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Rotation in living systems
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Rotation in living systems encompasses two modes of locomotion: rolling, and spinning about a fixed axle in the manner of a wheel or propeller. While many living systems move by means of rolling rotation, and despite the fact that the wheel has played an integral role in locomotion of vehicles designed by humans, wheels do not appear to play any role in the locomotion of biological systems. This lack of biological "wheels" has been a frequent topic of semi-serious debate among biologists, including noted evolutionary biologist Stephen Jay Gould.
Given the apparent utility of the wheel in human technology, and the existence of other technologies with biological analogues (such as wings and lenses), it might seem odd that nothing like a wheel has evolved naturally, but there are several likely explanations for this phenomenon.
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Rotation in living systems encompasses two modes of locomotion: rolling, and spinning about a fixed axle in the manner of a wheel or propeller. While many living systems move by means of rolling rotation, and despite the fact that the wheel has played an integral role in locomotion of vehicles designed by humans, wheels do not appear to play any role in the locomotion of biological systems. This lack of biological "wheels" has been a frequent topic of semi-serious debate among biologists, including noted evolutionary biologist Stephen Jay Gould.
Given the apparent utility of the wheel in human technology, and the existence of other technologies with biological analogues (such as wings and lenses), it might seem odd that nothing like a wheel has evolved naturally, but there are several likely explanations for this phenomenon. Firstly, there are several potential stumbling blocks to the evolution of a wheel by natural selection, and secondly, wheels do not necessarily carry a competitive advantage over other means of surface propulsion (such as walking, running, or slithering) for the environments in which ambulatory species have evolved. This latter fact explains why wheels have not found use in some human civilizations, despite those civilizations being aware of the wheel.
Rotation in nature
Rolling locomotion
Some organisms use rolling as a means of locomotion. These examples do not constitute the use of a wheel, as the entire organism rotates itself, with no fixed axle.
A species of caterpillar known as Pleuroptya ruralis, the Mother-Of-Pearl Moth, curls into a ring and rolls away when threatened. A species of mantis shrimp, Nannosquilla decemspinosa, performs partially rolling somersaults. The golden wheel spider Carparachne aureoflava, of the Namib Desert, escapes parasitic wasps by flipping onto its side and cartwheeling down sand dunes. A phylum of microscopic and near-microscopic pseudocoelomate animals called rotifers use cilia to sweep food into their mouths, and to propel themselves through the water. The phylum got its name, which is derived from Latin and meanings "wheel-bearer", for the wheel-like appearance of these cilia, although their motion is in fact reciprocal.
Other animals which roll their bodies, either actively or passively, include hedgehogs, armadillos, lizards such as Cordylus cataphractus, amphibians such as Taricha granulosa and Echinotriton chinhaiensis, isopods, myriapods, and fossilized trilobites.The salamander Hydromantes platycephalus and the pangolin also use rolling locomotion. The tumbleweed, Corispermum hyssopifolium uses passive rolling, powered by wind, to distribute its seeds. The dung beetle uses rolling to transport the feces on which it feeds.
Keratinocytes (a type of skin cells) migrate with a rolling motion during the process of wound healing.
Wheel-like rotation
While no known multi-cellular organism is able to spin part of its body freely relative to another part of its body, there are two clear examples of rotating molecular structures used by living cells. ATP synthase is an enzyme used in the process of energy storage and transfer, notably in photosynthesis and oxidative phosphorylation. It bears some similarity to flagellar motors. The evolution of ATP synthase is thought to be an example of modular evolution, where two subunits with their own functions have become associated and gained 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, propeller-like tail used by single-celled prokaryotes for propulsion. The bacterial flagellum is the best known example. 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. Archaea, a group of prokaryotes distict from bacteria, also feature flagella driven by motor proteins, though they are structurally and evolutionarily unique from bacterial flagella. While bacterial flagella evolved from the bacterial Type III secretion system, archaeal flagella appear to have evolved from type IV pili. Some eukaryotic cells, 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.
