What is Olfaction?
Olfaction = sense of smell.
It's the process where we detect airborne chemicals (called odorants) and our brain interprets them as
specific smells.
Why Does Smell Matter?
• In animals: Smell is critical for survival.
It helps them:
o Find food
o Know if something is safe to eat
o Detect predators (danger!)
o Recognize family (parents, mates, offspring)
o Communicate socially (mark territory, attract mates)
• In humans:
We still use smell, though maybe not as obviously.
o Smell can trigger emotions, memories
o It can influence partner selection (perfume industry plays on this!)
o We use it to detect danger too (e.g., gas leaks, spoiled food)
Historical Views on Smell (Why People Thought It Wasn't Important)
• Ancient Greeks & Immanuel Kant:
Thought smell was "vulgar and inferior".
➜ Meaning: They saw it as primitive, animal-like, not sophisticated.
• Broca (a famous brain scientist):
Said humans have "learned to suppress their animalistic odor impulses".
➜ Suggesting we don’t act on smells like animals do. - pushed away
• William James (psychologist):
Thought smell was boring: "almost nothing of psychological interest is known concerning them".
➜ He dismissed it, basically saying it didn’t matter much.
• Freud:
Said smell was "uncivilized".
➜ He thought being influenced by smell made you less refined.
Key point: They saw smell as inferior because they thought it connected humans to "primitive"
animal instincts—something they believed "civilized" people should avoid.
,Is Our Sense of Smell Really Inferior?
Our smell is inferior = having weak sensitivity for discrimination and detection so we should have a
relative hard time picking up smells and discriminated between smells.
➡️ Some people thought humans were bad at smelling.
But that’s not true! Let’s look at evidence.
Evidence: What Can Humans Do?
1. Detection Abilities:
Humans can detect tiny amounts of certain chemicals.
➜ Example: Ethyl mercaptan (added to gas to make it smell).
Only two drops in an Olympic pool of water and we can smell it!
➜ Not bad, right?
2. Scent Tracking (Porter et al., 2007 Study):
a. Humans followed a 10-meter scent trail on their hands and knees in an open field!
b. 21 out of 32 people succeeded with just their noses.
c. They got better and faster with practice. - scent tracing
d. Dogs are better at this, but humans can do it, too.
Nostrils Work Like Eyes!
• We use two nostrils, like two eyes for depth perception.
• Our nostrils compare scents from two sides, which helps us track where a smell is coming from!
• Studies showed each nostril samples a different area in space, so you can tell direction by
comparing them.
Why Don’t We Rely on Smell More?
We're vision-based creatures!
• Our survival and social life depend more on what we see.
• So we use smell less than animals like dogs.
• But that doesn’t mean smell is bad or weak—we just don’t use it as much.
Key Takeaways
• Smell (olfaction) is better in humans than people used to think.
• We can detect, discriminate, and track smells.
• We don’t rely on it much because we’re visual animals.
• Historical views treated smell as primitive, but science says it's actually powerful, just underused.
Confusing Word What It Means
, Vulgar Seen as primitive or animal-like, not refined.
Assumed to be weak or less important—scientifically means less sensitive or
Inferior
accurate, which isn't true for smell!
Humans don't act on smells like animals do (e.g., we don’t usually sniff each
Suppress impulses
other!).
Uncivilized Thought to be tied to raw instincts or less “refined” behavior.
The Big Idea (Theory/Context)
• Some scientists propose a theory that olfaction (smell) is inferior to vision, especially in humans
and other primates and that vision is the reason that olfaction is inferior.
• The idea is that as animals develop better vision (e.g., color vision), they rely less on their sense
of smell.
• As a result, the olfactory system becomes less important or even obsolete in species that rely
more on their eyes.
2. The Hypothesis
• If an animal relies more on vision, it won't need as strong a sense of smell.
• In terms of genes, this means:
➤ Animals that rely on vision will have more “non-functional” olfactory receptor (OR) genes,
called pseudogenes. - you don't express the genes that are related to olfaction anymore
➤ Pseudogenes are genes that exist but don't work—they are not making the proteins needed
for smell.
Gene Data in Different Species - Here’s where it gets interesting! - Olfactory R eceptor ( OR ) Genes
• These are genes responsible for d etecting sm ells.
