Biological Wheel

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Rotating locomotion encompasses two distinct modes of locomotion: simple rolling, and spinning relative to a fixed axle or body in the manner of a wheel or propeller. Several organisms move by rolling.

However, despite the integral role that the wheel has played in locomotion of vehicles designed by humans, true wheels do not appear to play any role in the locomotion of biological systems.

Given the apparent utility of the wheel in human technology, and the existence of biological analogues of many other human technologies (such as wings and lenses), it might seem odd that a true wheel has never evolved naturally, but this lack of biological wheels is typically explained by two main factors: first, there are several developmental and evolutionary obstacles to the evolution of a wheel by natural selection, and secondly, wheels often do not carry a competitive advantage over other means of propulsion (such as walking, running, or slithering) in the environments in which ambulatory species have evolved. (This lack of an advantage in certain environments incidentally explains why some human civilizations have eschewed wheels, despite their awareness of the technology.)

Several species of elongate organisms will form their body into a loop in order to roll, including caterpillars, tiger beetle larvae, mantis shrimp, and salamanders, while other species adopt more spherical postures, as in pangolins, hedgehogs, armadillos, isopods, the wheel spider, and fossilized trilobites. These species may roll passively (under the influence of gravity or wind) or actively, typically by altering their shape to generate a propulsive force. Tumbleweeds are the aboveground portion of certain plants, which separate from their root structure and roll in the wind to distribute their seeds.

Dung beetles form spherical balls of animal excrement, which they roll with their bodies. Although it is the dung ball that rolls rather than the beetle itself, the beetles face many of the same mechanical difficulties that rolling organisms contend with. Keratinocytes, a type of skin cell, migrate with a rolling motion during the process of wound healing. Rotifers, though their Latin name means ‘wheel-bearer,’ do not actually possess any rotating structures, but rather a ring of rhythmically beating cilia used for feeding and propulsion.

Though no known multicellular organism is able to spin part of its body freely relative to another part of its body, there are two known examples of rotating molecular structures used by living cells. ATP synthase is an enzyme used in the process of energy storage and transfer. It bears some similarity to flagellar motors, and is thought to be an example of modular evolution, in which two subunits with their own functions have become associated and gained a new functionality.

The only known example of a biological ‘wheel’—a system capable of providing continuous propulsive torque about a fixed body—is the flagellum, a propeller-like tail used by single-celled prokaryotes for propulsion. About half of all known bacteria have at least one flagellum, indicating that rotation may in fact be the most common form of locomotion in living systems. At the base of the bacterial flagellum, where it enters the cell membrane, a motor protein acts as a rotary engine. The engine is powered by proton motive force, i.e., by the flow of protons (hydrogen ions) across the bacterial cell membrane due to a concentration gradient set up by the cell’s metabolism. Flagella are quite efficient, allowing bacteria to move at speeds up to 60 cell lengths per second. The rotary motor at the base of the flagellum is similar in structure to that of ATP synthase.

Archaea, a group of prokaryotes (unicellular organisms that lack a nucleus) distinct from bacteria, also feature flagella driven by rotary motor proteins, though they are structurally and evolutionarily distinct from bacterial flagella. Some eukaryotic cells (nucleated), such as the protist Euglena, also have a flagellum, but eukaryotic flagella do not rotate at the base; rather, they bend in such a way that the tip of the flagellum whips in a circle. The eukaryotic flagellum, also called a cilium or undulipodium, is structurally and evolutionarily distinct from prokaryotic flagella.

The processes of evolution, as they are presently understood, can help explain why wheeled locomotion has not evolved in multicellular organisms; simply put, a complex structure or system will not evolve if its incomplete form provides no benefit to an organism. According to the modern evolutionary synthesis, adaptations are produced incrementally through natural selection, so major genetic changes will usually spread within populations only if they do not decrease the fitness of individuals. Although neutral changes that provide no benefit can spread through genetic drift, and detrimental changes can spread under some circumstances, large changes that require multiple steps will occur only if the intermediate stages increase fitness. Evolutionary biologist Richard Dawkins describes this situation as follows: ‘The wheel may be one of those cases where the engineering solution can be seen in plain view, yet be unattainable in evolution because it lies [on] the other side of a deep valley, cutting unbridgeably across the massif of Mount Improbable.’ In such a fitness landscape, wheels might be a highly beneficial ‘peak,’ but the valley around such a peak is too low or wide for the gene pool to move across by genetic drift or natural selection.

