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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?
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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.
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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.
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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.
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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.