Home > Biology, Uncategorized > The nerve that lost it

The nerve that lost it

I remember Richard Dawkins saying “Your own existence is the most astonishing fact you will ever have to face. Don’t ever get used to it”. By asking us not to get used to it, he is exhorting us understand how great the fortune of existence that has been bestowed upon us is and to make the fullest of it. I was reminded of this quote, a few days back, while commuting to office on train reading “Just Six Numbers”, by Martin Rees. The book talks about how there are 6 basic numerical values that determine the fate of the universe. In one of the chapters he explains how early stars cook basic atoms like Hydrogen and Helium into atoms higher up the periodic table. As I was reading this I saw three people seated next to me discussing ideas to win a photography contest in their company. The discussion itself was not very interesting, but since I was reading about how the atoms that make us were themselves made, I suddenly realised how astounding it is that 7,000,000,000,000,000,000,000,000,000 atoms came together temporarily* to form a group, called Martin Rees, learnt enough to write a book, which is read by another group of about the same humongous number of atoms (7 followed by 27 zeroes), which, called Madhav by other similar groups in the universe, understands it, and not only sees 3 more similar groups of atoms discussing something as abstract as ideas for a photography contest, but also understands to a reasonable extent how those and other similar groups of atoms came about starting right from their origin dating back to about 13,750,000,000 years ago till date. I want to say I was moved, but am holding back because it would have been a ridiculous understatement.

Even though evolution by natural selection is a simple idea, there are some beautiful things it explains which otherwise would have left us baffled. In today’s post I would like to discuss one such fac(e)t of evolution. That is, the short-sightedness of evolution and the consequent imperfect designs of the bodies of individuals. We will also see how this feature of evolution explains a fascinating and famous fact.

The fact first. There is a nerve called the recurrent laryngeal nerve which runs from our brain to our voice box and is present in all mammals. The word recurrent means it branches off from another main nerve. This nerve connects our brain and our voice box (larynx). The curious thing about this nerve is that, in order to go from the brain to our larynx (in out throat), it goes down to our heart (past our voice box) takes a u-turn and then comes back up to enter our voice box in our throat. It is as though it was following a long detour to avoid a one-way road in Chennai. Why should it go down and then come up? Why cant it go directly from the brain to the voice box?

To understand why those questions are important and to realise how bad the design is, we must look at Giraffes. In the giraffe, with its very long neck, the voice box is in the top of the neck, and its heart in its body below the neck. Imagine the nerve that needs to go from brain to its voice box, at the top if its neck, going all the way down the neck to the heart and then coming back up. In Giraffes, the detour by this nerve is as much as 15 feet. That is more than two and half times the height of an average adult human. What a waste of raw material that went into making the nerve, and the energy needed to keep it running? If you don’t trust me, you can see the real nerve, in this dissection of a Giraffe’s neck, done in a classroom. (The video is not for those who are averse to dissections, and hence avoided Biology in their 12th standard)

I intend to explain this fact using an analogy. All of you would have played the game of Tetris. In it, blocks, each consisting of 4 squares, but arranged in 7 different ways (called tetrominoes), keep falling from top, with which we have to keep completing lines at the bottom. The goal of the game is to arrange the blocks in such a way that no gaps exist. The key constraints of the game are that, you cannot undo what you have done, and you do not know what will come next. You have to play the game with what is on hand.

Naturally, you would do a much better job were you given all the blocks upfront, instead of one by one, and are also allowed to go back and change any of your moves, or even start from scratch if you feel you have gone wrong. With the usual Tetris, you would see that many decisions look stupid in hindsight, but since you cant predict what is coming (the blocks are falling randomly), and neither can you undo what you have done already, that is the best you could have done in that situation.

