Name: When Fish First Breathed Air
Uploaded: 2021-05-03T14:10:58.000Z
Duration: 9 min 8 s
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385 million years ago, the land was all but empty.

The continents were home to only some sparse plants and fungi, and a few pioneering arthropods.

Life in the oceans, however, was booming.

The waters teemed with squid, bony fish, and sea scorpions.

But there was one group of fish that was about to go its own way.

In time, these enterprising fish would undertake one of the most important journeys in the

They'd follow in the footsteps of the arthropods, to become the first vertebrates to live on

First, they had to acquire the ability to breathe air.

The giant leap between fish that breathed through water and fish that breathed air has

been really hard for us to study and understand.

That's partly because the organs that did the work of breathing -- either gills or lungs

But it's also because, until pretty recently, there just haven't been a lot of fossils

Fish began their transition onto land 400 million years ago, during in the Devonian

And for a long time, one of the few fossils that experts had to work with was a fish named

Eusthenopteron, originally found in Quebec in the 1880s.

It lived in shallow, estuary waters around 380 million years ago.

And like the famous Coelocanth, it was a “lobe-finned fish.”

Instead of having long, delicate fins with lots of joints in them, like you'd see in a

goldfish, its fins were shorter and stronger.

These hardy limbs could have helped Eusthenopteron move itself along in the shallows as it hunted

other fish, but its fins probably weren't strong enough to let it walk on land.

Another major find came about in the 1930s, when scientists uncovered Ichthyostega,

a meter-long creature found in rocks in Greenland dating back 364 million years.

Ichthyostega had a body that was a lot more salamander-like, including a fully-developed

But Ichthyostega still probably dragged itself around with its front legs, a bit like how

And it also had a thick, paddle-like tail for swimming, which means it likely spent

But perhaps neither of these fossils is as key to this story as Tiktaalik,

a 375 million year old animal found on Ellesmere Island, Canada in 2006.

Tiktaalik had a fishy body, but a head like a salamander's, and stiff, leg-like fins

that could have supported its weight outside of the water.

And most importantly, it also had bigger primitive hips, so its hind limbs had something to

anchor on to, an important step in becoming a fully four-legged animal.

However, Tiktaalik's travels on land were probably still stuck in front-wheel drive.

Even though its legs and hips were bigger than in Eusthenopteron, they weren't strong

enough to bear the strain of walking on land.

With traits that seem halfway between a fish, like Eusthernopteron, and a four-legged animal,

like Ichthyostega, Tiktaalik is a textbook example of a transitional fossil from this

And despite their differences, all three of these fossils are considered to be tetrapodomorphs,

a group that includes early four-footed animals and the lobe-finned fish that are closely

Now, one big trick to living on land, of course, is … breathing.

So how did these animals, which had been adapted for millennia to life underwater, start

Well, we know that all three of our friends -- Eusthenopteron, Tiktaalik and Ichthyostega

Because, even though gills themselves don't fossilize very well, the bony arches that

And each of these animals had gill arches.

But even though they still had gills, that doesn't mean they couldn't breathe air.

It turns out that the most important clue for when fish started breathing air isn't

Instead, it's the shape and location of a little hole in the skull.

This hole can still be found in many fish today.

It's the opening of a tube, called the spiracular tract, that's used to bring water in toward

This feature is really handy, because it allows fish to breathe when their mouths are busy

And this little skull hole can tell us a lot about when tetrapodomorphs first became able

All you have to do is compare where it shows up in fossils, with where it appears in different

Now, in most modern fish, the opening to the tract appears on the sides of the face, near

the front of the skull, which puts it pretty much right on top of the gills.

But there are also fish today that breathe air.

And in one of these fish -- the bichir from Africa -- the hole is bigger, and sits

on the top of its skull, farther back, kind of like a blowhole.

What's more, the opening also sits at an angle, not straight up and down.

That's an important clue, because this angle creates a more direct path for air to travel

Now, compare all of that with the skull openings in tetrapodomorphs.

In Eusthenopteron, the spiracular tract opens up near the front of the skull, just like

But, the hole is on the /top/ of the skull, not the side.

In this way, it kind of resembles the bichir, which uses its head-hole to breathe air from

So Eusthenopteron was maybe breathing air in addition to water, and since the hole was

close to the front of the face, they probably used their gills to handle both.

In both Ichthyostega and Tiktaalik, that hole is much bigger, is closer to the back of the

skull,  and sits at an angle, just like it does in the bichir.

This means the tract in these ancient animals was pointing right to where a primitive set

It's not the clear-cut evidence that a nice set of fossilized lungs would be.

But having a larger passageway that sits closer to where the lungs are in today's lunged

fish is a good sign that both Tiktaalik and Ichthyostega had some sort of primitive lungs.

As for where these lungs came from, experts think they derived from an organ that many

modern fish still have today: a swim bladder.

Swim bladders are often filled with air, which fish gulp down to help keep them buoyant.

And the ancestors of Tiktaalik and Ichthyostega probably had them.

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When Fish First Breathed Air

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