April 23, 2025 • 13 mins
Professor Turi King discusses the results of a scientific paper about the evolution of hearing, which explores the combined impact of biological and environmental factors on how our ears respond to sound. The study was the brainchild of Dr. Patricia Balaresque who got really interested in the different factors, such as our sex or environment, that might have shaped our hearing sensitivity. This groundbreaking study, ‘Sex, and Environment Shape Cochlear Sensitivity in Human Populations Worldwide,’ is published in Scientific reports. It was led by Dr Patricia Balaresque at the Centre for Biodiversity and Environmental Research (CRBE) in Toulouse, supported by the Eco-Anthropology unit (EA-CNRS Muséum National d'Histoire Naturelle / University of Paris) and the Toulouse Institute of Research in Computer Science (IRIT-CNRS / University of Toulouse) and others.
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Episode Transcript
Available transcripts are automatically generated. Complete accuracy is not guaranteed.
(00:01):
This episode about a just-published scientific paper about the evolution
of hearing which explores the combined impact of biological and environmental
factors on how our ears respond to sound.The study was the brainchild of Dr. Patricia
Balaresque who got really interested in the different factors, such as our sex or environment,
(00:22):
that might have shaped our hearing sensitivity.This groundbreaking study, ‘Sex, and Environment
Shape Cochlear Sensitivity in Human Populations Worldwide’, is published in Scientific reports.
It was led by Dr Patricia Balaresque at the Centre for Biodiversity and Environmental
Research (CRBE) in Toulouse, supported by the Eco-Anthropology unit (EA-CNRS Muséum National
(00:52):
d'Histoire Naturelle at the University of Paris) and the Toulouse Institute of Research in Computer
Science (IRIT-CNRS at the University of Toulouse) and others, including me at the University of Bath
and in my previous role at the University of Leicester. So how did the study start.
(01:12):
Despite being one of the traditionally recognised senses, the others being taste, sight, touch and
smell, hearing remains an underexplored aspect of human evolution. We know that generally,
our hearing declines with age and we know that our modern world is contributing to hearing issues
around the world. Recreational noise and ototoxic substances, chemicals, such as tobacco smoke, that
(01:39):
have an effect on the ear or it’s nerve supply, can cause damage to different parts of the ear.
We also know that exposure to chemicals can make ears more sensitive to the harmful effects of
noise. And we know that hearing damage from the environment in the form of noise pollution is
on the rise (01:55):
this can be the background noise of traffic and everyday activities such as household
appliances like vacuum cleaners, or lawnmowers through to more one-off high-noise level events
like concerts or sporting events, and so on.So, what have been the various factors that
have shaped our hearing and our diversity of hearing sensitivities in the human population.
(02:20):
I think it’s probably a good idea to start with how hearing works. Put more simply, our ear
collects sound and then turns that into signals for the brain to interpret. There are three parts
to our ear, the outer, the middle and the inner. The outer part of our ear, which is made up of
the pinna (the bit of the ear you can see) and the ear canal, collect sounds from our environment and
(02:45):
channel them down the ear canal to the middle ear. Here, they cause the ear drum to vibrate. On the
other side of the ear drum are three tiny bones, called the by the Latin name’s malleus, the incus,
and the stapes – these translate to hammer, anvil, and stirrup. They’re lined up in a configuration
(03:06):
with the malleus next to the ear drum picking up its vibrations, the incus in the middle, and the
last one, the stapes, transmitting the vibrations to the inner ear. The inner ear is made up the
vestibule and the cochlea. The cochlea looks a bit like a snail shell, the sound vibrations
enter the opening of the cochlea, travel down it, and the cochlea turns this information into
(03:29):
nerve signals which then travel to the brain to be interpreted. Currently your brain will be
interpreting you listening to this podcast.There’s two main measurements of sound:
frequency and amplitude. Another name for frequency is pitch: drums have a low sound
pitch where a whistle is high-pitched. Frequency is measured using a unit known as a Hertz and
(03:54):
human hearing range is between 20 HZ and 20,000 HZ. Anything above 20,000 HZ is known as ultrasound.
Amplitude is a term for loudness and that is measured in decibels. Normal
conversation tends to be at 60-65 decibels where vacuum cleaners are around 75 decibels.