Something similar happened with the recurrent laryngeal nerve. All we mammals evolved from fishes. In fishes this nerve branch went from the brain to one part of their gills directly. There was one artery, a tube that carries blood from one’s heart to other parts of the body, which was then out of the pathway of this nerve. Thus the nerve went directly from the brain to the gills. But with evolution, this artery, that was initially out of the way, slowly started coming in between the brain and the connecting part of the gill. And it was this part of the gill that later becomes the larynx. So this artery started pushing the nerve slowly, away from the shortest path from the brain to the gill (later larynx). Like a marathon winner running through the winning ribbon, the artery started going through the path between the brain and the voice box, thus stretching the connecting nerve into a sort of a horizontal “U”. That explains, to a certain extent, why the nerve takes that detour.

But what do I mean, when I say that the artery pushes the nerve away from its shortest path. Does it push some extent daily, or say, hourly? No. In evolution, when we say something happened, it means it happened across many generations. That is, if the present generation has the nerve extended a bit, its children, or grand children, or somewhere down the line, it gets pushed further. And some more, even more further down the line. That is how the “pushing” happens.

Though it is somewhat easy to see, how the stretching happens, what is the analogy with the Tetris game? More specifically, seeing what are the constraints on evolution that are similar to the two constraints in the game explains why the stretching happened.

The first constraint is obvious. One would never know what the next mutation is going to be. Nor can one know how the environment is going to change tomorrow. This is the counterpart of the constraint in Tetris, where you cannot predict what block is going to come down next. This feature of not being able to see into the future, is one constraint faced by evolution.**

The other constraint in Tetris is that you cannot undo what has been done. Understanding this in evolution needs a bit of explanation. In every generation from fishes to mammals, the neck might have stretched only very very gradually, and in most cases not at all. But the way evolution proceeds is by natural selection. A change will survive if it is not detrimental to the body. The very small stretching of this nerve, would not have given any great disadvantage to the individual when compared to its peers. When comparing fishes and mammals you see that the detour of the nerve is a waste of resources. But when seen from parent to child, it would not have caused a huge disadvantage (it wouldn’t have been even noticeable). So it was not “selected out” of the species. You could argue, that a mutation could have come up where the nerve did not pass around the artery, but went straight from brain to heart, which could have been selected “for”. But the point is that, that would have been too huge a  change to have come about with one mutation. And any mutation that attempted it would have been detrimental to the individual involved. Thus individuals whose nerve stretched a little bit was preferred over individuals whose nerve “attempted” to take a short cut.

The problem is that to move from the point where the nerve is stretched with a detour, to the point where the short cut is used, a lot of intermediate generations were needed. But, for these intermediate generations, the change would have been dangerous. To see that, let us assume, that this change to use the short cut needs two steps. One is a nerve cut off, and the other is a reconnection using the short cut. But the first generation that has its nerve cut off, will have lesser probability to survive (or at least not be able to survive long enough to reproduce), because its voice box will not function. Thus the second change cannot arise. Of course, my two-step assumption is very simplistic, as the actual change will need lot more steps, but it serves the purpose of realising why achieving this short cut, is tough. Thus evolution continued to work by just stretching the nerve on its detour and that is how the “no undo” constraint plays out**. The key is that evolution always has to work starting from where the earlier generation left off. It cant create something radically new overnight. It has to work on what was given to it by the earlier generation.

Here is a video that explains this. I don’t know who it is in the video, though.

And thus it is, that we have ended up with less than perfect design. There are many more examples of this in our body. Another of those “flaws” is the blind spot in each of our eyes. We will discuss that some other time.

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* – This fascinating fact becomes even more fascinating, when we learn that the atoms that we are made of as a baby are not the same atoms that stay with us throughout our life. Our body is made of different sets of atoms at different points of time. Atoms keep entering our body, replacing other atoms that move out. It is somewhat surprising to think, that even though the same material is not used to make our body throughout our life, we still retain all the memories, functions, diseases etc. The analogy can be made to a  Company that when started off would have had different employees, but employees keep moving out and coming in, keeping the Company as a unit. Something similar happens in our body.

** – Undoing is very tough, though not always impossible. Life originated in the seas, and then some species moved to land. But later, some of them went back to the sea, and again came back to the land. So undoing happens, but it is tough.

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  1. March 26, 2011 at 20:30

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