One of my favorite “proofs” of evolution is the recurrent laryngeal nerve (RLN)—the nerve that innervates the larynx from the brain, helping us speak and swallow. It takes a very circuitous course, looping from the brainstem down around the aorta and then back up to the larynx. Here’s its course in humans:
It’s a prime example of “bad design”, that is, of the ham-handedness of any creator that was responsible for designing organisms. Of course, we aren’t designed, but evolved from very different ancestors. That’s why our bodies are full of glitches and kludges, and this nerve is one of them. It’s much longer than it need be, taking a tortuous route several feet longer than the direct path from brain to neck.
I’ve talked about the evolutionary reasons for this many times; you can see the full explanation in WEIT or read about it here. This diagram shows how it worked: the nerve used to line up with a blood vessel, both servicing the gills of our fishy ancestors. When the vessel moved backwards during evoution, the RLN was constrained to remain behind it, still retaining its connection to the larynx, which evolved from a gill arch. The nerve could not “break” to attain the shortest route, for that would not be possible by natural selection: it would interrupt the nerve transmission and be maladaptive. Click to enlarge:
As I also point out in WEIT, this poor design reaches ludicrous heights, so to speak, in giraffes, whose long neck makes the RLN take a 15-foot detour:
Do remember that this nerve consists of a bundle of nerve cells, each of which travels the entire length of the nerve. Thus the giraffe must have nerve cells (including the axons) about fifteen feet long. That is a very long cell!
But there are even longer. In a new paper now in press at Acta Palaeontologica Polnoica (free at the link, reference below), anatomist Mathew J. Wedel simply thought a bit more about the nerve: what would it look like in animals with even longer necks? Those, of course, would include the sauropod dinosaurs. And in some of them, like the gynormous Supersaurus, the recurrent laryngeal nerve, and its included cells, could have been longer than 28 meters (92 feet). Here’s a diagram from Wedel’s paper, which is very clear and well written (he also explains it in a post at his website, Sauropod Vertebra Picture of the Week):
There’s little doubt, by the way, that dinosaurs did have recurrent laryngeal nerves. All living tetrapods do, whether they be amphibians, reptiles, or mammals. This suggests very strongly that the RLN is an ancestral condition in tetrapods, resulting from their mutual evolution from fishy ancestors.
But wait, there’s more! (Does this sound like an ad for Ginsu knives?) Even longer cells can be found in living organisms, in particular the blue whale, Balaenoptera musculus. Wedel speculates, reasonably (I doubt that dissections have been done), that nerves running from the whale’s brain to the flukes of its tail might be 30 meters or more (98 feet) in length.
But there’s still more! The sauropod dinosaur Amphicoelias fragillimus was, arguably, as long as 49-58 meters (161-190 feet) from snout to tail. The nerves innervating the tail could have been only a meter or so shorter than that. Now we’re not sure about the neck length of A. fragillimus: its RLN could have been 38 meters long (124 feet) or more; remember that the nerve has to run the length of the 19-meter neck twice.
So what was the longest cell in the history of life? Our best guess is 40-50 meters (130-160 feet!) for nerves innervating the tail in the longest sauropods:
Wedel points out that cells this long pose some obvious problems to the physiologist:
- Pain signals traveling along the RLN in sauropods could have moved extremely slowly. Wedel notes that “unmyelinated vagal afferent fibers have conduction velocities as low as 0.5 m/s, and some unmyelinated fibers are present even in the recurrent laryngeal nerve of the giraffe. Unless selected for faster response, similar unmyelinated fibers would have taken almost a full minute to relay ‘slow pain’ signals to the brain of Supersaurus!” Wedel notes, though, that an injury to the dinosaur’s throat could have been detected more quickly from damage to nerves in the skin of the throat, whose path to the brain was only a meter long.
- A bigger problem: nerve cells must transport material from the cell body itself to the tips of the axons. It’s done through a process called “axoplasmic streaming”, which carries different molecules at different rates. Neurotransmitters and enzymes, for example, travel 200-400 mm (8-16 inches) per day, but the transport of some proteins is slower: 0.1 -1.0 mm per day! As Wedel notes, “Even at 1 mm/day, slow axoplasmic streaming would take more than four decades to move proteins from the nerve cell body to the axon terminals in the longest neurons of large whales. This, of course, is not feasible, and Wedel suggests that either axoplasmic streaming must be much faster in whales (and in dinosaurs), or there is some other way they transport proteins through nerve cells. Here’s a fertile field for cell biologists!
