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.