One of my favorite little facts: many scientists believe whales may have evolved from relatives of hippos; see also here. (They used to think they may have evolved from wolf-like creatures.) You can watch this video to see the evidence.
It makes sense: hippos stay under water for long periods. You can imagine a hippo wandering farther and farther out, down the river, the fateful step across the continental shelf, falling into the dark of the deep -- something brand new and terrifying, yet a return of sorts.
Many of the missing-link creatures are weird walking-whales. Steadily the arms shrink and become flippers (with the skeleton of a land mammal's limbs still inside), the legs come together and fuse (this is why cetaceans (whales, dolphins, porpoises) have horizontal tail fins). They swim like otters, by undulating their spines up and down. Fish, on the other hand, have vertical tail fins, and swim by undulating from side to side.
The return to the water has allowed cetaceans to achieve great size (the blue whale) and intelligence (dolphins). No one is quite sure how intelligent dolphins are. The interesting thing is that their brain structure has evolved in quite a different manner than primate brain structure. Dolphins are second only to humans in brain to body mass ratio. Their lobes are shaped differently, although, like humans, they have a large frontal lobe.
EEGs show alternating hemispheric assymetry in slow waves during sleep, with occasional sleep-like waves from both hemispheres. This result has been interpreted to mean that dolphins sleep only one hemisphere of their brain at a time, possibly to control their voluntary respiration system or to be vigilant for predators.Wikipedia.
Dolphin brain stem transmission time is faster than that normally found in humans, and is roughly equivalent to the speed found in rats. As echo-location is the dolphin's primary means of sensing its environment -- analogous to eyes in primates -- and since sound travels four and a half times faster in water than in air, scientists speculate that the faster brain stem transmission time, and perhaps the paralimbic lobe as well, support speedy processing of sound. The dolphin's dependence on speedy sound processing is evident in the structure of its brain: its neural area devoted to visual imaging is only about one-tenth that of the human brain, while the area devoted to acoustical imaging is about 10 times that of the human brain. (Which is unsurprising: primate brains devote far more volume to visual processing than almost any other animals, and human brains more than other primates.) Sensory experiments suggest a high degree of cross-modal integration in the processing of shapes between echolocative and visual areas of the brain.
That's right, dolphins never fully sleep: it appears half their brain is on all the time. It is amazing to imagine what kind of picture of the world dolphins (and other cetaceans) create with all of that brain processing power devoted to "acoustical imaging" (producing images of the world from the echoes of their clicks bouncing back toward them). Apparently there's some innate math involved, as dolphins figure out the shape, size, distance, speed, and texture of objects based on the echo of the click, which they apparently read through their peculiarly spaced teeth. I just get AM radio.
What about that narwhal and its gigantic tooth sensor/weather station?