Plato famously described the human psyche as two horses and a charioteer: One horse represented instincts, the other represented emotions, and the charioteer was the rational mind that controlled them. Astronomer Carl Sagan continued this idea of a three-layer, “triune brain” in his 1977 book The Dragons of Eden.

But leading neuroscientist Lisa Feldman Barrett challenges this idea of the brain evolving in three layers, instead revealing a common brain plan shared by all mammals and vertebrates. The development of sensory systems led to the emergence of the brain, and hunting and predation may have initiated an arms race to become more efficient and powerful predators.

Despite advances in neuroscience and genetics, the question of why the brain evolved remains elusive. But Feldman Barrett’s fascinating exploration of the brain’s evolution offers insights into the most important functions of this complex organ, and invites us to think more deeply about the origins of our own intelligence.

Lisa Feldman Barrett: 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.