Neuroscientist Paul MacLean suggested in 1967 that the human brain functions as three separate “brains,” each of which represents a stage in evolutionary development. He referred to the three-way unity as humanity’s triune brain. Through evolution’s penchant for preserving genetic code that proves useful for survival and discarding mutations that prove useless, MacLean suggested that human brains evolved by adding to successful brain structures of earlier vertebrates. Thus, both fish and dogs have brain structures in common with people. But instead of the evolutionary structures being uniformly mixed throughout the human brain, they nest one inside another like Russian dolls. The most primitive lies deepest in the brain, under more modern layers.

Charles Darwin observed that domesticated animals have thinner cortical layers than their wild cousins in the forest. Wild animals’ exposure to a wider variety of environmental stimuli may create richer neural connections.



MacLean’s first “brain” is the R-complex, which takes its name from its resemblance to the simple brains of reptiles. The R-complex formed from an extension of the upper brain stem. It’s enough to keep a snake or a salamander alive as well as ensure the continuation of the species. The R-complex oversees sleeping and waking, breathing and heartbeat, temperature regulation, and automatic muscle movements. It also plays a crucial role in the processing of sensory signals from the peripheral nervous system. MacLean’s experiments with a variety of animals demonstrated that the neural connections in the R-complex provide sufficient mental firepower for hunting, mating, establishing territory, and fighting. In other words, everything necessary for finding food, competing with other animals for survival, and passing along the genes of the dominant, strongest individuals. Humans may think of themselves as being far above turtles and alligators, but their brain shares the same mechanics for regulating basic body functions. Further-more, whenever humans engage in a schoolyard scuffie or compete for the affections of another, they’re exercising the reptilian cores of their brain.


The second “brain” is the limbic, or paleo mammalian, system. It’s common to all mammals, including humans, but is lacking in reptiles. The limbic system coordinates and refines movement. It gives rise to emotions and simple memory, as well as the rudimentary social behaviors they make possible. When MacLean destroyed part of the limbic system in the brain of young mammals, their behavior regressed toward the reptilian. They stopped playing and exhibited weaker mother-offspring bonds. Humans who flush with anger when they get slapped across the face, or glow with happiness when kissed, are using their limbic systems. If they choose to ignore the slap or the kiss, however, they need to exercise the third and highest level of the brain.

Swinging through forest has been linked in theory to brain hemisphere specialization.
Swinging through forest has been linked in theory to brain hemisphere specialization.

The third “brain” is the cerebral cortex. Many mammals possess a cortex, but it is most highly developed in humans. It adds the benefits of problem solving and both long-term and complex working memory to the lower two “brains.” The neomammalian brain, as MacLean dubbed it, gives humanity its capacity for language, culture, memory of the past, and anticipation of the future. It also makes humans the first species with empathy, the ability to see the world through the eyes of others.

“It is this new development that makes possible the insight required to plan for the needs of others as well as the self … In creating for the first time a creature with a concern for all living things, nature accomplished a 180-degree turn-about from what had previously been a reptile-eat-reptile and dog- eat-dog world,” MacLean said.

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