Dogs’ reputation as super smellers is overrated, while ours is underrated. Olfaction, or the sense of smell, is thought to be reduced in humans compared to other animals—but in fact there is little to support this notion. In truth, the evidence points to humans possessing a rather refined sense of smell compared to other mammals. I am often asked, as an olfactory scientist, if the reputation dogs have as superior smellers is true. This question, and the conventional wisdom behind it, has always puzzled me. It seems to me that if dogs had such a miraculously good sense of smell then they could perform their customary method of greeting—sticking their noses quite directly into the private parts of some other unknown dog—from a greater distance, like at least a foot or two.

There is actually a perfectly good reason for this friendly if invasive practice. Dogs, and other animals, are simply willing to put their noses where the odors are. The single greatest challenge to our sense of smell is that we stand upright—an anatomical fact that places our noses some five feet or so in the air. Odor molecules have some weight and tend to accumulate near the ground. This is why, when dogs catch some interesting scent, they put their noses directly on the ground to follow where the smell is coming from. When we humans meet a stranger, we may shake hands. Social decorum (thankfully) prevents us from sniffing each other’s armpits, where there is much more olfactory information to be found. Even canine breeds that have been bred for a more refined sense of smell and to detect some scents at greater distances, like bloodhounds, need to put their nose to the ground and will sniff very close to an object or trail.

There is one other interesting example that demonstrates our refined sense of smell, and that is flavor. Not taste, which refers only to sweet, sour, salty, bitter, and savory (or umami, as it was named in Japan). Flavor, on the other hand, is a blend of these tastes with smells, and is the result of an evolutionary adaptation specific to humans. An estimated 80–90% of flavor is determined by olfaction. When our jaws became shorter and our faces flatter, a change in the pharyngeal bones led to an opening between our oral cavity and our nasal organ. This conduit, known as the retronasal pathway, is how odor molecules released by chewing food find their way to the olfactory epithelium, the thin tissue at the very top of our nasal cavity where the cells that detect odors are located. When we eat, stimuli from food—odor molecules—and the nasal organ are in close proximity, facilitating the olfactory sense. Humans possess what is likely the most discriminating palette of any animal on the planet. Not to pick on our poor canine companions again, but as any dog owner knows, they will eat just about anything.

Our sense of smell, compared to our dogs’, is actually quite good. Scientists working in this area tend to place humans right about in the middle of the pack of terrestrial animals. Not as good as some, but not as bad as many.

But what exactly are we smelling? What are we detecting? Or more simply put, what is an odor? The answer is also not as simple as it seems.

Almost all odors are organic molecules, which means they are combinations, sometimes very complex combinations, of just three atoms: carbon, oxygen and hydrogen. Occasionally among the fouler odors there may be a bit of nitrogen or sulphur, but that’s about it. You wouldn’t think just three to five atoms could be put together in so many varied ways, but theoretical models show that there are likely to be trillions of organic molecules. We ourselves are mostly made up of organic molecules, often quite large ones. DNA, RNA and proteins are all organic molecules with nothing but chains of these five atoms strung together, by the thousands, in incredibly complex ways. An entire branch of chemistry, organic chemistry, is devoted to molecules made up of these five atoms. Carbon is the linchpin of sorts because a single carbon atom is able to form bonds, stable connections, to as many as four other atoms, the most of any atom in the entire periodic table. An oxygen atom can form two bonds and a hydrogen atom just one. This is why organic chemistry is often called the chemistry of carbon—and why scientists looking for signs of extraterrestrial life elsewhere in our galaxy do so by searching for complex carbon chemistry.

Odor molecules are on the simpler side of organic chemistry, typically consisting of between three and 50 atoms of carbon, oxygen, and hydrogen. Even with this number of atoms, it is possible to construct many millions of complicated molecules. And your nose can detect and discriminate between several thousands of these molecules. One could make the case, in fact, that your nose is the best chemical detector on the planet.

The nose’s receptors are proteins embedded in the surface membrane of the cells that live in your nasal epithelium (that aforementioned tissue that lines the upper nasal cavity). They are called olfactory neurons because they are actually brain cells that have evolved to detect odors and send signals back to the rest of the brain. Each protein receptor has a differently shaped pocket that is capable of capturing differently shaped odor molecules—a kind of lock-and-key arrangement where the receptors are the locks and the odor molecules are the keys. If the key (odor) fits into the lock (receptor) then it will activate the olfactory neuron and send a signal to the brain that such and such a molecule must be in the area. It’s not quite that simple, because most of the things we smell are actually mixtures of many different molecules, but that’s pretty close to the general idea.

When molecules activate many different receptors at once, complications arise. For example, coffee is known to contain 780 odorous molecules. Some of them will activate the same receptor because they have similar shapes or properties, but it is still likely that sniffing coffee grounds will activate a fair number of the receptors in your nose.

One might wonder how different two molecules have to be in order for our olfactory system to register their smells as distinct. The answer lies in the diagram above, which details one way chemists show the structure of a molecule. Each of the little balls represents an atom—gray for carbon, red for oxygen and white for hydrogen. They are connected by bars that represent the bonds between them. In this case, the two molecules consist of a chain of carbon atoms with some hydrogens attached to them and what is known as a “functional group” at one end where the red oxygens are. The functional group shown here is called an ester. Among the sorts of organic molecules that make up odors there are about 25 different types of functional groups. Esters are among the most common of odor molecules.

If you count the number of carbon atoms in the two chains (from the red oxygens to the left) you will see that one of them has six carbons attached to the ester group and the other has seven. In other words, they are identical except for this difference in one carbon atom. Now here’s the kicker. The 6-carbon molecule (known as hexyl acetate) smells like bananas, while the 7-carbon molecule (known as heptyl acetate) smells like pears. While both are fruity smells, I doubt you would ever mistake a banana for a pear. This means that you can smell the difference of one carbon atom! And you aren’t even a trained perfumer.

Our noses can make a discrimination that only the most sophisticated chemistry equipment can make—a single carbon atom in a chain of only six or seven atoms. Your remarkable nose performs many feats like this dozens of times a day: at every meal, when you enter a room, when you take a walk in the park or down a narrow street on garbage day in the summer. It tells you what to avoid and what to seek out, bringing sensual pleasure to many simple activities. And all this without requiring you to think much about it at all. Next time you’re at the grocery store, go to the fruit section and close your eyes. Smell the bananas and the pears, and impress yourself with the amazing power of your olfactory sense. ♦