Humans can discriminate a trillion smells — wait, what?
What does it even mean to detect so many smells?
Smell has always seemed quite different from the other senses. In Kant’s Reflexionen zur Anthropologie, he distinguishes the senses which rational beings use to know things — such as sight, sound, touch — and those which are more carnal in nature, such as scent. However, he points out that classification of these smells will be difficult. Whereas in sight and sound all our vocabulary is of those senses, for odor we must borrow from others. Something smells sour: that is not a scent, that is a taste. Are there any words in the English language exclusively devoted to smell?
Smells are peculiar in a way that other senses are not — somehow, our primal responses to odors resist classification. This is not a problem that we have for sight, sound, touch or taste. For instance, in vision we know that every color is made of some mixture of red, blue or green. Every shade of purple or black — or even, say, razmatazz — can be given a unique combinatoric of RGB. This isn’t some fundamental physical property of the universe, though: it is a result of the structure of the photoreceptors on your retina. This visual description of the world may just be a quirk of fate, as a substantial minority of people are red/green colorblind; many animals can see only one or two colors. But there is also a group of people who can see four fundamental colors. Which is to say, the way we describe our world is contingent on our nervous system, not some property of the world itself.
What makes smell so different? It begins with the fundamental processing of odors. We know from extensive work in neuroscience that smell — olfactory — processing begins in the olfactory epithelium before descending into the olfactory bulb. Here, odors are picked up by small fibrous cells that send their threads into a tangled bag called the glomerulus. These baggy structures are the basic unit of response in the olfactory bulb, where the unique combination of responding glomeruli represent a precise odor.
In the area of the brain that first performs vision, there seems to be a clear structure to the neurons: things that are near each other in visual space cause neurons that are physically near each other to respond. Neurons that are near each other respond to similar edge orientations, and so on. But the baggy glomerulus willfully defies this order, and which odor it will respond to is seemingly random. There is no fundamental description that we will get from this; there is no structure that we know of.
In What the Nose Knows, Avery Gilbert describes the history of the people who have tried to force an order onto smells. It begins, somehow predictably, with the godfather of scientific systemization, Linnaeus. The man who had brought us the taxonomic classification for animals, with names such as Felis catus and Caenorhabditis elegans, attempted to do the same for odors. He decided there must be a discrete set of classes of odors, which include fragrant, spicy, musky, garlicky, goaty, foul, and nauseating. This was later refined by Hendrik Zwaardermaker who added the classes ethereal and empyreumatic, as well as adding subclasses for each class. Next came Hans Henning, who decided that smells lie on an odor prism. Each vertex was a specific quality of odor — flowery, foul, fruity, spicy, burnt, and resinous — and the distance of a small from each vertex was the relative contribution of that quality to the odor. This gave odors a space and direction and possibly even dynamics from one point to another. But it also didn’t work.
It was in this tradition that Ernest Crocker and Lloyd Henderson began yet another system of classification. This time, they reduced smell to four primal odors: fragrant, acid, burnt, and caprylic. Any smell could be assessed on each of these dimensions on a nine point scale: freshly roasted coffee, for instance, was a seven fragrant, six acid, eight burnt (depends on your coffee!), and a three caprylic. This gave 9*9*9*9, or 6561 possible odors; people considered this number too difficult to remember and decided to just round up to 10,000.
It is in this tradition that Caroline Bushdid and colleagues threw up their hands and just asked: how many odors can we discriminate? Forget fundamental odor classes. How many odors are there to us? Combining and mixing distinct chemicals whose properties are already known, they asked people to sniff. These tinctures were not all the same but had some overlap in their components. When the overlap was too much, the odors were indiscriminable from one another. In a poetic touch, they imagined these odors packed together in a sensory space, overlapping just slightly. Like oranges on display in a market, they were able to determine that over a trillion smells could fit in this sensorium.
Yet this number is probably an understatement, and probably unimportant anyway. As humans, when we label specific colors we can discriminate them better: we can see more variations and shades of these colors. The structure of the brain opens up and allows more neurons to represent those colors. Surely, too, would a sommelier learn to smell more odors. In the fly, there are two structures for smells: one that has hardwired responses, and one that learns.
But what does it even mean to smell so much? If you tried to count up to one trillion, it would take over 30,000 years. Perhaps we could think of it like sight; it is hard for us to say, there are five miillion colors that you could see. Yet look at the image above: one has more than one and a half million colors, and another has five hundred and twelve. The world is less rich, less vivid. It may be hard to realize at first how many different colors we discriminate on a regular basis, but there they are.
And each smell is a combination of many different odors. The smell of burnt toast in the morning has flecks of wheat and sugar and butter and little bits of dying carbon. The basis of color is three whereas the basis of smell is many. And used in ways that you never would expect, or think about again. When the Neanderthal genome was to be sequenced, a Neanderthal bone was popped open and asked to give up its DNA. Pääbo, its discoverer, savored the smell of the burnt bone as it was being cut because he could tell from the odor that its inner collagen had survived.
The world of the smell is a richer place than you imagined.
