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  • [ intro ]

  • Chances are you don't give your tongue the credit it deserves.

  • That fleshy muscle in your mouth is so much more than a food-tasting organ.

  • Without tongues, we might still be swimming around

  • and staring longingly at the dry land we and all other backboned animals never managed

  • to conquer.

  • So today, let's take a trip through evolutionary history to see how the humble tongue was key

  • to world domination.

  • The earliest vertebrates were fish, and fish don't have tongues.

  • I know, you swear you've seen a fish with a tongue.

  • But I promise, you haven't.

  • Many fish have a structure called a basihyal:

  • a raised lump of bone or cartilage sits at the bottom of the mouth.

  • It can look sort of like a tongue.

  • But it doesn't really move, and it has no taste buds.

  • So it's not a true tongue

  • it's just a tough shield to protect sensitive nerves and blood vessels.

  • And this all makes sense when you realize that most animals with tongues use them for

  • catching and swallowing food,

  • and fish don't need a mouth appendage for that.

  • Most have a neat trick called suction feeding.

  • When a fish spots a tasty morsel,

  • it swims up to it and explosively expands its mouth.

  • The top of the skull moves upward, the lower jaw swings open,

  • and a bony throat structure called the hyoid moves down into the throat.

  • All this causes a quick drop in water pressure inside the fish's mouth,

  • and water rushes in, dragging the tasty morsel into the mouth.

  • Even fish that take bites of their prey often employ a bit of suction to move things toward

  • the throat.

  • And those that don't employ suction can just open their mouths and swim to let water

  • push things backwards.

  • But, while all of this works great in water, air is much less dense,

  • so it doesn't provide the force needed to drag or shove prey down the throat.

  • So fish were faced a problem as they expanded their horizons.

  • We know from the fossil record that one group of bony fishesthe sarcopterygians -

  • made the move to land between 400 and 350 million years ago, during the devonian period.

  • Tetrapodsthe legged animals that they became, including all amphibians, reptiles, birds,

  • and mammals

  • would go on to conquer the land, the sky, and arguably, with us included, space.

  • But first, they had to get out of the water.

  • Fossils of early tetrapods like the famous 'fishapod' Tiktaalik reveal just how much

  • needed to be tinkered with to make this transition,

  • including changing up their limbs, backbones, skulls, eyes, lungs and more.

  • But they also had to change how they ate.

  • Unfortunately, not a lot of fossil evidence has been found for the early evolution of

  • tongues,

  • but by comparing fish to land-lubbers, we can see where the change happened.

  • It all has to do with that throat structure I mentioned earlier: the hyoid.

  • In fish, hyoid bones largely support muscles of the gills.

  • But tetrapods lack gills for the most part.

  • So, the hyoid supports the muscles of the tongue instead.

  • We can even see this transition in action in modern-day amphibians!

  • Larval salamanders live in the water, and their hyoid bones are part of their gill structure.

  • When they metamorphose into adults, these bones switch to supporting the tongue!

  • As for how this shift from gill to tongue happenedwell, there are two main ideas.

  • One hypothesis is that tongues first evolved to help food move from the mouth into the

  • throat.

  • Picture an early tetrapod crawling along a Devonian shoreline like a weird slimy crocodile.

  • It snaps its jaws onto a small critter and tilts its head back to drop the food into

  • its mouth.

  • A simple tongue would have been very helpful just to move food to the right place.

  • In salamanders that feed this way nowadays, the hyoid drops down into the throat to move

  • the tongue.

  • Since this is the same motion the hyoid makes during suction feeding in fish,

  • this seems like a pretty straightforward evolutionary step.

  • But there's another hypothesis that suggests tongues first evolved to capture food instead.

  • Lots of animals, such as salamanders and lizards, poke out their sticky tongues to snatch up

  • their food.

  • A 2015 study found a similar strategy in an unusual fish that uses a hydrodynamic tongue.

  • That is, a tongue made of water!

  • This study found that mudskippers carry along a mouthful of water on their trips onto land.

  • When they find food on the ground, they spit the water all over it, then suck it all back

  • up.

  • It's sort of a loophole that lets them suction-feed on land.

  • Kind of.

  • And slow-motion X-ray video of the process revealed that when the mudskippers spit the

  • water out,

  • their hyoid bone moved upward toward the mouth.

  • That's similar to the movement the hyoid makes in salamanders that catch food by sticking

  • out their tongues.

  • So, perhaps early tetrapods developed a forward-moving hyoid for water tongues, mudskipper style!

  • Then, a soft, fleshy tongue eventually replaced the fluid.

  • As of now, we can't say for sure if either hypothesis is totally correct.

  • It could even be that tongues as we know them evolved multiple times!

  • But we do know once our tetrapod ancestors were armed with proper tongues,

  • they were ready to feast all along ancient shorelines.

  • That still left whole continents to conquer!

  • Like modern-day amphibians, early tetrapods would have been restricted to environments

  • with plenty of water available,

  • partially because their eggs need to be laid in water.

  • But around 300 million years ago, one group of tetrapods developed eggs with sturdy shells

  • that could be laid in drier places.

  • This group is known as the amniotes, and ultimately gave rise to all reptiles and mammals.

  • And as early amniotes moved into new environments, they needed new tongues!

  • Most amphibians have wet, squishy tongues.

  • If they dry out, they can't eat properly.

  • Reptiles and mammals, on the other hand, tend to have rough or scaly tongues covered in

  • keratin, the same material that makes up your hair and fingernails.

  • This protects the tongue from losing moisture, so these animals can feed in less humid places.

  • So tongues allowed vertebrates to move onto wet land, then wholly onto dry land, and thereby

  • conquer every environment on Earth.

  • So, be kind to your tongue.

  • You owe it a lot.

  • Thanks for watching this episode of SciShow!

  • If you liked learning about tongues and how weirdly important they are, you might like

  • our episode looking at 7 of the weirdest ones around today.

  • And as always, don't forget to subscribe!

  • [ outro ]

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舌頭如何幫助脊椎動物征服陸地? (How Tongues Helped Vertebrates Conquer Land)

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    林宜悉 發佈於 2021 年 01 月 14 日
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