(Ancient genetic signals can be resurrected after lying
dormant for millions of years, awaiting only the right
trigger to release them.)
In the early Carboniferous there arose an organism
with, for the very first time,
more information in its brain than in its genes,
and the brain grew more dominant over time.
The modern brain's blueprint is a story
of successive accretion and specialization,
which freed us from the tyranny
of DNA's monopoly on total information.
The three interconnected parts of the modern brain
correspond to the evolutionary emergence
of reptiles, mammals and primates -
especially humans -
and represent a major burst forward
in overall brain development,
each step adding a new function and a new layer.
And since these were laid down over existing systems,
the preexisting systems had to be accomodated
because making basic change deep within the fabric of life
very frequently turns out to be fatal.
Adding new layers while retaining the old ones
allows major change consistent with survival.
Today's triune, or three part brain
consists of the R-complex at its core,
surrounded and surmounted by the limbic system,
which is enveloped by the massive neo-cortex.
The R-complex is the most ancient part,
the dinosaur component of human nature,
seat of reptilian ritual and aggression,
territorial and hierarchical behavior.
No carefully-weighed decisions happen here,
no nuances of emotion's wrenching contradictions,
just the stolid acquiescence to the dictates of its genes,
no thought or action taken of its own volition.
The R-complex lies below the limbic system,
which was added during the development of mammals.
It generates emotions, changes of mood and states of mind
and the maternal instinct found in warm-blooded animals.
Emotions can be triggered by hormonal exudations
of endocrine systems such as the amygdala.
Stimulating the amygdala can make a placid animal frenzied
or cause a cat to cower from a mouse in abject terror.
The limbic system is the seat of subtle emotions
which we think uniquely ours among the animals,
but altruism may have gotten started here
and love may be an invention of the mammals.
The neo-cortex is layered over the limbic system,
and accounts for 85% of the brain's mass.
Its frontal lobes most likely are connected
with deliberation and regulation of our actions.
Transforming experience into symbolic languages
is a primary function of the neo-cortex,
the extrasomatic manifestation of knowledge
expressed as reading, writing, speaking and mathematics.
It may control anticipation of the future
and cause worry about the possibility of disaster,
but in our attempts to regulate the future we produced
ethics, science, legal codes and magic.