Regardless, the recurrent laryngeal nerve of the long-necked dinosaurs is not only a “monument of inefficiency,” as Wedel notes in his title, but an even better monument to evolution.
___________
Wedel, M.J. 2011. A monument of inefficiency: the presumed course of the recurrent laryngeal nerve in sauropod dinosaurs. Acta Palaeontologica Polonica, in press. DOI:10.4202/app.2011.0019
From Wedel’s website:
It’s not a nerve cell, and it isn’t exactly long so much as large, but these guys will give any cell a run for its money in terms of sheer size – http://www.newscientist.com/article/dn18468-zoologger-living-beach-ball-is-giant-single-cell.html
Oh, that’s cool.
But, but….JESUS WANTED IT THAT WAY….and the ways of THE ALMIGHTY are not always ours to understand. 🙂
Understood!
Since when was Jesus the Creator!
e.g. John 1:3,10 Colossians 1:16 Hebrew 1:2
motor proteins and microtubules, for example?
Is that what’s meant by “streaming”? It’s not really clear (to me).
Always interesting to learn about dinosaurs! But I’m confused about one thing: if the length of the nerve cells in the RLN were the full length of the RLN itself, then why would it take longer for pain signals to travel to the brain?
I’m sure I’m misunderstanding how pain signals travel, but it seems like it would only be a slow process if the nerve cells were small and had to pass along the information one to another across the length of the RLN. But if the cells are actually as long as the RLN, wouldn’t the pain signal still be more or less instantaneously passed from the damaged area to the brain? Or does the signal still need to pass “through” the length of the cell?
Hope that made some sense.
No, a signal actually has to physically travel (or, more precisely, be propagated) along the membrane of a neuron from one end to the other. It’s true that this is slow in small unmyelinated ‘slow-pain’ neurons.
What’s not mentioned here is that their function is for longer-term, chronic ‘soreness’ pain, not the sharp ‘fast pain’ of a new injury. There are larger, myelinated pain neurons that run in parallel.
…and that carry signals much, much faster than the neurons mentioned.
Neuronal signal propagation
In a single neuron, the signal still needs to travel the length of the axon. If that signal can travel at 1 m/s, and the RLN is 15ft (~4.5m), that is a noticable delay. And only in one direction!
Just what we need: another excuse for the Japanese to hunt whales.
The long delays in neuronal transmission in dinosaurs may be the reason why large sauropids had a second ‘brain’ in the pelvis, to exercise local control. This prompted the delightful poem by Bert Leston Taylor (1866-1921):
Behold the mighty dinosaur,
Famous in prehistoric lore,
Not only for his power and strength
But for his intellectual length.
You will observe by these remains
The creature had two sets of brains –
One in his head (the usual place),
The other at his spinal base,
Thus he could reason A priori
As well as A posteriori.
No problem bothered him a bit
He made both head and tail of it.
So wise was he, so wise and solemn,
Each thought filled just a spinal column.
If one brain found the pressure strong
It passed a few ideas along.
If something slipped his forward mind
‘Twas rescued by the one behind.
And if in error he was caught
He had a saving afterthought.
As he thought twice before he spoke
He had no judgment to revoke.
Thus he could think without congestion
Upon both sides of every question.
Oh, gaze upon this model beast,
Defunct ten million years at least.
I know it’s only Wiki Richard, but here it says this:
So I guess the second ‘brain’ might be a speculation too far ?
The poem, however, remains delightful!
There is an error in the figure 3-11. There is a label pointing to the 1igamentum arteriosum–the remnant of the ductus arteriosus, present only in fetal life. This constricts after birth to become the ligament around which the nerve loops. The pulmonary artery IS the pulmonary artery–labeled the “pulmonary trunk” here. Since we are all about the facts here.
Love the post! I remember seeing the dissection of a giraffe’s RLN in one of Richard Dawkins’ books. Thanks for all you do with your blog.
Good spot!
Very interesting – I’ve been wondering about how this fault crept in and how many others of God’s creatures shared in its design flaw. Now I know, thank you.
I believe the correct spelling is “ginormous.” The alternate “gynormous” looks etymologically related to, for example, “gynecology.”
