Introduction
Scent – the most underestimated yet deeply embedded human sense. While we close our eyes and sharpen our hearing, the nose quietly continues its work in the background, unnoticed but indispensable. It is a paradox: in our culture, smell is often seen as the least important sense, yet it profoundly shapes our wellbeing, our memories, and even our survival.
Why Can Humans Smell? The Biology of Scent
The Olfactory System
Smelling is a biological masterpiece. When odor molecules in the air enter the nose, they bind to specialized receptors in the olfactory epithelium – the sensitive tissue high up in the nasal cavity. This binding triggers electrical signals that are sent via the olfactory nerve directly to the olfactory bulb in the brain.
The human body has about 6 million olfactory receptor cells, and in the olfactory epithelium thousands of different odor receptors can be found. These receptors work as a “lock-and-key” system: each receptor type recognizes only certain types of odor molecules.
The Direct Connection to the Emotional Brain
The olfactory bulb sends signals directly to key brain regions that define us as human beings: the piriform cortex (which identifies smells), the amygdala (the emotion center), and the hippocampus (the memory center). This is fundamentally different from other senses. While vision and hearing first pass through the thalamus (the brain’s relay station), smell goes straight into our emotion and memory centers.
This means that scent reaches the brain faster than thought – an odor can trigger a memory or feeling before we even have time to think.
Evolutionary Advantage
Why do we have this system? Smell is one of the oldest senses in evolution. Even primitive life forms, from insects to mammals, rely on scent for survival. Fossil research shows that early whales – which now have very little sense of smell because they adapted to water – had a very well-developed olfactory bulb.
Although humans have fewer olfactory receptor genes than many other mammals (about 400 functional receptors compared to thousands in rats), our advanced brains compensate for this. Our larger brains can extract much more information from the same olfactory input.
How Important Is Scent for Us? Multiple and Deep Effects
Smell is not just another sense – it is a vital system with multiple functions.
1. Food Intake and Flavor
The first temptation of a meal does not come from taste, but from scent. Taste – strictly speaking – is limited to just five basic qualities: sweet, sour, salty, bitter, and umami. All the richness of flavor that we associate with “taste” – the complexity of a chocolate truffle, the subtlety of a wine, the distinctive note of cinnamon – comes from smell.
When odor molecules travel from the mouth to the nasal cavity (retronasal olfaction), they enrich the basic sense of taste with countless nuances. Without smell, eating becomes mere refueling – nutrition without joy.
2. Warning and Safety
Smell warns us of danger. The sharp scent of spoiled food, the smell of gas, the smoke of a fire – smell protects us against invisible threats. People without a sense of smell have two to three times more household accidents and poisonings than people with a normal sense of smell.
3. Emotion and Psychological Wellbeing
This may be the most remarkable aspect. The direct connection between smell and the limbic system – the emotional center – means that certain scents can instantly change our mood. Research shows that specific aromatic compounds, such as alpha-pinene (found in pine oil), can reduce anxiety by influencing BDNF gene expression in the olfactory bulb and hippocampus.
4. Social Functioning and Relationships
Body odors and personal scent play a crucial role in human interactions and romantic attraction. This goes far beyond romance alone – the smell of a loved one, your children, your home – these are emotional anchor points.
5. Memory and Identity
A particular scent can bring back a memory in seconds that has been buried for decades – the perfume of your grandmother, your mother’s kitchen, grass after the rain. These “scent memories” are often stronger and more emotional than visual or auditory memories.
The Hierarchy of the Senses: Where Does Smell Really Stand?
The Historical Myth
There is a widespread belief that the senses can be ranked in a hierarchy: sight (first), hearing (second), touch (third), taste (fourth), and smell (fifth). This hierarchy goes back to the work of the French neuroscientist Paul Broca in the 19th century, who observed that humans have smaller olfactory structures compared to animals.
The Modern Reality
This classic ranking is misleading. Research from the University of York showed that this hierarchy is not universal – it varies between cultures and contexts. Moreover, modern neuroscience shows that although human olfactory organs are relatively small, our advanced brains can do more with the information they receive.
