The Evolution of Electrical Engineering:  An Imaginary Tale

Nerves carry electrical impulses.  Ipso facto, they are subject to laws of physics concerning conductance, capacitance, and resistance.  Getting a signal from one end of an animal to the other in time can be a matter of life and death.  In order to maintain optimum levels of electrical conductivity to meet their lifestyle requirements, animals possess numerous adaptations to increase throughput.  In a paper in Current Biology,¹ D. K. Hartline (U of Hawaii) and D. R. Colman (McGill U, Quebec) described how these adaptations fall into two main categories:

Nervous systems have evolved two basic mechanisms for increasing the conduction speed of the electrical impulse.  The first is through axon gigantism: using axons several times larger in diameter than the norm for other large axons, as for example in the well-known case of the squid giant axon.  The second is through encasing axons in helical or concentrically wrapped multilamellar sheets of insulating plasma membrane – the myelin sheath.  Each mechanism, alone or in combination, is employed in nervous systems of many taxa, both vertebrate and invertebrate.  Myelin is a unique way to increase conduction speeds along axons of relatively small caliber.  It seems to have arisen independently in evolution several times in vertebrates, annelids and crustacea.  Myelinated nerves, regardless of their source, have in common a multilamellar membrane wrapping, and long myelinated segments interspersed with ‘nodal’ loci where the myelin terminates and the nerve impulse propagates along the axon by ‘saltatory’ conduction.  For all of the differences in detail among the morphologies and biochemistries of the sheath in the different myelinated animal classes, the function is remarkably universal.

Hartline and Colman went on to describe how the insulation provided by myelin increases throughput dramatically:

Myelin sheaths are frequently associated with rapid reactions, especially in invertebrate taxa.  For fibers of a few microns or more in diameter, myelin speeds the conduction of nerve impulses by a factor of ten or more compared to unmyelinated fibers of the same diameter.  This increases the nervous system’s information processing capacity and delivery speeds, decreasing reaction times to stimuli, increasing temporal precision, more closely synchronizing spatially distributed targets (such as different regions of a muscle sheet), and providing for shorter delays in feedback loops (for example in muscle control).  Because less current is needed to satisfy the charging needs of myelinated fibers, mean sodium channel densities averaged over the length of a fiber are much lower than for unmyelinated ones.  This results in a smaller ionic imbalance that must be restored after an impulse passes and confers a several hundred-fold improvement in metabolic efficiency for recouping the energy cost of nerve impulse traffic.  For a nervous system such as ours, which already accounts for 20% of the body’s resting metabolic energy budget, this is not an inconsequential advantage.  Another advantage is economy of space: to achieve the same ten-fold improvement on conduction speed through increasing axonal diameter, axons would have to be 100 times larger (with a comparable scale-up in soma size to accommodate the metabolic needs).  Imagine yourself with a 100-fold thicker spinal cord!

    The authors were also interested in how these adaptations could have evolved.  Consider the scope of their puzzle:

• Both mechanisms (axon gigantism and insulation) are dispersed throughout the animal kingdom.
• There are great evolutionary distances between similar adaptations.
• There are no transitional forms.
• There are no clues from fossils.

So despite confidence that these adaptations did evolve, they admitted they had only speculation about natural selection might have achieved it:

So ancient is its evident appearance in each of these lines, and so sophisticated its morphological and chemical structure, that its exact origin in most of those lines is hard to establish.  Even in vertebrates there is a great evolutionary distance between the unmyelinated hyperoartia (lampreys) and the gnathostomes.
    The initial steps in the evolution of myelination may not, however, be that difficult to reconstruct.  Electrically sealing together two apposed membrane surfaces over a small region of axon decreases its transverse capacitance and proportionately speeds impulse propagation along it.  The sealing can be achieved by narrowing the conductive space, either cytoplasmic or extracytoplasmic, between adjacent axonal and/or glial membranes … or through impermeable specializations at margins, for example precursors of septate junctions.  Even the random sealing of patches of single-layer glial membrane over half of an axon’s surface is predicted to increase conduction speed by about 20%.  Once such a process has started, it is not difficult to imagine a sequence of small improvements driven by natural selection that would ultimately lead to the complex structures we see today.  This is speculative, however; no cases have been described so far of ‘intermediate stages’ in extant groups.  Developmental sequences, the lack of fossil records and the paucity of candidate molecular precursors so far identified have made the task more difficult.  Perhaps better insight will be gained through increased attention to myelin evolved in the invertebrates. (Bold and underlining added.)

