The first cetaceans on Earth were not much like the whales, dolphins, and porpoises of today. They appear in the fossil record in the early Eocene (about 55 million years ago). Cetaceans evolved from land animals that made a gradual transition to water. The earliest cetaceans were semi-aquatic, living along the shores of the ancient Tethys Sea. Using all four legs, they probably half-swam, half-crawled through the water like hippos. By the mid-Eocene (about 45 million years ago), cetaceans became more seaworthy, with long noses, blowholes, webbed toes, strong tails, and flexible backbones for swimming. Front legs gradually evolved into flippers, and hind legs got smaller. It wasn’t until the Oligocene (about 35 million years ago) that hind legs disappeared into the body as remnant (vestigial) bones. Cetaceans were fully aquatic by the late Eocene and eventually developed the big brains and thick blubber that we see in today’s whales.

It might seem odd to group Cetaceans (whales) with Artiodactyls (hooved animals such as pigs, deer, camels, llamas, and hippos). But that is exactly what the combination of their scientific names into Cetartiodactyla is about. For some time, scientists had proposed that whales descended from land mammals, with the focus on the hooved fossil Mesonychia. In the 1990s, DNA sequences from many different genes revealed a closer relationship between whales and hippos than between hippos and any other hooved mammals. While it is not yet known what common ancestor whales and hippos share, the newly discovered relationship is getting attention. It appears that whales and hippos may have branched off from Artiodactyls as long as 60 million years ago. Scientists have proposed a new group called Whippomorpha (wh[ale] + hippo[potamus]; morphe = form) to include whales and hippopotamus and exclude other hooved animals.