When Thomas Hummel catches a hint of an unripe green tomato, he’s instantly transported back to his childhood in Bavaria. In the small bedroom he shared with his brothers, they had three beds and a simple table. “My mother would put those green tomatoes on the cupboard to ripen,” recalls Hummel, who studies olfaction at a German university. “They have a very distinct smell.”
The scent is often grassy, pungent, and somewhat bitter, he explains. Even today, when he walks past a tomato bin at the market, “it’s always somewhat emotional,” he notes. “Every smell carries some emotion.”
Smell is intricately linked to emotion and memory. A whiff of lavender might remind you of a dear friend, while the odor of cheap vodka could bring on groans of nostalgia from college days. Similarly, that particular laundry detergent scent can evoke tears from childhood memories with grandparents.
It turns out smell is our oldest sense, dating back billions of years to ancient chemical-sensing cells. Interestingly, even though there’s a wealth of knowledge about hearing and vision, science hasn’t delved as deeply into the realm of smell. This could be because smell has long been viewed as unimportant for survival—humans have been labeled “bad smellers” for quite some time. Plus, studying it poses its own set of challenges.
“It’s a complex sense,” mentions Valentina Parma, an olfactory researcher in Philadelphia. “We’re still figuring out how different chemicals translate to what we perceive.” But progress is being made to systematically analyze and quantify our sense of smell, breaking it down to its core components—from the odor molecules entering our noses to how those are processed by neurons in the brain.
New databases, including one recently featured in the journal Scientific Data, aim to create a common scientific language to describe molecular scents—essentially, what each molecule “smells like” to humans. On another front, researchers have detailed in Nature how these scent molecules convert into a neural language, activating emotions and memories.
These combined efforts add depth to our understanding of smell, challenging the long-held notion that it’s our least important sense.
Anosmatique
The belief that humans are poor at smelling stems from a century-old misunderstanding.
Back in the late 19th century, French neuroanatomist Paul Broca was exploring why humans possess free will while other animals do not, despite their similar brain structures. He discovered that olfactory bulbs—the key brain areas for processing smells—are relatively small in humans compared to other species. In contrast, the olfactory bulbs in mice and horses are quite large when compared to their overall brain size.
Broca concluded that while animals are driven by smell, humans have the ability to choose to react or ignore these scents. He deemed humans as anosmatique, or “non-smellers,” while labeling some animals as osmatique, meaning “smellers.” This distinction suggested that the control we possess over our sense of smell elevated us beyond mere animals. “He formed this sweeping conclusion,” notes John McGann, an olfactory researcher, “and then he almost immediately passed away.”
However, in no time, English anatomist Sir William Turner misinterpreted Broca’s findings. He mistakenly thought Broca was discussing the ability to smell rather than addressing free will, thus framing humans as poor smellers and dogs as proficient ones.
“The idea transformed through many retellings until people began to claim, ‘Oh, humans just don’t need to smell,’” says Sarah Cormiea, a postdoctoral researcher at the University of Pennsylvania. Freud didn’t help the situation either; he frequently suggested that smell was a primitive sense that lingered from our ancient past.
He might have partially been right. Studies trace the mammalian sense of smell back three billion years to ancient bacteria that used chemical detection to find food. Molecules in the water would attach themselves to bacterial cell membranes, prompting these organisms to move toward or retreat from certain chemicals. This rudimentary form of smell, known as chemosensation, shares similarities with olfactory systems found in complex mammals.
In many ways, our sense of smell is our most ancient interface with the environment, says zoologist Matthias Laska from Sweden. “No single cell can see or hear, but they can all respond to chemicals.”
Today, our chemical sense is incredibly sophisticated. In the 1990s, Nobel Prize winners Linda Buck and Richard Axel revealed genes responsible for odorant receptors in mammals. Additional research indicated that humans have about 400 types of olfactory receptors within their noses, with millions lining the nasal passages. Each receptor is a protein capable of recognizing various odorants—those tiny molecules that evaporate from everything from coffee to damp grass.
When you smell a rose, over 800 distinct odorants engage your olfactory receptors, which transmit patterns to the brain. There are approximately 5.8 million molecules on Earth that could serve as detectable odorants for humans; however, it’s unlikely anyone could realistically determine all of them, Laska notes. Yet, we often underestimate our olfactory capabilities because our vocabulary for it is limited, adds Antonie Bierling, who studies olfaction at a German university. Visually, we might describe a pineapple with its yellow and green characteristics, but can we articulate what it truly smells like?
“This smells like a pineapple,” Hummel muses. “But what does pineapple actually smell like?” Often, our descriptors for smell are tied to their sources—like saying something smells grassy or akin to a wet dog. He poses the question: “What actually makes the pineapple distinct?”
This challenge in articulating smells, particularly in languages like English, has hampered research on human olfaction. Various research teams are now addressing the issue systematically. The connection between the chemical structure of an odor molecule and how it smells is still largely enigmatic, Bierling remarks. “The only way we can change that is by generating data.”
Single-Molecule Smells
If we perceive light at a certain wavelength, we describe it as red. If a sound vibrates at a specific frequency, we hear an F sharp. But there isn’t an easy way to map odors. They often reach our noses as a complex blend of different molecules, and that blend might smell different to each individual based on their prior experiences with that particular scent.
“Real odors are intricate and layered,” Cormiea explains. “People aren’t really clear on which features of an odor, such as the molecules, contribute to different perceptual experiences.”
What makes a flower smell like a flower? What defines cheese’s distinct aroma? Odorous molecules have several characteristics that influence their scent. Are they large or small? What other substances are they interacting with? Do they carry a charge? Even molecules that are mirror images of each other can emit entirely different smells. For example, pine and citrus scents are mirrored forms of the molecule limonene, yet they are perceived quite differently.
