Rotating locomotion – it’s simple and efficient, a brilliant means of getting around. There are two kinds of rotating locomotion. One of them is cultural in origin: the wheel.
The wheel is a human invention, a product of hominid imagination of incalculable value to our species. The other type of rotating locomotion is rolling. Rolling, as opposed to the spinning of a wheel, is a naturally occurring mode of transportation – a product of evolution.
Surely wheels are as effective a solution as rolling. A parsimonious package of transportative efficiency, the wheel makes smooth and quick forward motion possible. What gives? Why did rolling evolve as an adaptation in biological organisms, but not the wheel? Wouldn’t it be great if we all had wheels as appendages?
<h2>Paradox of the Wheel</h2>
A wheel is a non-motorized tool for making work easier – a classic example of a simple machine. Other simple machines, such as screws, levers and pulleys, are found in complex organisms, especially at their joints. Why not fixed-axle wheels?
Think about it: the wheel has to be both independent and connected. A true rotation can’t just end with a little twist. It has to continuously shift, exceeding 360 degrees again and again. This characteristic already presents some problems from a biological perspective. How would the wheel metabolize energy with the rest of the body, taking in nutrients and getting rid of wastes? How would the nerves and blood vessels not get all tangled up?
<h2>Wheels vs. Evolution</h2>
The wheel sounds simple, but it is actually complex because it involves several component parts. Specifically, a wheel cannot function without an axle. It is defined by a round part spinning on a fixed axle or body. This is a problem given the nature of evolution. Because a wheel is a complex system, its characteristics are beyond the scope of known evolutionary mechanisms.
Biologists note that evolution always occurs gradually, part by part. Each part must have utility or provide some kind of advantage to an organism. But what advantage would be provided by a singular proto-wheel without an axle or a proto-axle without a wheel? Evolutionary biologists can’t think of anything.
Such a useless appendage simply wouldn’t stick around long enough to evolve into the complex wheel-and-axle system. And since organisms don’t spontaneously evolve complex systems in a single bound, the likelihood of evolution producing a true bio-wheel is quite low.
There is one significant exception, though, and it can be found all around us. The sole true biological wheel-and-axle system that we know of is found in prokaryotic bacteria – the simplest living organisms, consisting only of a single cell. Single-celled prokaryotes evolved billions of years ago; they are among the most abundant organisms on Earth.
These bacteria get along by means of a flagellum – a whip-like appendage or ‘tail’ that extends from the cell wall. Like a propeller, the flagellum is turned, literally, by a rotary motor located where it connects to the bacterium’s body. This molecular ‘stepper’ motor consists of a joint that locks the flagellum on and a series of discs and studs that divide the rotational work into several steps. Transportation power comes from a simple protein gradient – the pumping of proteins across the cell membrane, the same way all cells breathe and metabolize energy.
If it sounds remarkable, that’s because it is. It’s the only biological instance of a true rotary engine and propulsive unit – an organic wheel-and-axle system. The mechanism is quite efficient in terms of its power-to-weight ratio, but we don’t see these structures anywhere else in the animal world. Nobody knows why.