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Binocular vision gives us a 3D perspective, and two ears let us locate the direction of a sound. But why two nostrils? (continued)
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Matthew Ravosa
Johns Hopkins University School of Medicine, Baltimore, Maryland, US
There are several reasons why two nostrils or “nares” are beneficial in terrestrial vertebrates. In contrast to fish, where the nostrils open to sensory cavities lined with chemoreceptors for detecting waterborne odours, the nostrils of terrestrial vertebrates are “doors” to both sensory cavities and passageways connected to the lungs. Accordingly, they serve multiple functions: they are involved in both chemosensation of airborne scents and inspiration (also called inhaling).
In humans, there is evidence that at any given time, one nostril is differentially focused on facilitating airflow into the nasal cavity as part of the initial phase of the breathing process. Simultaneously, the other nostril is tasked with sampling airborne odours and chemical cues, more colloquially referred to as smelling or olfaction. The dominant nostril devoted to breathing inhales air more rapidly, with this asymmetry in functional patterns switching between left and right sides every few hours.
Because bilateral symmetry is such a successful body plan, it must convey significant selective benefits
Jonathan Wallace
Newcastle upon Tyne, UK
It should be borne in mind that we have evolved from an extremely long lineage of bilaterally symmetrical animals extending right back to distant invertebrate ancestors. Bilateral symmetry in animals gives rise to paired organs on either side of the longitudinal axis, as we can see when we look at the wings and legs of an insect or most of the organs in the human body. It might be more appropriate, therefore, to ask for an explanation for those cases that diverge from this symmetry, such as toothed whales with their single nostril, as such instances require a break away from the ancestral body plan.
Because bilateral symmetry is such a successful body plan, evolutionarily speaking, it is reasonable to conclude that it must convey significant selective benefits. Animals with symmetrical, paired body parts may benefit in various ways, such as the binocular vision and stereo hearing suggested by the questioner, redundancy (lose one kidney to injury or disease and you can still function with the other), ability to fly and development of a streamlined body shape, among others. But the adaptive benefits of bilateral symmetry that we see in extant animals may mostly be secondary benefits that weren’t involved in the original evolution of our first bilaterally symmetrical ancestor.
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