And “gynecology” looks wrong to me ‘cos you left the ‘a’ out.
You Americans with your logical, rational approach spelling. Where’s the fun in that?
Not my area of experience. But my impression has become that gynecology has no fun in it, regardless of spelling!?
No fun? They do say that a cardiac surgeon has your life in his hands, but a gynaecologist has his hands in your wife.
HAHHAHAHAHHAHAA (y)
My Hellenic friend who studies Homer would no doubt complain about using the Latin alphabet!
😉
I’m confused.
Maybe my biology (or memory of it, more likely :-/) has not kept up with the times, but I thought neurons handed over to each other in the larger bundles in the same way as in the brain. (Subject to the constraint that signal transmission over synapses is slower than axion transmission.)
In either case, but especially if neurons are really forced to elongate over meter lengths, it raises some interesting questions:
AFAIK even myelinated pain fibers forms part of reflex arcs to cut down on reaction speed. (No need to tell the brain a muscle group needs to withdraw an arm when potentially hurt.)
I see the handover [it exists, of sorts, then!? but we are talking about at least partly different signal pathways] are located in the spinal cord, which in dogs at least is claimed to go halfway down the tail.
I don’t think there is any reason that the tail couldn’t be whipped aside by a reflex arc in the longer mammals described. The point being that not the signal path length doesn’t always map to the entire neural “organ” length.
There is a “not” that not belongs in the last sentence.
Gah! Also, an axion is a (putative) particle, not the nerve’s axon at all.
Most of the length of all nerves consist of axon bundles. Nerve cell bodies are found in the central nervous system and in ganglia such as the dorsal root ganglia and sympathetic chain ganglia.
In the RLN, then, there will be nerves with the cell body in the brainstem and an axon running the entire length of the nerve to the target.
The sensory nerve of a sauropod’s tail tip will have its cell body in a dorsal root ganglion beside it’s spinal cord somewhere around hip level. It’s axon splits into two branches. One will go down the tail, while the other (if it is a temperature or light touch neuron) will travel all the way up the spinal cord to the brainstem before synapsing there. So the single nerve will stretch from base if the skull to tip if the tail. Sensory neurons for other pain modalities aren’t as long. Vibration and pressure ones synapse in the spinal cord, for example.
I can think of several hypotheses to address the axoplasmic question other than increased axoplasmic transport rate:
1. On-site production. Rather than rely on axoplasmic transport, proteins are produced in special areas along the axon. This would require a “daughter” nucleus or some other means of gene storage be present. Probably a very low probability on this one being correct as I’m not aware of this ever being observed.
2. Manufacture of needed proteins by support cells. This addresses the problem with hypothesis 1. Much like in the CNS with glial cells, PNS glial-like cells could produce the necessary proteins needed by the neuron along the axon.
4. The nerves don’t actually run the whole length but synapse with an interneuron after all. Again, not yet observed but still plausible.
That’s what I get for deleting a comment and not editing.
Hypothesis 3 – see #4.
This called to mind Richard Dawkins “The Greatest Show On Earth” so I rushed back to see the actual photo of a dissection of a giraffe and the laryngeal nerve. But I was dissapointed; somehow I had built a mental pcture in color like his other phantastic photos but it was in black and white. He followed with the route of the “vas deferans” and to me this diagram always get’s me to laughing; what a sad, droopy left testicle. But maybe God HAD started this design as a means of turning men’s seed on and off, but of course his favorite in Genesis was opening and closing wombs. A master gynecologist and “practicer” of birth control.
You may have been thinking about the National Geographic show “Inside the Giraffe”. http://channel.nationalgeographic.com/episode/inside-the-giraffe-4308
Richard Dawkins made a guest appearance on the show in which the anatomist highlighted the RLN. Unfortunately, the vidoes on the NG web site don’t show that part.
Yep – this giraffe definitely has a sore throat.
It was a stunning colour photo with lots of red. But somebody at the publishers’ objected that it was too gory, they couldn’t bear to look at it. To be fair to her, Darwin himself gave up a medical career on similar grounds. But I’ve always regretted the decision to print it in b/w.
