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Why we have two nostrils instead of one big hole
(www.popsci.com)
We have stereo smell to help with locating smells. There's also the nasal cycle. One nostril/sinus handles most of the airflow, then they swap (the sinuses are separate until they get to the throat). That way one can recover moisture, plus some smells are more easily detected with fast airflow and others with slow. So the nostrils functioning differently gives us a broader range of odor detection. What else? Umm, bilateral symmetry and redundancy is useful.