A Q-tip is a remarkable invention. It is a paper stalk with cotton batting on each end. They are sold by the millions, perhaps the hundreds of millions. The box lists all kinds of interesting uses for them, such as cleaning your computer keyboard. But we all know what they are really for: cleaning out the ears. For that they are excellent.
I find it particularly felicitous that there is a cotton tip at each end of the stalk, for a total of two, and we happen to have exactly two ears that need cleaning! What are the odds of that?
What if we had three ears? I don’t think Q-tips would sell very well then, because you would need a minimum of two Q-tips to do the job and would end up throwing away one of the Q-tips having used only one end of it. It just would not seem right and I don’t think people would use Q-tips so readily.
So someone would have to come up with a three-headed Q-tip, which is not inconceivable, but no matter what it looked like, it simply would not be as elegant as the simple double-ended Q-tip we enjoy today. It would cost a lot more to produce and would never work as well.
Having two ears is technically useful, especially when they are separated by the distance of the head, as in our case. That allows good echolocation, finding the source of a sound in space. You could do it with one ear, as a rotating or oscillating radar dish does, but that is technically complicated. You could have one fixed ear and scan it by moving your head from side to side, but you can’t move your head at the speed of sound, so precision would suffer. You would simply miss a lot of sounds.
Having a third ear would not give you any particular advantage over the two you already have, and would complicate the wiring quite a bit. The evolutionary cost would be high for very little gain.
So it turns out that two ears is just right: elegant, simple, economical, efficient. Just like a Q-tip, but for different reasons.
Bilateral symmetry in a body does not seem very complicated. The double helix itself is bilaterally symmetrical. So if you’re going to have one ear, you might as well have two. The incremental cost is negligible. But three is too many.
It just happens that a stick has two ends, so each end of a Q-tip can have a cotton swab. There is no a priori reason why that topological fact about sticks should fit so nicely with the symmetry of our developmental morphology.
There are a lot of forms in nature that are not stick-shaped, like loops and branches and ovals. Stick shapes are not terribly common. And of the stick shapes, many, like tails and antennae, do not have two free ends. And even of those that do have two free ends, the ends may not be symmetrical, as in a picked flower or a femur.
There is something non-obvious, even paradoxical, about purpose-built devices for the body. The body allows expression of human intentionality and yet we are perfectly capable of objectifying it to make devices like eyeglasses that hook over the ears. Convenient!
What good would t-shirts be if we didn’t have shoulders? Would scissors ever have existed if our thumbs weren’t just as they are? And isn’t it amazing that Q-tips have exactly two tips! Who thought of that?
We should appreciate Q-tips more for the elegant design they illustrate.