Is there a smell that has the power to transport you back in time? I bet there is. For me, just one whiff of an old-style canvas tent and I’m back camping with my family as a young kid. There are other evocative odours too. The yeasty aroma of bread dough rising in the sun and an Autumn bonfire piled high with damp plane tree leaves. Our noses are exquisitely sensitive and we are beginning to understand how smells are linked to emotion and memory.

A Sunday roast? Grandma’s apple cake? Freshly-cut hay? Sometimes just a whiff of a familiar smell and we’re transported back in time. Image credit: Anaïs via Flickr

The science of smell

Smells are complex. For example, the scent of chocolate is made up of hundreds of different odour molecules. Our noses have 1,000 smell receptors, which occur in a small area in the upper part of the lining of our nostrils. These receptors detect odour molecules and send messages to the brain, resulting in us smelling a particular smell. There are also around 1000 genes involved in the process and the research that uncovered this process was awarded a Nobel Prize back in 2004.

Early estimates put the number of different smells our noses could distinguish at around 10,000. But research published last year upped that figure to one trillion different smells. That figure is an average – the range is 80 million to a thousand trillion unique scents for different people. These findings give our noses first prize for most sensitive organ in our bodies (in terms of the number of different inputs it can tell apart).

Why are our noses so sensitive? It’s possible we evolved to be great at detecting smells for safety. Scientists suggest our sense of smell may have been just as important as language in giving us modern humans an evolutionary leg up. The argument: it was a distinct advantage for our ancestors to be able to smell fire, tasty but hard-to-find food or food that had gone off.

Indeed, many women (myself included) will tell you they knew they were pregnant way before any pregnancy test could confirm it because of their enhanced sense of smell. It makes sense we have evolved to be particularly good at detecting food that could cause miscarriage. It’s also been proposed morning sickness nausea is a result of this heightened sense of smell.

And in fact without smell, we can’t even taste food. Next time you are congested, notice how little taste your food has. Or try this simple experiment: pinch your nose shut with your fingers and eat a jelly bean. It will taste a little sweet, but you won’t be able to work out what colour your jelly bean. Not until you open your nose, swallow, and allow the smell of the jelly bean to enter your nostrils.

Why smells evoke memories

Smells are surer than sounds or sights to make your heart-strings crack Rudyard Kipling

This Kipling line comes from the poem Lichtenberg in which an Australian soldier is transported home in his mind during the Boer war by the smell of wattle blossoms. I can relate. Pretty much the only time I felt homesick during a year living as an exchange student in Europe as a 16-year old was when I came across gum trees in Italy. The power of that smell to make me miss home took me by complete surprise. I completely agree that our noses are emotional time machines.

Scientists call this experience of particular smells making us immediately recall childhood memories olfactory-evoked recall. The effect is also known as the Proust phenomenon after a description in Proust’s In Search of Lost Time. In the scene in question, the author suddenly vividly recalls long-forgotten childhood memories while smelling a tea-soaked madeleine biscuit.

The most common nostalgic smells are associated with baking bread and cakes but other cooking smells are also common. And smells are not only good at evoking memories in general, they are particularly linked to emotional memories.

There are a number of theories as to why smell, memory and emotion are so tightly linked. A huge clue comes from the fact the olfactory bulb (the part of the brain responsible for processing information about smells) and the amygdala and hippocampus (regions of the brain associated with emotion and memory) are all located in close proximity.

Smells are connected to memories by networks of nerves in our brains and it has been shown when we smell something during an emotional experience, that smell is neurally woven together with the memory in the same region of the brain. Once the link has initially formed, a distinct smell has an amazing capacity to trigger the associated memory for many years to come.

What else do smells tell us?

Smells are powerful in other ways too. Your sweat can signal whether you are happy, and it’s no myth you can smell a person’s fear.

Research shows we are also good at sniffing out compatible romantic partners. Many animals, including humans, can identify potential partners that are genetically distinct from ourselves. Among other positives, this generally results in children with efficient immune systems. Men are also able to determine when a woman is most fertile based on her smell.

Businesses use scents to lure us into shops and to encourage us to be in the mood for shopping. If you google scent branding you’ll see what I mean. Something else to be aware of next time you go shopping.

For now, I say close your eyes and take a long, deep breath. How many different scents can you smell? How many of them can you put a name to? Marvel for a moment at the amazing capacity of your humble nose.

Links and stuff

• Listen to an interview with Leslie Vosshall whose research resulted in the 1 trillion smells estimate
• The Science of Scent: Luca Turin’s wonderful TED talk
• What’s that smell? Totally cerebral podcast episode
• Scents and Sensibility – the role of scent in our love lives

Radio on demand

 This post accompanies a radio segment on Triple R’s Breakfasters program on Wednesday 6 May 2015.