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, The Miracle of the Cell, will be published in September.

The evidence overall suggests strongly that, for fundamental reasons, the maximum power stroke of any sort of molecular motor cannot be much greater than it is. And since the packing of the myosin motors in muscle tissue is virtually crystalline and just about as tight as possible, muscles cannot be designed, on biological principles, to generate any greater degree of power. If either the tightness of packaging or the power of the motors had to be less for some reason in a counterfactual world, then organisms of our size and weight would not be feasible because their muscles would be unable to generate the necessary mechanical forces to lift their bodies off the ground and perhaps no movement of any sort would be possible. 

Diminishing Returns

Increasing the percentage of the body’s mass devoted to muscle is also not an option. As it is, mammals invest 40 percent of their mass in muscle,¹ and — as every medical student comes to learn when first dissecting the human body at medical school — our limbs are almost entirely composed of muscles. It would be simply impossible to redesign the human body to compensate for muscles only half as powerful by increasing the proportion of muscles. Such a strategy would be a matter of diminishing returns, because as the volume of the muscles increased, their weight would increase proportional to L³ while their strength would only increase by L². No large terrestrial organism built on biological principles could be designed to move with muscles much less than half as powerful as they are. And muscles cannot be redesigned to generated greater force per unit volume. 

Major Design Problems

Even muscles only slightly less powerful would create major design problems. For example, the strength of the grip of the human fingers is generated by extrinsic muscles in the forearm and not by the small muscles in the hand itself. Given the existing contractile power of muscle, this placement of the grip muscles in the forearm is not in the least bit gratuitous but of absolute necessity. The muscle bulk necessary to provide the required strength of grip cannot be accommodated in the hand. The fact that it is necessary, even with the strength of muscles as they are, to place the muscle generating grip in the forearm indicates the tremendous difficulties that would be encountered in attempting to redesign the human frame to handle fire and to inhabit a planet the size of the Earth if muscles were even slightly less powerful. It is astonishing that the design of the musculature in the arm of man and even the placement of specific muscle groups can be explained to a very large degree from consideration of the force delivered by one individual molecular motor. 

The Heart and Its Design

 The strength of muscles is not just relevant to the movement of our limbs and the maintaining of an upright posture. Muscles based on the same basic design provide the heart with its ability and the strength to pump the blood. And it is muscles that move the chest during respiration. If the basic myosin power stroke was significantly less powerful, then the circulatory and respiratory system in beings of our size and weight would be impossible. We would be unable to stand or breathe or pump the blood around the body. 

The power stroke of the myosin motors must not only exert the force it does, but the energy requirements to drive it must also be close to what they are. The delivery of oxygen to the tissues in an organism like man is constrained by atmospheric conditions (the danger of fire and oxygen toxicity if the partial pressure of 160 mm Hg of oxygen in the atmosphere were significantly higher) and the necessity for an area the size of a tennis court (about 100 square meters)²for gaseous exchange in the lungs, and the constraints on capillary design and function. Based on these varied constraints, it is virtually impossible to envisage any sort of radical redesign of either the circulatory or respiratory systems in complex organisms which would double or triple the delivery of oxygen to muscle tissues and the production of metabolic energy. 

As it is, during strenuous activity, much of the volume of active muscles is made up of blood capillaries. If the power stroke of our molecular motors was cut in half or was a third as efficient in terms of energy utilization, i.e., if they required two or three times more ATP or metabolic energy, then large complex forms of life dependent on muscles for motility would in all probability be impossible. 

Given that muscle power (force per cross sectional area) cannot be increased and is virtually the same throughout the animal kingdom, and given that mass increases proportional to L³, it is clear that organisms of our size are near the limit of what is practical given the power of muscles. Again, it’s a close call. 

A Miniature Human

It is evident, then, that to stand upright our muscles and our dimensions must be very close to what they are. A miniature human, built on the same biological principles but only one half or one-third our size (possessed of only a fraction of our strength) would have considerable difficulty in cutting and manipulating logs of much more than a few kilos in weight. Such a being would be restricted to making fires using small twigs and whether the heat and sustainability of such fires would have sufficed for the discovery of metals and for the development of metallurgy is open to question. As we have seen, metallurgy necessitates high temperatures of many hundreds of degrees and this requires properly designed kilns and the use of large quantities of wood or charcoal.³

Tomorrow, “For Fire, Nature Obliges Us with Rapid Reflexes.”

Notes

1. Vogel, 390. It is not just the muscles which occupy the same proportion of the body in mammals. The circulatory system occupies approximately the same proportion of body mass in birds and mammals, see Vogel, 187. Heart size also occupies same proportion of body mass in small and large mammals, see Schmidt-Nielsen (1997) Chapter Three, fig 3.9, and the lungs in mammals also occupy the same proportion of body mass in all mammals, see “Mammalian Lungs,” fig 1.15.
2. Schmidt- Nielsen (1997), Chapter One, see section “Mammalian Lungs.” 
3. Arthur Wilson, The Living Rock: The Story of Metals Since Earliest Times and Their Impact on Civilization (Cambridge: Woodhead Publishing Limited, 1994), Chapter Two, 10-16.