Richard
Aha– that explains it. My oldest was still quite taken with that photo, despite it’s being black and white. I could hardly get through GSOE, in fact. Every time I turned around, one of my kids had nicked it to look at the photos! (I went back and got my copies of WEIT and GSOE out to read the bits about the RLN again, too:-)
Wonderful footage. I’ve just completed “Why Evolution is True” and the section on that nerve I found particularly fascinating. It’s one thing to have the path described, it’s quite another to see exactly how it is laid out.
More overwhelming evidence – not that it was required!
So there are research grounds for cutting up whales? Dammit, I’m in the wrong business.
Fascinating insights & show us how ignorant we really are about the world.
i wonder how large the aortas on those sauropods must have been.
“it’s a bad design”
so u are implying that u know what is good design? and all possible reasons for this nerve to exist?
Maybe the nerve is so long because there is delay needed for some reason we don’t know yet.
u don’t know everything.
It’s long because it’s caught under the aorta. Look at how the otherside is caught under the carotid artery.
This makes sense from an evolutionary perspective. The fact that the heart itself isn’t centered in the chest cavity should be another clue as to lack of design because this causes uneven blood pressure on both sides of the body.
Body parts are clearly jammed/cramped inside. A good designer would have worked around this by making parts smaller to keep symmetry.
Calling God a good designer is belittling good designers. Good designers would not have left broken genes (vitamin c) in the body either.
Maybe God is an evil tinkerer?
Evolutionary history seems to describe the state of things the best.
Live with it.
“This makes sense from an evolutionary perspective.”
or embryo development perspectrive.
But beseides i don’t think that internal organs should be beautiful and symetrical by design. they are ugly for purpose to shock u when u see it. some fish IIRC throw up their internal organs to shock their predators.
what a beautiful liver you have my dear. 🙂
Yes kurva, maybe god sat up in his heaven 14 billion years ago and just thought it all up. Like he did with elephants testicles. “Hmm, you know, I just feel the need to not let the elephants balls hang free.”
you are funny , u should be comedian
How old is the Earth? What is your basis for your answer?
i don’t know.
what is the basis for random questions?
you think that if you can put me in some category that then you can decide if what I’m saying is legit?
i wish if it was that easy.
It does seem to be a design flaw to you with your very limited perspective analysisng it with an organ no bigger than your fist which you use to adjudicate that this is a waste of time. Maybe, just maybe, there is lots that you do not know about & that there is/was/will be a very good reason for this design. A little bit of humilty would not be out of place.
Maybe, just maybe, evolution is true.
I’d like see an Intel chip design where most of the chip has really tight wiring but then they deside to run a couple of the wires 10ft outside of chip, just for the hell of it. Whatever engineer would do this would get their asses canned.
If it was just the RNL, perhaps you could argue that there’s a reason for it being like that which is currently not known. But the human body has many flaws. Even our eyes, which are used as an argument against evolution by many misguided creationists, are full of flaws: we can see only a tiny portion of the electromagnetic spectrum; we have a “blind spot” where the nerves (which inexplicably run in front of the retina) gather and run out the back of the eye; we cannot distinguish between pure colors and mixed versions of the same colors (metamerism); we cannot detect polarization; we see no detail outside the center of our vision, requiring us to move our eyes around to see; and on top of that our angle of vision is fairly limited. There are animals that don’t have some of these limitations, and others that have better eyesight in other ways (e.g. hawks, which can see much more detail than we can). If we are the pinnacle of God’s creation, why do squids lack a blind spot while we have one? Again, one such question is no big challenge to God, but there are hundreds of them, all of them pointing to a lack of a designer.
Wasn’t there a suggestion that there may have been a nucleus (or some nerve cluster) at the root of the tail which provided some control (perhaps entirely automatic)? For example, if T rex bit on your tail, this would allow you to start moving it before the information got to the brain.
(I know we have similar feedback loops, but I’m sure I saw something suggesting a sort of secondary brain…)
Read comment 6 & my reply to comment 6
Wait, I’m missing something. So, some organisms have ridiculously long nerves, and this is evidence for evolution. I understand and agree. But here’s the part I don’t get: does every nerve contain nerve cells that are the same length as the nerve itself? If that’s true, I’m surprised I never learned it before. Can someone confirm, please, and offer some sources? Thank you!
>“axoplasmic streaming”, which carries different molecules at different rates. Neurotransmitters and enzymes, for example, travel 200-400 mm (8-16 inches)
I’m curious: might this provide an explanation for why it takes several days to recover from heavy indulgences like sex or drugs?
