The Brain from the Inside Out

Let’s start with a bizarre thought: the discipline of neuroscience is actually the brain coming to understand itself. To try and understand the folded, overlapping, brilliant mess of the structure of the brain, biologist Paul MacLean developed the famous triune brain theory based on the evolutionary processes that formed it. According to the theory, the brain was built from the inside out, with three main parts layering on top of each other over time: the reptilian, the limbic, and the neocortex. The reptilian brain is the oldest and most primal, first emerging in fish nearly 500 million years ago. It controls basic but vital biological functions such as heart rate, breathing, and temperature, and it’s also responsible for aggressive territorial behaviours and social heirarchies. The limbic brain emerged next, 150 million years ago with the first mammals—it’s the major source of human emotions, moods, judgements, self-awareness, and concern for our young. Lastly, the necortex evolved with the primates (and the genus Homo) just 2–3 million years ago. It is this outer layer that truly makes us human. It’s responsible for the development of language, imagination, astract thought, and consciousness, and is the reason human culture developed—the reason poetry and mathematics and music and science exist. The three parts don’t operate independently, but instead have developed trillions of neural connections in a complex network. It’s like the brain is an ancient city that has expanded and developed over time, but roads and tunnels have sprung up to connect the old to the new.