Editor’s note: We are pleased to present a series adapted from biologist Michael Denton’s book, Fire-Maker: How Humans Were Designed to Harness Fire and Transform Our Planet, from Discovery Institute Press. Find the whole series here. Dr. Denton’s forthcoming book, TheMiracle of the Cell, will be published in September.

There is another condition that must be satisfied if an organism the size of Homo sapiens is to stand tall and intelligently manipulate the environment and handle fire. Yes, we must be the right size and android design; yes, we must have muscles sufficiently powerful to resist gravity and raise us off the ground. But, in addition, our muscular activity must be finely controlled. This necessitates fast reflexes and fast nerve conduction velocities. Fortuitously, nature obliges again. 

Catching a ball, rowing a canoe, dodging a wave as it breaks on the shore, blinking an eye to prevent small objects from impacting on the cornea, coordinating the various muscles groups involved in movement, coordinating eye movements to maintain focus on a moving object or to compensate for motion while walking or running, handling and manipulating moving embers of a fire — all these require rapid reflexes. 

Your Eye on the Ball

One area where very fast nerve conduction is vital is vision, more specifically, in keeping the eyes fixed on some object in the field of vision while in motion. With each step, the head moves and so do the eyes. If it were not for speed of what is known as vestibular-ocular reflex (VOR), vision in motion would be impossible. 

But it’s not just keeping the eyes on a target that necessitates very fast nerve conduction. Just keeping our balance when we walk requires continual second-by-second assessments of the position of the limbs in space and continual simultaneous coordinated contraction and relaxation of different muscle groups. Patients with the disease hereditary spastic paraplegia, for example, have slow nerve conduction speeds due to degeneration of the peripheral afferent nerves carrying sensory information to the brain. This causes them to have great difficulty balancing and walking.¹

Just Fast Enough!

The rapid reflexes necessary for an organism the size of a human to carry out finely coordinated motor activities (especially for very fast reflexes like the VOR) are only possible because the speed of nerve conduction in vertebrates is very rapid. Nerve conduction speeds in different organism vary over more than three orders of magnitude, from 10 centimeters per second in simple invertebrates to a maximum of 120 meters per second in the nervous system of mammals.² Obviously the fine control and coordination of muscular activity and motion is only possible because such relatively rapid speeds of nerve impulse conduction are in fact possible. This is fast enough to enable rapid reflexes in animals of our size. But just fast enough! No less than muscle power, the speed of nerve conduction imposes an absolute limit on the maximum size that an animal can attain. No animal can be 100 meters long and at the same time be nimble. Even at the fastest conduction speeds, in a 100-meter long organism, a nerve impulse will take two seconds to travel from the brain to its extremities and back. An organism our size could never handle fire or undertake any sophisticated manipulation or exploration of the world if the maximum speed of nerve conduction was ten or a hundred times less. Indeed, we would probably be unable to function in any way imaginable to us.

Tomorrow, “A Crucial Design Difference in Vertebrate Nerves.”

Notes

1. Jorik Nonnekes, Mark de Niet, Lars B. Oude Nijhuis, Susanne T. de Bot, Bart P. C. van de Warrenburg, Bastiaan R. Bloem, Alexander C. Geurts, Vivian Weerdesteyn, “Mechanisms of Postural Instability in Hereditary Spastic Paraplegia,” Journal of Neurology 260, no. 9 (September 2013): 2387–2395. doi:10.1007/s00415-013-7002-3; see also Tina M. Weatherby, April D. Davis, Daniel K. Hartline, Petra H. Lenz, “The Need for Speed. II. Myelin in Calanoid Copepods,” Journal of Comparative Physiology. A, Sensory, Neural, and Behavioral Physiology 186, no. 4 (April 2000): 347–357. 
2. Schmidt-Nielsen, (1997) Chapter Eleven, see table 11.4.