What is smell?

Smelling is a complex set of chemical reactions that helps us make sense of the world. The ability to smell developed much earlier than our other sensory perceptions, and it actually is the first sense you use when you’re born.
When you smell, air filled with volatile molecules goes through your nose. Different materials and entities have different levels of volatile substances, which is why you'll find a plant very fragrant, and steel not so much. This is also why it is strongly advisable not to smell something stinky for a long time.

How does smell work exactly?

At the very back of our noses, there is a region called the olfactory epithelium – that’s where our olfactory receptor cells are. There are specific receptors reacting to specific smells, and these receptors actually are the only part of your nervous system that is in direct contact with the outside world. Different regions of our brains then convert these chemical stimuli, and make sense of them.

Our sense of smell is first processed in our limbic lobe, which is deeply connected to our memories, impulses and emotions. Odors can trigger memories or influence how you feel before you become aware of smelling the specific scent that triggered it. This is the reason why you might have undefinable memory flashes or feelings when exposed to some smells, or why you’ll react emotionally when someone is wearing your ex’s perfume.

Smell and taste are also intimately connected. Actually, much of the things that we taste are in fact smells. The taste receptors on our tongues only taste bitter, sweet, salty, sour and umami. All the tastes more complicated than that are smells. The taste / smell of a fruit that has gone bad for instance is hard to describe: it might not be sour of bitter, and certainly not umami – and yet, you taste it. Thus, our ability to smell is classified in ortho-nasal olfaction (odorants through the nose), and retro-nasal olfaction (odorants reaching our nose through our mouths).

Because of taste and smells' strong link, smells have shaped the evolution of different species quite a bit. Cats for instance have lost their ability to smell anything sweet because of multiple generations of carnivores. Dolphins can’t taste or smell anything sweet, umami or bitter – they don’t actually taste their food at all before swallowing it. Even bacteria can smell and taste to a certain degree. (1)

Different animals make different sense of different scents, but the smell of smoke apparently is a warning signal for all mammalian species.

All people don't smell the same things

Even if we as humans all have the same olfactory neurons, it’s interesting to note that not everybody can smell the same things. One famous example of this is the so called asparagus pee. About 25 % of the population smells a strong, unpleasant smell in their urine after eating asparagus. 75 % won’t notice the difference. The results vary depending on the sample of population, apparently (2). A study has also found that depending on your genes, androstenone – a derivative of testosterone –might smell like vanilla, or urine (again). This brought an answer to a question that had been tormenting me for a long time. I have always found that some men smell foul for no apparent reason, but now I know why.

Things also smell different in different places and temperatures, as heat affects molecular volatility. The nose works best in humid, warm air because dry, cold air doesn’t trap fragrant odorants as effectively. This is one of the reasons why everything smells and tastes better in the tropics, and mountain air feels less fragrant.

What does it all mean?

What we know is that initially, our olfactory abilities evolved to tell us things about the world that our other senses couldn’t. Our sense of smell warns us not to eat something that has gone bad, or not to stay near bas smells. Even if we rarely think about smelling, we do it constantly, as we breathe.
Smells have not been a popular scientific research subject, in part because smell was so hard to measure until recently, and because we simply didn't understand how our smell receptors worked until 2004 (the discovery won a Nobel Prize). Yet, we have known for a long time that they play a really important role in our biology and cultures. Noam Sobel, a Stanford University neuroscientist specialised in olfactory studies put it well: our western societies might not value smell, but regarding our life related to food and sex, it's our primitive brains that make the decisions (3).

We are a visually dominant species, but our sense of smell is not as bad as we might think and we use it for more than we think. We are all cognitively affected by odors whether we we understand and recognise their influence or not.We will never smell as accurately as, say, dogs, but we do have bigger brains. Our initial olfactory signals are less finely tuned, but our brains can make more sense of what we smell, and thus also make better use of the things that we smell. Smells are not just smells to us, but rather signals that we interpret, and attach meanings to. Only humans could come to the conclusion than the stinkier the cheese, the better it is.