You may have heard that in moments of high stress or sudden anger, a primitive part of our brain takes over. And that this irrational side of us doesn’t come from the highly evolved, human part of us; it’s down to the residue of our reptilian ancestors that evolution left within our brain, untouched. Your ‘lizard brain’, some say, is to blame.
The lizard brain theory was coined in the 1960s by neuroscientist Paul MacLean. MacLean was studying the brains of humans and other animals in the hopes of understanding where negative emotions come from.
During his research, he noticed there were some behaviours common in both reptiles and mammals – ‘primitive’ behaviours associated with basic survival, such as enacting routines and protecting territory – and those that were unique to mammals.
This, along with developments in neuroscience that meant MacLean could identify similarities and differences in brain structures, led him to conclude that the human brain had evolved from the reptilian one and, crucially, still had within it the old lizard brain.
According to MacLean, there were actually three brains within the one human brain, which he dubbed the ‘triune brain.’ First there was the oldest, reptilian brain. Then, as mammals evolved, new structures grew around the lizard brain to build the ‘second’ brain, called the paleomammalian complex or limbic system.
Finally, as the higher primates evolved, a third brain grew around the existing two that gave us and our closest animal relatives more complex cognitive functions, like language and reasoning. The triune brain theory became immensely popular.
Astronomer Carl Sagan references MacLean’s work in his 1977 Pulitzer Prize-winning book, The Dragons of Eden. Even today, there are countless articles that blame all number of actions on the lizard brain.
The thing is, you don’t have a lizard brain. As one 2020 paper puts it, “your brain isn’t an onion with a tiny reptile inside.” We know this because neuroanatomists can see commonalities and differences in the brains of animals that the triune theory doesn’t allow for.
For example, if each of the three brains is the result of evolution developing new structures atop old, we’d expect the first and second ‘brains’ to look the same in higher primates as they do in other mammals.
Yet the amygdala, which is located within the limbic system MacLean identified, is much more developed in monkeys than it is in rats. Evolution doesn’t simply add more stuff on to a species, keeping the old unchanged. Nor does it follow a law of progression such that the older animals are more basic and newer (in evolutionary terms) more complex.
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