The history of the wheel is one of many quiet revolutions, says Dr Kai A James from Georgia Institute of Technology.
Imagine you’re a copper miner in southeastern Europe in the year 3900 BCE. Day after day you haul copper ore through the mine’s sweltering tunnels.
You’ve resigned yourself to the gruelling monotony of mining life. Then one afternoon, you witness a fellow worker doing something remarkable.
With an odd-looking contraption, he casually transports the equivalent of three times his body weight on a single trip. As he returns to the mine to fetch another load, it suddenly dawns on you that your chosen profession is about to get far less taxing and much more lucrative.
What you don’t realise: you’re witnessing something that will change the course of history – not just for your tiny mining community, but for all of humanity.
Despite the wheel’s immeasurable impact, no one is certain as to who invented it, or when and where it was first conceived. The hypothetical scenario described above is based on a 2015 theory that miners in the Carpathian Mountains – in present-day Hungary – first invented the wheel nearly 6,000 years ago as a means to transport copper ore.
The theory is supported by the discovery of more than 150 miniaturised wagons by archaeologists working in the region. These pint-sized, four-wheeled models were made from clay, and their outer surfaces were engraved with a wickerwork pattern reminiscent of the basketry used by mining communities at the time. Carbon dating later revealed that these wagons are the earliest known depictions of wheeled transport to date.
This theory also raises a question of particular interest to me, an aerospace engineer who studies the science of engineering design. How did an obscure, scientifically naive mining society discover the wheel, when highly advanced civilisations, such as the ancient Egyptians, did not?
A controversial idea
It has long been assumed that wheels evolved from simple wooden rollers. But until recently no one could explain how or why this transformation took place. What’s more, beginning in the 1960s, some researchers started to express strong doubts about the roller-to-wheel theory.
After all, for rollers to be useful, they require flat, firm terrain and a path free of inclines and sharp curves. Furthermore, once the cart passes them, used rollers need to be continually brought around to the front of the line to keep the cargo moving. For all these reasons, the ancient world used rollers sparingly. According to the sceptics, rollers were too rare and too impractical to have been the starting point for the evolution of the wheel.
But a mine – with its enclosed, human-made passageways – would have provided favorable conditions for rollers. This factor, among others, compelled my team to revisit the roller hypothesis.
A turning point
The transition from rollers to wheels requires two key innovations. The first is a modification of the cart that carries the cargo. The cart’s base must be outfitted with semicircular sockets, which hold the rollers in place. This way, as the operator pulls the cart, the rollers are pulled along with it.
This innovation may have been motivated by the confined nature of the mine environment, where having to periodically carry used rollers back around to the front of the cart would have been especially onerous.
The discovery of socketed rollers represented a turning point in the evolution of the wheel and paved the way for the second and most important innovation. This next step involved a change to the rollers themselves. To understand how and why this change occurred, we turned to physics and computer-aided engineering.
Simulating the wheel’s evolution
To begin our investigation, we created a computer program designed to simulate the evolution from a roller to a wheel. Our hypothesis was that this transformation was driven by a phenomenon called “mechanical advantage”. This same principle allows pliers to amplify a user’s grip strength by providing added leverage. Similarly, if we could modify the shape of the roller to generate mechanical advantage, this would amplify the user’s pushing force, making it easier to advance the cart.
Our algorithm worked by modelling hundreds of potential roller shapes and evaluating how each one performed, both in terms of mechanical advantage and structural strength. The latter was used to determine whether a given roller would break under the weight of the cargo. As predicted, the algorithm ultimately converged upon the familiar wheel-and-axle shape, which it determined to be optimal.
During the execution of the algorithm, each new design performed slightly better than its predecessor. We believe a similar evolutionary process played out with the miners 6,000 years ago.
It is unclear what initially prompted the miners to explore alternative roller shapes. One possibility is that friction at the roller-socket interface caused the surrounding wood to wear away, leading to a slight narrowing of the roller at the point of contact. Another theory is that the miners began thinning out the rollers so that their carts could pass over small obstructions on the ground.
Either way, thanks to mechanical advantage, this narrowing of the axle region made the carts easier to push. As time passed, better-performing designs were repeatedly favoured over the others, and new rollers were crafted to mimic these top performers.
Consequently, the rollers became more and more narrow, until all that remained was a slender bar capped on both ends by large discs. This rudimentary structure marks the birth of what we now refer to as ‘the wheel’.
According to our theory, there was no precise moment at which the wheel was invented. Rather, just like the evolution of species, the wheel emerged gradually from an accumulation of small improvements.
This is just one of the many chapters in the wheel’s long and ongoing evolution. More than 5,000 years after the contributions of the Carpathian miners, a Parisian bicycle mechanic invented radial ball bearings, which once again revolutionised wheeled transportation.
Ironically, ball bearings are conceptually identical to rollers, the wheel’s evolutionary precursor. Ball bearings form a ring around the axle, creating a rolling interface between the axle and the wheel hub, thereby circumventing friction. With this innovation, the evolution of the wheel came full circle.
This example also shows how the wheel’s evolution, much like its iconic shape, traces a circuitous path – one with no clear beginning, no end and countless quiet revolutions along the way.
Dr Kai A James is a professor of aerospace engineering at Georgia Institute of Technology. His research interests are in the areas of computational mechanics and multidisciplinary design optimisation, with a particular focus on problems involving complex, nonlinear structures and mechanisms.
Don’t miss out on the knowledge you need to succeed. Sign up for the Daily Brief, Silicon Republic’s digest of need-to-know sci-tech news.
