An evolutionary history of the human brain, in 7 minutes | Lisa Feldman Barrett

Summary

The script delves into the evolution of the human brain, debunking the 'Triune brain' theory and highlighting that brains did not evolve in layers. It suggests that the brain's development is more about the duration of developmental stages, leading to the significant cerebral cortex in humans. The script also explores the possible reasons behind the brain's evolution, proposing that it may have emerged due to the predatory behavior of early animals, which initiated an evolutionary arms race for sensory and motor coordination, emphasizing the importance of metabolic efficiency for survival and reproduction.

Takeaways

• 🧠 The human brain is a metabolically expensive organ, consuming about 20% of our body's energy budget despite being only three pounds in size.
• 🐉 The 'Triune brain' theory, which suggests the brain evolved in three layers—reptilian, limbic, and neocortex—has been largely debunked by modern neuroscience.
• 🔬 Molecular genetic techniques have revealed that the brain does not evolve in distinct layers but follows a common plan among all mammals, with variations in developmental timing.
• 🐟 Both reptiles and mammals evolved from fish, indicating that the idea of an 'inner lizard' in humans is not supported by evolutionary biology.
• 📚 The concept of the triune brain can be traced back to ancient Greek philosophy, specifically Plato's allegory of the charioteer and two horses representing human psyche.
• 🏆 Carl Sagan's book 'The Dragons of Eden,' which popularized the triune brain theory, won a Pulitzer Prize and had a significant impact on popular culture.
• 🧬 Modern neuroscience has shown that the brain's development is not stratified but rather a coordinated growth of various regions based on genetic instructions.
• 🌐 The evolution of the brain is thought to be linked to the emergence of predation and the subsequent 'arms race' for better sensory and motor systems to hunt or avoid being hunted.
• 🦈 The first step towards brain evolution may have been the act of one animal deliberately hunting another, leading to the development of more complex sensory and motor systems.
• 🤔 The 'why' of evolution, including brain evolution, is challenging to answer definitively due to the nature of teleology, but we can understand the functions and pressures that drive evolution.
• 🌿 Metabolic efficiency is a critical factor in natural selection, and the brain serves as the control center for coordinating various body parts in an energy-efficient manner to ensure survival and reproduction.

Q & A

Why are brains considered expensive in terms of metabolic budget?

Brains are considered expensive because they constitute only about three pounds of our body weight but consume approximately 20% of our metabolic budget, indicating their high energy demand relative to their size.

What is the 'Triune brain' theory and how does it describe the evolution of the brain?

The 'Triune brain' theory suggests that the human brain evolved in three layers: the inner 'lizard brain' for instincts, the limbic system for emotions, and the cerebral cortex for rationality. This theory was once dominant but has since been challenged by more recent scientific findings.

Why does the idea of having an 'inner lizard' not make sense from an evolutionary perspective?

The idea of having an 'inner lizard' does not make sense because reptiles and mammals evolved from fish on different branches, and the only animal with a 'lizard brain' is a lizard itself. This concept is more of a metaphor rather than a literal evolutionary description.

What is the origin of the idea that the human psyche can be described as two horses and a charioteer?

This idea originates from the philosopher Plato, who used it as a metaphor to describe the human psyche, with the horses representing instincts and emotions, and the charioteer symbolizing the control over these aspects.

How did the 'Triune brain' concept become popularized in modern times?

The 'Triune brain' concept became popularized through Carl Sagan's 1977 book 'The Dragons of Eden,' which won a Pulitzer Prize and was widely read, spreading the narrative of the triune brain to a broader audience.

What scientific advancements have challenged the 'Triune brain' theory?

Molecular genetic techniques have allowed neuroscientists to trace the genes that form brain cells, revealing that the brain did not evolve in layers as previously thought. Instead, there is a common brain plan for all mammals, differing only in the duration of developmental stages.

What is the common brain plan among all mammals, and how does it differ from the 'Triune brain' theory?

The common brain plan among all mammals suggests that the brain's structure and function are not layered but rather follow a similar developmental pattern. The differences lie in the length of time each developmental stage persists, affecting the relative size of brain components like the cerebral cortex.