Neuroscientist John McGann adds that despite humans having only about 400 olfactory receptors (much fewer than many other mammals), these still offer enormous capabilities. “There are very few smells that people cannot detect, even though we have far fewer receptors than rats, mice, and dogs,” says McGann. What matters is not the number of receptors, but what the brain does with them. Human olfaction is essential for health and wellbeing, and losing it has significant consequences.
Smell in Animals: Another Universe
The Overwhelming Superiority of Animal Smell
While humans have about 6 million olfactory receptor cells, dogs have more than 100 million. Even more impressive: bloodhounds can use their sense of smell up to 100 million times more sensitively than humans. A typical dog can smell something from about 800 meters away, and some specialized breeds can track scent trails that are weeks old.
The processing of scent in their brain is also very different. While in humans roughly 5% of brain volume is devoted to olfactory processing, in dogs it is about 33%.
Smell as a Primary Sense for Animals
For animals, smell is not just a supporting sense – it is their primary lens on the world. A dog “sees” its world through smell, with scents forming a continuous narrative of its surroundings.
This goes far beyond detecting food. Smell enables animals to:
- Navigate: Many animals follow scent trails for long-distance navigation
- Find mates: Pheromones communicate sexual readiness over kilometers
- Mark territory: Scent marking establishes territorial boundaries and social hierarchies
- Detect threats: Animals smell predators long before they see them
For insects such as moths, pheromones from females can be detected by males over long distances – an absolutely crucial reproductive mechanism.
Evolutionarily Fundamental
Research shows that for animals in the wild, olfactory ability can literally be the difference between life and death. In fruit fly larvae (Drosophila), a functioning olfactory system is necessary for effective food searching and survival to adulthood, especially in competitive environments.
What Do Humans Miss When We Cannot Smell Well?
The question “What do we miss without smell?” is easier to answer when listening to people who have lost it.
Loss of Pleasure in Eating and Nutritional Quality
Patients with anosmia (complete loss of smell) report that food loses much of its pleasure. One review summarized it as: “Food becomes nutrition without joy.” Some patients eat less because their interest in food declines. Others avoid cooking altogether because they cannot tell whether food has spoiled or because the cooking process no longer feels rewarding.
This can have serious consequences: weight loss, nutritional deficiencies, and a decline in general health may follow.
Psychological and Emotional Consequences
The impact on mental health is profound. Around 25–33% of patients who lose their sense of smell show depressive symptoms. And it is not a one-way street – there is a reciprocal relationship: people with major depression also often have reduced olfactory sensitivity.
Neurologically, this makes sense: anosmia reduces the amount of sensory input reaching the limbic system from the olfactory bulb, making it harder to regulate emotions and increasing feelings of anxiety and sadness.
One person with congenital anosmia described it like this: “I feel like I’m behind a barrier. Behind glass. It feels like that emotionally with people, too. I know they’re there, and they are people I love, but there is a connection that’s missing.”
Social Isolation
Smell contributes to intimate human contact. Losing it can feel like a meaningful separation. A father with anosmia put it very simply: “My kids are grown now, but they still smell like my kids. Yet every time I see them and hug them, I don’t get that connection I used to have.”
Studies show that anosmia is linked to increased social insecurity and reduced romantic relationships.
Risks and Safety
Without smell, people cannot detect dangerous situations: gas leaks, fire, spoiled food. This is not just theoretical – patients with anosmia demonstrably have more household accidents.
Professional Consequences
For some professions, anosmia can be career-changing. Chefs, perfumers, sommeliers, nurses – they all depend on an intact sense of smell. Up to 60% of patients must adapt their job roles, and around 5% have to change careers entirely.
Cognitive Decline
Here it becomes especially worrying. Research shows that anosmia or hyposmia (reduced smell sensitivity) is associated with:
- Alzheimer’s disease and cognitive decline: Impaired smell predicts cognitive decline more strongly than episodic memory tests. The link is literal: amyloid-beta (the pathological hallmark of Alzheimer’s) accumulates in olfactory regions.
- Depression and anxiety disorders: Beyond smell loss itself, broader cognitive and emotional consequences can appear.
