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Author Topic:   Recurrent Laryngeal Nerve: Part II
Granny Magda
Member (Idle past 123 days)
Posts: 2462
From: UK
Joined: 11-12-2007


Message 39 of 75 (572893)
08-08-2010 11:55 AM
Reply to: Message 30 by ICdesign
08-08-2010 1:07 AM


Re: Doh....!!
Hi ICDesign,
The problem is I haven't seen any books that explain the details of where the RLN came from...
Correction; you haven't read any books that explain the RLN. You need to actually open them up and read them.
Obviously, none of you have a clue either according to your empty posts.
Just as CD has observed, this has already been covered in the previous RLN thread. From Message 37
Granny writes:
The basic version is that the RLN is a branch of the vagus nerve, the fourth branch. Now trace our evolution back as far as fish and this branch took a path between the gill arches. This took it back behind the sixth gill arch. This is what we see in modern fish.
Now in a fish this isn't a problem. The gill arches are close together and the nerve only covers a short distance - it all lines up, with each nerve branch going through each gill slit in turn. The problem is that in mammals, the "sixth gill arch" is homologous to and has evolved into the ductus arteriosus, a small channel that allows the blood in a developing foetus to bypass the lungs (this duct closes up soon after birth - usually). The RLN has to go around this. That's why it must take so torturous a route around the aortic arch. Here is a diagram showing the nerve in both fish and mammals (sorry it's a bit fuzzy);
Now this makes sense from an evolutionary perspective. If the fourth vagus branch originally went around the far side of the sixth gill arch, then the modern RLN must do the same with regards to the ductus arteriosus. Why? Because one thing that evolution absolutely cannot do is evolve through a stage which, though might have a beneficial effect in a million years time, is lethal in the short term.
For an engineer the problem is simple. The RLN doesn't need to go so far down into the chest. It's unnecessary and it exposes the nerve to a greater risk of injury (just ask any heart surgeon what they think of the of the RLN - the damn thing's in the way!). The obvious solution is to sever the nerve and re-attach it higher up, in the neck, where it needs to be, where it can be much shorter. It doesn't need to go around the ductus arteriosus; the duct serves no function in adults anyway. Problem solved. Here is an illustrative example;
The obvious solution is for the gardener to walk around the tree and back round to the flowerbed from the other side.
Evolution can't do that. Evolution works by mutation and a mutation that broke the nerve would kill the organism. There is no way for the RLN to evolve its way around to the other side of the ductus. (going back to our hapless gardener, it's as though the hose were attached at both ends, stuck around the tree) It's stuck back there, constrained by the limits of what evolution is able to do. So evolution does what it can. It stretches the nerve out, making it longer and longer (equivalent to our gardener lengthening the hose; a poor solution I hope you'll agree). This jury-rigged arrangement is typical of an evolutionary solution, doing what it can, modifying what it is given.
That's what leads to a giraffe with an essential nerve that takes a pointless fifteen foot detour. No engineer would design something wit such an obvious flaw.
That post full enough for you?
Mutate and Survive

"A curious aspect of the theory of evolution is that everybody thinks he understands it." - Jacques Monod

This message is a reply to:
 Message 30 by ICdesign, posted 08-08-2010 1:07 AM ICdesign has replied

Replies to this message:
 Message 40 by ICdesign, posted 08-08-2010 12:16 PM Granny Magda has replied

  
Granny Magda
Member (Idle past 123 days)
Posts: 2462
From: UK
Joined: 11-12-2007


Message 44 of 75 (572902)
08-08-2010 12:25 PM
Reply to: Message 40 by ICdesign
08-08-2010 12:16 PM


Re: Doh....!!
Hi ICD,
Full of it is exactly what I was thinking, thank you!
Thanks. I hope that after responding to a thorough post with a flippant one-liner, you'll not be complaining about posts being too thin on content again.
How would evolution know the organism would die if that nerve broke? Are you telling me evolution has reasoning ability?
Er... no. Huntard has it right. There are some things that evolution is good at doing and some that it is bad at doing. This is the latter.
Mutate and Survive

This message is a reply to:
 Message 40 by ICdesign, posted 08-08-2010 12:16 PM ICdesign has not replied

  
Granny Magda
Member (Idle past 123 days)
Posts: 2462
From: UK
Joined: 11-12-2007


Message 57 of 75 (573065)
08-09-2010 3:47 PM
Reply to: Message 50 by ICdesign
08-09-2010 5:32 AM


Re: Moderator Comment: You can't get blood from a stone.
Hi ICD,
I think I have a fairly sound grasp on what the theory of evolution is claiming. I was simply asking where the RLN originated from and how it came to serve such obvious purpose.
Okay. The thing is that your question comes across as a little bit... odd... for a person who understands the ToE. It isn't the kind of question that anyone with a firm grasp of the theory would be likely to ask, save as a rhetorical question.
As for where the the RLN originated, I think I made a pretty good effort towards explaining that in Message 39. Do you have any points you want to bring up about that explanation?
I was also asking how evolution could know the nerve was in jeopardy of breaking.
And I answered you. Evolution doesn't "know" anything. The problem is not about knowing. The barrier to re-engineering the RLN is that evolution works without a goal and it does so by small stepwise changes - every one of which must be beneficial (or at least neutral) in the short term (evolution is unable to plan for the long term). Since the right-side RLN is effectively hooked behind the aortic arch, it can't progress back up around the other side of the arch without either severing the nerve or severing the arch - either one of which would prove lethal to the organism.
This doesn't mean that such a mutation couldn't arise; it could. The problem is that any individual that carried such a mutation which severed the RLN would not survive. Dead organisms don't get to add their mutations to the wider gene pool.
What evolution can do is to lengthen the nerve little by little. Each step would be small enough to achievable by mutation, safe enough to occur without harming the organism. It's not a perfect solution, but that's exactly what we would expect from an unguided evolutionary process; jury rigged, slightly shoddy solutions that are just good enough. That is exactly what we see in the RLN.
Mutate and Survive

"A curious aspect of the theory of evolution is that everybody thinks he understands it." - Jacques Monod

This message is a reply to:
 Message 50 by ICdesign, posted 08-09-2010 5:32 AM ICdesign has not replied

  
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