Dear Peyton,

When you think about a whale, you probably picture an enormous sea creature without legs. But what if I told you the first whale had four legs and could walk on land?

I talked about whales and other marine mammals with my friend Kevin Turner. He teaches marine biology at Washington State University.

He told me marine mammals have horizontal tails because they flex their bodies up and down to move. Fish flex their bodies side to side. Scientists think it has to do with the way different animals evolved.

“Life began in the oceans then wandered out onto land,” Turner said. “Then some organisms wandered back into the oceans and readapted to that aquatic environment. So, their evolutionary history of having lived on land comes with them.”

The marine mammals that moved back to the ocean full time include whales, dolphins, porpoises and sea cows—which are also called manatees and dugongs. Seals, sea lions and their relatives moved back to the ocean, too. They’re marine mammals that live on land some of the time.

More than 500 million years ago, fish evolved from worm-like ancestors. They wiggled side-to-side to get around. So, fish inherited strong side muscles and bodies that flex side-to-side. Eventually, they developed tail fins to help them move better.

But life took a wild turn for one group of ancient fish. Scientists estimate that about 400 million years ago, they began to evolve to live on land. They used strong, fleshy fins to walk. Over time, animals with stronger, more leg-like fins were able to survive better on land. They had babies with strong legs, too. That’s how you and I wound up with our strong legs.

But legs are just part of what it takes to move on land. Think about how a cheetah runs. It bends its spine to stretch out its front legs and push the ground hard with its back legs. As it runs, its spine flexes up and down.

That’s how marine mammal spines move, too. When their ancestors returned to the ocean, marine mammals kept that motion.

Let’s look at whales for example.

About 50 million years ago, a land mammal called Pakicetus hunted in shallow waters. It had four legs and webbed feet. It had a long snout. It was about the size of a wolf.

A resin cast of a Pakicetus skeleton at the Canadian Museum of Nature, Photo: Kevin Guertin/Wiki

Even though it had legs, the bones in Pakicetus’ head were different from the head bones of land mammals. They were like the bones whales have in their heads today.

That’s because Pakicetus was the first whale.

Over about 10 million years, whales moved back to the water full time. Their front legs adapted into flippers. Their back legs disappeared. They developed a wide tail to swim faster and push up to the surface to breathe air.

“These organisms already had strong back and belly muscles to flex the spine while running,” Turner said. “So, when they start wandering back into the oceans, those are the muscles they had to work with.”

Other marine mammals have similar evolutionary histories. The cool thing about evolution is that sometimes an animal’s body tells the story of what life was like for the animal’s ancestors.

For marine mammals, it’s a whale of a tale.


Dr. Universe