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I will tolerate recurrent laryngeal nerve slander no longer! It's actually the result of several elegant solutions to difficult problems in embryology, and the length is a non-issue. A

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Peyman Milanfar

@docmilanfar

Oct 27

My meager education in biology and evolution gave me the mistaken impression that evolution optimized everything. But it didn't. One example is the recurrent laryngeal nerve (RLN). It goes from behind your ear, loops down below your aorta, and then back up to the voice-box 1/2

9:28 AM · Oct 28, 2024

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Space Koala

@culpable_mink

13h

The fundamental problem in embryology is that you have to get from a single cell to a complex organism that is not symmetrical in two different axes, and about 10 orders of magnitude. Oh, and everything must work at all times while growing. 2/13

As the cells in the embryo divide, it seperates into two spaces with a flat disc in between. This disc becomes the template for how your body is structured from top to bottom, and side to side. 3/13

So how does this play out? Well, now that there's a disc with two layers, two tubes start to form. The neural tube will become your spine and brain. The endoderm will become your digestive and respiratory tracts. 4/13

But how do you make sure you don't get a head at each end of the body? Because you start this process at the the end that will become your head (caudal), and propagates down towards the end of your spine. And at the very caudal edge, the heart starts to form. 5/13

So absolute essentials start at the caudal end (which makes sense), but now you have an issue: Your heart is not on the top of your head. That means it needs to move. And it does, twisting down into your chest. 6/13

So now you can see why the recurrent laryngeal nerve is in a position to get caught by the aorta as it descends: the aorta is heading where it belongs, and moves through the space between the neural tube and the forming GI and respiratory tracts. 7/13

But why not wait on growing the nerve that will become the recurrent laryngeal nerve until after the aorta has already passed? For that we need to get into how innervation works. And that involves chemotaxis. 8/13

Basically, the tissues to be innervated are putting out signalling molecules that attract the nerve cells to extend to connect to the tissue (motor neurons send axons and sensory neurons send dendrites). 9/13

But because chemotaxis relies on gradients of signalling molecules, it works best over short distances. But once a nerve makes the connection to the tissue, it doesn't really care how much longer the axon gets as the organism grows. 10/13

In order to wait for the aorta to pass, you would have to put in embryological development signalling pathways to delay growth, then connect later when the tissues needing connected to are now even further away. You add needless complexity, and make innervation harder. 11/13

And it's not like there's any real downside for the organism having the nerve run down and back. It's deep inside and nestled next to structures far more essential for survival. And any organism that large has to be able to tolerate nerves of comparable lengths anyways. 12/13

So stop hating on the recurrent laryngeal nerve. If you see something in nature that doesn't make sense, maybe that's a sign that it's optimizing for problems you're not even thinking about that actually matter more during development. 13/13

GIF

I would learn 10,000 more things like this quite happily. Do you have any good embryology books or YouTube channels etc?

I've personally got an old copy of Langman's Medical Embryology, and more recent editions can be found for fairly cheap. But this thread is thoughts I've had rolling around in my head for years. Embryology tends to talk what happens without commentary on the why's.

Are there any animals that have a similar nerve that does not make the detour, or is it common across all animals? Is there an analog of the cephalopod eye and its lack of retina blind spot?

It's fairly well conserved, but animals like fish tend to have their hearts closer to their brains so the effect isn't as pronounced. Keep in mind the recurrent laryngeal gets snagged in the first month of human development, so it's before you can really distinguish most animals

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AdvancingDialysis.org

@AdvDialysis

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The length is a non-issue if you're proportioned like a human. If you're a giraffe...

The still have to run neurons from their brain all the way down to the end of their spine, and neurons from their spine to the bottoms of their feet.

2.4K

Paul Otto

@Paulnternet

I learned a lot, thanks for posting

Haha. I hated all my developmental bio classes (seriously, ocular nerve development, what the fuck) but im a bigger hater of when engineers talk about biology Like That. Ty for this thread, much needed

Trying to figure out why all the wacky stuff in embryology was happening is what kept me sane during those classes. Tends to be a lot of "this is what's happening, good luck on any framework to understand the why" when most people teach it.

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Nedi Sawego Yogya

@NediSawego

Do you forget about the superior laryngeal nerve? Because the development has proven that something like superior laryngeal is possible, Ii you have a blank canvas design, this is not optimal. But I know what you mean, we evolve from something so evolution can't get around it.

909

Space Koala

@culpable_mink

The superior laryngeal nerve simply forms after the aorta passes.

