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|  | | The Triune Brain/Limbic System Model—What To Keep, What To Discard |
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| | | The Quest for the “Emotional Brain” |
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| | Hominization, or The History of the Human Lineage | |
The human brain as we know it today is like a city with a long history. It has its old sections where, in ancient times, the activities required for survival took place. It also has other, newer sections that developed around the older ones. Lastly, it has the modern section as we know it now, which was often built on the foundations of the other sections.
The reptilian brain first appeared in fish, nearly 500 million years ago. It continued to develop in amphibians and reached its most advanced stage in reptiles, roughly 250 million years ago.
The limbic system first appeared in small mammals, about 150 million years ago.
Lastly, the neo-cortex began its spectacular expansion in primates, scarcely 2 or 3 million years ago, as the genus hom*o emerged.
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| THE EVOLUTIONARY LAYERS OF THE HUMAN BRAIN |
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The first time you observe the anatomy of the human brain, its many folds and overlapping structures can seem very confusing, and you may wonder what they all mean. But just like the anatomy of any other organ or organism, the anatomy of the brain becomes much clearer and more meaningful when you examine it in light of the evolutionary processes that created it. Probably the best known model for understanding the structure of the brain in relation to its evolutionary history is the famous triune brain theory, which was developed by Paul MacLean and became very influential in the 1960s. Over the years since, however, several elements of this model have had to be revised in light of more recent neuroanatomical studies (see the first two history modules, to the left).
Keeping this in mind, MacLean's original model distinguished three different brains that appeared successively during evolution :
| The reptilian brain, the oldest of the three, controls the body's vital functions such as heart rate, breathing, body temperature and balance. Our reptilian brain includes the main structures found in a reptile's brain: the brainstem and the cerebellum. The reptilian brain is reliable but tends to be somewhat rigid and compulsive. | The limbic brain emerged in the first mammals. It can record memories of behaviours that produced agreeable and disagreeable experiences, so it is responsible for what are called emotions in human beings. The main structures of the limbic brain are the hippocampus, the amygdala, and the hypothalamus. The limbic brain is the seat of the value judgments that we make, often unconsciously, that exert such a strong influence on our behaviour. | The neocortex first assumed importance in primates and culminated in the human brain with its two large cerebral hemispheres that play such a dominant role. These hemispheres have been responsible for the development of human language, abstract thought, imagination, and consciousness. The neocortex is flexible and has almost infinite learning abilities. The neocortex is also what has enabled human cultures to develop. |
These three parts of the brain do not operate independently of one another. They have established numerous interconnections through which they influence one another. The neural pathways from the limbic system to the cortex, for example, are especially well developed.
For more about how these structures of the mammalian brain evolved together, click this link.
