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  • [MUSIC]

  • 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: Theyre the only ones in your body directly exposed to the

  • environment, and theyre 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, were 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, were 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 theyresmellingfor, 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 eyesresolution, 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 earsfrom 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.

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B2 US smell olfactory receptor odor scent nerve

How Many Smells Can You Smell?

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    Eating posted on 2015/02/03
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