Cetaceans include whales, dolphins, and porpoises. While they appear drastically different from their land-dwelling ancestors, fossil evidence clearly shows they evolved from a four-legged mammal that lived on land about 50 million years ago.
Here's why:
* Shared anatomical features: Cetaceans have similar bone structures in their flippers and tails to the legs and feet of artiodactyls.
* Genetic similarities: DNA analysis shows a close genetic relationship between cetaceans and hippos, making them the closest living relatives of cetaceans.
* Fossil evidence: A series of fossil discoveries have shown a clear evolutionary path from land-dwelling artiodactyls to the fully aquatic cetaceans we see today.