Imagine a lungfish in a pond.

It's the dry season, and the pond is shrinking.

It will soon disappear.

But even though this fish is about to be out of water, it isn't totally out of luck.

When the water gets too shallow, the lungfish can gulp air at the surface using its lungs.

And when the pond becomes too small, it can use its sturdy fins to crawl across the ground in search of a new one.

These abilities seem almost tailor-made for surviving on land.

You'd think this fish was just one evolutionary step away from becoming terrestrial.

Ok but here's the thing: this lungfish has no interest in land.

Those "terrestrial" adaptations exist to help it survive long enough to get back to the water.

Over a century ago, one paleontologist wondered if the lungfish strategy held the key to understanding one of the biggest evolutionary moments in history: why our own ancestors left the water.

Maybe, he thought, the earliest tetrapods also had no interest in colonizing land.

And while parts of his hypothesis turned out not to hold water, it looks like lungfish have something to teach us after all.

Because it's beginning to look like our ancestors might have originally evolved "terrestrial" traits in order to...stay fish.

Hundreds of millions of years ago, the group of animals we belong to, four-limbed vertebrates called tetrapods, conquered the land.

They evolved from a group of fish called sarcopterygians, which include the ancestors of tetrapods: the tetrapodomorphs.

And for over a century, scientists have been trying to solve an important puzzle: how and why did the earliest tetrapods evolve from these ancestors, making the monumental leap onto land?

Through the fossil record, we can track what happened during this transition, more or less.

But why it happened has been much harder to answer.

During the early twentieth century, researchers contemplating what drove our fish ancestors to leave the water considered a bunch of ideas... Like, maybe the fish were escaping from predators in the water or taking advantage of food resources on land.

But in 1917, paleontologist Richard Lull offered a different idea.

He looked at lungfish and wondered if scientists had been thinking about this problem backwards.

Lull's idea, later championed by paleontologist Alfred Sherwood Romer, became known as the "shrinking pond hypothesis."

Lull and Romer pointed to how African and South American lungfish living in arid, seasonal environments use a range of adaptations to cope with changing amounts of water... ...Like entering a dormant state underground to escape the dry season, as well as using those skills of breathing air and sometimes crawling around on land.

And they pointed out that, while those traits could be confused for having evolved for life on land, their real function was just to sustain the fish long enough to get back to the water.

Lull and Romer viewed these fish as possible proxies for our tetrapod ancestors, because lungfish are also sarcopterygians closely related to the ancestors of tetrapods.

Maybe those ancestors also lived in similar environments - that changed seasonally with bodies of water shrinking and disappearing.

And maybe, this led those tetrapodomorphs to evolve seemingly land-friendly traits, for the same reason: to survive long enough to find water again.

But as new fossils emerged over the next few decades, the shrinking pond hypothesis began to fall apart.

For example, by the 1940s, the evidence suggested that lungs actually evolved long before the fish-tetrapod transition.

51 00:03:00,041 --> 00:03:04,291 before the arrival of tetrapods, meaning they must have developed to serve a different purpose.

Then, the early 2000s brought a ton of new discoveries of transitional tetrapodomorphs, giving us more insight into the environments where they lived.

Like the famous tetrapod ancestor Tiktaalik, which fun fact was not named after TikTok but lived during the Devonian Period in what's now northern Canada, around 375 million years ago.

Tiktaalik and its relatives- including Panderichthys and Elpistostege - are sometimes called "fishapods," because they look sort of halfway between a fish and a tetrapod.

And among their more tetrapod-like traits were limb-like fins that even had the precursors to toes.

Tiktaalik was also one of the first to evolve a neck, giving it a distinct head and torso, unlike the classic fish body plan with no neck.

This allowed for greater mobility of its head, and it's a key trait that distinguishes the tetrapod body plan from that of fish.

But as more fishapod fossils began to surface, and their habitats were studied, scientists realized they lived in shallow water environments like tidal channels, braided streams, and swamps.

These weren't the seasonal, drought-prone landscapes that Lull and Romer had envisioned.

