As a student, I was introduced to the architecture of the brain through a specific phylogenetic lens, using the terms archicortex, paleocortex, and neocortex. While the term archaeocortex was still in use at the time, it has since largely been superseded by archicortex. These designations were more than mere labels; they reflected a chronological narrative of evolutionary appearance, categorised from oldest to newest.
Archicortex: The phylogenetically oldest component of the cerebral cortex, primarily represented by the hippocampus and the dentate gyrus.
Paleocortex: Considered the intermediate or "old" cortex, it is fundamentally associated with the olfactory system.
Neocortex: The newest and most sophisticated addition, appearing first in early mammals and now accounting for approximately 90% of the human cerebral cortex.
While this three-part schema remains a useful pedagogical starting point, it is, in truth, a gross simplification. Each section consists of distinct neuronal layers, and the phylogenetic order itself is a matter of debate. While many sources cite the archicortex as the most ancient, some neuroanatomists argue that the olfactory system appeared first, or at least concurrently. I suspect the professor who taught us this order belonged to the latter school of thought. In a lecture on the cranial nerves, he adopted a similarly phylogenetic approach: he argued that the olfactory nerves were the oldest not just anatomically, but in terms of how life first "contended" with its environment. The first interaction was chemical; what we call "smell" is, in effect, the detection of airborne molecules. It requires little imagination to see this as a primary evolutionary necessity.
He extended this logic to the remaining cranial nerves. If the first challenge was chemistry, the second was electromagnetic radiation—specifically, the spectrum we call light. Thus, the optic nerve followed. Whether this specific evolutionary sequence is entirely accurate is perhaps a moot point; its value lies in providing an entry into the brain’s daunting complexity. It suggests an underlying logic to biological structure, reinforcing the idea that the brain is not a single, unitary object, but a record of evolutionary history.
This perspective aligned with the Triune Brain hypothesis, a theory that was particularly popular during my student years. Originated in the 1960s by Paul MacLean and popularised by Carl Sagan in his 1977 book The Dragons of Eden, the hypothesis used the archicortex, paleocortex, and neocortex as the biological foundation for a theory of three distinct evolutionary "brains" residing within the human skull. MacLean’s model was an expansion of these cortical categories; he integrated them with deeper structures, such as the basal ganglia (or "R-complex"), to suggest a functional hierarchy of behaviour that moved from primal survival to complex reasoning.
The enduring popularity of the Triune Brain in the public imagination is a testament to the power of compelling narrative in popular science. Through Sagan’s prose, a hypothesis was "frozen" in the public consciousness, persisting long after the scientific community had moved toward more integrated models. Today, neuroscience has largely moved past these "additive" layers in favour of network neuroscience and neuroplasticity, which view the brain as a highly interconnected system rather than a series of autonomous chronological strata.
In the decades since, the Triune Brain hypothesis has fallen out of favour. However, I wonder if the theory still holds value as a pedagogical "thinking tool" or an object for critical analysis. It could serve as a rigorous exercise for students—not merely as a historical footnote, but as a challenge. It is a rare and perhaps necessary experience in scientific education to be handed a once-revered hypothesis with the instruction: "Explain exactly why this is now considered incorrect."
