Once upon a time whales used to walk on four legs

Whales are expert swimmers and perfectly adapted to living underwater. But unlike fish, they do not lay eggs and are actually mammals who produce babies. Once upon a time, whales and a whole category of marine mammals, had lived on land and walked on four legs! Thankfully, that was a solid 50 million years ago. We are going to trace that journey from land to water today.  

Who were the ancestors of whales? 
Back in the age of dinosaurs, lived the Pakicetus, a goat-sized, four-legged creature that scientists think was one of the ancestors of cetaceans, i.e. the group of marine animals including dolphins and whales. While it did not at all look like a dolphin, this prehistoric goat was at ease in water. Scientists have traced it to lakes and riverbanks of what are now India and Pakistan. The Pakicetus hunted small land animals and freshwater fish. Eventually, relatives of this animal started venturing further into water. 

Which were the first water mammals? 
One related species of  Pakicetus was the Ambulocetus. It lived in or near river estuaries about 50 to 48 million years ago, swimming out to the ocean and occasionally coming to land. It was much larger than goats. It had large, flipper like feet, and also used its tail for swimming. Then 40 to 33 million years ago came the Dorudon. It was about 5 metres long, with both flippers and tiny hind legs. It gave birth underwater. Over the next 10 million years, it evolved completely from a land mammal to a water mammal. This sounds like a long time, but terms of evolutionary history, it’s surprisingly fast. Imagine humans growing extra arms or breathing underwater in 10 million years!  

What happened to the eating habits of whales?  
The descendants of Dorudon have long evolved into modern whales. About 34 million years ago, a group of whales again evolved, this time in terms of food habits. They developed flatter skulls and feeding filters in their mouths. They came to be called baleen whales, and include present day blue whales and humpback whales. These creatures don’t have teeth! Other whales kept their teeth, such as orcas and sperm whales, as did the dolphins and porpoises.