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 Homo emerged.
THE EVOLUTIONARY LAYERS OF THE HUMAN BRAIN
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
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
abilities. The neocortex is also what has enabled human cultures to