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.
The most efficient model for understanding the brain in terms of its evolutionary
history is the famous triune brain theory developed by Paul MacLean. According
to this theory, the following three distinct brains emerged successively in the
course of evolution and now co-inhabit the human skull:
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.