(04:18):
In terms of the evolution of communication and hearing, the thinking around animals is the
Acoustic Adaptation Hypothesis. It predicts that the sound properties of communication would have
been selected for the optimal transmission for the habitats the animals are living in. For example,
(04:38):
animals living in habitats like jungles are going to make and hear different sounds than those in
grasslands versus those who live in the ocean.So that got Pat thinking, it would be interesting
to look at factors that affect human hearing. The cochlea, as we know,
plays a central role in hearing, converting sound waves into neural signals. You can
(05:02):
measure ‘cochlear sensitivity’ using something known as Transient-Evoked Otoacoustic Emissions
(TEOAE) techniques – which measure the ear’s reaction or sensitivity when it receives sound:
more specifically the cochlea’s ability to produce and transmit an acoustic response after being
(05:24):
sound-stimulated. It is often used by Ear Nose and Throat specialists, as one measure of hearing
sensitivity, often to check hearing in newborns.We already know that the sensitivity of the
cochlea varies across individuals due to biological factors such as age,
which ear it is, left or right, and sex. For example, we know that hearing declines with
(05:46):
age. We know that the right ear is typically more sensitive than the left and women seem
to have higher sensitivity than men particularly above 2000 HZ, at least in European populations,
since other populations have not investigated to the same degree. We know that ethnicity can
potentially also play a factor, though this too is not well studied, but suggests that there may be
(06:10):
evolutionary adaptation to local environments. But so far, the influence of these on hearing
sensitivity in humans has been underexplored.This study aimed to look at cochlear sensitivity
through TEOEA measurements across global human populations and evaluate the relative impact
(06:31):
of both endogenous factors such as age, sex, and ear side as well as exogenous
factors such as language, environment, and ethnicity, on hearing variability.
We analysed the hearing of 450 individuals across 13 populations in Ecuador, England, Gabon, South
(06:51):
Africa, and Uzbekistan. These populations were selected to capture a wide range of ecological and
cultural contexts, including underrepresented rural and non-European groups. We were
interested in whether people were living in urban environments, rural forest, plains, steppes, or at
(07:11):
altitude. We categorised participants into groups based on their age, and sex, language family,
ethnicity, and the environment they’re living in.So, what did we find.
Sex was found to be the leading factor explaining cochlear sensitivity differences:
with an average of 2 dB difference between men and women across all populations, this biological
(07:37):
factor best accounts for variations between individuals. Following this, the ecological
environment in which individuals live best explains the remaining differences, which
can reach an average of 3.5 dB between landscapes.We saw significant differences between populations
(07:58):
in a forest environment and populations at high altitude with the forest populations
having higher sensitivities than those at high altitude who had the lowest sensitivities.
Of the endogenous factors, sex emerges as the most influential factor with women
consistently showing higher sensitivity across the entire frequency spectrum
(08:21):
tested and across all populations. Several hypotheses can help explain these findings.
First, sex differences may be partly attributable to varying levels of hormone exposure during
development in the womb. There’s also structural differences in cochlear anatomy between men and
(08:41):
women although these don’t fully account for the differences seen. In addition to heightened
cochlear sensitivity, women also perform better in other hearing tests and speech perception,
indicating superior function in both the peripheral auditory system (the ear) and
the central auditory pathways which is the nerve signals and the brain interpreting the sounds.
(09:08):
Women show a higher prevalence of hyperacusis which is a disorder in loudness perception.
Given the well-documented detrimental effects of noise on overall health,
such as reduced sleep quality and increased cardiovascular disease risk,
maintaining heightened cochlear sensitivity in noisy environments has potential implications
(09:30):
for understanding sensory processing and health in diverse environments.
While a decline in cochlear sensitivity with age is seen, its influence is not as strong as
sex on hearing sensitivity and ear side is the least influential factor in explaining hearing
sensitivity, though does reaffirm a subtle right ear advantage. This is likely due to the right
(09:57):
ear’s primary role in auditory processing within the left hemisphere of the brain,
which is crucial for human abilities such as speech perception and production.
