~LUCA TURIN, author of The Secret of Scent: Adventures in Perfume and the Science of Smell
The sense of smell is probably nature’s oldest. We can trace its odoriferous origins back to the single-celled organisms who sampled Earth’s most primordial perfumes. Humans depend mostly on sight and sound to navigate our world, but on every step of that journey we are led by our noses. Literally, because it’s on the front of your face. So how many smells can you smell?
An aromatic evolution
Our eyes and ears can be fooled by illusions or noise, but not our noses. And compared to the spectrum of smell, our tastebuds are crude, like looking at Van Gogh on an Atari.
Of the human genome’s 20,000 or so genes, almost a thousand code for olfactory receptors. Those genes have duplicated and specialized throughout our aromatic evolution, and they’re found on all but two of your chromosomes. Being the eyeball-dependent apes that we are, only about 400 of those olfactory receptor genes are still functional, each of them is specific to certain chemicals.
During each of the 20,000 or so breaths that you take every day, air molecules float back and land on mucus-covered tissue the area of a couple postage stamps, they’re packed with about 40 million special scent-detecting nerves.
These nerves are unique: They’re the only ones in your body directly exposed to the environment, and they’re regularly replaced about once a month. Each nerve ending is covered in just one of your 400 or so receptor types. One group of nerves for a Christmas tree, another for cinnamon. If an odor molecule has just the right shape, it can fit like a tiny lock and key and trigger that nerve. But because each lock can accept many shapes of keys, we’re able to detect lots more than 400 chemicals odors.
Sometimes, molecules with completely different 3D shapes can trigger the same receptor, so some scientists think molecular vibrations might determine which keys unlock which receptors.
When we smell, say, a rose, we’re experiencing a mosaic of more than 200 chemicals. We can detect some of them at concentrations as low as two molecules in a billion. We can piece apart a smell with chemical instruments, but that doesn’t equate to the experience of the smell.
Olfactory nerve impulses enter the brain near the amygdala and other regions that process emotion and memory. And people rate memories evoked by smell as more as more emotional than those triggered by sight and sound.
Our smell-associated memories tend to peak around age 5, which is why the first whiff of a scent is our most memorable one, which is why no matter how many old ladies I meet that wear grandma’s perfume, I always picture her.
Smell is the first sense that all of us use
As babies, we can sniff out our parents before we ever lay eyes on them. You might have smelled your first smell even before that. Human sperm are covered in the same type of odor detection proteins as we find inside our noses. Scientists aren’t sure exactly what they’re smelling for, but wherever that chemical quest ended, you began.
The scents we encounter are molecular mixtures, though, not individual odors. So how many diffent smells can we smell?
Compared to our eyes and ears, our sense of smell may be the most boundless. Taking into account the wavelengths of light we can detect using our three types of color receptors and our eyes’ resolution, scientists think we can distinguish between two and a half and seven and a half million colors. For sounds, that number is just 340,000.
For a long time, scientists put our scent resolution at a measly 10,000 different smells but it turns out no one ever really tested that. I guess you’d say they sort of pulled it out of thin air. But this year scientists upped our smell estimat to 1 trillion. That enough scratch and sniff stickers to reach from here and back to the moon 32 times.
Truth is, though, we don’t really know enough about how our brain processes mixtures of odor molecules, or how many dimensions of smell we can recognize at once, to even know if that’s a good guess. It might be lower, or even higher.
The speed of smell, riding the breeze to reach our noses, might be slower than light or sound. But it can deliver messages from way beyond the limits of our eyes and ears…from miles, or even years away.