Imagine a lungfish in a pond.

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

It will soon disappear.

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

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

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

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

You’d think this   fish was just one evolutionary step聽 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聽 long enough to get back to the water.

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

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

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

Because it’s beginning to look like  our ancestors might have originally聽聽 evolved “terrestrial” traits  in order to…stay fish.

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

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

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

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

But why it happened聽 has been much harder to answer.

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

But in 1917, paleontologist Richard聽 Lull offered a different idea.聽聽 He looked at lungfish and wondered if scientists聽 had been thinking about this problem backwards.

Lull’s idea, later championed by  paleontologist Alfred Sherwood Romer,聽聽 became known as the “shrinking pond hypothesis.” Lull and Romer pointed to how African and聽 South American lungfish living in arid,聽聽 seasonal environments use a range of adaptations聽 to cope with changing amounts of water… …Like entering a dormant state  underground to escape the dry season,聽聽 as well as using those skills of breathing聽 air and sometimes crawling around on land.

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

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

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

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

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

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

In fact, lungs likely appeared millions of years聽聽 before the arrival of tetrapods, meaning they聽 must have developed to serve a different purpose.

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

Like the famous tetrapod ancestor聽 Tiktaalik, which fun fact was not named after TikTok but lived during the聽聽 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聽聽 were limb-like fins that even聽 had the precursors to toes.

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

This allowed for greater mobility of its head,聽聽 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聽 their habitats were studied, scientists realized聽聽 they lived in shallow water environments like聽 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聽 Romer might have been onto something,聽聽 even if they had the wrong habitat.

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

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

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

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

That is, its movement was powered primarily聽 by its back limbs instead of its front limbs.聽聽 This form of locomotion would one day聽 become a feature common to tetrapods.

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

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

And when Tiktaalik was described around this聽 time, its discoverers suggested that its聽聽 tetrapod-like traits might have evolved聽 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聽 occasionally, most of its time was probably聽聽 spent navigating stream beds and shallow聽 channels cluttered with plants and debris.

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

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

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

And more evidence supporting this聽 aquatic picture continued to emerge.

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

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

Not exactly the most effective evolutionary聽聽 strategy.

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

Then, a 2013 study found that聽 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,聽聽 the fishapods would have been mostly聽 incapable of taking advantage of prey there.

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

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

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

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

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

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

There was probably tons of variation.

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

So Lull and Romer were right聽 about one crucial thing:聽聽 the colonization of land was聽 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聽 that later helped tetrapodomorphs聽聽 conquer continents initially evolved to聽 navigate the shallows, not escape them.

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

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

Our聽 success on land, and that of all tetrapods,聽聽