Scientists are story weavers. They pull together clues and evidence to create tales that sometimes seem fantastic. Is it a tall tale or a fact that animals who once roamed on land found their way to live in the ocean? Scientists think that happened with the largest animals that have ever existed on the planet — the cetaceans. Cetaceans are aquatic mammals that we know as whales, dolphins and porpoises. Scientists have put together the clues to tell the tale of how their ancestors left land to live in the water. 
Cetaceans are large animals. The largest cetacean is the blue whale. It can be longer than 90 feet. But their ancestors were small, deer-like creatures called Indohyus (in-doe-HI-us). Scientists know this because of the fossil record and other clues. Indohyus once crawled and waded through swamps on four legs. When scientists X-ray a dolphin’s front flippers, they see bones that look like hands. When they X-ray a dolphin’s tail, the bones look like legs. The bones are a clue that dolphins evolved from animals that once lived on land. 
About 50 million years ago, Indohyus, an animal that looked like a deer and was the size of a cat, lived in what is today India and Pakistan. Indohyus was an even-toed ungulate, meaning it had feet that look like those of giraffes, horses and pigs. The Indohyus of 50 million years ago (in the Eocene age) has a distant relative living today. It is the deer-like water chevrotain, or African mouse-deer. The mouse-deer lives in central and southern Africa. 
Scientists studied the fossils of Indohyus and learned they had dense bones. This was a clue that they spent a lot of time in the water. The hippopotamus is the closest living relative of whales. It lives on land and in water. Its dense bones help it to walk along the bottom of lakes and rivers. As Indohyus evolved, or changed to adapt to its environment, it spent more time in the water. At first, Indohyus ate a plant-based diet. Its descendants adapted to spending time in the water and began to eat meat. Scientists think this meat-eating animal, Pakicetus (pak-ee-cee-tus), sat in the water and waited for prey. Like the crocodiles of today, they waited for animals to come to the water to drink. Then they pounced.
About 48 million to 42 million years ago, the first ancestors of whales appeared and began to go into the ocean. Their fossils are found around the world, from Pakistan to the eastern U.S. and Peru. Researchers think these animals looked more like sea lions. In the next evolutionary stage, creatures that resembled sea snakes evolved. The largest were the size of a bus and had small legs and front forearms. These forearms evolved into flippers. Another group of creatures had a fluke at the end of their tails. These are the same flukes, or flipper tails, found on the cetaceans of today. 
About 41 million years ago, baleen whales emerged. About 34 million years ago, the ancestors of orcas, dolphins, porpoises, sperm whales, beaked whales and belugas appeared. The descendants of these creatures began to have streamlined heads and noses. They came up from underwater to breathe air. Their noses moved closer to the back of their heads. This made it easier for them to breathe. Cetaceans come up above water to breathe. They have blowholes on top of their heads that open when they are above water. This opening allows them to take a breath of air. When they go back under water, the blowhole closes. The blowhole method of breathing is another clue that cetaceans once lived on land.
The story of how dolphins came to live in the oceans is a long one. It is a tale well told. Scientists have found many clues by looking at fossil records of different eras. By putting together clues and pieces of evidence, they discovered an amazing story. Animals that once lived on land now live in the ocean.