¹D.K. Hartline and D.R. Colman, “Rapid Conduction and the Evolution of Giant Axons and Myelinated Fibers,” Current Biology, Vol 17, R29-R35, 09 January 2007.

These guys started right off the bat with the BAD strategy (brazen assertions of dogmatism): “Nervous systems have evolved…”  Ahem–there are a lot of people who do not accept that.  Your evidence, please?  Some fossils, perhaps?  A long sequence of intermediate steps, each with increased survival value?  A clear phylogenetic pattern?  None of the above.  So here is their argument: “it is not difficult to imagine…”
    If you thought science was about evidence and proof, welcome to the Storybook Land of the Darwinist (see 12/22/2003 commentary).  Instead of launching into another sermon against imagination in science as a substitute for evidence, this time we will let the Good Book do it for us.  Substitute Darwin and natural selection where appropriate:

• God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually. (Genesis 6:5)
• the imagination of man’s heart is evil from his youth. (Genesis 8:15)
• Why do the heathen rage, and the people imagine a vain thing? (Psalm 2:1)
• they imagined a mischievous device, which they are not able to perform. (Psalm 21:11)
• They also … imagine deceits all the day long. (Psalm 38:12)
• Which imagine mischiefs in their heart (Psalm 140:2)
• Deceit is in the heart of them that imagine evil (Proverbs 12:20)
• neither shall they walk any more after the imagination of their evil heart. (Jeremiah 3:17)
• But they hearkened not, nor inclined their ear, but walked in the counsels and in the imagination of their evil heart, and went backward, and not forward. (Jeremiah 7:24)
• But have walked after the imagination of their own heart, and after Baalim, which their fathers taught them. (Jeremiah 9:24; for fun, try substituting Darwin for Baalim here)
• Yet they obeyed not, nor inclined their ear, but walked every one in the imagination of their evil heart (Jeremiah 11:8)
• This evil people, which refuse to hear my words, which walk in the imagination of their heart (Jeremiah 13:10)
• And ye have done worse than your fathers; for, behold, ye walk every one after the imagination of his evil heart (Jeremiah 16:12)
• And they said, There is no hope: but we will walk after our own devices, and we will every one do the imagination of his evil heart. (Jeremiah 18:12)
• Thou hast seen all their vengeance and all their imaginations against me. (Lamentations 3:60, 61)
• What do ye imagine against the Lord? (Nahum 1:9)
• He … hath scattered the proud in the imagination of their hearts. (Mary, praising God in Luke 1:51)
• Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened. (Paul, in Romans 1:21)

Paul continues, “professing themselves to be wise, they became fools.”  Here you have witnessed academic elite scientists claiming, out of thin air, without any evidence, that sophisticated electrical engineering emerged by mistake!  Does the shoe fit, or what?  They have glorified their own speculations.  They have imagined a fable that runs 180 degrees contrary to the evidence.  They have totally refused to bend their stiff necks to alternative explanations, like intelligent design.  They have shown themselves to be dogmatist wolves in scientist sheep’s clothing, walking after the imaginations of their own foolish hearts.
    So we end with more advice from the Apostle Paul: words that would have been understood as a mission statement by many of the founders of science: “Casting down imaginations, and every high thing that exalteth itself against the knowledge of God” (II Corinthians 10:4).