Olfaction Is the Oldest Sense

When complex life established a foothold on Earth 1.5 billion years ago, smell was the first sense that evolution developed. Chemoreception — detecting chemicals in the environment by scent — is a common trait among all animals, and even single-celled organisms. Smell is vital not only for finding food but also for finding a mate, a pretty key ingredient for furthering your particular limb on the tree of life. But although this sense is ubiquitous throughout the various phyla, kingdoms, and domains, not all organisms smell the same way. Single-celled organisms use a protein in the cell wall to detect chemicals, while plants use mechanisms baked in their genes to detect volatile organic compounds. Snakes, meanwhile, use their forked tongues to “grab” scents and then quickly return them to the olfactory bulb located at the roof of their mouths. This allows snakes to discern the direction of smells, which scientists describe as “smelling in stereo.” When it comes to life’s oldest sense, evolution has had more than enough time to create a variety of techniques.

Our Nose Can Sense 1 Trillion Odors

Humans are actually better than dogs when it comes to detecting certain smells, and our noses can actually sniff out a staggering 1 trillion (yes, with a “t”) odors. The impressive nature of our noses is a relatively recent discovery, however, which may explain why the human sense of smell long got a bad rap. Nearly a century ago, scientists pegged the human nose’s olfactory abilities at about 10,000 distinct smells — not bad, but far less impressive than our eye’s ability to glimpse 1 million colors (or more, for those lucky tetrachromats). In 2014, researchers from Rockefeller University in New York City decided to take a closer look at the nose’s true powers, and found that the human nose was much more capable than we imagined, giving the phrase “the nose knows” a whole new level of credibility.

Smell Is Closely Entwined With Memory

Sometimes smells like fresh-cut grass, a delicious baked pie, or a particular deodorant will bring back a long-forgotten recollection that sends you strolling wistfully down memory lane. Well, that’s by biological design, and it has to do with the way the human brain is wired. Unlike our four other best-known senses — which are first routed through the thalamus before reaching the hippocampus, the area of the brain associated with memory — smells are sent directly to the olfactory bulb located above the nasal cavity. While this bulb is directly tied into the hippocampus, it’s also connected with the emotion-processing amygdala, which is why smells can elicit such potent memories. Because smells can deliver these powerful whiffs of nostalgia, companies including Nike, Verizon, and many others have developed — and sometimes even trademarked — certain smells associated with their retail stores and products.

Some Women Don’t Have Olfactory Bulbs (But Smell Fine)

The olfactory bulb, as the name suggests, is central to our sense of smell. With millions of olfactory receptor cells, the bulb helps translate smells into signals the brain can interpret, so no olfactory bulb means no sense of smell — or so we thought. In 2019, scientists at the Weizmann Institute of Science in Israel were studying why some people have such a particularly strong sense of smell when they discovered in an MRI scan that one 29-year-old participant was completely missing her olfactory bulb. The woman was also left-handed — a factor that when combined with a missing olfactory bulb has a known effect on the organization of the brain. After poring over more data, the scientists estimated that 0.6% of women (and 4.25% of left-handed women) don’t have an olfactory bulb, but nevertheless can smell as well as — and in some cases even better than — those with one. Strangely, missing bulbs were not found in men.

How is this possible? Well, we don’t really know (yet). It’s possible that functions associated with the bulb were disorganized in these particular southpaws, meaning the necessary receptors are still there, just arranged in ways imperceptible to MRI scans. For now, the mystery remains.

Our Sense of Smell Is Strongest in the Evening

Although it’s imperceptible to the average person, smell actually fluctuates throughout the day. Research conducted by Brown University in 2017 studied 37 teenagers for a week and measured their sense of smell in relation to levels of melatonin, a hormone that helps induce sleep. The study found that our sense of smell is intimately entwined with our circadian rhythm — the natural cycle our bodies experience every day. When participants were approaching “biological night” around 9 p.m., their sense of smell was heightened, but strangely the opposite was true between 3 a.m. and 9 a.m. Although scientists don’t know the reason for this sniffing discrepancy, one theory harkens back to our evolution. In our hunter-gatherer days, the body might’ve ramped up our sense of smell right before sleep in an effort to hunt (or forage) for that last meal or to detect any nearby threats before bedding down for the night. An increased sense of smell might’ve also encouraged some pheromone-induced mating. Whatever the reason, humanity’s sense of smell appears to be a bit of a night owl.

Humans Are Wired to Smell Petrichor

You know that earthy smell that always accompanies rain after a long dry spell? That specific aroma has a name, “petrichor.” Coined in 1964 by Australian scientists (who would know a thing or two about dry spells), “petrichor” is a portmanteau of “petros” (stone) and “ichor,” which is a name for a bloodlike “ethereal fluid” of the gods in Greek mythology. The smell comes from actinobacteria that release organic compounds known as geosmin into the air when it rains. What’s strange is that humans are incredibly sensitive to the stuff — our ability to smell petrichor is far more sensitive than the ability of sharks to smell blood in the water. Many of humanity’s modern biological oddities can be explained by the hundreds of thousands of years spent living in hunter-gatherer tribes, and our keen sense for petrichor is another one to add to the list. Some scientists theorize our noses are so fine-tuned to sniff out this smell because finding water and rainy weather was often a matter of survival — and where there’s petrichor, there’s water. So the next time that telltale smell tickles your nostrils, sit back and marvel at the meticulous machinations of evolution that made such a moment possible.

You Can Smell Emotions

Most people are familiar with the power of pheromones, but research is hazy at best regarding what role they play in human behavior and sexual attraction. Bees, for example, possess a vomeronasal organ that detects pheromones and sends signals to the brain. While some humans possess the remnants of this organ, it’s vestigial and nonfunctional. However, scientists expect that other parts of the olfactory system might’ve picked up the slack, making chemoreception between humans possible. A study conducted in 2012 collected sweat from male participants as they watched fear- or disgust-inducing movies. When women participants were asked to do a visual task while exposed to the sweat samples, scientists monitored their facial expressions and discovered that women matched the emotion that originally elicited the sample — a sign that something in the sweat activated some form of chemoreception locked away in the human mind.