Why we evolved two nostrils (hint: it's all about domination)

There are sound evolutionary reasons why we have two eyes and two ears. By why do we have two nostrils? Researchers at Stanford University have the answer - after doing a study that revealed the weird reason why we don't have one big hole in our nose.

Face holes tend to come in pairs. We have two eyes, and we need both of them. The slightly different picture that each eye gives us is integrated by the brain to give us a three dimensional picture of the world. Because of this, we don't just see, we see depth. We have two ears, and need them. The slight difference in the time sound takes to make it to each ear lets us know the direction of the sound. We don't just hear, we hear direction. And then there's the nose. The nose splits a single passage from the lungs into two tubes right next to each other. Our sense of smell just isn't good enough that the split in the passages helps us orient ourselves. Why do we have two nose holes? Sheer piercing possibilities?

Stanford University kicked off an Olfactory Research Project that took a look at this puzzling phenomenon and found that certain nostrils are, temporarily, better at detecting different smells. Your smell perception changes depending on what nostril you're sniffing through. At any point in time, there is a dominant nostril. The body sucks more air, more quickly, in through one nostril than it does the other. This dominance will actually switch back and forth during the day. What each nostril smells depends on whether or not it's currently dominant.

Researchers tested this by mixing two substances, l-carvone, which smells like peppermint, and octane, which smells like anise. They had people sniff the concoction, first with their low-flow nostril, and then with their high-flow one. Seventeen out of twenty people noticed the octane in their low-flow nostril, and the l-carvone in their high-flow nostril. When the subjects were re-tested a few hours later, their high and low flow nostrils had switched, but the high-flow nostrils still smelled the l-carvone, and the low-flow ones still smelled the octane.

The different smells lie in the different absorption rates of the two chemicals. In order to be detected, the smells don't just have to enter the nose, they have to drill through the mucus to reach the olfactory receptors. A chemical that doesn't absorb quickly can be in the lungs before it even gets a chance to be absorbed by the high-flow nostril. Meanwhile, a slow draw of air into the nostril gives it time to be well and truly sniffed. But why doesn't a highly absorbent molecule get sniffed in both nostrils? Scientists think that the quickly-flowing air through the high-flow nostril embeds the molecule up and down the length of the nose, letting it hit all the olfactory receptors and really make its presence felt. If the molecule absorbs quickly, though, in the low-flow nostril, it will be completely absorbed into the mucus while still just inside the nose. With the quickly absorbed molecules used up, the rest of the nostril will be clear for more slow-absorbing scents.

Two nostrils are better than one, because the different rates of flow might let your brain separate out complicated scents and appreciate them all. You don't just smell, you smell a full chemical analysis.

Image: Great Sea

Via The Cognitive Neurosciences and Nature.