A multidisciplinary team has proposed a process by which the wheel might have been invented that does not rely on a single moment of insight from some ancient genius. Instead, they suggest that the invention could have come in a series of steps (or perhaps we should say were made along a gradual slope). The authors even think they have identified where this happened first, and the conditions that facilitated it.

There’s a reason the invention of the wheel has embedded itself in our language – the world changed so much when it happened there was no need to do it again. Nevertheless, we don’t know how or exactly when the wheel was invented, let alone by who. Now, two engineers and a historian have combined to offer answers to some of these questions (although without a time machine, it is likely we will never know if they are right).

Arguably, wheels predate humanity, as spiders in the Namib desert contort themselves into a disk so they can roll down sand dunes to escape predators. Clever as this may be from an evolutionary standpoint, it lacks one of the key aspects that makes wheels useful to us: the axle. Just rolling a stone downhill at your enemies doesn’t really capture the full utility.

To identify the origins of the wheel-and-axle combination, Dr Lee Alacoque, Professor Richard Bullieet, and Dr Kai James started with the most detailed descriptions they could find of ancient wheel systems and sought to identify their precise functions. When they fed likely functions such a system needed to perform into an original design algorithm, the authors write that it; “Automatically synthesizes a wheel-and-axle structure despite being given little prior information about the system’s geometry.”

Based on this, the authors propose that the wheel evolved from what they call “free rollers”, cylindrical poles placed under heavy loads for transportation over short distances. Such poles could easily have been made from the trunks of trees with a tendency to grow straight and without low branches.

These rollers would have reduced friction compared to sliding (snow sledding aside). However, unless you have a very short distance to travel or an immense number of rollers, you need to constantly collect some from behind the load and rush them around to the front.

That creates a lot of extra work, but is practical when you have a large enough workforce, such as for the builders of the pyramids. The trio note, however, that it’s not an option within a typical mine, since the sides are so narrow there is no room to pass the rollers around. 

Consequently, the authors argue, the next stage in wheel development occurred among miners seeking to bring ore up steep slopes. This was to carve sockets out of the base of the vessel carrying their load. The shape of the socket kept the roller in place. A necessity in mines improved performance elsewhere as well, saving on the constant need to ferry rollers forward, and the risk of losing momentum if one did not arrive quickly enough. They refer to this advance as unilateral rolling.

Nevertheless, unilateral rolling comes with a drawback, as sliding occurs where the roller meets the socket, introducing extra friction. However, by smoothing the socket, and introducing lubricants, this could be kept to a minimum.

Over time, the authors propose, users of this technology would have realized that if the rollers operated only at the sides of the cart, and were thinned in the middle, the system gained clearance. Small obstacles needed to be removed from where the roller touched the ground but could remain in the middle. 

Such central thinning also made the rollers lighter, and therefore easier to transport. The axle could therefore have developed gradually. Despite lacking the physics concepts to understand why, those pushing the loads would have discovered carts with axles much thinner than their wheel diameters were easier to push, because of what we now call a mechanical advantage.

This is not all new work. Previous historians of the wheel have already reached the conclusion that unilateral rollers represented a stage in its development, despite a lack of archaeological evidence. However, the authors of this paper claim; “Our theory differs from earlier theories in that previous authors did not address the transition to unilateral rolling, nor did they relate the evolution of the axle to mechanical advantage.”

Initially, wheels and axles would have been carved from single pieces of wood. Eventually, some ancient engineers familiar with this system likely experimented with separating the wheel from the axle and found the product better still, particularly when turning, since the wheels could rotate at different speeds.

Once again, the authors conclude the advantages of the change would have been greatest in mines, which are often forced to turn to follow ore deposits or avoid outcrops of harder rock.

The authors argue the mines where these innovations (particularly the last one) occurred were in the Carpathian Mountains, where clay models of four-wheeled wagons dating to 3600 BCE have been found. The models are thought to have been used as drinking mugs. The fact that 150 of these models have survived to be discovered by archaeologists suggests they were once exceptionally common. This is probably a sign that the region’s inhabitants not only really liked beer, but were very proud of a technology unknown to their neighbors. 

Copper mining is known to have occurred in the mountains, but only after it was well established in the Balkans. The authors propose the Boleráz culture, responsible for the mugs, had access to lower quality ores, forcing them to transport more ore to the smelter than their southern counterparts, providing more incentive to find better ways to move it.

Wheel design has been optimized over the subsequent millennia, so the research doesn’t help us build better ones. On the other hand, it carries two important messages. Firstly, new technologies seldom appear fully formed, instead being developed in stages, likely with input by many people, even when a single individual gets the credit. 

Moreover, the authors add; “Our findings also demonstrate the critical role that environmental factors played in the creation of wheeled technology.”. If they are right, the Boleráz people did not invent the wheel because they were smarter than members of any other culture, but because they faced specific challenges in bringing their ore to the surface. These opened up a path to invent the wheel one roll at a time.

The paper is published open access in the journal Royal Society Open Science.