How Our Brains Were Built

At first glance, the human brain might appear to be a marvel of engineering—a seamless interface for sensory input, cognitive control, and motor output. But if you peer beneath its sophisticated functions, one quickly sees a structure resembling something more like London's winding streets: layered, circuitous, and often baffling in its logic. This resemblance is no accident. Like old cities, our brains weren’t planned; they evolved with new features layered over older foundations, constrained by what was already in place. Because of this, the brain's evolutionary history has given rise to a complex and seemingly disordered system of neural architecture.

Building on Bedrock: The Reptilian Roots

To understand why the brain appears so convoluted, one must start from the bottom, literally and evolutionarily. The most ancient part of our brain, sometimes referred to as the "reptilian brain," comprises structures such as the brainstem and basal ganglia. These regions regulate fundamental survival processes, including heart rate, respiration, and primitive reflexes. Such features are found in most vertebrates and lay the groundwork for later developments (Holloway, 1967).

As evolution progressed, particularly with the emergence of mammals, newer regions, such as the limbic system, developed. This system allowed for more complex social behaviors and emotional processing. Rather than redesigning the entire brain to integrate these new features, evolution did what it does best—it repurposed and built upon what was already effective. Like narrow alleyways transformed into highways or cottages refitted as skyscrapers, new neural pathways arose atop old ones (Hofman, 2014).

The Cortical Expansion: A Tangle of Innovation

The neocortex is the brain's most recent and significant evolutionary addition. This "new brain" expanded significantly in size during hominid evolution, becoming responsible for higher-order thinking, language, and conscious decision-making. Instead of replacing older structures, it was added to the top. This cortical ballooning created new possibilities but also led to increased interconnection, redundancy, and convolution. Each new adaptation had to work with, not against, its more primitive predecessors.

According to Hofman (2014), the evolution of larger brains, particularly in primates, was more about scaling up existing systems than developing new ones. Consequently, the brain's internal wiring became less efficient in absolute terms. Signals traveling between distant parts of the brain now had to traverse increasingly long and indirect routes, much like a person trying to travel diagonally across a city grid designed in concentric circles.

Development Mirrors Evolution

An elegant demonstration of this evolutionary layering is provided by developmental neuroscience. How our brains develop in the womb recapitulates their evolutionary history—a process known as recapitulation. Early in fetal development, the brain resembles a fish or reptile. Only later do the structures responsible for human-specific functions, like the frontal lobes, emerge (Neubauer & Hublin, 2012). This layering indicates that even as newer, more sophisticated structures develop, they must connect with and rely on more ancient systems that weren’t designed with such complexity in mind.

Finlay and colleagues (2001) highlight that this developmental sequence is governed by a conserved schedule: brain regions develop in a relatively fixed order, and the duration of development is extended in larger-brained species, such as humans. The result? A brain that’s functionally richer but structurally more tangled—like a city that’s grown too quickly to be mapped.

The Price of Plasticity

Why didn’t evolution “clean up” the brain's wiring along the way? The answer lies in the principle of plasticity. Brains evolve to be adaptable, not perfect. This flexibility is evident in how various brain regions can take on new functions when damage occurs—a trait that would be impossible in a more rigid, modular architecture.

Moreover, complexity itself has evolutionary advantages. Bassett and Gazzaniga (2011) state that the brain operates as a complex network where redundancy, distributed processing, and nonlinear pathways enhance resilience and flexibility. In such a system, what appears convoluted may be a feature, not a flaw.

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Conclusion: The Brain’s Long Road of Change

Why is the brain similar to London’s streets? Both have evolved through historical layering—adaptive, gradual, and constrained by their past. The brain may seem inefficient, but this is not the result of poor design; rather, it reflects the evolutionary process, utilizing available resources in a patchwork manner. Like the ancient Roman roads that support present-day London routes, the reptilian brain forms the foundation for complex human cognition. Our brains are chaotic, remarkable, and, fundamentally, rooted in history.