A simple google search deconstructs your poorly thought out conclusions :
Human-designed devices, such as radios and computers, do not need to function until their assembly is complete. By contrast, living organisms must function to a high degree in order to thrive during every developmental stage from a single-cell zygote to adult. The embryo as a whole must be a fully functioning system in its specific environment during every second of its entire development. For this reason, adult anatomy can be understood only in the light of development. An analogy Blechschmidt uses to help elucidate this fact is the course of a river, which “cannot be explained on the basis of a knowledge of its sources, its tributaries, or the specific locations of the harbors at its mouth. It is only the total topographical circumstances that determine the river’s course.”
Thanks for the nuanced reply on the need for continuity. By the same token, the “evolution” of certain other features also seem to imply a knowledge of the end-result for it to work.
Whether or not we agree on any form of “creator”, the facts also seem to suggest that the dice are loaded. i.e. non-random mutation.
“…the facts…seem to suggest that the dice are loaded. i.e. non-random mutation.”
I’d be very interested in learning about such facts. Can you provide examples?
Thank you for this post – I also enjoyed this section in the book. I really appreciate the time and care you take to explain biology to those who have possibly never even had a course in it (such as myself). When you are surrounded by people with similar education and in similar fields it would seem easy to forget how little the general public actually understands.
Why I’m sure the argument a creationist would make goes something like, “This way all our words have to come from the heart to be spoken.”
Really satisfying post. I hope have fun with this at my church picnic tomorrow. other facts of evolution have too much unproven. But this is staring us right in the face.
Has any scientist ever measured how long it takes for reality to travel along the sheer nerve of a creationist?
Is the larynx the only structure supplied by the recurrent laryngeal nerve ?
Could the course of the nerve be due to a developmental consideration rather than an evolutionary phenomenon ?
with respect to the comment by Jerry Kindall “that we only see a small portion of the em spectrum” so the eye is poorly designed does this mean that Mercedes Benz cars are poorly designed because they do not fly ?
This reasoning is flawed. The assumption that longer is not better is a tenuous one. Without knowing everything there is to know about the LNR how can anyone make an assumption that a longer nerve is not a superior arraingement?The RLN serves more than the larynx and there other nerves that serve it. New developmental evidence shows that having a looping nerve may have a very important function a earlier developmental stage than a direct route.
Without knowing the complete mind of a creator it is hard to make assumption what the best arraignment truly is. Trying to do so puts us in the position of playing God.
“Without knowing the complete mind of a creator it is hard to make assumption what the best arraignment truly is.”
And that is why any ‘explanation’ that invokes God is outside science: it cannot be tested. No such ‘explanation’ can be tested because whatever is found can then be said to be the way God wanted it. In this case, we see it illustrated by some folk assuming ‘God must have had some reason.’ but let’s face it, even if it could be conclusively proven that the recurrence of the RLN is nothing but bad, bad bad bad bad, folk would still be able to say ‘God wanted it that way for reasons we cannot understand.’
So, ‘God made it that way.’ is a hypothesis that can’t be tested by reason applied to evidence and is not science.
Do you think these people care whether or not it is science?
So tempting…..
Pray, give us a link to the above-referenced developmental evidence. Also, perhaps you would care to similarly reflect here on why the giraffe’s neck is the length it is, as well as the various lengths of the necks (and un-necks) of the multitude of creatures on Earth.
Do you know the mind of God? If not, then on what basis can you assume that all of his/her creations would be efficient? How do you know that God is not capricious, or evil, or simply a curious tinkerer? Your hypothesis has to be more fleshed out than “unknowable creator”.
I should add that when an issue like this is raised and the opposition has nothing to say except “you don’t know everything about this, so therefore maybe it is necessary in some way”, then the opposition really has nothing to say. Sure, it goes without saying that this might not be an artifact of common descent, just as it goes without saying that the Earth’s core might be ranch dressing instead of molten rock. At the very least, though, you’re going to need an actual argument against the molten rock hypothesis.
nice read 🙂
This post make us think about.. Really nice post..
I think we should look for the embryological development of the nerve…the nerve and the artery were at the same level early in embryo and the artery arching around the nerve..and when the artery elongate it pulls with it the nerve making it look like this