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 (in Vibrio species there are two kinds of flagella, lateral and polar, and some are driven by a sodium ion pump rather than a proton pump). Flagella are quite efficient, allowing bacteria to move at speeds up to 60 cell lengths/second. The rotary motor at the base of the flagellum is similar in structure to that of ATP synthase.
Constraints of evolutionary processes
The processes of evolution, as they are presently understood, can help explain why wheeled locomotion has not evolved in multi-cellular 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 only usually spread within populations 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 only occur if the intermediate stages increase fitness. 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 its lies the other side of a deep valley, cutting unbridgeably across the massif of Mount Improbable." In such a picture of a fitness landscape, wheels might be a highly-beneficial "peak", but the valley around such a peak is too low for organisms to move across by genetic drift or natural selection. As Gould explains, biological adaptation is limited to working with available components, saying "wheels work well, but animals are debarred from building them by structural constraints inherited as an evolutionary legacy".
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 one known example of a freely rotating propulsive system in biology. In the case of the flagellum, individual components were recruited from other structures, where they performed tasks unrelated to propulsion. For example, in the evolution of flagella the basal body that is now the rotary motor may have evolved from a structure used by the bacterium to inject toxins into other cells. The recruitment of existing structures to serve a new purpose in evolution is called exaptation.
Impediments to the evolution of a biological wheel
The major potential problem with multi-cellular organisms having wheels 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, the axle cannot be attached to the rest of the machine or organism. No true multi-cellular organism is known to grow tissue or organ structures which 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, which may be electric, turbine-driven, combustion-driven, pneumatic, hydraulic, or of some other type. Torque may also be provided by human power, as in the case of a bicycle. In animals, motion is 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 can not 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 must be used to reduce the friction between the 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 reduction 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 a lubricant called synovial fluid, which has very low viscosity. Scholtz asserts that a similar excreted lubricant or dead cellular material could allow a biological wheel to rotate freely, though such a mechanism is not found in nature.
One other potential problem at the interface is the ability to transfer materials across it. If the tissues which make up a wheel are living, they will need to be supplied with oxygen and nutrients and have wastes removed. A typical animal circulatory system, composed of blood vessels, would not be able to provide transportation across the interface. Lacking circulation, oxygen and nutrients would have to be able to diffuse across the interface, a process which would be greatly limited by the available partial pressure and surface area. For large multi-cellular animals, diffusion would be insufficient. Alternately, a wheel could be composed of excreted, non-living material, such as keratin, of which hair and nails are composed.
Mechanical disadvantages of rotating locomotion
Rotating locomotion incurs mechanical disadvantages in certain environments and situations which may help to explain why multi-cellular life has not evolved wheels for locomotion.
Wheels can be considered to fall into two types: passive and driven. A passive wheel simply rolls over a surface, reducing friction when compared with dragging. A driven wheel is powered, and transmits energy to the surface as a means of achieving locomotion. Wheels are typically round, or nearly so, but this is not strictly necessary for a wheel to be an effective form of locomotion, as seen in the example of the square wheel.
Efficiency
Jared Diamond notes that while ship propellers typically have efficiencies around 60%, and aircraft propellers near 90%, much higher efficiencies, in the range of 96%–98%, can be achieved with an oscillating flexible foil, like a fish tail or bird wing.
Although wheels are more energy efficient than other means of locomotion when traveling over hard, level terrain (such as paved roads), wheels have several distinct disadvantages when compared to limbed locomotion which make them unlikely to replace limbed locomotion of animals. These disadvantages stem largely from the fact that many natural environments are ill-suited to the use of wheels.
Rolling resistance
Wheels are not especially efficient on soft terrain such as soils, because they are vulnerable to rolling resistance. In rolling resistance, the wheel is robbed of energy by the deformation of the wheel and the surface on which it is rolling. Smaller wheels are especially suceptible to rolling resistance. Softer surfaces deform more and recover less than firm surfaces, resulting in greater resistance. Rolling resistance in sand, for example, is ten times higher than that for concrete.