So that's that then, right?.

The shrinking pond hypothesis is wrong.

Well, maybe not entirely.

Lull and Romer might have been onto something, even if they had the wrong habitat.

See, amid the new tetrapodomorph discoveries of the early 2000s, another researcher suggested that the tetrapod body plan might have evolved mostly or entirely in the water, not on land.

One animal she pointed to was Acanthostega, from the swamps of what is now Greenland during the Late Devonian Period, 365 million years ago.

While it wasn't a true tetrapod, it was getting close, with a salamander-like body and four limbs instead of fins.

This was also one of the first tetrapodomorphs known to evolve a rear-powered mode of locomotion.

That is, its movement was powered primarily by its back limbs instead of its front limbs.

This form of locomotion would one day become a feature common to tetrapods.

But while Acanthostega looked like a tetrapod, its arms and weakly built ribcage seem unlikely to have been able to support its weight on land, suggesting it was entirely aquatic.

Those seemingly land-friendly features, like its hands, were instead probably being used for navigating the waters of its swampy habitat and holding onto plants in the water.

And when Tiktaalik was described around this time, its discoverers suggested that its tetrapod-like traits might have evolved in order to "walk" underwater as well.

Actually it'd be more like this because it has four limbs While they thought it may have come ashore occasionally, most of its time was probably spent navigating stream beds and shallow channels cluttered with plants and debris.

It was looking like some of the key pieces of the tetrapod body plan represented an exaptation - a trait that initially evolved as an adaptation for one thing, but later became used for something else.

In this case, our fish ancestors evolved tetrapod traits while adapting for life in the shallows- and only later was that body plan co-opted for life on land.

It was the lungfish paradox all over again-traits that looked like adaptations for land were actually just better ways to be fish.

And more evidence supporting this aquatic picture continued to emerge.

For example, a 2012 study reexamined the limbs of Ichthyostega, another tetrapodomorph that shared Acanthostega's environment.

Scientists originally thought its limbs could have supported its body on land, but these researchers found that their range of motion would have limited Ichthyostega's terrestrial movement to dragging its body like a seal.

Not exactly the most effective evolutionary strategy.

Ichthyostega probably did spend some time on land, but not a lot.

Then, a 2013 study found that jaws of earlier "fishapods" like Tiktaalik wouldn't have allowed them to swallow prey on land.

And that's a big deal, because it means that, even if they did occasionally venture onto land, the fishapods would have been mostly incapable of taking advantage of prey there.

This suggests that, at this point in their evolution, tetrapodomorphs were still very much shallow water dwellers and the abundant resources on land probably played a minimal role in their evolution.

And most recently, there was a study from 2021 that analyzed bone microstructure of tetrapods from Nova Scotia that lived in the early Carboniferous period, several million years after Acanthostega and Ichthyostega.

It found that the stress patterns in their bones were consistent with aquatic habits, and inconsistent with movement on land.

So it looks like not only were most tetrapodomorph fishes mostly or fully aquatic, but that even the later, first true tetrapods still weren't living on land.

Ok but now, we should be careful about oversimplifying the water to land transition overall.

Not all tetrapodomorphs form a perfectly neat progression in their traits or through time, and their habitats weren't all identical.

There was probably tons of variation.

But many scientists are beginning to think that most transitional tetrapodomorphs were primarily shaped by shallow water habitats, choked with plants and other debris.

So Lull and Romer were right about one crucial thing: the colonization of land was probably an evolutionary accident.

So these animals crawl onto land by accident and now I have to pay taxes Those limbs, necks, and other adaptations that later helped tetrapodomorphs conquer continents initially evolved to navigate the shallows, not escape them.

And possessing these traits beforehand probably made the transition to life on land much easier for our ancestors when they eventually did come ashore and stayed there.

So in the end, or rather in the beginning, our ancestors didn't set out to revolutionize life on Earth.

Our success on land, and that of all tetrapods, from frogs to dogs to dinosaurs, was just a lucky side-effect of fish trying to stay fish.