Our results offer pioneering insights into the role of non-biological, so exogenous,
factors in shaping cochlear sensitivities. We found that the environment, in particular,
(10:20):
not only affects the response to volume but also to the range of frequencies of sound
perceived. In fact, population, environment, and language all significantly contribute to
the variation in hearing across human groups.We aren’t quite sure why. It could be that
the entire body is affected by the environment (elevation and temperature) and that then affects
(10:44):
the cochlea. Or maybe it’s long-term adaptations to varying soundscapes. Or maybe it’s different
levels of noise exposure and exposure to chemical compounds that are toxic to the ear in some way.
The greatest contrast was found between populations living in sheltered forest
environments and high-altitude Andean populations in Ecuador. The forest environment, characterised
(11:09):
by rich biophony, sounds generated by non-humans and minimal sounds generated by humans,
suggests that higher cochlear sensitivity in these populations may reflect a number of things, it
could be an innate sensitivity to non-human sound environments, or an inherited trait favouring
heightened sensitivity in biophonic environments, where vigilance is essential for survival.
(11:34):
It could also be being reinforced by the absence of noise pollution
or ototoxic substances such as chemicals.Those populations at high altitudes had lower
response levels. This reduced sensitivity could be due to a number of factors, the effect of
(11:54):
atmospheric pressure on TEOAE measures; potential sound reduction in high-altitude environments,
which might lower the need for high auditory acuity; or physiological adaptation to hypoxia.
We do know that high-altitude causes low blood oxygen levels which affects the body’s physiology
(12:17):
and that may include damage to the cochlea. There is also a difference between urban and
rural populations with a shift towards higher frequencies in urban populations which may be
a response to the low frequency traffic noise prevalent in urban populations. We know there is
also a similar shift in bird vocalisations in areas where there are more humans.
(12:41):
These results highlight the diversity of mechanisms through which we perceive and process
sound. The differences between men and women could be explained by anatomical, hormonal, or genetic
differences. Differences according to ecological landscapes, on the other hand, could result from
two pressures (12:59):
either ancient adaptations from human colonisation of these environments long ago,
for example altitude, or that there is plasticity in our hearing sensitivity such that it can change
in response to an environment that has changed more recently such as urban versus rural. While
age is a known factor in hearing decline this study shows that this factor is overshadowed
(13:24):
by the effects of sex and the environment.This study highlights the crucial influence
of environmental factors on cochlear sensitivity and underscores the need for further research to
determine why. We know that we humans are continuing to evolve so the question here
is are these differences just our ability for hearing to change in response more generally to
(13:48):
different environments or are there new genetic adaptations that are involved. Watch this space!
This episode about a just-published scientific paper about the evolution
of hearing which explores the combined impact of biological and environmental
factors on how our ears respond to sound.The study was the brainchild of Dr. Patricia
Balaresque who got really interested in the different factors, such as our sex or environment,
(00:22):
that might have shaped our hearing sensitivity.This groundbreaking study, ‘Sex, and Environment
Shape Cochlear Sensitivity in Human Populations Worldwide’, is published in Scientific reports.
It was led by Dr Patricia Balaresque at the Centre for Biodiversity and Environmental
Research (CRBE) in Toulouse, supported by the Eco-Anthropology unit (EA-CNRS Muséum National
(00:52):
d'Histoire Naturelle at the University of Paris) and the Toulouse Institute of Research in Computer
Science (IRIT-CNRS at the University of Toulouse) and others, including me at the University of Bath
and in my previous role at the University of Leicester. So how did the study start.
(01:12):
Despite being one of the traditionally recognised senses, the others being taste, sight, touch and
smell, hearing remains an underexplored aspect of human evolution. We know that generally,
our hearing declines with age and we know that our modern world is contributing to hearing issues
around the world. Recreational noise and ototoxic substances, chemicals, such as tobacco smoke, that
(01:39):
have an effect on the ear or it’s nerve supply, can cause damage to different parts of the ear.
We also know that exposure to chemicals can make ears more sensitive to the harmful effects of
noise. And we know that hearing damage from the environment in the form of noise pollution is
on the rise (01:55):
this can be the background noise of traffic and everyday activities such as household
appliances like vacuum cleaners, or lawnmowers through to more one-off high-noise level events
like concerts or sporting events, and so on.So, what have been the various factors that
have shaped our hearing and our diversity of hearing sensitivities in the human population.