Natural selection therefore explains why wheels have not appeared, as a wheel missing one or more of its key components would probably not impart an advantage to an organism. The same cannot, however, be said of the flagellum, the only known example of a freely rotating propulsive system in biology. In the evolution of flagella, individual components were recruited from other structures, where they performed tasks unrelated to propulsion. The basal body that is now the rotary motor might have evolved from a structure used by the bacterium to inject toxins into other cells. This recruitment of previously evolved structures to serve new functions is called ‘exaptation.’ Molecular biologist Robin Holliday has written that the apparent lack of biological wheels argues against creationist or intelligent design accounts of the diversity of life—for, free of the limitations imposed by evolution, an intelligent creator would be expected to deploy wheels wherever they would be of use.

The greatest anatomical impediment to wheeled multicellular organisms is the interface between the static and rotating components of the wheel. In either a passive or driven case, the wheel, or wheel and axle, must be able to rotate freely relative to the rest of the machine or organism. Unlike animal joints, which have a limited range of motion, a wheel must be able to rotate through an arbitrary angle without ever having to be ‘unwound.’ As such, a wheel cannot be permanently attached to the axle or shaft about which it rotates (or if the axle and wheel are fixed together, the axle cannot be affixed to the rest of the machine or organism). No true multicellular organism is known to grow tissue or organ structures that are not attached in some way to the rest of the organism.

In the case of a driven wheel, some type of torque must be applied to the axle to generate the locomotive force. For human-made technology, this torque is generally provided by an engine, or human power (as in the case of a bicycle). In animals, motion is typically achieved by the use of skeletal muscles, which derive their energy from the metabolism of nutrients from food. Because these muscles are attached with connective tissue to both of the components which must move relative to each other, they would not be an effective means of directly driving a biological wheel. In addition, animals suffer degraded energy efficiency because their propulsive cycles employ only periodic accelerations (repeated flexion and extension of joints). Large animals cannot produce high rates of acceleration, because as animal size increases, it becomes more difficult for muscles to quickly generate high enough stress to overcome relative inertia.

In typical mechanical systems, some sort of bearing and/or lubricant must be used to reduce friction at the interface between two components. Reducing friction is vital for minimizing wear on components, and preventing overheating. As the relative speed of the components increases, and as the force of contact between the components increases, the importance of friction mitigation increases as well. In biological joints such as the human knee, friction is reduced by means of cartilage with a very low friction coefficient, as well as lubricating synovial fluid, which has very low viscosity. Gerhard Scholtz, professor at the Institute for Biology and Comparative Zoology in Berlin, asserts that a similar excreted lubricant or dead cellular material could allow a biological wheel to rotate freely, though such a mechanism has not been found in nature.

Another potential problem that arises at the interface between wheel and axle is the ability of an organism to transfer materials across this interface. If the tissues that make up a wheel are living, they will need to be supplied with oxygen and nutrients and have wastes removed in order to sustain metabolism. A typical animal circulatory system, composed of blood vessels, would not be able to provide transportation across the interface. In the absence of circulation, oxygen and nutrients would need to diffuse across the interface, a process that would be greatly limited by the available partial pressure and surface area. For large multicellular animals, diffusion would be insufficient. Alternately, a wheel could be composed of excreted, nonliving material, such as keratin, of which hair and nails are composed.

Wheels incur mechanical and other disadvantages in certain environments and situations, which would represent a decreased fitness when compared with limbed locomotion. These disadvantages suggest that, even barring the biological constraints discussed above, the absence of wheels in multicellular life may not, in fact, be the ‘missed opportunity’ of biology that it first seems. On the contrary, given the mechanical disadvantages and restricted usefulness of wheels compared with limbs, the central question can be reversed: not ‘Why doesn’t nature produce wheels?’, but rather, ‘Why don’t human vehicles make more use of limbs?’ The use of wheels, rather than limbs, in many engineered vehicles can likely be attributed to the complexity of design required to construct and control limbs, rather than to a consistent functional advantage of wheels over limbs.