How do amphioxus illustrate the simplicity of early life forms without a brain?

Amphioxus, simple creatures that have existed for around 500 million years, lack a brain, eyes, ears, and a head, demonstrating that early life forms could survive with minimal sensory capabilities and coordination.

What evolutionary event is believed to have triggered the development of brains in animals?

Scientists believe that the development of brains was triggered by the emergence of predation, where one animal began to hunt and eat another, leading to an evolutionary arms race to improve sensory and motor systems for both predation and evasion.

Why is the question of 'why' something evolved considered difficult to answer in scientific terms?

The 'why' question is difficult because it involves teleology, which is the explanation of phenomena in terms of their purpose or goal. Scientists can describe the processes and functions but cannot definitively establish the purpose or intention behind an evolutionary development.

How does metabolic efficiency play a role in the evolution of the brain?

Metabolic efficiency is a major selection pressure in evolution. A brain acts as a control center to coordinate various body parts in an energy-efficient manner, which is crucial for survival and reproduction.

Outlines

00:00

🧠 The Evolution and Metabolic Cost of Brains

This paragraph delves into the concept of the 'Triune brain,' which suggests that the human brain evolved in three distinct layers: the 'lizard brain' for instincts, the limbic system for emotions, and the cerebral cortex for rationality. The narrative traces the idea back to Plato's chariot allegory and its adoption by early neuroscientists. However, modern molecular genetic research has debunked this theory, revealing that brains did not evolve in layers but share a common plan among all mammals. The paragraph also touches on the high metabolic cost of brains and the evolutionary significance of their development, hinting at the role of predation in driving brain evolution.

05:02

🐉 The Arms Race of Sensory Systems and Brain Development

The second paragraph explores the idea that the evolution of brains was triggered by the predatory behavior of early animals, leading to an 'arms race' for sensory and motor development. As animals developed more sophisticated ways to hunt and avoid being hunted, the need for a coordinating brain became essential. The paragraph discusses the philosophical difficulty of attributing a 'why' to evolution, known as 'teleology,' and emphasizes the importance of metabolic efficiency in the survival and reproduction of species. It suggests that brains evolved as a control center to ensure the efficient coordination of an organism's parts, which is crucial for survival and the continuation of genetic lineage.

Keywords

💡Metabolic Budget

The term 'metabolic budget' refers to the proportion of an organism's total energy that is allocated to a specific function or organ. In the context of the video, it is used to highlight the high energy cost of maintaining the brain, which consumes about 20% of our body's energy despite being only three pounds in size. This emphasizes the brain's significance and the evolutionary investment in cognitive development.

💡Triune Brain

The 'Triune Brain' concept suggests that the human brain evolved in three distinct layers: the 'lizard brain' for instincts, the limbic system for emotions, and the cerebral cortex for rationality. Although this model was once dominant, the video explains that it has been largely debunked by modern neuroscience, which shows that the brain does not evolve in such clear, stratified layers.

💡Limbic System

The 'limbic system' is a set of brain structures involved in emotion, behavior, and motivation. In the video, it is mentioned as part of the outdated triune brain model, suggesting it evolved in ancient mammals and is layered on top of the 'lizard brain.' However, the video also clarifies that this concept does not hold up under modern scientific scrutiny.

💡Cerebral Cortex

The 'cerebral cortex' is the outer layer of the brain responsible for higher cognitive functions, including perception, thought, awareness, and conscious behavior. The video contrasts the size of the cerebral cortex in humans with that in other mammals, illustrating how the extended developmental period of this structure in humans has resulted in its large size relative to the rest of the brain.

💡Evolutionary Arms Race

The 'evolutionary arms race' is a concept that describes the competitive evolution of traits between different species, such as predators and prey. The video suggests that the development of the brain may have been driven by such an arms race, where the ability to hunt and the need to avoid being hunted led to the evolution of more advanced sensory and motor systems, necessitating a more complex brain.

