Despite our best intentions, scientists sometimes make a very basic mistake: we look for what makes humans unique.
Certainly, humans are not just unique, but extraordinary. Nothing else in the known universe has produced art, science, technology, or civilization. But, our history of searching for how, precisely, we came to be exceptional has often led to bad science – and to popular acceptance of bad science. Nowhere is that clearer than in the hugely popular – and entirely wrong – theory called the Triune Brain Hypothesis.
You may have heard of it as the proposal that we have “lizard brains.”
The triune brain hypothesis, developed by the neuroscientist Paul MacLean between the 1960s and 1990s and widely popularized by the astronomer Carl Sagan, asserts that we have a “lizard brain” under our “mammal brain,” and that our “mammal brain” is itself under our primate/human brain. Under this hypothesis, brain evolution is an additive process: new layers of brain tissue emerge on top of old layers, leading to a tenuous but effective coexistence between the “old brain” and the “new brain.”
MacLean proposed his (incorrect) theory after he made some curious observations about the effects of cutting out what he called the “reptilian complex” of a monkey’s brain (so named because he thought it looked similar to the tissue that made up most of a reptile’s brain). When MacLean took out this part of a male monkey’s brain, the monkey stopped aggressively gesturing at its own reflection (which it thought was another male monkey). This behavioral change seemed to fit MacLean’s hunch that he had taken out a “reptile”-like part of the monkey’s brain, since he thought that aggressive gesturing is a typical example of “reptilian behavior.”
It’s unclear why cutting out this part of the monkey’s brain made the monkeys stop showing aggressive displays, but this brain area, more commonly called the globus pallidus, is known to be involved in an enormous variety of processes. Also, to my knowledge, MacLean’s original observations have not been replicated. What’s more, MacLean’s claim about the prominence of the globus pallidus in the reptilian brain is false: it forms just one part of reptiles’ brains, exactly as it does in the monkey brain.
Based on these loose observations, MacLean argued that we might have a “lizard” brain inside of our brain. In other words, he thought that we never got rid of the “reptilian” brain we inherited from our reptile ancestors, but instead evolved new brain structures on top of our old reptile brain.
Based on these shaky foundations, together with other loose observations regarding what he considered to be uniquely mammalian behavior, MacLean went on to develop a full-blown theory of human brain evolution. The theory held that inside our brains there is a primitive reptilian complex, which is surrounded by an “old” mammalian structure called the limbic system, which is itself surrounded by a “new” mammalian structure called the neocortex. The neocortex was, MacLean asserted, the crowning jewel of brain evolution – the structure, in other words, which made humans (and perhaps other intelligent mammals) unique.
Over the last few decades, MacLean’s theory has become part of the cultural zeitgeist. Clickbait articles bashing the “basic ‘lizard brain’ psychology” of an opponent political group appear on mainstream news websites. Articles with headlines like “Your Lizard Brain” and “Don’t Listen to Your Lizard Brain” get featured on Psychology Today, a magazine whose sales have soared to the top 10 in the nation. The triune brain theory has even been featured prominently in a blog article on Scientific American, an award-winning and massively popular science magazine. Except perhaps for the political clickbait, these are all publications that make an honest and serious attempt to get the scientific facts right. And this popularity can’t just be pinned on major media: I’ve seen the triune brain theory pop up in college psychology textbooks (e.g this one, this one, and this one), and a search for #triunebrain on Twitter yields a litany of casual references to the idea that we have a lizard brain.
But MacLean’s triune brain theory is completely wrong – and neuroscientists have known it’s wrong for decades.
The theory is wrong for a simple reason: our brains aren’t fundamentally different from those of reptiles, or even from those of fish. Every mammal has a neocortex (not just the really intelligent ones), and all vertebrates, including reptiles, birds, amphibians, and fish, have analogues of a cortex.
In fact, the very idea that new brain structures emerge on top of old ones is fundamentally at odds with how evolution usually works: biological structures are typically just modified versions of older structures. For example, the mammalian neocortex isn’t a completely new structure like MacLean thought it was, but instead is a modification of the repitilian cortex. As the evolutionary neuroscientist Terrence Deacon explains: “Adding on is almost certainly not the way the brain has evolved. Instead, the same structures have become modified in different ways in different lineages.” This fact is illustrated quite nicely in this figure:
Notice that the cortex and its analogues (colored here in blue) are found in all vertebrates, and isn’t unique to mammals. What’s more, all the major structures of the mammal brain can also be found in the reptile brain, and even in the fish brain.
So what’s gone wrong here? Why is the triune brain theory widely believed, even among psychologists, while evolutionary neuroscience abandoned the theory decades ago (and never took it very seriously in the first place)?
The problem starts, of course, with MacLean. I think it’s fairly clear that MacLean wanted to find what makes humans (and mammals more broadly) unique. And that desire to identify our uniqueness led him to judge his available evidence poorly. MacLean should have considered alternative hypotheses, such as the possibility that differences between our brains and those of other vertebrates are a matter of degree, rather than kind. And he should have asked whether those alternative hypotheses could explain his evidence as well as his own theory could. This sort of self-questioning is key to doing good science: we need to work especially hard to try to prove ourselves wrong. Fortunately, science is structured such that if we can’t (or won’t) prove ourselves wrong, our colleagues most certainly will. And other scientists did prove MacLean wrong, as detailed thoroughly in Terrence Deacon’s paper on what’s known about mammalian brain evolution.
But the evidence that MacLean’s theory was wrong never seemed to make it out of the small world of evolutionary neuroscience. And for that, I think that some of the blame lies with one of my heroes, Carl Sagan.
The triune brain theory played a starring role in Carl Sagan’s bestseller and Pulitzer Prize winner, The Dragons of Eden. In The Dragons of Eden, Sagan drew on MacLean’s theory to account for how humans evolved to produce science, art, math, and technology – the features of our mind, in other words, which make us unique. Underneath our thinking neocortex, Sagan wrote, is a sea of primitive mammal emotions and even more primitive reptilian proclivities toward hierarchy and aggression. But, he argued, humans are special because our neocortex is particularly well-developed, and so, unlike other animals, we can reason our way out of our primitive instincts.
To be fair, Sagan was honest and careful in his writing about the triune brain theory, and peppered his explanations with qualifying and cautious language (e.g. “if this theory is correct…”). He also stressed that the model is “an oversimplification” and that it may be nothing more than “a metaphor of great utility and depth.” But Sagan’s enthusiasm for the theory was clear in both his writing and television programs, which were, as always, beautiful and captivating – and had a huge audience. It should therefore come as no surprise that, partly by way of Sagan’s eloquence and popularity, MacLean’s faulty ideas made their way into the cultural mainstream.
It’s unclear how to undo the damage done, except through honest communication of what’s known. Evolutionary neuroscientists guessed from the start the the triune brain theory probably wasn’t right, and now they know it’s not right. But the word hasn’t gotten around. And that’s where you and I come in.
For my part as a neuroscientist, all I can do is point out what we do have good evidence for: that new brain structures are typically just modified versions of old brain structures, and that we don’t have a lizard brain inside our mammal brain.
But you have a part to play in this too, since now you also know that our brain is simply a vertebrate brain, just like that of every fish, amphibian, reptile, bird, and mammal. Help make that astounding and beautiful fact part of our cultural zeitgeist.