Though stiff wheels are more energy efficient than other means of locomotion when traveling over hard, level terrain (such as paved roads), wheels are not especially efficient on soft terrain such as soils, because they are vulnerable to rolling resistance. In rolling resistance, a vehicle loses energy to the deformation of its wheels and the surface on which they are rolling. Smaller wheels are especially susceptible to rolling resistance. Softer surfaces deform more and recover less than firm surfaces, resulting in greater resistance. Compared with rolling on concrete, resistance on medium-hard soil can be five to eight times greater, and on sand can be ten to fifteen times greater. While wheels must deform the surface along their entire path, limbs only induce a small, localized deformation at the point of foot contact.

Rolling resistance is also the reason wheels are not seen in certain human civilizations. During the Roman Empire, wheeled chariots were common in the Middle East and North Africa; yet when the Empire collapsed, wheels fell out of favor with the local populations, who turned to camels to transport goods in the sandy desert climate. Biologist Stephen Jay Gould discusses this curiosity of history in his book ‘Hen’s Teeth and Horse’s Toes,’ asserting that in the absence of maintained roads, camels required less manpower and water than a cart pulled by oxen.

In aquatic locomotion, rotating systems carry an efficiency advantage only at extremely high viscosities, such as those experienced by bacterial flagella, whereas oscillating systems have the advantage in lower viscosity fluids. Whereas ship propellers typically have efficiencies around 60%, and aircraft propellers up to around 80% (achieving 88% in the human-powered Gossamer Condor), much higher efficiencies, in the range of 96%–98%, can be achieved with an oscillating flexible foil, like a fish tail or bird wing.

Wheels are prone to slipping—an inability to generate traction—on loose or slippery terrain. Slipping wastes energy, and can potentially lead to a loss of control or becoming stuck, as with an automobile on mud, ice, or snow. This disadvantage of wheels is apparent in the realm of human technology; in an example of biologically inspired engineering, legged vehicles find use in the logging industry, where they allow access to more challenging terrain than wheeled vehicles can navigate. Tracked vehicles suffer less from slipping than wheeled vehicles, due to their larger contact area with the ground—but they tend to have larger turning radii than wheeled vehicles, and are less efficient and more mechanically complex.

Standard methods of steering, which do not provide individual wheel articulation, are limited in their achievable turning radius, thus limiting the ability of such vehicles to navigate an environment with a high obstacle frequency. Biologist Michael LaBarbera illustrates the poor maneuverability of wheels by comparing the turning radii of walking and wheelchair-bound humans. As science author Jared Diamond points out, most biological examples of rolling are found in wide open, hard-packed terrain, including the use of rolling by dung beetles and tumbleweeds.

Wheels are poor at dealing with vertical obstacles, especially obstacles on the same scale as the wheel itself. Assuming a vehicle or animal can shift its center of mass, the limiting height of vertical obstacles for a passive wheel is equal to the radius of the wheel. If the center of mass cannot be shifted, the highest obstacle a vehicle can surmount is one quarter to one half the radius of the wheel. Because of these limitations, wheels intended for rough terrain require a larger diameter. 

In addition, without articulation, a wheeled vehicle can become stuck on top of an obstacle, with the obstacle between the wheels, preventing them from contacting the ground. Limbs, in contrast, are useful for climbing, and equipped to deal with uneven terrain. For unarticulated wheels, climbing obstacles will cause the body of the vehicle to tilt. If the vehicle’s center of mass moves outside of the wheelbase or axle track, the vehicle will become statically unstable and tip over. At high speeds, a vehicle can become dynamically unstable, meaning that it can be tipped over by an obstacle smaller than its static stability limit, or by excessive acceleration or tight turning. Without articulation, this can be an impossible position from which to recover.

Additionally, articulated limbs used by animals for locomotion over terrain are frequently also used for other purposes, such as grasping, climbing, branch-swinging, swimming, digging, jumping, and kicking. With a lack of articulation, wheels would not be as useful as limbs in these roles.

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