General Question

kingpinlovesyou's avatar

Why do we have two nostrils?

Asked by kingpinlovesyou (312points) July 7th, 2011

What evolutionary purpose do we have for two nostrils and not one?

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34 Answers

SavoirFaire's avatar

In case one gets plugged up, we have a spare.

ANef_is_Enuf's avatar

We only breathe through one at a time.

Brian1946's avatar

Because we have two lungs, and each one is accessed by its own respiratory starting point.

I’d say that we have two lungs because breathing is a critical process, and for that reason we have pulmonary redundancy.


I guess because if one gives out, the other one can take over, sort of like having two kidneys, two eyeballs, two testicles, two lungs, two arms, two legs, etc.

dappled_leaves's avatar

It’s all about the symmetry.

kingpinlovesyou's avatar

But one nostril would be symmetrical, assuming it was in the middle

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marinelife's avatar

For the same reason that we have two kidneys. The functionality is so important to the body that we have a redundancy built in.

syz's avatar

Because we are (largely) bilaterally symmetrical.

gasman's avatar

There doesn’t need to be a “reason” for it—the respiratory tract develops from paired bilateral structures which includes the nostrils. Embryonic / fetal development uses the genetic “toolkit” already at hand from ancestors. It evolved to be “good enough”—from the standpoint of natural selection, “if it ain’t broke don’t fix it.”

raven860's avatar

Why don’t we have two hearts or minds or stomachs…?

Coloma's avatar

Most mammals have two nostrils, most likely as @gasman says.
Unless you were born a whale.
My daughter was a whale fan as a little kid, she called her nostrils blowholes. lol

thorninmud's avatar

The nostrils are an evolutionary relic of our fish ancestry.

Fish nostrils have nothing to do with breathing. In most species, they are used only for smelling and don’t even connect to the respiratory system. So our fish ancestors had 4 nostrils: 2 forward nostrils through which water entered, and two rear nostrils through which it exited. The passages between fore and aft contained the olfactory organs.

At some point in our evolution, the rear nostrils got moved inward and became the choanae, where the two nasal passages join at the very top of the throat. From there all the air travels down a single passage, the trachea, until it branches again to the two lungs (each lung does not have its own nostril).

If you look at our evolutionary history, you could say that the mouth is one big breathing aperture. It just so happened that the nasal passages ended up also feeding into that system. As far as breathing goes, we can really do pretty well without any nostrils at all, which is why we don’t die when we get colds.

Since the nostrils evolved primarily as smelling apertures, there might be some advantage to having two of them in animals, like fish, that depend heavily on smell. Just as having two ears and two eyes provides useful spacial information about stimuli, maybe the difference in the sensation of the two nostrils on either side of a fish’s head conveys some subtle sense of direction or proximity.

MilkyWay's avatar

The same reason why we have two ‘everything else’.

LuckyGuy's avatar

If I was in charge of designing the system I’d give everyone 4. Two large and two small. The large nostrils would would be closed most of the time and would only open when partaking in extreme physical activity and when the other two were plugged. I’d also make it easy to install replaceable pollen filters and would include a wash-out / flush feature via a port in the mouth.

Women would find men with large openings desirable because they imply the ability to engage in athletic amorous activities.
Women would outline their nostrils with black mascara to make them look larger. There might even be a tattoo service that specializes in permanent nostril makeup

And of course the rappers would install fake gold tubes to match the other obligatory bling that defines their individuality.

MilkyWay's avatar

@worriedguy An eccentric but great answer.

Lightlyseared's avatar

Nature likes symmetry.

Coloma's avatar


As long as you are revising the evolutionary blueprint, would you redesign womens reproductive cycle to twice a year, like dogs?

I always thought this would be more than enough to whelp those little pups.

SavoirFaire's avatar

@raven860 If your body had two minds, how would either mind find out about the other?

thorninmud's avatar

Just did a bit of research and found an interesting study that looked at the ability of humans to track scents by comparing the sensory input of the two nostrils. If you can’t access the full article, here’s the abstract:

“Whether mammalian scent-tracking is aided by inter-nostril comparisons is unknown. We assessed this in humans and found that (i) humans can scent-track, (ii) they improve with practice, (iii) the human nostrils sample spatially distinct regions separated by approx3.5 cm and, critically, (iv) scent-tracking is aided by inter-nostril comparisons. These findings reveal fundamental mechanisms of scent-tracking and suggest that the poor reputation of human olfaction may reflect, in part, behavioral demands rather than ultimate abilities.”

Zaku's avatar

@raven860 We do have a two-part brain, and a four-part heart.

@SavoirFaire It is also possible to receive brain damage where the two parts of the brain have reduced or no communication between each other.

It is also pretty common for people to have one side of their brain more or less dominant than the other. Roughly, logic-dominated (left-brained) people versus artistic-dominated (right-brained) people.

The nervous system crosses and hooks up to the opposite side of the brain. Having a nostril and smell system for each nostril thus gives each half of the brain its own smell information (though you could just have one hole and two sides of smellers inside, but still).

Finally, since some people over-reason evolution and require survival “reasons”, consider how attractive you would find someone with only one nostril. ;-) If it’s a turn-on, I’m sure the cosmetic surgery would be easy… ;-P

SavoirFaire's avatar

@Zaku It was a joke…

Also, two-part brain ≠ two minds.

