Secondary systems amplify our sense of smell

Although we can discriminate between an estimated 1 trillion different odors, human olfaction compared to that of other species is much maligned. Professionals who consider detecting and identifying aromas part of their stock in trade would be far more confident about their abilities if they had a better understanding of how such aromas are interpreted by our bodies.

Over the years I’ve discussed olfaction from many different angles, but I’ve never drilled down much past transduction— the conversion of volatile aroma compounds into the electrochemical signals our brains process—to understand what’s actually taking place within our expanded olfactory system.

Interestingly, the human olfactory genome contains a large number of what are known as pseudogenes, or products of gene replication, which suggests that olfaction became less important over the course of our evolution. More than 70% of the olfactory receptors encoding genes in humans are actually pseudogenes, differentiating us from rats or primates, which have less than 5%. The African elephant tops the mammalian olfactory gene chart with 2,000 genes; dogs can possess up to 1,200, while humans have only about 400.

Despite this, accessory functions gained during evolution allow us to
identify thousands of aromas. Once early humans became bipedal, their noses were farther from the ground, making smells easier to perceive individually. Although our olfactory organs grew smaller, they did so without many sacrifices to sensation.

Meanwhile, our adoption of fire radically diversified the odors and tastes of food. It can be said that no other species of mammal or primate ever benefited from this type of olfactory stimulation during their period of evolution.

Anthropologists initially thought that when the size of our brains increased, our olfactory receptor organs became secondary to the senses of vision and hearing. But they overlooked the integration areas of olfaction, which are many. For example, olfaction is enhanced by a secondary system through the sensitive branches of our trigeminal nerve, and many odorants can produce sensations that are transmitted by them. My favorite example of this is camphor, which produces a cold sensation via the trigeminal nerve. Researchers postulate that about 70% of odors also stimulate the trigeminal nerve, albeit with less intensity than our receptors.

Cognition amplifies olfaction

The cognitive component of olfaction, which according to researchers is not found in other species, includes the ability to discriminate between and compare odors. The process of analyzing olfactory perception is a synergistic one that uses not only smell but also taste, language, and memory. In other words, our reduced
repertoire of genes for olfactory receptors is compensated for by the great processing capacity of our brains.

While our sense of smell is no longer necessarily vital to survival in humans, it plays a part in attaching emotional resonance to things (think homecooked meals). That said, it is more powerful when paired with sight. Humans may use their sense of sight more, but through smell, we can determine such things as the quality and consistency of the foods and beverages we are about to ingest.

The maxim “the nose smells what the eyes see” refers to cross-modal interactions in which our senses communicate. Visual cues can strongly influence our perception of smells; they can make us think we smell something even if the odor isn’t there (a phenomenon known as phantosmia).

Conversely, smells can alter what we see: Through the phenomenon of spatial perception manipulation, for instance, cool scents such as cucumber can make a room seem larger, while warm ones like barbecue spice can make it seem smaller. These make for a richer but sometimes misleading olfactory experience.