just chemicals?

Richard Hall rhall at uvi.edu
Thu Jan 9 10:20:17 EST 1997


Survival depends on the ability to act appropriately.

Animals possess magnificent chemical engines (muscles and glands) that need
continuous control. In most multicellular animals, motor control is a
neural process organized by a central nervous system of proportionate size
and complexity. The animal brain seems well suited toward pattern
recognition and one fundamental aspect of successful thinking is relating
patterns to possible outcomes.  Put the two processes-motor controls and
awareness of outcomes- together you have a machine that makes active
choices.

The major distinction between, say, insect and mammalian brain functions
lie in the complexity of patterns involved and the mode of learning
patterns.  Insects recognize and respond to relatively simple external cues
because they have limited pattern recognition capabilities. Social insects
such as bees and ants may have rudimentary symbols to communicate the
location of food and to activate defensive behaviors.  I am convinced that
the male mosquito whine is an auditory cue to invite the blood sucking
female to dine and mate. Because mammals recognize more complex patterns,
we have evolved with more complex behaviors.  Humans actively use symbols
(words and figures) to quantify and record patterns and spend considerable
time teaching these patterns to each other.  From an evolutionary
perspective, the pattern recognition systems of an insect are no worse or
better than those of primates. Both systems work just fine.

What does this have to do with the just a bunch of chemicals string?

Motor systems are essentially reductionistic.  Reductionism is a convenient
process for imposing order on systems, but it also reduces the complexity
of any pattern to common elements.  At that point, the pattern loses
informational value. This is not fatal but it can be inconvenient if the
animal needs to perform complex tasks. Humans have perhaps one trillion
neurons in the CNS, perhaps 10 million sensory receptive fields (not
sensory neurons-receptive fields), and one or two million motor neurons.
As the animal brain became more complex, the ability to generate internal
patterns augmented sensory function allowing something more than simple
reflex behaviors as internal patterns also modify the control of motor
function. No matter how complex brain pattern recognition might be, we have
relatively few motor options controlled by only a few motor neurons. So,
how can complex motor behaviors evolve?

While motor controls in the brain stem are "somewhat" similar in all
tetrapods, cerebral systems show remarkable evolutionary trends.  In the
mammalian brain, most conscious interpertation of somatic senses begins in
the somatosensory cortex. Surface body sensations are mapped according to
the location of sensory receptors. A pain in the butt maps to a different
region than a pain in the neck.  Conscious control of motor function is
also mapped in a similar array. According to Kandel, Schwartz, and Jessell
(Principles of Neuroscience), the sensory and motor maps are essentially
mirror images.  What is remarkable is that in the lowly hedgehog, the
somatosensory and primary motor cortex map to the same area!  The overlap
is almost 100%.  A phylogenetic survey of motor and somatosensory motor
maps shows that the overlap has diminished as the cerebral cortex has
become more extensive.  Forgive the simplification, but overlap in a horse
might be 70%, a dog 50%, a primate 20%.  Further, association cortices have
also become more extensive which presumably increases the complexity of
patterns that can be incorporated into sensory and motor functions.  Thus,
in the evolution of mammals incremental increases in the capacity for
complex thought result by segregating motor and sensory function and
increasing association of complex patterns with each.

(I would love to know of similar analyses in fish which have incredible
diversity, a much longer evolutionary history, but very little appendicular
motor function.  In a similar fashion, an interesting comparison of motor
and sensory segregation may be made between the volumes of the red nucleus
of the midbrain [motor] and the cuneate and gracile nuclei [sensory] of the
medulla of amphibians, reptiles, birds, and mammals.)

The consequences of more complex thought are not incremental nor strictly
biological.

In humans and some tetrapod animals, amplification of motor function has
occurred via the use of "tools". The novelity of human behavior has been
expanded with the creation of additional motor tools which increase the
complexity of our motor responses. What began with twigs and stone tools
has led to computers and hydraulic systems that greatly expand our motor
skills and allow greater expression of internally generated patterns. The
copnsequences have been striking and rapid.

In the past six to 20 thousand years, human civilizations have developed
around the use of tools. Those civilizations have fueled significant social
and intellectual sophistication with little evidence of actual brain
evolution. We have no evidence that humans are more intelligent today than
they were 100 or 10,000 years ago.  We have developed better tools to
express our intrinsic intellect and have articulated more complex models of
behavior.

To paraphrase Darwin's Lion, Julian Huxley: We can only do better with what
we have because we are social creatures with choices framed by our
evolution.  Sadly, those choices are only voluntary.

rlh




Richard Hall
Comparative Animal Physiologist
Division of Sciences and Mathematics
University of the Virgin Islands
St. Thomas, USVI  00802

809-693-1386
rhall at uvi.edu





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