Species have different num bers of:
1. F unctional genes = working smell receptors
2. P seud ogenes = nonworking smell receptors
Sp ecies F unctional Genes P seud ogenes
Dogs 811 278
Hum ans 396 425
Mice 1130 236
Chim p anzees 380 414
R ats 1207 508
Key P oint
• Hum ans have m ore p seudogenes and fewer functional sm ell recep tors than many other
animals.
, • Dogs and rats, which rely more on sm ell, have a higher num ber of working recep tors .
This supports the idea that anim als relying on vision (like humans and primates with color vision)
have m ore olfactory p seud ogenes .
3. The Evidence (Gene Comparisons)
• Olfactory receptor (OR) genes are responsible for detecting smells.
• All mammals have lots of OR genes, but some are functional and some are pseudogenes
(nonfunctional).
• Example numbers (from Figure 14.9 in the book):
➤ Dogs have 811 functional OR genes, and only about 25% are pseudogenes.
➤ Humans have fewer than 1000 OR genes, but 52% are pseudogenes!
➤ Mice and rats have more OR genes and fewer pseudogenes than humans.
• The more an animal relies on vision, the less it relies on smell, and more of its OR genes
become pseudogenes.
4. Examples Across Species
• Humans: Have high reliance on vision, so lots of OR pseudogenes.
• Monkeys and Apes:
➤ Old World monkeys (like baboons) and apes have more pseudogenes than New World
monkeys, because they also developed better color vision (trichromatic vision).
➤ Howler monkeys (a New World monkey) are an exception because they have trichromatic
color vision, and they also have more OR pseudogenes, just like Old World monkeys.
5. Color Vision Connection
• Better color vision is based on having more cone types in the eye.
• Animals with trichromatic vision (three cone types) can discriminate more colors.
• The theory suggests that as color vision improved, animals depended less on their noses, so
olfaction declined.
• Some women have a second L-cone type (you can get 2 X chromosomes that have different L cone
type, so you get an extra tuning curve, better at discriminate red colors)– red and green blindness
more by men because the l-cone is on the X chromosome.
• Better colour vision, defined as better colour discrimination, as based on the number of case
type one has
6. But… It’s Not That Simple! (Caveats)
• better color vision does not always mean fewer functional OR genes.
Olfaction = sense of smell.
It's the process where we detect airborne chemicals (called odorants) and our brain interprets them as
specific smells.
Why Does Smell Matter?
• In animals: Smell is critical for survival.
It helps them:
o Find food
o Know if something is safe to eat
o Detect predators (danger!)
o Recognize family (parents, mates, offspring)
o Communicate socially (mark territory, attract mates)
• In humans:
We still use smell, though maybe not as obviously.
o Smell can trigger emotions, memories
o It can influence partner selection (perfume industry plays on this!)
o We use it to detect danger too (e.g., gas leaks, spoiled food)
Historical Views on Smell (Why People Thought It Wasn't Important)
• Ancient Greeks & Immanuel Kant:
Thought smell was "vulgar and inferior".
➜ Meaning: They saw it as primitive, animal-like, not sophisticated.
• Broca (a famous brain scientist):
Said humans have "learned to suppress their animalistic odor impulses".
➜ Suggesting we don’t act on smells like animals do. - pushed away
• William James (psychologist):
Thought smell was boring: "almost nothing of psychological interest is known concerning them".
➜ He dismissed it, basically saying it didn’t matter much.
• Freud:
Said smell was "uncivilized".
➜ He thought being influenced by smell made you less refined.
Key point: They saw smell as inferior because they thought it connected humans to "primitive"
animal instincts—something they believed "civilized" people should avoid.
,Is Our Sense of Smell Really Inferior?
Our smell is inferior = having weak sensitivity for discrimination and detection so we should have a
relative hard time picking up smells and discriminated between smells.
➡️ Some people thought humans were bad at smelling.
But that’s not true! Let’s look at evidence.
Evidence: What Can Humans Do?
1. Detection Abilities:
Humans can detect tiny amounts of certain chemicals.
➜ Example: Ethyl mercaptan (added to gas to make it smell).
Only two drops in an Olympic pool of water and we can smell it!
➜ Not bad, right?
2. Scent Tracking (Porter et al., 2007 Study):
a. Humans followed a 10-meter scent trail on their hands and knees in an open field!
b. 21 out of 32 people succeeded with just their noses.
c. They got better and faster with practice. - scent tracing
d. Dogs are better at this, but humans can do it, too.