💡Ampioxus

Ampioxus, also known as lancelets, are simple chordate animals that lack a brain, eyes, ears, and a defined head. The video uses amphioxus as an example to illustrate the evolutionary journey from simple organisms to complex ones with brains, highlighting the dramatic changes in sensory and motor capabilities over time.

💡Sensory Systems

Sensory systems are biological components that allow organisms to perceive their environment through senses such as sight, hearing, and touch. The video explains that the evolution of sensory systems was a key factor in the development of the brain, as these systems provided the necessary input for more complex behaviors and survival strategies.

💡Motor Coordination

Motor coordination refers to the brain's ability to control and synchronize the body's movements. The video suggests that as animals evolved more complex bodies and sensory systems, the need for motor coordination increased, which in turn drove the evolution of a more sophisticated brain.

💡Metabolically Efficient

Being 'metabolically efficient' means that an organism's energy use is optimized for survival and reproduction. The video emphasizes that the brain's role in coordinating the body's functions in a metabolically efficient way is crucial for survival and the continuation of genetic lines.

💡Genetics of Cells

The 'genetics of cells' pertains to the study of genetic material within cells, which can reveal insights into the development and function of organisms. The video mentions that molecular genetic techniques have helped scientists understand that the brain did not evolve in layers, challenging the triune brain theory.

💡Common Brain Plan

The 'common brain plan' refers to the shared developmental blueprint found across all mammals, and possibly all vertebrates. The video explains that while the basic structure of the brain is consistent across species, the differences lie in the duration of developmental stages, leading to variations in brain size and complexity.

Highlights

Brains are the most metabolically expensive organ in the human body, consuming about 20% of our metabolic budget despite being only three pounds in size.

The concept of the 'Triune brain' suggests that the brain evolved in three distinct layers: the 'lizard brain' for instincts, the limbic system from ancient mammals, and the cerebral cortex for rationality.

The 'Triune brain' theory, once dominant, has been challenged by modern neuroscience, which shows that brains did not evolve in layers but share a common plan among all mammals.

The evolutionary origin of the brain can be traced back to ancient Greek philosophy, with Plato's metaphor of two horses and a charioteer representing human psyche.

Carl Sagan's book 'The Dragons of Eden' popularized the 'Triune brain' concept, which was later debunked by molecular genetic research.

Molecular genetic techniques revealed that the brain's development is not layered but follows a common plan with variations in developmental stage duration.

The size of the cerebral cortex relative to the rest of the brain varies among species, indicating a difference in developmental timing rather than layered evolution.

The story of brain evolution is likened to a science fiction narrative, highlighting the fascinating journey from simple creatures to complex organisms with advanced sensory systems.

Ampioxus, a simple creature without a brain, eyes, or ears, represents an early stage of life on Earth and raises questions about the necessity and evolution of sensory organs.

The evolution of brains is theorized to have been triggered by the emergence of predation, leading to an 'arms race' for better sensory and motor systems.

The development of sensory systems and the need for coordination among various body parts are identified as key factors driving the evolution of the brain.

Metabolically efficient coordination of body parts for survival and reproduction is a major selection pressure that has shaped the evolution of the brain.

The 'why' of evolution is a complex philosophical question, with any explanation being a narrative rather than a verifiable truth.

Brains can be viewed as the control center for coordinating the body's parts in an efficient manner to ensure survival and reproduction.

The transcript challenges traditional views on brain evolution and presents a more nuanced understanding based on recent scientific findings.

The discussion on brain evolution emphasizes the importance of considering both genetic and environmental factors in understanding the development of complex organisms.

The evolutionary journey of the brain underscores the continuous interplay between survival needs and the development of advanced cognitive functions.

Transcripts

- Brains are the most expensive organ

we have in our whole body.

Brains cost about 20% of our metabolic budget-

and they're only three pounds in size.

So they're really, really expensive.

So I sort of wondered,

"Well, why did brains evolve in the first place?"

Like, what is a brain good for?

The 'Triune brain' is this idea

that your brain evolved in three layers:

You have the inner "lizard brain,"

which is supposed to contain circuits for instincts.

