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