Marine Mammals Once Walked on Land

“Basilosaurus was indeed a whale, but one with two delicate hind legs, each the size of a three-year-old girl’s leg, protruding from its flanks. These winsome little limbs—perfectly formed yet useless, at least for walking—are a crucial clue to understanding how modern whales, supremely adapted swimming machines, descended from land mammals that once walked on all fours.”
–Excerpted from Valley of the Whales by Tom Mueller, National Geographic Magazine, August 2012.

All animals that inhabit the land, from worms and slugs to insects, reptiles, birds and mammals, evolved from creatures that emerged from the sea. Animals with back bones, vertebrates, descended from large salamander-like animals that crawled from the water some 360 million years ago. That little salamander-shaped creature evolved into a huge variety of animals. Eventually, the evolutionary web included mammals. But one form of mammal returned to the sea, eventually adapting to a fully aquatic life. Whales.

So, here’s how land mammals evolved into whales
Between 49 and 50 million years ago a mammal called a pakicetus lived along the shores. Fossil bones suggest that it may have lived like sea lions do today—resting and giving birth on land, but swimming and even eating in the water for much of its life. Because short, wide fins are better than long arms and legs for swimming, the pakicetus adapted to its environment over time. Eventually, the arms and legs shrank, fingers and toes became webbed fins, and the spine grew long and powerful. Nostrils moved up on the head and became a blow hole. And Voila, by 38 to 36 million years ago, pakicetus was a whale. . . or almost. That’s when scientists believe it had become a fully aquatic mammal and a precursor to modern whales which appeard a couple of million years later.

(Side note: Dolphins and porpoises belong to the same Order as whales – Cetacea—so they are often referred to as whales in the scientific world.)

Seals, Sea Lions and Walruss, too!
When  Alannis’s father shows her the seal fin, she sees how much the bones look like a skeletal hand. She was five, but even she was able to make the connection. Pinnipeds (seals, sea lions, walrusses) evolved from land animals. Bear-like animals, in fact. Not hard to grasp when you consider how well adapted polar bears are to swimming and hunting underwater. Perhaps the drive for pinniped evolution began like that.

One hundred and fifty years ago, Darwin wrote that upon watching bears swim, it was easy to imagine that, over time, a similar creature could evolve into a whale. You were close Sir Darwin, so very close.

How do we know this stuff?
Well, it’s been written about in lots of science journals (which we keep tabs on), but when it comes to things written in normal language that everyone can understand, we trust information from universities, museums and respected publications like National Geographic, Smithsonian, Scientific American, and sources like that.

For some of the facts in this article, we would like to thank:

Tom Mueller and National Geographic:

Research scientists Natalia Rybczynski, Mary R. Dawson & Richard H. Tedford, and the Journal Nature:

The Smithsonian’s National Museum of Natural History:


Leave a Reply

Please log in using one of these methods to post your comment: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s