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What is the Reptilian Brain?

By Alan Rankin
Updated Mar 03, 2024
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The reptilian brain, according to a classic theory of brain science, has corresponding structures in the brains of mammals, including humans. According to the “triune brain” theory, the reptilian brain, concerned with instinct and survival, developed first in evolutionary history. Creatures such as mammals developed more complicated brain structures on the foundation of the reptilian brain, allowing for thought, emotion and self-awareness. Brain studies have since shown that the triune brain theory is oversimplified at best; however, it remains popular with the media and the general public.

During the 1960s, neuroscientist and physician Paul D. MacLean’s research into brain structures revealed that the basal ganglia, a group of structures in the base of the human brain, resembled the brain of lizards and other reptiles. This, coupled with the knowledge that the basal ganglia are strongly involved in motor functions, led MacLean to believe brain development corresponded to evolutionary development. Reptiles developed first in evolutionary history, followed by mammals and then humans, so he reasoned that the brain could likewise be divided into sections based on developmental complexity.

In MacLean’s theory, the basal ganglia, which he called the reptilian brain, controlled baser instincts such as aggression and territoriality, behavior that can be observed in reptiles as well as mammals, including humans. The intermediate brain structures, which he called the “limbic system,” controlled higher functions necessary to rearing the young but were not necessary in reptiles, which generally lay eggs rather than give birth to and raise live young. The neocortex, found only in higher mammals, allowed the development of language, reasoning, and conscious thought in humans.

Subsequent discoveries in brain and animal science have shown the triune brain theory is not a precise model. Creatures such as birds, for example, are capable of using rudimentary tools and language, despite their lack of a neocortex. Some brain functions once believed to be controlled by the reptilian brain have since been found to involve various areas of the brain. Evolutionary development is also not as simple as once thought, further disputing MacLean’s developmental model.

The triune brain and the reptilian brain remain fixtures of popular culture and belief about brain functions. Astrophysicist Carl Sagan’s popular science bestseller “The Dragons of Eden” gave the triune brain theory wide exposure during the 1970s. In his groundbreaking graphic novel “Elektra Assassin,” comics artist Frank Miller gave his character Elektra the ability to function only with her “reptilian brain,” allowing her to act instinctively and ruthlessly in the presence of danger.

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Discussion Comments

By stl156 — On Dec 25, 2011

@Izzy78 - I think I could agree with you on the reptile idea. After the mass extinction, there were obviously some reptiles left as well as whatever other things were on Earth at that time. Since they didn't have to compete against such large animals, I guess there wasn't really any reason to keep growing so big. I'm not positive on this, but I think most of the reptiles that survived were aquatic, so maybe that had something to do with it, too.

More to the idea of the brain differences between mammals and reptiles, though, I would be very interested to see what would happen if you could have a reptilian brain in humans. Clearly, this could never happen, both from physical and ethical standpoints, but what would be the hypothetical outcome if a mammal could be born with a reptile-type brain?

What instincts would take over? Would the mammal just be concerned with finding food and shelter? Another good question is, since part of being a mammal is being raised by a parent, would it be able to learn anything from the parent?

By Izzy78 — On Dec 24, 2011

How does the amphibian brain fit into all this? I know they are one step below the reptiles in terms of development, but what is their brain like? If the reptiles just have the ability to protect their territory and things like that, how could the amphibians be any less developed, because wouldn't territory just be a basic animal instinct? Even insects protect their territory, and they don't have any sort of a brain, just nerves.

I was also curious if scientists had, through the discovery of dinosaurs, any idea about how the reptile brains had developed over the past several million years. Are the reptiles now smarter than the dinosaurs? In what ways? They are definitely smaller, so my guess would be less about them getting smarter per se, and more about them protecting themselves against new threats.

By TreeMan — On Dec 24, 2011

@jcraig - I think that is where the real discussion comes into play. It is widely know that the spinal cord controls movement and the frontal lobe is in charge of different reasoning skills. Obviously, if one of those two areas is injured, its skills will be diminished. What I think isn't fully known, though, is whether the mammal brain would have been able to develop without the reptile brain that came before it.

Also, I think another part of the question is,does every animal that can have reasoning skills need a frontal lobe similar to humans? Birds obviously have some sort of ability to do things like that, but how did they end up with it?

I am not very familiar with the evolutionary tree, but I would assume that a lot of the answers could be found there for someone who knew how to efficiently utilize it.

By jcraig — On Dec 23, 2011

Interesting. I had never heard of the triune brain system before. I think it sounds like a worthwhile concept even if it doesn't always hold true. That being said, though, if scientists have found that birds can do certain actions without a neocortex, then what is the real source of those instincts?

Although the article doesn't directly mention it, there is still proof that certain areas of the brain are responsible for different skills. Even if the parts discovered by MacLean aren't the perfect solution, it still seems like he was on the right track to really discovering the basic functions of the brain. For example, people can get brain injuries from things like car wrecks, and depending on the part of the brain that was injured, they might lose movement, or they might instead lose different reasoning skills.

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