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Uploaded:2019-07-22
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You might not think much of your tongue, but without it, we may have never conquered dry land and the world as we know it.

Hosted by: Michael Aranda

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Sources:

References:
http://science.sciencemag.org/content/362/6413/460
https://www.merriam-webster.com/medical/basihyal
https://www.floridamuseum.ufl.edu/discover-fish/sharks/shark-biology/
https://www.sciencefocus.com/nature/do-fish-have-tongues/
https://www.merriam-webster.com/dictionary/hyoid
https://www.sciencedirect.com/science/article/pii/S1546509805230029
https://www.merriam-webster.com/dictionary/sarcopterygian
https://www.dictionary.com/browse/devonian
https://royalsocietypublishing.org/doi/full/10.1098/rspb.2015.0057
https://www.newscientist.com/article/dn27181-tongues-may-have-evolved-from-a-mouthful-of-water/
http://www.ucmp.berkeley.edu/vertebrates/tetrapods/tetraintro.html
https://tiktaalik.uchicago.edu/meetTik.html
http://jeb.biologists.org/content/jexbio/221/8/jeb154427.full.pdf
http://www.ucmp.berkeley.edu/vertebrates/tetrapods/amniota.html https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1570891/
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0198078
https://news.nationalgeographic.com/2018/06/dinosaur-trex-tongue-alligator-spd/
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.22.3754&rep=rep1&type=pdf
https://www.the-scientist.com/features/why-human-speech-is-special--64351
https://www.nidcd.nih.gov/health/dysphagia
https://www.biorxiv.org/content/biorxiv/early/2017/02/14/106955.full.pdf


IMAGES

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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 fishes – the sarcopterygians - made the move to land between 400 and 350 million years ago, during the Devonian period. Tetrapods—the 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 happened… well, 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 ].