Humans have a curious attachment to the number three. From The Three Little Pigs and The Three Musketeers to “reduce, reuse, recycle” and the Holy Trinity, three-part structures have long helped us organise ideas and make them easier to remember.
Neuroscience, it turns out, has its own famous three-part theory.
In 1959, American neuroscientist Paul D. MacLean proposed what became known as the “triune brain” model. His theory suggested that the human brain consists of three evolutionary layers, each associated with a different set of functions.
At the centre was what became popularly known as the “reptilian” or “lizard brain”. According to the theory, this ancient part of the brain was responsible for basic survival instincts, including feeding, reproduction and other automatic behaviours.
Surrounding it was the so-called mammalian brain, containing the limbic system, which MacLean associated with emotions, social behaviour, attachment and parental care.
The outermost layer was the “neomammalian” brain, essentially the cerebral cortex. This was presented as the seat of sophisticated human abilities such as language, reasoning and conscious thought.
The model offered an appealing explanation for human behaviour. Our primitive instincts, emotional responses and rational thinking appeared to be competing systems, with the supposedly superior cortex capable of restraining the impulses of the more primitive brain underneath it.
The idea proved enormously popular. Expressions such as “reptilian instincts” became common in psychology, business, self-help and popular culture. The concept was also used to explain impulsive behaviour, aggression, short-term thinking and other supposedly primitive tendencies.
There was only one problem: the science did not support the metaphor as neatly as the story suggested.
Modern neuroscience has largely rejected the idea that the human brain consists of three distinct evolutionary layers stacked on top of one another.
Evolution does not work like the construction of a building, with new mental abilities simply being added on top of older ones. Instead, brain structures have evolved, adapted, and become interconnected over extraordinarily long periods.
MacLean’s research involved comparative studies and dissections of different animals, but many of his interpretations have since been challenged or discarded.
For example, structures once regarded as uniquely associated with mammals are also found in reptiles. Likewise, characteristics once considered exclusive to humans or other “higher” mammals can be found in a variety of other species.
Animal behaviour also presents a problem for the traditional hierarchy. Many species demonstrate sophisticated learning, problem-solving, communication, social cooperation and responses to emotional stimuli.
The evidence therefore makes it difficult to divide behaviour neatly into three categories, instinct, emotion and reason, and assign each to a separate evolutionary brain.
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As neurologist Guy Leschziner has noted, it is certainly true that different parts of the brain perform different functions. But describing humans as possessing a distinct “lizard brain” responsible for basic instincts is not supported by either evolutionary or physiological evidence.
The evolutionary history of the brain is considerably more complicated.
Contemporary researchers such as psychologist Lisa Feldman Barrett have further challenged the idea that the brain operates as a battlefield between primitive instincts, emotions, and rational thought.
Instead, the brain functions as an integrated and dynamic system. Its fundamental task is to regulate the body, anticipate what is likely to happen, and manage the energy required to keep the organism alive.
This does not mean that the brain lacks specialised regions or that instinctive behaviour does not exist. Rather, it means that those functions cannot be understood as the activities of three independent brains competing for control.
Despite the scientific evidence, however, the lizard-brain idea has proved remarkably difficult to eliminate.
Part of its appeal is simplicity. The model offers an intuitive explanation for why humans sometimes behave impulsively or emotionally: an ancient part of the brain has supposedly taken control.
It also fits neatly with the way people like to tell stories. Three distinct layers make for a memorable narrative, particularly when they are presented as a progression from primitive to sophisticated.
But the model carries a deeper assumption that deserves scrutiny: that humans sit at the top of an evolutionary hierarchy.
The idea of a primitive reptilian brain beneath a uniquely sophisticated human cortex reinforces the belief that our species is fundamentally different from other animals, more rational, more advanced, and somehow separate from the natural world.
Modern neuroscience paints a different picture.
Human beings undoubtedly possess extraordinary cognitive abilities, but many of the underlying mechanisms of learning, emotion, communication and social behaviour have deep evolutionary roots and are shared, to varying degrees, with other animals.
That does not diminish what makes humans remarkable. If anything, it gives us a more fascinating understanding of our place in nature.
The brain is not an onion with a tiny reptile hidden at its centre. It is an extraordinarily interconnected organ shaped by millions of years of evolutionary change.
The “lizard brain” remains a powerful metaphor because it makes complicated neuroscience easy to understand. But a useful metaphor is not necessarily a scientific fact.
It may be time to retire the reptile, and replace the neat three-part story with a more accurate one: the human brain is not a collection of competing evolutionary layers, but an integrated system whose abilities emerged through a complex history shared, in many ways, with the rest of the animal kingdom.