- Reduced body awareness: Anosmia is associated with poorer interoceptive awareness (the perception of internal bodily signals).
A World Without Scent: The Scenario
Imagine what a world without scent would look like.
Everyday Pleasures Lost
Every meal would have roughly the same structure, the same nutrients – the same boredom. The rituals we have built around food – the aperitif, the gastronomic experience, the smell of home – would lose much of their meaning.
Social and Romantic Bonds
Dating would be more limited. Human attraction is inseparably linked to scent – the classic “chemistry” between two people is quite literally chemistry. This invisible dimension of attraction would disappear.
Loss of Identity and Memory
The sensory texture of life – the scent of your homeland, the smells of your childhood, the aromas that remind you of loved ones – would vanish. Memories without an olfactory context would feel much flatter and less vivid.
Increased Physical and Psychological Risks
Without the early-warning system of smell, accidents would become more frequent. Rates of depression would likely rise. Overall wellbeing would noticeably decline.
Ecological Loss
Importantly, a scentless world would not just affect humans. In the natural world, without olfactory communication, pollination and many reproductive mechanisms would collapse. As climate change alters the scents of flowers (a process already observed), the interaction between plant and animal begins to break down – a core relationship in many ecosystems.
Does Poor Smell Also Affect Our Other Senses?
This is a fascinating field of research. The answer is yes – in several ways.
Multisensory Compensation
When the olfactory system is impaired, other senses take on a greater role. Studies show that people with anosmia display enhanced audiovisual integration – their brains combine sound and sight more efficiently to compensate for the missing sense of smell.
Interestingly, congenital anosmia (present from birth) leads to larger benefits from having two sensory inputs in visual tasks.
Visual Effects
Vision and smell are linked in perception. Research indicates that when we smell something, our eyes automatically move toward the source of that scent. Without smell, this automatic visual guidance is lost.
Changes in Touch Perception
Perception of texture and mouthfeel is altered in the absence of smell. This is particularly noticeable around food: eating is not only taste and smell – it is also texture, temperature, and sound (the crunch of a crisp bite).
Interoceptive Changes
Anosmia has been linked to reduced interoceptive awareness – the ability to sense internal bodily signals. This can contribute to eating disturbances and a disrupted sense of one’s own body.
It Is Not a Simple Adaptation
It is important to stress that although other senses can “pick up the slack,” this does not mean the system is intact. It is more of a reorganization – a form of neuroplasticity where other systems are strengthened, but something is always lost.
Scientific Insights Summarized
In short, research shows:
- Neurological: Smell has unique, direct neurochemical pathways to emotion and memory centers that no other sense shares.
- Evolutionary: Even though humans have fewer olfactory receptors than many mammals, our advanced brains process scent in a highly sophisticated way.
- Health: Olfactory loss predicts Alzheimer’s disease, cognitive decline, depression, and reduced quality of life.
- Everyday Life: Smell strongly shapes food intake, emotional wellbeing, social functioning, and safety.
- Psychophysiological: There is a bidirectional relationship between anosmia and depression – loss of smell can trigger depression, and depression can reduce olfactory sensitivity.
Conclusion: Appreciating the Silent Sense
Smell may be the most undervalued human sense. Because it operates quietly in the background, we easily forget how crucial it is to our lives. Yet when it is lost, the impact is overwhelming.
The essence of scent is this: it is not just a sense. It is a bridge between the physical world and our inner emotional landscape. It is what turns nutrition into experience, contact into bonding, and memories into meaning.
If there were more appreciation for this remarkable system – more gratitude for the scented moments that define and connect us – life would likely feel fuller, richer, and more alive.
Neuroscientist Sandeep Robert Datta of Harvard put it this way: “It is now clear that our sense of smell, while not as robust as that of a mouse or a bloodhound, is deeply connected to our cognitive centers, our emotional centers, and our memory centers. We rely on it for a sense of wellbeing and of being centered in the world.”
Today, take a moment to smell something – notice how it affects you, how it makes you feel in your body. This quiet sense deserves our attention and appreciation.