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.

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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 detect lots more than 400 chemical 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 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

different 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. Sorry. That joke kind of stunk.

But this year scientists upped our smell estimate 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.

Is there a smell that conjures up a special memory for you? Let me know down in the comments,

and stay curious.