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 Roman Empire collapsed, wheels fell out of favor with the local populations, who turned to camels to transport goods in the sandy desert climate. Stephen Jay Gould discusses this curiosity of history in his book Hen's Teeth and Horse's Toes, asserting that in the absence maintained roads, camels required less manpower and water than a cart pulled by oxen.
Obstacles
Wheels are poor at dealing with vertical obstacles, especially obstacles on the same scale as the wheel itself. The highest obstacle a passive-wheeled vehicle can surmount, assuming the vehicle cannot change its center of mass, is one quarter to one half the radius of the wheel. Even if the vehicle can move its center of mass, the limiting obstacle height for a passive wheel is one radius. Without articulation, a wheeled vehicle may 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 rotate. If the rotation angle is too high, the vehicle will become statically unstable and tip over. At high speeds, a vehicle can become dynamically unstable, able to be tipped over by an obstacle smaller than its static stability limit. Without articulation, this can be an impossible position from which to recover.
Turning
Most methods of steering wheeled vehicles involve some degree of skidding, and may place limits on the achievable turning radius, thus limiting the ability of an vehicle to navigate around obstacles in areas with a high obstacle frequency. As 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.
Limited versatility
Articulated limbs used by animals for locomotion are frequently also used for other purposes, such as grasping and kicking. With a lack of articulation, wheels would not be as versatile in this regard.
Rolling and wheeled creatures in fiction and legend
Rolling
The hoop snake is a legendary creature of the United States and Australia. The snake is said to grasp its tail in its mouth and roll like a wheel towards its prey. The Japanese Tsuchinoko is a similar mythical creature.
The Dutch graphic artist M. C. Escher invented a creature he called Pedalternorotandomovens centroculatus articulosus, which was capable of rolling itself forward like a wheel. He illustrated this creature in his 1951 lithograph Wentelteefje, or in English, Curl-up.
The 1944 science-fiction short story "Arena", by Fredric Brown, features a telepathic, alien creature called an "Outsider" which is roughly spherical and moves by rolling. The story was the basis for the 1967 Star Trek episode of the same name, and a 1964 Outer Limits episode entitled "Fun and Games".
Tuf Voyaging, a science fiction novel by George R. R. Martin, first published in 1986, features an alien species called the "Rolleram", which kills its prey by rolling over it.
The 1995 short story "Microbe", by Kenyon College biologist and feminist science fiction writer Joan Slonczewski, describes an exploratory expedition to an alien world, whose plant and animal life consists entirely of doughnut-shaped organisms. It turns out that the planet's biology is based on triple-helix DNA, rather than the double-helix molecule on which life on Earth is based.
Wheeled
L. Frank Baum's 1907 children's book Ozma of Oz features humanoid creatures with wheels instead of hands and feet, called "Wheelers".
The 1968 novel The Goblin Reservation by Clifford D. Simak features an intelligent alien race which uses biological wheels.
David Brin's Uplift Universe includes a wheeled species called the G'Kek, which are described in some detail in the novel Brightness Reef. The G'Kek are described as looking like "a squid in a wheelchair." They suffer from arthritic axles in their old age, particularly when living in a high gravity environment.
The 2000 novel The Amber Spyglass, by English author Philip Pullman, features an alien race known as the Mulefa, who use large, disc-shaped seed pods as wheels. They grip the pods with two limbs, while propelling themselves with two other limbs. The Mulefa have a symbiotic relationship with the seed pod trees, which depend on the rolling action to crack open the pods and allow the seeds to emerge.
In the 2000 novel Wheelers, by English mathematician Ian Stewart and reproductive biologist Jack Cohen, an alien species called "blimps" has developed the ability to biologically produce machines called wheelers, which use wheels for locomotion.
See also
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