(02:20):
I think it’s probably a good idea to start with how hearing works. Put more simply, our ear
collects sound and then turns that into signals for the brain to interpret. There are three parts
to our ear, the outer, the middle and the inner. The outer part of our ear, which is made up of
the pinna (the bit of the ear you can see) and the ear canal, collect sounds from our environment and
(02:45):
channel them down the ear canal to the middle ear. Here, they cause the ear drum to vibrate. On the
other side of the ear drum are three tiny bones, called the by the Latin name’s malleus, the incus,
and the stapes – these translate to hammer, anvil, and stirrup. They’re lined up in a configuration
(03:06):
with the malleus next to the ear drum picking up its vibrations, the incus in the middle, and the
last one, the stapes, transmitting the vibrations to the inner ear. The inner ear is made up the
vestibule and the cochlea. The cochlea looks a bit like a snail shell, the sound vibrations
enter the opening of the cochlea, travel down it, and the cochlea turns this information into
(03:29):
nerve signals which then travel to the brain to be interpreted. Currently your brain will be
interpreting you listening to this podcast.There’s two main measurements of sound:
frequency and amplitude. Another name for frequency is pitch: drums have a low sound
pitch where a whistle is high-pitched. Frequency is measured using a unit known as a Hertz and
(03:54):
human hearing range is between 20 HZ and 20,000 HZ. Anything above 20,000 HZ is known as ultrasound.
Amplitude is a term for loudness and that is measured in decibels. Normal
conversation tends to be at 60-65 decibels where vacuum cleaners are around 75 decibels.
(04:18):
In terms of the evolution of communication and hearing, the thinking around animals is the
Acoustic Adaptation Hypothesis. It predicts that the sound properties of communication would have
been selected for the optimal transmission for the habitats the animals are living in. For example,
(04:38):
animals living in habitats like jungles are going to make and hear different sounds than those in
grasslands versus those who live in the ocean.So that got Pat thinking, it would be interesting
to look at factors that affect human hearing. The cochlea, as we know,
plays a central role in hearing, converting sound waves into neural signals. You can
(05:02):
measure ‘cochlear sensitivity’ using something known as Transient-Evoked Otoacoustic Emissions
(TEOAE) techniques – which measure the ear’s reaction or sensitivity when it receives sound:
more specifically the cochlea’s ability to produce and transmit an acoustic response after being
(05:24):
sound-stimulated. It is often used by Ear Nose and Throat specialists, as one measure of hearing
sensitivity, often to check hearing in newborns.We already know that the sensitivity of the
cochlea varies across individuals due to biological factors such as age,
which ear it is, left or right, and sex. For example, we know that hearing declines with
(05:46):
age. We know that the right ear is typically more sensitive than the left and women seem
to have higher sensitivity than men particularly above 2000 HZ, at least in European populations,
since other populations have not investigated to the same degree. We know that ethnicity can
potentially also play a factor, though this too is not well studied, but suggests that there may be
(06:10):
evolutionary adaptation to local environments. But so far, the influence of these on hearing
sensitivity in humans has been underexplored.This study aimed to look at cochlear sensitivity
through TEOEA measurements across global human populations and evaluate the relative impact
(06:31):
of both endogenous factors such as age, sex, and ear side as well as exogenous
factors such as language, environment, and ethnicity, on hearing variability.
We analysed the hearing of 450 individuals across 13 populations in Ecuador, England, Gabon, South
(06:51):
Africa, and Uzbekistan. These populations were selected to capture a wide range of ecological and
cultural contexts, including underrepresented rural and non-European groups. We were
interested in whether people were living in urban environments, rural forest, plains, steppes, or at
(07:11):
altitude. We categorised participants into groups based on their age, and sex, language family,
ethnicity, and the environment they’re living in.So, what did we find.
Sex was found to be the leading factor explaining cochlear sensitivity differences:
with an average of 2 dB difference between men and women across all populations, this biological
(07:37):
factor best accounts for variations between individuals. Following this, the ecological
environment in which individuals live best explains the remaining differences, which
can reach an average of 3.5 dB between landscapes.We saw significant differences between populations
(07:58):
in a forest environment and populations at high altitude with the forest populations
having higher sensitivities than those at high altitude who had the lowest sensitivities.