Sell that to a giraffe

The giraffe neither knows nor cares how long it's recurrent laryngeal nerve is. The adaptions they need to manage the blood flow and pressure issues with such long necks and overall height are a way bigger issues then keeping a myelinated nerve impulse going a few more feet.

I remember back around ~2000 there was a lot of talk in pop-sci spaces about "junk DNA". I remember at the time I was like "you fucking idiots you don't know whether its junk or not" But I guess I wasn't smart enough to apply the same idea here

delian

@zebulgar

followed because of this thread

893

AdvancingDialysis.org

@AdvDialysis

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Bernie Buss

@BernieBuss

You nicely explain the developmental program, however this is not a proof that evolution couldn’t have solved it differently as well. The issue with local minima is very real in an uncontrolled evolutionary developmental program.

418

Julian Gough is writing The Egg & the Rock

@juliangough

10h

What a splendid thread. Thank you.

652

Ronald Rosenthal

@Roezeeee

Evolution is chaotic, it is messy and random because the mutations and genetic changes that drive it are not planned. Most significant mutations are not beneficial and many are lethal. Rarely, they are beneficial and with long time frames species evolve

590

Henning Sittler

@HenningSittler

Parallel optimization?

290

Gregory Besharov

@GregoryBesharov

How about our blind spots? Developmental story for them too or just bad luck?

Great thread

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I love this kind of observation-counterobservation procession. It's great to get some meaningful perspective contra our exploratory intuitions!

439

Aki

@aki_thinker

That was a good explanation! Thank you for sharing it!

I always suspected something like this was going on. People tend to assume that when something looks different from what they would have designed, it's some sort of mistake. But evolution is (generally) smarter than the generation & selection process going on in your head!

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jin ( de / fi )

@xkonjin

this an insanely good explanation

228

goblin costanza in limbo

@goblinodds

11h

oh this was a fun read ty!!

681

Dr Diego Caleiro /dd

@diegocaleiro

I knew coming in this was gonna be good. Didn't know it was gonna be this good.

Fascinating!!!

Insta-follow…

Seek Discomfort! Look for situations that challenge you... Look for challenges that are NOT easy to complete. Not only is this more enjoyable... It also keeps you growing.

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Bonjour, you can read it here: threadreaderapp.com/thread/1850937 Talk to you soon.

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-Cantīde- is e/vibelove.

Excellent thread, thank you!! The “plate” phase is .. remarkable.

161

I’m Fancy

@CMassner

This earned a a follow!

sidereal

@sidereal111

this is most ridiculous cope I've ever read

302

Bil Dye

@willdye

I asked Grok for a picture of an evolutionary biology death metal band named "Recurrent Laryngeal Nerve Slander".

238

Pledges of Demilitarization

UBI

@PageLyndon

If I have any kind of surgery I am going to ask the doc to straighten that out. Somebody needs to calculate the unnecessary energy waste by inefficient evolutionary structures. Not unexplainable but still inefficient.

309

Michael C. Grant

@michael_c_grant

I blame the partisan media

174

QuixoteSword

@QuixoteSword

I've legitimately wondered about this for years. Thank you for the detailed explanation.

372

@Toberworld

10h

bro your vagus nerve is showing

247

Evan Daphne אבן דפנה

@Allmeridians

I don’t understand even one bit of this argument but I am ecstatic that it exists.

AdvancingDialysis.org

@AdvDialysis

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I think the two takes are not mutually exclusive. While you described why the nerve is the way it is, the end result can still be considered silly from an engineering perspective.

271

Enea Zarelli

@PJINKX

This is a great explanation, thanks! But as a Latinist I wonder about the use of “caudal” for the head part. “Cauda” means tail. I wonder if the term isn’t “capital end or edge”vs “caudal end” for the tail end.

barry goldman

@barrygoldman1

11h

love it! thanks! we dont' know how to think about engineering systems that function WHILE we are building them. reminds me of niven and pournell's "mote in gods eye" where this the aleins had this little teddybear/engineer caste that constantly crawl all over ur cars, 1/n

381

Fred

@FredWitting

Thank you for this. These two threads reminded me today of why I used Twitter. Wonderful learning opportunities .

103

MarcoPolo

@Marco777Polo

Sounds exactly like evolution. There would have to be a jump to come up with a new concept, and there seems no additional payoff for the final setup. Isn't this example normally used as in "good enough" and to show the common ancestors?

UrsoBruto

@urso_bruto

I want to express my gratitude for the wonderfully clear explanation you've provided. You took me from the edge of understanding to what I would now call a well-grounded comprehension. I deeply appreciate the insight that you’ve shared. Thank you.

Sabrdance

@sabrdance

This was throughly fascinating.

