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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

  • history of life.

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

  • dry ground.

  • But they couldn’t just move onto land.

  • Not yet.

  • 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

  • -- don’t fossilize well.

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

  • from that time for us to study.

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

  • period.

  • 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

  • pelvis, strong limbs, and even fingers.

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

  • a mudskipper moves today.

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

  • a lot of its time in the water.

  • 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

  • time.

  • 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

  • related to them.

  • Now, one big trick to living on land, of course, isbreathing.

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

  • to breathe air?

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

  • -- had gills.

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

  • support the gills do.

  • 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

  • the absence of gills.

  • 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

  • the gills.

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

  • eating.

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

  • to breathe air.

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

  • kinds of modern fish.

  • 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

  • to the bichir’s lungs.

  • 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

  • in most water-breathing fish with gills.

  • 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

  • the surface of the water.

  • 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.

  • But!

  • 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

  • of lungs would have been.

  • 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.

  • The thinking is that, among some lobe-finned fish, the swim bladder became bigger and contained

  • more blood vessels, so it became better at putting oxygen into the bloodstream.

  • In time, this organ took on a different function, finding a new use for the air that was already

  • there: breathing.

  • This kind of makes sense if you look at air-breathing fish today, like the lungfish and bichir.

  • Their swim bladders are split into two, and so full of blood vessels that they look basically

  • just like our lungs do.

  • But, why would any fish, modern or fossil, bother with breathing air, when there’s all that

  • lovely water around?

  • Well, during the Devonian, things werecomplicated.

  • And all of those complications led to a steady drop in the amount of oxygen in the oceans.

  • On land, new plant species were diversifying, which you’d think would be good for oxygen

  • levels.

  • Except, all of these new plants were also dying on land, and then getting washed into

  • the ocean.

  • All of that organic material fueled huge blooms of algae, and then bacteria, which in turn

  • sucked up oxygen from the ocean.

  • So by the late Devonianright when fish start to transition onto land - oxygen in

  • the air was really low: some estimates go as low as 13%, compared to almost 21% today.

  • For animals, low oxygen is generally bad.

  • But for organisms that live in the water, it's even worse, because oxygen concentrations

  • are always lower in water than they are in the air.

  • So when Oxygen levels first started to fall 385 million years ago, the benefit of transitioning

  • onto land was pretty clearno more gasping for breath in the water.

  • Being able to breathe air made Tiktaalik, Ichthyostega and maybe even Eusthenopteron

  • more energetic and better able to hunt their food.

  • And these first air-breathers eventually gave rise to the true tetrapods -- the first vertebrates

  • to live on dry land, full time.

  • Eventually, they lost their gills.

  • And by the time the Devonian Period had ended and the Carboniferous was underway, they had

  • also lost their spiracular tracts, and started using a totally different kind of skull-hole

  • for breathing, called nostrils.

  • But those lobe-finned fish that were our ancestors weren’t the only fish that figured out how

  • to breathe air.

  • Today, there are lungfish, mudskippers, bowfin and bichir.

  • Theyre not direct descendants of the likes of Tiktaalik.

  • Instead, they each acquired that ability independently, at different times.

  • It just goes to show you that breathing air has turned out to be very convenient for a

  • lot of us over time.

  • So remember: For all of the breaths that youre taking today, and the way that youre taking

  • them, you owe your fishy ancestors a debt of thanks.

  • Thanks for joining me today!

  • And I have big news!

  • Eons is now on Patreon!

  • Patreon is a voluntary subscription service that helps keep these videos coming.

  • So if you’d like to support the show, head over to patreon.com/eons and sign up at any

  • level you want!

  • Now let me know what you want to learn about!

  • Leave a comment down below, and if you haven’t already, go to youtube.com/eons and subscribe.

385 million years ago, the land was all but empty.

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鱼第一次呼吸空气时(When Fish First Breathed Air)

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    joey joey 發佈於 2021 年 05 月 03 日
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