Layered on top of that is the limbic system;

limbic meaning border.

And this was supposedly evolved in ancient mammals.

And then layered on top of that is the big cerebral cortex-

that's supposed to be the home of circuitry for rationality.

For many years, this was the kind of dominant view

of brain evolution and brain function

and brain development, really.

Both reptiles and mammals evolved from fish;

they're actually on different branches.

And so the idea that you would have an inner lizard

actually doesn't make any sense

from an evolutionary standpoint.

The only animal on this planet

that has a lizard brain is a lizard.

This idea of brain evolution and brain function,

actually, you can trace its roots all the way back

to ancient Greece to the philosopher Plato.

Plato's idea was that the human psyche could be described

as two horses and a charioteer who controlled the horses.

One of the horses represented your instincts;

one of the horses represented emotions;

and then the charioteer who controlled them both.

And this morality tale really survived through millennia.

When neuroscience began as a proper scientific field,

neuroscientists used this metaphor

to describe the structure and function of the brain.

The triune brain idea was around

for a long time in popular culture,

but it became really, really entrenched in a particular book

that was written by Carl Sagan in 1977

called "The Dragons of Eden."

That book won a Pulitzer Prize,

and it was explosive in popularity,

and it contained the narrative of the triune brain.

And then neuroscientists developed the ability

to peer deep into genetics of the cells themselves.

Using molecular genetic techniques,

they were able to trace the genes

that helped to form these cells.

And by doing this, they realized

that actually the brain didn't evolve in layers.

This work, which was really, I would say,

established and replicated in the 1970s,

really blew apart the idea that the brain developed,

was structured and functioned in sedimentary layers.

In fact, there's a common brain plan for all mammals,

and maybe to some extent all vertebrates.

So what differs is the length of time

that each developmental stage runs for,

and that's what makes it look like

a mouse doesn't have a very big cerebral cortex

relative to the rest of its brain.

Whereas it looks like we have a very big cerebral cortex

relative to the size of our brains.

I think the story of brain evolution

is actually the coolest story.

You know, it's like a sort of like

a science fiction story actually.

There are creatures around today, they're called amphioxus.

They almost look like little worms, but they're not worms;

they have gill slits.

They've been around really for 500 million years,

more or less.

But what's really cool

about these animals is they have no brain.

They have no eyes. They have no ears.

They basically have no head.

They're just little stomachs on a stick.

So they're really, really simple animals

that managed to keep themselves alive

without being able to sense anything really,

or very, very little about their environment.

I found it really interesting to understand

that these creatures, and creatures like them,

really dominated the Earth for millions of years.

And at some point in evolutionary history, things changed.

Why did a brain evolve? How did it evolve?

And it turns out that scientists' best guess at the moment,

is that brains emerged on the evolutionary scene

because, at some point, one animal ate another animal,

deliberately-

hunted another animal.

So, hunting turns animals into predators and prey.

And this, scientists think, launched an arms race,

or you might think of it as an eye, ear, nose, and fin race,

to become more powerful predators,

and more efficient to avoid becoming prey.

And so, what you see happening over evolutionary time,

that all of a sudden,

animals start developing actual sensory systems.

When you have more sensory systems to sense at a distance,

and you have more motor parts that you have to coordinate,

and then you have a bigger body so you need internal organs-

now, there are a lot of parts to coordinate,

and that requires a brain.

It turns out that you can never really say

why anything evolved.

The "why?" question is a really tough question;

it's what philosophers refer to as 'teleology.'

Anything that any scientist or philosopher or historian

tells you about why something evolved, is just a story.

We can never really verify the truth value of that story.

But we can understand something about how organs evolve.

We can also understand something

about the most important functions of that organ.

If you think about brains as the control center

of all of these parts that have to be coordinated

in a metabolically efficient way in order to survive.

And it turns out that metabolic efficiency is a major,

major selection pressure.

If your parts aren't working efficiently

in a coordinated way, you don't have enough energy

to do really what is your ultimate job-

which is to produce offspring, to pass your genes along.