Nostrils Work Like Eyes!
• We use two nostrils, like two eyes for depth perception.
• Our nostrils compare scents from two sides, which helps us track where a smell is coming from!
• Studies showed each nostril samples a different area in space, so you can tell direction by
comparing them.
Why Don’t We Rely on Smell More?
We're vision-based creatures!
• Our survival and social life depend more on what we see.
• So we use smell less than animals like dogs.
• But that doesn’t mean smell is bad or weak—we just don’t use it as much.
Key Takeaways
• Smell (olfaction) is better in humans than people used to think.
• We can detect, discriminate, and track smells.
• We don’t rely on it much because we’re visual animals.
• Historical views treated smell as primitive, but science says it's actually powerful, just underused.
Confusing Word What It Means
, Vulgar Seen as primitive or animal-like, not refined.
Assumed to be weak or less important—scientifically means less sensitive or
Inferior
accurate, which isn't true for smell!
Humans don't act on smells like animals do (e.g., we don’t usually sniff each
Suppress impulses
other!).
Uncivilized Thought to be tied to raw instincts or less “refined” behavior.
The Big Idea (Theory/Context)
• Some scientists propose a theory that olfaction (smell) is inferior to vision, especially in humans
and other primates and that vision is the reason that olfaction is inferior.
• The idea is that as animals develop better vision (e.g., color vision), they rely less on their sense
of smell.
• As a result, the olfactory system becomes less important or even obsolete in species that rely
more on their eyes.
2. The Hypothesis
• If an animal relies more on vision, it won't need as strong a sense of smell.
• In terms of genes, this means:
➤ Animals that rely on vision will have more “non-functional” olfactory receptor (OR) genes,
called pseudogenes. - you don't express the genes that are related to olfaction anymore
➤ Pseudogenes are genes that exist but don't work—they are not making the proteins needed
for smell.
Gene Data in Different Species - Here’s where it gets interesting! - Olfactory R eceptor ( OR ) Genes
• These are genes responsible for d etecting sm ells.
Species have different num bers of:
1. F unctional genes = working smell receptors
2. P seud ogenes = nonworking smell receptors
Sp ecies F unctional Genes P seud ogenes
Dogs 811 278
Hum ans 396 425
Mice 1130 236
Chim p anzees 380 414
R ats 1207 508
Key P oint
• Hum ans have m ore p seudogenes and fewer functional sm ell recep tors than many other
animals.
, • Dogs and rats, which rely more on sm ell, have a higher num ber of working recep tors .
This supports the idea that anim als relying on vision (like humans and primates with color vision)
have m ore olfactory p seud ogenes .
3. The Evidence (Gene Comparisons)
• Olfactory receptor (OR) genes are responsible for detecting smells.
• All mammals have lots of OR genes, but some are functional and some are pseudogenes
(nonfunctional).
• Example numbers (from Figure 14.9 in the book):
➤ Dogs have 811 functional OR genes, and only about 25% are pseudogenes.
➤ Humans have fewer than 1000 OR genes, but 52% are pseudogenes!
➤ Mice and rats have more OR genes and fewer pseudogenes than humans.
• The more an animal relies on vision, the less it relies on smell, and more of its OR genes
become pseudogenes.
4. Examples Across Species
• Humans: Have high reliance on vision, so lots of OR pseudogenes.
• Monkeys and Apes:
➤ Old World monkeys (like baboons) and apes have more pseudogenes than New World
monkeys, because they also developed better color vision (trichromatic vision).
➤ Howler monkeys (a New World monkey) are an exception because they have trichromatic
color vision, and they also have more OR pseudogenes, just like Old World monkeys.
5. Color Vision Connection
• Better color vision is based on having more cone types in the eye.
• Animals with trichromatic vision (three cone types) can discriminate more colors.
• The theory suggests that as color vision improved, animals depended less on their noses, so
olfaction declined.
• Some women have a second L-cone type (you can get 2 X chromosomes that have different L cone
type, so you get an extra tuning curve, better at discriminate red colors)– red and green blindness
more by men because the l-cone is on the X chromosome.
• Better colour vision, defined as better colour discrimination, as based on the number of case
type one has
6. But… It’s Not That Simple! (Caveats)
• better color vision does not always mean fewer functional OR genes.