Uebagi

@uebagi

It annoys me whenever I see this pointed out. Fish do not have this issue because they dont have the same neckbody anatomy. There is no pressure to rewire the nerve, which is why terrestrial vertebrates haven't evolved a human idealized solution for this, even ones with long neck

Canles

@canlesofficial

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I loved both the original post and your rebuttal. What does this say about me?

186

ام عبدالله

@shameypop

Thanks for this. There also must be a reason in terms of heart function. And evidence indicates that it does affect heart rate. I think it’s beautiful. Because the pathway of ear —> heart—-> speech is highly connected and necessary .. in a spiritual sense.

Jarred Barber

@MyNameIsTooLon

Sounds like you are describing exactly a local optimum

Sir Winston Leonard Spencer Churchill

@WinnieChurchiIl

I didn't even know this was knowable information. We truly are wonderfully made.

And that’s a follow

Would it increase gestation time if the long nerve was avoided, or is it simply a matter of lower complexity being more stable?

Luukas

@Mishima9keh

God created us perfectly in his image!

Harriet Kismet

@heathertwiggy

ngl the body is kind of gross

eanna_mceanna

@EMceanna

10h

This is an amazing thread

145

Jude Mayburn

@rhumverveine

This is fascinating. Thanks!

Mrremoraman

@mrremoraman

This is very interesting! I never took embryology, only Developmental Biology, which is like everything you learned in cell bio plus everything you learned in zoology on steroids

Serving Suggestion: Kamala Harris, FOR THE PEOPLE

@PaulKentAllen

Fascinating. Thanks!

Mike Renwick

@runonthespot

Brilliant thread

Your Decode Makes Baby Jesus Cry

@DecodeCry

There are only two certain things. Death and chemotaxis.

VelliJanta

@VelliJanta9211

A well earned follow!

Post³

@postcub3

that's amazing koala

doomy with a silent y

Really great thread

Woah

omg ty. is there a similar explanation for optical nerves causing blind spots, or is that one just a mistake

117

drew (disambiguation)

@aeiendewhy

Thank you!!!!!!!!!!!!

Canles

@canlesofficial

Tired of squeezing your feet into narrow shoes? Canles Earthtreks are designed for wide feet! Ditch the discomfort and experience the freedom of a spacious toe box and a comfy, barefoot feel.

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Damn.

can we get a laryngeal nerve fancam?

141

Psychohistorian

@BoydsArchitect

Thank you. TIL like 5 things.

Greenfield Jr r

@el_swamo

Someone has to stand up to laryngeal nerve slander!

Uncle Easy

@rogergbh

And not design. No slander at all.

Evelyn Kokemoor

@EKokemoor

13h

If I were designing a mature organism I would deliberately include a nerve like this for timing purposes.

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Brad DB

@BDBSoCal

Cool! Next do a thread kn what it would take for the entire complex eve to evolve feom single-celled organisms.

Voynich Manuscript audiobook

@Frogisis

It’s all amazingly clever, but if we’ve figured out how all this shit grows why are we still stuck clinically on a few simple stunt cases here & there over the last *20 years*? Fire up the organ farms already! I wanna see so many spare human hearts they’re ending up in hot dogs!

Best girl Tsukky

@bestgirltsukky

Super cool thread as I just start to grow a little baby. Feeling the itch to learn some embryology to understand the process better.

Ken

@whatabouttheken

Dunno, it still sounds like an intelligent designer could have come up with a modified fetal development sequence to avoid the nerve detour, which is the point: we share lots of our embryology with fish because of evolution.

zzszzzszz

@Tripleoblack

This is a cool explanation of how it works but nothing here actually says it has to work this way If you were designing this yourself you could simply sort the order of forming tissues differently, or develop a way for the nerve to pass through that gap before it's closed, or...

The Scarlett Pimpernell

@WiscJohnson

Bravo! This is the coolest, most mind blowing thing I’ve read on X in a while!

Lord’s Army

@reallordsarmy

yEaH bUt NeRvE LoNg ShOuLd bE sHoRt

Silence

Good and interesting thread just want to point out you've got the head and tail backward. Caudal isn't the head part its the tail. Cephalad is the head part

Punished Medium Spicy Pepe

@PunshdSpicyPepe

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Punished Medium Spicy Pepe

@PunshdSpicyPepe

14h

Replying to @docmilanfar

The chances of this "oopsie" actually being a good design decision and you being just a human retard are about 1000000:1.

Punished Medium Spicy Pepe

@PunshdSpicyPepe

Once you grok that evolution is smarter than any human by many orders of magnitude, you can only see these "well atchually" evolutionary gotcha guys as retards.

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