We know whales as graceful giants.

Some are powerful hunters.

Some are gentle filter feeders.

But no matter what they eat or how they live, whales -- as we know them -- are bound to the sea.

But!

There was actually a time en whales could walk.

The tale of whale evolution is a story about one of the most remarkable transitions in the history of mammals.

The fossil record shows how these animals transformed from tiny, four-legged plant-eaters no bigger than house cats to the sea-faring giants we know today.

This change was dramatic, and ... kinda fast.

Fossils from over the past 50 million years have revealed whale-like animals of all shapes and sizes, each like a piece in the puzzle of whales' evolution.

Smack in the middle of this amazing transformation is Ambulocetus: a toothy predator the size of a sea lion -- and a striking example of a mammal order in transition.

Ambulocetus lived about 48 million years ago, in what's now northern India and Pakistan.

And its full name, Ambulocetus natans, literally means the walking, swimming whale.

But scientists will tell you that it wasn't really great at either.

In the water, it was a powerful swimmer, but not very fast or efficient.

On land it was clumsy too, with legs that splayed out to the sides, a belly that almost dragged on the ground, and a snout that was so long and heavy, it looked like it could barely lift its head.

But Ambulocetus was perfectly equipped for its environment.

It lived in partly freshwater environments, like river deltas, where it lurked in the shallows and grabbed whatever prey that came near its giant snout.

Now, if a long, aquatic ambush predator sounds kind of familiar, that's because Ambulocetus is basically the mammal version of...a crocodile.

It lived a lifestyle that was a lot like a crocodylian's -- ideal for an animal that lives between land and water.

But despite their similarities, crocodiles and whales are not directly related at all.

In fact, the group of mammals that includes whales and dolphins -- known as cetaceans - - are so different from other living mammals that it's been hard to figure out what extly Interestingly, research done both in the field and in the lab revealed some surprises.

First, in the 1980s and 90s, a set of genetic studies took sequences of DNA from whes and compared them to the same sequences in other living anima.

And these comparisons showed that cetaceans are actually most closely related to a group known as artiodactyls, hoofed mammals that includes hippos, pigs, and deer.

Then, a number of fossils found a little later seemed to support this same conclusion.

In 2007, paleontologists in Kashmir, India, found the fossil of a 47 million year old hoofed creature the size of a house cat that they named Indyus.

But, it turned out that this tiny mammal had a specialized, thickened ear bone that, until this discovery, has only been found in whales.

The bone -- called an involucrum-- helps aquatic mammals hear underwater, and it shows up even in the earliest cetaceans.

It also had other adaptations for life in water, like really dense leg bones, a trait that helps keep mammals like hippos weighted down when they're walking through a river.

But!

Indohyus wasn't a cetacean.

It had four legs and hooves for crying out loud!

It even had a special ankle bone, called an astragalus, shaped kind of like a pulley.

And that feature is only found in artiodactyls.

Some very early cetaceans have this ankle bone, too, which tells us that cetaceans evolved from artiodactyls.

So, Indohyus is now largely considered the closest non-cetacean relative of whales.

Unlike Ambulocetus, it's not a member of the immediate whale family, but it shares a common ancestor with them, helping to connect today's artiodacty.

In other words, if Ambulocetus represents the transition from land to water, then Indohyus In other words, if Ambulocetus represents the transition from from artiodactyls to whales.

By the time the first recognizable whales, like Basilosaurus, show up in the fossil record about 40 million years ago, this group of mammals would never come out of the water again.

But there's still the question of ... why.

Why would cute little deer-things end up leading a whole order of mammals to life in the deep sea?

That's a question that remains unanswered.

Maybe there were fewer predators in the sea than on land 50 million years ago.

Or maybe there was more food in the oceans, and less competition for it.

After all, from Indohyus to Ambulocetus, there are many adaptations that show that the diet of these animals changed from land-based sources to aquatic pre But food probably isn't the whole reason.

Whales are predators, but the only other mammals that moved from land to water are manatees and dugongs, and they're both herbivores.

So, as in many other areas of natural history, we don't have all the answers yet.

But still, let's just pause to appreciate the fact that it took less than 20 million years -- about the evolutionary equivalent of a lunch break!

-- for this entire, astonishing transition to take place.

And there in the middle is the walking swimming whale, linking whales as we know them to tiny, cat-sized deer-things, just dipping their toes in the water for the first time.