The Universe of Discourse

Mon, 16 Oct 2006

Why two ears?
Aaron Swartz, remembering my earlier article about interesting science questions, sent me a reference to an interesting article about odd questions asked of students applying for admission to Cambridge and Oxford universities.

The example given in the article that I found most interesting was "Why don't we just have one ear in the middle of our face?". As I said earlier, I think the mark of a good question is that it's quick to ask and long to answer. I've been thinking about this one for several days now, and seems pretty long to answer.

Any reasonable answer to this question is going to be based on evolutionary and adaptive considerations, I think. When you answer from evolutionary considerations, there are only a few kinds of answers you can give:

  1. It's that way because it confers a survival or reproductive advantage.
  2. It's that way because that's the only way it can be made to work.
  3. It's that way because it doesn't really matter, and that's just the way it happened to come out.
All of these, I think, have a role to play here. Having two ears is useful for redundancy: if you lose one, you can still hear, so there is a survival advantage to having two ears, just as there is for having two eyes and two kidneys. Why two eyes? In case you lose one. Why two kidneys? In case one fails. Why two nostrils? So you can still breathe even when one is clogged.

(Why only one heart? There's no benefit to having two; if you lose 50% of your cardiac capacity, you'll die anyway. Why one mouth? It needs to be big enough to eat with, and anyway, you can't lose it. Why one liver? No reason; that's just the way it's made; two livers would work just as well as one. Why two lungs? I'm not sure; I suppose it's a combination between "no reason, that's the way it's made" (#3 above) and "because that way you can still breathe even if one lung gets clogged up" (#1).)

The positioning of your ears is important. Having two ears far apart on the sides of your head allows you to locate sounds by triangulation. Triangulation requires at least two ears, and requires that they be as far apart as possible. This also explains why the ears are on the sides rather than the front.

Consider what would go wrong if the positions of the eyes and ears were switched. The ears would be pointed in the same direction, which would impede the triangulation-by-sound process. The eyes would be pointed in opposite directions, which would completely ruin the triangulation-by-sight process; you would completely lose your depth perception. So the differing position of the eyes and ears can be seen a response to the differing physical properties of light and sound: light travels in straight lines; sound does not.

The countervailing benefit to losing your depth perception would be that you would be able to see almost 180 degrees around you. Many animals do have their eyes on the side of their heads: antelopes, rabbits, and so forth. Prey, in other words. Predators have eyes on the fronts of their heads so that they can see the prey they are sneaking up on. Prey have eyes on the sides of their heads so that predators can't sneak up on their flanks. Congratulations: you're predator, not prey.

Animals do have exactly one nose in the middle of their face. Why not two? Here, triangulation is not an issue at all. Having one nose on each side of your head would not help you at all to locate the source of an odor. So the nose is stuck in the middle of the head, I suppose for mostly mechanical reasons: animals with noses evolved from animals with a long breathing tube down the middle of their bodies. The nose arises as sensors stuck in the end of the tube. This is another explanation for the one mouth.

Another consideration is symmetry. The body is symmetric, so if you want two ears, you have to put one on each side. Why is this? I used to argue that it was to save information space in the genome: there is only so much room in your chromosomes for instructions about how to build your body, so the information must be compressed. One excellent way to compress it is to make some parts like other parts and then express the differences as diffs. This, I used to say, is why the body is symmetric, why your feet look like your hands, and why men's and women's bodies are approximately the same.

I now think this is wrong. Well, wrong and right, essentially right, but mostly wrong. The fact is, there is plenty of space in the chromosomes for instructions about all sorts of stuff. Chromosomes are really big, and full of redundancy and junk. And if it's so important to save space in the chromosome, why is the inside of your body so very asymmetric?

I now think the reason for symmetries and homologies between body parts is less to do with data compression and storage space in the chromosome, and more to do with the shortness of the distance between points in information space. Suppose you are an animal with two limbs, each of which has a hand on the end. Then a freak mutation occurs so that your descendants now have four limbs. The four limbs will all have similar hands, because mutation cannot invent an entirely new kind of hand out of thin air. Your genome contains only one set of instructions for appendages that go on the ends of limbs, so these are the instructions that are available to your descendants. These instructions can be duplicated and modified, but again, there is no natural process by which a new set of instructions for a new kind of appendage can be invented from whole cloth. So your descendants' hands will look something like their feet for quite a long time.

Similarly, there is a certain probability, say p, of an earless species evolving something that functions as an ear. The number p is small, and ears arise only because of natural selection in favor of having ears. The chance that the species will simultaneously and independently evolve two completely different kinds of ear structures is no more than p2, which is vanishingly small. And once the species has something earlike, the selection pressure in favor of the second sort of ear is absent. So a species gets one kind of ear. If having two ears is beneficial, it is extremely unlikely to arise through independent evolution, and much more likely to arise through a much smaller mutation that directs the same structure, the one for which complete instructions already exist in the genome, to appear on each side of the head.

So this is the reason for bodily symmetry. Think of (A) an earless organism, (B) an organism with two completely different ears, and (C) an organism with two identical ears. Think of these as three points in the space of all possible organisms. The path from point A to C is both much shorter than the path from A to B, and also much more likely to be supported by selection processes.

Now, why is the outside of the body symmetric while the inside is not? I haven't finished thinking this through yet. But I think it's because the outside interacts with the gross physical world to a much greater extent than the inside, and symmetry confers an advantage in large-scale physical interactions. Consider your legs, for example. They are approximately the same length. This is important for walking. If you had a choice between having both legs shortened six inches each, and having one leg shortened by six inches, you would certainly choose the former. (Unless you were a sidehill winder.) Similarly, having two different ears would mess up your hearing, particularly your ability to locate sounds. On the other hand, suppose one of your kidneys were much larger than the other. Big deal. Or suppose you had one giant liver on your right side and none on the left. So what? As long as your body is generally balanced, it is not going to matter, because the liver's interactions with the world are mostly on a chemical level.

So I think that's why you have an ear on each side, instead of one ear in the middle of your head: first, it wouldn't work as well to have one. Second, symmetry is favored by natural selection for information-conserving reasons.

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