The Roll of the Wheel in Nature

It’s seven in the morning, and you’re half awake, overwhelmed by the noise pollution of the subway station, when the sudden, loud screech of the train pierces your ears. Almost everyone is unfazed by the auditory clutter that they experience daily, hurrying to board before the train car overflows. You glance at the wheels of the train, quickly sighing before boarding after the other people. These wheels are the foundation of our lives, moving us from home to work or school. Around 3500 BC, the role of these wheels also revolved around people, though in a much different way. Originating as pottery wheels and turntables, these basic rotating devices would not be what we consider wheels until 1,500 years later. As early humans progressed towards an agricultural society, the emergence of the spoked wheel would mark the beginning of the new age of man. 

The spoked wheel in 2000 BC and the wheels of bicycles today can be summed up in three major components: the axle, the wheel around the axle, and the complex that holds the components together. Though ancient spoked wheels consisted of far simpler parts, while modern-day wheels have more supportive structures and designs, all spoked wheels rely on axles to function. Axles are strong rods that turn with the rotation of the wheels, using friction to establish control of the wheel by regulating speed while simultaneously bearing the load above the wheel. Friction also works to help with the movement of the wheel, as the friction of the ground and the wheel allows the wheel to push off the road and generate force. However, the axle cannot connect to both the wheel and the complex, as only one part can be joined to keep this transfer of friction from the ground to the wheel. As the wheel rotates and makes contact with the ground, it reduces friction between the ground and the machine by concentrating friction at the point where the wheel and the axle come in contact. This contact zone is designed to be equally smooth so that, as the two parts turn, the wheel turns properly without displacing friction in other parts of the wheel. 

By concentrating locations for the friction, the wheels are able to easily move the load above them in many directions. They also uniquely utilize leverage in a way that allows heavier loads to be transported quickly and without too much manual effort. In this case, leverage is the concept that the rotation of the wheel outputs much more force than required to move the load, allowing for the application of less force overall. Force is amplified by the radius of the wheel during rotation, and this force is built up to turn the axle, which then moves the load much faster compared to the initial push. 

Given that wheels are able to provide good control of direction, speed, and movement of weight, how is it that they haven’t developed in nature at all? The absence of wheels is due to three main factors: the poor net energy gained from this adaptation, the unlikeliness of natural pressures to promote the development and success of wheels, and the fact that organisms’ growth may conflict with how the wheels’ parts interact.

In normal circumstances, all adaptations of a surviving organism are passed down to offspring because of how instrumental they are in helping the organism live its life. Wheels, despite their ingenuity, are extremely costly and do not produce the same level of energy conservation or ecological success. Wheels would require multiple parts, such as the axle and the wheel itself, which both must be isolated from the rest of the body of the organism. These parts aren’t complex in their basic form, but in how they develop. The disconnected status of the wheel would make its organic development almost impossible, as structures like bones rely on glands from inside the body to be physically attached to grow new cells. However, the separation of the different parts of the wheel would warrant isolation from the circulatory system of the organism. These factors create a critical issue of being unable to self-repair, as no new materials can be delivered to the wheel segments after they are fully developed—if they’re even able to properly develop. As stated above, wheels can only function when the axle is connected to only one of two parts, either only to the complex or only to the wheel. When both are connected, which is how organic parts tend to grow, the wheel would not work. 

Though the possibility of wheels in nature may sound bleak after all the conditions needed for their development, there are two ways organisms navigate this challenge. The first special case is flagella, which may be the only true wheel to occur organically. Found in a plethora of cells from eukaryotic to prokaryotic, flagella function as a way to move the cell around. Flagella rely on microtubules, which are the largest part of the cytoskeleton and are composed of tubulin proteins which hold the cell’s structure and also function as a road for motor proteins to run along. Flagella are composed of a hook attached to the cell surface and a rotary formation of microtubules to turn the flagellum in a clockwise or counterclockwise motion, with a central pair of microtubules functioning as an axle and the microtubules around the axle functioning as a wheel. 

Using adenosine triphosphate (ATP) or an ion gradient in the case of bacteria, the microtubule bundle is coordinated to slide in a direction guided by surrounding proteins. One of the ATP’s three phosphate groups is removed, causing a reaction to occur which releases high amounts of energy. This energy powers the dynein motor proteins to walk along the microtubules to create a sliding motion between the filaments, which then convert into an overall bending of the flagellum. In the case of an ion gradient, a similar process occurs. Utilizing the movement of ions into the cell through stator units, protein complexes drive motor proteins to undergo movements similar to the dynein motor proteins in order to contract and bend the flagellum.

While flagella may be the only wheel-adjacent structure discovered to this day, some organisms use a second loophole to overcome this limitation. Ultimately, rolling is the better way of locomotion compared to wheels, being low-cost and versatile. For example, pangolins and tumbleweeds can rotate around like wheels by simply developing into a spherical form. Instead of taking the complex path of developing true wheels, rolling fulfills the same goal through easier means and, in the case of pangolins, provides both physically and behaviorally defensive measures on top of locomotion. 

The wheel emerged in human society from more artistic means, being the basis of pottery or even just an element of home decoration. Over time, humans have innovated the wheel from its humble agricultural origins to a core part of society, from the gears that turn our machinery to the little motifs in our artwork. Wheels today may seem more industrial and boring than their origins, but others may find inspiration in them. Perhaps in the distant future, with never-before-seen conditions, wheels might emerge in something completely novel beyond conventional biology and find new loopholes to exist.