Why do we have two nostrils?

Bilateral symmetry.

A square is always a rectangle, but a rectangle isn't always a square.

-edit-

Hmm, that's maybe not the best analogy I could have made, but it more or less works. If something is radially symmetrical, it wouldn't be classified as bilaterally symmetrical, biologically speaking (at least, that is how I learned it). Aaand I realized I got sidetracked because you said circle, and disregarded the mouth and tongue. Never mind.

Anyhoo, to give a less snarky reason, I would say it follows to have two nostrils since the sinus cavities in your head come in pairs. Now, as to why those are divided, well, I can only offer to you my first post.

Redundancy? Congestion could block the effectiveness of a nostril and you could still smell through the other one.

I suspect the evolutionary pressure would favor the septum being present in order to keep the nasal opening open even when pressure is applied to the face from any direction with a flat surface, yet still remain flexible enough that, as a protrusion, it's not likely to cause problems when hit.

Go ahead and try it. Your head isn't heavy enough, and with the nose protruding, you can't relax on a flat surface in any position and block your breathing through your nose - your head would always roll to one side, and even if it was twisted in a way to force the face flat into the surface it's very hard to completely block the nose.

With a single nasal opening you don't really have that feature, and it would be a very, very strong evolutionary pressure - those without a septum are very likely to die at a higher rate than those with one.

I'm only guessing though. I can't think of any other reason for it. It almost seems like it would be even better if we had one nasal opening on each side of our face, but on the other hand that would increase the total length of the nasal cavity, leading to other problems.

Directional sensitivity doesn't seem to come into play. You can sense the direction of danger using two eyes or two ears, but can you tell what direction a particular smell comes from?

Because how else are you going to stick two straws up your nose and pretend to be a walrus?

ThatNickGuy said:

Because how else are you going to stick two straws up your nose and pretend to be a walrus?

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... those go in your mouth.

This is why you're a terrible walrus.

My mom says I'm a great walrus.

I like Stienman's explanation, but my intuition tells me it has something to do with symmetry and simplicity of design. Think of a fish-like shape: it just seems easier to poke a hole on each side than design a hole in the middle, at a pointy cusp.

I'm more inclined to go with redundancy, just in case one nostril is blocked.

Stieny's septum theory would only work for humans, and possibly other primates. Mammals such as elephants and giraffes also have two nostrils, but very different nasal structures.

The hammerhead shark's nostrils are wide apart, which I believe gives it some directional sensitivity when detecting blood. As far as I can recall, though, such wide nostrils are in the vast minority in nature, so the hammerhead is probably the exception rather than the rule these days. That is not to say, however, that some common evolutionary ancestor didn't use two wide-set nostrils for directional smelling, though.

Perhaps because we evolved from a common group of prehistoric animals that had much better senses of smell, and could use dual nostrils to sense direction and whatnot the same way we humans can still do with our ears and eyes.

ThatNickGuy said:

My mom says I'm a great walrus.

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You're mom said I was the best kisser in the 8th grade.

gryfter seems to have given (an) answer that makes sense to me.

Other than that, I think you're al lbeing a bit anthropocentric. We don't smell directionally, a dog or a deer, for example, can and does. So I'd sya it made sense at some point in evolution, and us humans just evolved other senses more. But hey, gryfter quotes a study that it still makes sense for us too, so...yeah.

Stereo input to detect the direction of who is farting silently.

Dave said:

So you can breath while digging for gold. Duh.

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I usually breathe from underneath my tongue, so I don't have to coome up for air. Am I doing it wrong? My girlfriend doesn't complain...

Why do gorillas have such large nostrils?

Bubble181 said:

I usually breathe from underneath my tongue, so I don't have to coome up for air. Am I doing it wrong? My girlfriend doesn't complain...

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HOW DO YOU DO THAT

tegid said:

HOW DO YOU DO THAT

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Tongue upwards against the teeth/roof of mouth, but don't let it stick in the corners/sides of your mouth. A wide tongue is suppsoedly better, but in-between, making your tongue slightly thinner/pointier helps breathing along the sides.

...what was this thread about again?

klew said:

Directional sensitivity doesn't seem to come into play. You can sense the direction of danger using two eyes or two ears, but can you tell what direction a particular smell comes from?

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I can tell what direction a smell is coming from. It smells stronger if I go toward it or not as strong if I am going in the opposite direction. When I go running in my neighborhood I can smell what people are cooking for breakfast or dinner and know where it is coming from because I can tell if it is a stronger or lighter odor.

You could do the same with only one functioning ear...