LuckyGuy's avatar

@Coloma That is a great idea! 2x per year is plenty in today’s overpopulated world. It would reduce the number of accidents to 1/6 the level of today.
On the male side, I would install a manually activated valve in the male vas deferens that was only accessible to the partner.

I think we should patent these ideas and start working on the implementation immediately. This will take a lot of field work….a lot of repeated field work. ;-)

Nimis's avatar

Along the same lines as what thorninmud said, I think anything involved with the five senses from afar (i.e. sight, smell and hearing) require two of each to add a spatial dimension. (e.g. depth perception with two eyes) We’re hunters by nature and need to know how to track our prey.

chyna's avatar

@worriedguy I’m kind of grossed out by all that.

Meego's avatar

We have 2 nostrils in case one fails…and in the rarest circumstance if they both fail we can breath through our 1 mouth, that’s weird.

If you want to be technical….
We have the
Frontal lobe
Parietal lobe
Occipital lobe
Temporal lobe…
The human brain controls everything so is it fair to say we have one mind? Some ppl believe they have 5 or 6 different ‘minds’ telling them what to do try to tell them they have “one mind”.

Qingu's avatar

I think the question is really about bilaterianism.

If you look at your body you’ll notice that you have two of most things. Two eyes, two ears, two arms, two legs, two lungs, two kidneys, even two clearly-defined halves to your brain.

Look at almost every other animal and you’ll notice the same thing. Even insects have two “sets” of legs. Almost all animals have a very particular symmetry, called bilaterianism, or two-sidedness. The only exceptions, in fact, are very simple animals like sponges and jellyfish (even starfish are born with bilaterian symmetry; they change into five-sided shapes later in life.)

So, why do we have it? The short answer is that bilaterian symmetry must have evolved very early in the animal kingdom. It is a basic template that virtually every complex animal is built around. You don’t have four nostrils or six hearts because evolution would need to throw out the entire basic structure for your body and start over from scratch to get there. Evolution likes small changes built onto rigid underlying structures, and bilaterianism is among the most rigid structure in the whole animal kingdom.

But that raises another question: why two sides and not three? The best explanation I’ve heard is that it has something to do with the digestive tract. Because that’s one of the other fundamental structures that all animals have in common: we’re almost all built around a tube that has a mouth at one end and an anus at the other. The only animals that aren’t—jellyfish and sponges—also aren’t bilaterially symmetrical! What does two sides have to do with a digestive tract? Such animals need a front (the part of their body with a mouth) and a back (the part of their body with an anus). That’s two sides. Nature built the symmetry around this basic function.

LuckyGuy's avatar

@thorninmud Scent tracking! That is fantastic. I know we humans can to that with hearing but it never occurred to me that we do it with scents. Great study. Thank you!

raven860's avatar


There are four chambers in the heart yes but they are essential for it to act as a pump that replaces bad blood with good.

I am not sure about the brain.

gasman's avatar

@qingu ”Such animals need a front (the part of their body with a mouth) and a back (the part of their body with an anus). That’s two sides. Nature built the symmetry around this basic function.

Similarly this Wikipedia article on Bilateria says: ”…bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back…

I respectfully suggest this is wrong. Bilateral symmetry refers to mirror symmetry on either side of a central plane, conferring left and right sides. It does not imply anything about top & bottom or front & back.

Wikipedia contradicts itself here with this defintion:
In bilateral symmetry (also called plane symmetry), only one plane, called the sagittal plane, will divide an organism into roughly mirror image halves…Thus there is approximate reflection symmetry. Often the two halves can meaningfully be referred to as the right and left halves, e.g. in the case of an animal with a main direction of motion in the plane of symmetry.

Echinoderms like sea stars seem to have radial symmetry, but are considered bilaterians because their larvae are bilaterally symmetric. And of course most animals have a distinct front and back for physiologic reasons, as well as top & bottom due to Earth’s gravity—meaning they lack symmetry of those kinds.

raven860's avatar

I think its safe to say that we need exactly two eyes. We can make out depth that way. 1 is 1 too less and 3 is one too many.

Similarly with ears, its easier to locate the origins of the sound with two ears than one.

My point is that in at least those two cases we don’t have backups but a dual-component system. Although at the loss of one eye or ear each system can function at a less effective rate.

Qingu's avatar

@gasman, yea, you’re right, I should have been more careful. The mouth-anus “sides” are different from the mirrored bilaterian sides. But bilaterianism seems to have emerged side-by-side (heh) with the mouth-anus digestive tract, which was the point I was trying to make. And if you think about the simplest way to “fill out” something that is basically a tube with a front and back, mirrored sides is the most economical.

neospeaker's avatar

1. Airflow is greater into one nostril than the other, due to slight swelling that obstructs airflow.
2. Switches nostrils several times per hour

Both part of nostril are designed as such that odorants absorb across nasal mucosa at different rates
a) high-absorption odorant – induces small response when airflow is low, and large one when airflow is high
b) low-absorption odorant – large response when airflow is low

odorants do indeed smell different in nostrils, depending on the air flow and absorption of the odorant! so thats why we have 2 nostril.

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