Indeed dolphins and all cetaceans like whales and porpoises evolved from animals that lived, reproduced and died on land. In fact, their closest relative alive is the hippopotamus, which in turn also relates them to the artiodactyls.
The common ancestor of dolphins and hippos is an animal called Pakicetus that roamed the Earth about 50 million years ago. Pakicetus had a four-leg functional anatomy, elongated snout, flexible neck and a very long tail, morphology that laid the foundation for the subsequent appearance of cetaceans.
Millions of years later, creatures like Ambulocetus and Kutchicetus emerged with body characteristics increasingly adapted to the marine world.
While the former is the commonly accepted theory, others suggest that cetaceans descend from the Mesonikia, prehistoric animals that lived on Earth about 55 million years ago. Its appearance was similar to that of a tiger, and the evolutionary change began while searching for food in the disappeared sea of Thetis.
The explanations about the evolution of dolphins are:
One. The scarcity of food on land forced some coastal populations to venture into the ocean for food, which gradually adapted their anatomy to their new habitat.
Two. The other theory has to do with the climatic changes of the era, which forced them to enter the water to get a convenient place with satisfactory temperature and abundant food.
Several indicators confirm that dolphins evolved from terrestrial animals, and these are the most evident:
1. They have a robust bone structure rather than a cartilaginous skeleton like sharks or fish.
2. They are mammals because they feed their young with breast milk from the mother.
3. Dolphins breathe air like us. They can not breathe underwater like fish because they have lungs instead of gills.
4. Dolphins have vestigial bones that once were legs and pelvic bones where they were attached.
5. These cetaceans are warm-blooded.
Even that some scientists still debate about the dolphin ancestors and their relation with other species, there are enough probes to confirm that these cetaceans undoubtedly descend from terrestrial animals.