Of the endogenous factors, sex emerges as the most influential factor with women
consistently showing higher sensitivity across the entire frequency spectrum
(08:21):
tested and across all populations. Several hypotheses can help explain these findings.
First, sex differences may be partly attributable to varying levels of hormone exposure during
development in the womb. There’s also structural differences in cochlear anatomy between men and
(08:41):
women although these don’t fully account for the differences seen. In addition to heightened
cochlear sensitivity, women also perform better in other hearing tests and speech perception,
indicating superior function in both the peripheral auditory system (the ear) and
the central auditory pathways which is the nerve signals and the brain interpreting the sounds.
(09:08):
Women show a higher prevalence of hyperacusis which is a disorder in loudness perception.
Given the well-documented detrimental effects of noise on overall health,
such as reduced sleep quality and increased cardiovascular disease risk,
maintaining heightened cochlear sensitivity in noisy environments has potential implications
(09:30):
for understanding sensory processing and health in diverse environments.
While a decline in cochlear sensitivity with age is seen, its influence is not as strong as
sex on hearing sensitivity and ear side is the least influential factor in explaining hearing
sensitivity, though does reaffirm a subtle right ear advantage. This is likely due to the right
(09:57):
ear’s primary role in auditory processing within the left hemisphere of the brain,
which is crucial for human abilities such as speech perception and production.
Our results offer pioneering insights into the role of non-biological, so exogenous,
factors in shaping cochlear sensitivities. We found that the environment, in particular,
(10:20):
not only affects the response to volume but also to the range of frequencies of sound
perceived. In fact, population, environment, and language all significantly contribute to
the variation in hearing across human groups.We aren’t quite sure why. It could be that
the entire body is affected by the environment (elevation and temperature) and that then affects
(10:44):
the cochlea. Or maybe it’s long-term adaptations to varying soundscapes. Or maybe it’s different
levels of noise exposure and exposure to chemical compounds that are toxic to the ear in some way.
The greatest contrast was found between populations living in sheltered forest
environments and high-altitude Andean populations in Ecuador. The forest environment, characterised
(11:09):
by rich biophony, sounds generated by non-humans and minimal sounds generated by humans,
suggests that higher cochlear sensitivity in these populations may reflect a number of things, it
could be an innate sensitivity to non-human sound environments, or an inherited trait favouring
heightened sensitivity in biophonic environments, where vigilance is essential for survival.
(11:34):
It could also be being reinforced by the absence of noise pollution
or ototoxic substances such as chemicals.Those populations at high altitudes had lower
response levels. This reduced sensitivity could be due to a number of factors, the effect of
(11:54):
atmospheric pressure on TEOAE measures; potential sound reduction in high-altitude environments,
which might lower the need for high auditory acuity; or physiological adaptation to hypoxia.
We do know that high-altitude causes low blood oxygen levels which affects the body’s physiology
(12:17):
and that may include damage to the cochlea. There is also a difference between urban and
rural populations with a shift towards higher frequencies in urban populations which may be
a response to the low frequency traffic noise prevalent in urban populations. We know there is
also a similar shift in bird vocalisations in areas where there are more humans.
(12:41):
These results highlight the diversity of mechanisms through which we perceive and process
sound. The differences between men and women could be explained by anatomical, hormonal, or genetic
differences. Differences according to ecological landscapes, on the other hand, could result from
two pressures (12:59):
either ancient adaptations from human colonisation of these environments long ago,
for example altitude, or that there is plasticity in our hearing sensitivity such that it can change
in response to an environment that has changed more recently such as urban versus rural. While
age is a known factor in hearing decline this study shows that this factor is overshadowed
(13:24):
by the effects of sex and the environment.This study highlights the crucial influence
of environmental factors on cochlear sensitivity and underscores the need for further research to
determine why. We know that we humans are continuing to evolve so the question here
is are these differences just our ability for hearing to change in response more generally to
(13:48):
different environments or are there new genetic adaptations that are involved. Watch this space!
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