Have Whales Always Been Fully Aquatic Mammals?

Seeing a whale in the open ocean can be a once-in-a-lifetime event, but if you had been around 50 million years ago, you wouldn't have had to leave dry land for the experience. That's because the world's first known cetaceans -- the group of marine mammals that includes whales -- were landlubbing creatures belonging to the extinct genus Pakicetus. The truth is that Pakicetus didn't look much like today's whales: It was about the size of a goat, walked on four legs, and didn't care much about water -- except for catching prey there. It would take the evolutionary process about 10 million years before the descendants of Pakicetus developed the necessary traits to move from a life on land to one in the water. It was then that their bodies changed and they began filtering food through the water, allowing their diets to drastically improve and enlarge. In time, they grew into the huge creatures that roam today's seas, such as humpback whales and blue whales.

A whale of a tale:

• Blue whales are the largest animals to have ever lived; a blue whale's tongue can weigh as much as an elephant.
• The Arctic-dwelling bowhead whale is covered by blubber that can be up to 20 inches (51 cm) thick.
• The sperm whale has the biggest brain of any animal, weighing up to 20 pounds (9 kg).