Around 50 million years ago, certain four legged land creatures started spending more and more of their time in water. (An animal that there are fossil records of from around this time named Pakicetus may represent this initial stage). For some reason, this change must have suited them, as they gradually evolved, and their bodies changed form, so that they eventually lost the ability to move onto the land at all. An interim stage in this development is represented by the Protocetids and examples of the first exclusively aquatic dwelling form are the Durudon and the 60 foot/18 metre long Basilosaurus. They lived some 38 million years ago.
Illustration source: Cetacean Palaeobiology, University of Bristol, Department of Earth Sciences
About 30 million years ago, Squalodon appeared; a toothed whale. Although it probably was not a direct ancestor of dolphins, it appeared to use echolocation, a key attribute that would become a vital skill later to modern dolphins.
During the early Miocene (about 20 million years ago), echolocation developed in its modern form. Various extinct dolphin-like families flourished belonging to Kentriodontidae, which were small to medium-sized toothed cetaceans and thought likely to include ancestors of some modern species. Kentriodontids date to the late Oligocene to late Miocene. Kentriodontines ate small fish and other organisms; they are thought to have been active echolocators, and might have formed schools.
The genus Tursiops, which bottlenose dolphins belong to, first appeared in the fossil record about five million years ago.
Film clips about the evolution of dolphins and whales
For some dolphin related fiction, try this:
In the novel Dolphin Way, the dolphins have their own culture and language with oral histories that explain their racial memories of how they and humans went their seperate ways, with radically different results.