual-communicative system used by modern dolphins and other odontocetes, Marino says. The second major change occurred in the origin of the superfamily Delphinoidea (oceanic dolphins, porpoises, belugas and narwhals) by about 15 million years ago. Both increases probably relate to changes in social ecology (the animals' social lifestyle) as well, Marino says. "

http://news.bio-medicine.org/biology-news-3/Emory-study-details-dolphin-brain-evolution-for-the-first-time-14129-2/

7-8 million years is when our ancestral line split from chimps. Cetaceans have been big-brained longer than primates, but not a long time at all by solar system habitability standards.