Final answer:
Cetaceans have been on a unique evolutionary path from other mammals for approximately 50 million years, with their brains reflecting increased size and complexity due to their highly adapted aquatic lifestyle.
Step-by-step explanation:
Cetaceans, which include dolphins and whales, have been on an independent evolutionary trajectory from other mammals for about 50 million years. The group known as Cetacea is believed to have diverged from other even-toed ungulates around that time. Cetaceans share a common ancestry with modern animals like the hippopotamus. Unlike other mammals, cetaceans have adapted to an aquatic lifestyle, an example of a derived trait in their phylogenetic lineage.
The brains of cetaceans have also followed a unique evolutionary path. Their relatively large brain size coupled with high levels of cortical folding have led to complex behaviors and high intelligence. Brain evolution in cetaceans is notably distinct in contrast to other vertebrates, and it is particularly intriguing because of the co-evolution of enlarged brain size and complex societies in this group of maritime mammals.
It's important to recognize that while all mammals show increased brain size compared to other vertebrates, the cetacean brain has gone through significant specialization due to the demands of their marine environment. From the point of divergence, cetacean brain development has progressed alongside their morphological and behavioral adaptations, culminating in the intelligent and socially complex animals we are familiar with today.