Why do we have two nostrils?
• Most animals, and many plants, are bilaterally symmetrical. The evolutionary origin of this trait can be traced back hundreds of millions of years. Our two nostrils are a consequence of this process.
What is much more interesting, though, is that there is little symmetry between the two nostrils, either in shape or function. For example, many people can smell different odours in each nostril, and you can test this yourself.
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I have also noticed that when I have a virus, it usually only causes a blockage in one nostril at a time, which I would think has to do with the fact that killing the host is not usually in the best interests of an infectious agent.
Many organisms are not truly bilaterally symmetrical, however. Our heart and liver do not conform, for example, and some flatfish become asymmetrical in adulthood. But like everything else, there are exceptions to prove the rule.
Tony Holkham, Boncath, Pembrokeshire, UK