A Meditation on Scoliosis

A Meditation on Scoliosis

by Erik Bendix

It is evident that people are vertebrates.  Our core of segmented bones following the lead of the head is a structure we share with all mammals, birds, reptiles, amphibians, and fish.  Of these, the fish are our most ancient brethren.  A quick comparison of a human skeleton with that of a fish shows that our limbs are more recent additions to the old design.  It is interesting that our limbs add onto the spine in the way that they do, symmetrically on either side of it, rather than, say, only on one side, or out the front or back.  We are symmetrical side to side, and it is worth asking why.

The symmetries we see in ourselves and in our vertebral kin are not just what can be seen on the surface. The inner structure of the human nervous system, for example, is bilaterally symmetric to a fault.  The cerebral cortex divides into equal right and left halves, so do the cerebellum and smaller subcortical structures, and so does the spinal cord itself, which has little central grooves front and back called median fissures which divide it neatly into lateral halves.  Why are we built this way?

These symmetries must have developed in response to some need.  Since all vertebrates share this symmetry of form, the needs that gave rise to it must have existed right from the beginning of vertebral life.  Fishes were the pioneers among vertebrates.  Prior to them, creatures in our yet more ancient line of ancestry could propel themselves at best very slowly and without much aim.  Fish changed this into targeted and self-propelled pursuits of food.  They began to hunt.  Their ability to travel in this way gave them their crucial edge in vying with other life forms.  A spine and muscle system capable of propulsion and a central nervous system capable of navigating had developed together to make such locomotion possible.  It is these anatomical systems that most markedly display bilateral symmetry.

Anyone looking at swimming fishes can see that they propel themselves by bending from side to side.  It is reasonable to suppose that something like this movement was how the very first vertebrates got around.  It is also worth asking whether such movement might have anything to do with the basic side-to-side symmetry we still display through most of our anatomy.  Could our bilateral symmetry of build in some way be a result of the first and most basic needs of vertebrate movement?

To answer such a question, and to understand such a naturally evolved movement, it is worth considering the energy expended to bring the movement about.  The harnessing and use of energy by organisms is one of life’s most precious accomplishments, and nature is not prone to waste it.  This is why natural movement so often seems beautiful to us, for its elegance lies in its superb efficiency.  So to understand the wriggling of fish, we must look to what makes such movement efficient, or not wasteful of energy.  The fact that fish do their moving inside of water cannot alone account for their mode of propulsion, for water surrounds them equally on all sides.  In principle at least, water affects all types of movement within it equally.  Whatever it is that could make one kind of fish movement more efficient than another must itself have a bias in some direction.  The most obvious candidate for such a bias is gravity, perhaps as it is expressed in how water becomes denser as it deepens.

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