Why does the natural world have wings, fins and legs rather than propellers and wheels?
Simply put, propellers and wheels work as follows: they convert rotational motion into thrust, which generates a pressure difference between the front and rear surfaces, which results in movement. Propellers are the go-to design for movement through air and water, and wheels for movement across land. At least, for us humans. But this is not the case in nature. How many organisms can you think of that fly or swim using a propeller, or move on wheels? Granted, there are a few organisms that use rotational motion for movement, such bacteria with their flagella, or the Wheel Spiders of Namibia, that folds its legs in to make itself a wheel shape to escape predators. But propellers and wheels are certainly not, by any means, a popular design for movement through air and water and over land in the natural world. Why is this the case?
Evolutionary constraints: an intermediate propeller?
There are a number of constraints that prevent the evolution of propellers in the natural world. Evolution by the process of natural selection means that genetic changes which enhance the ability of an organism to pass on its genes, known as fitness, spread within populations. Although both neutral and detrimental genetic changes can spread in a population under some circumstances, large changes that require multiple steps tend to only spread in a population if the intermediate stages also incrementally increase fitness. Imagine what the intermediate stages may look like in producing a propeller; they are unlikely to benefit an organism. Therefore, an organism cannot gain the potential benefits of evolving a full propeller because they must go through a stage where they are at a disadvantage first and are less fit in comparison to the individuals without an intermediate propeller. Think of it like a landscape. An organism cannot get from Peak A (no propeller) to Peak B (propeller) without descending down a valley first; therefore, the forces of natural selection push the population back towards Peak A (no propeller). This makes propellers, and wheels extremely difficult to evolve.
Keeping the moving part alive?
A further reason as to why a propeller or wheel would be difficult to evolve is the need to keep the rotating part alive. To keep a part alive, it needs to receive nutrients and oxygen. For a bacterium with a flagellum, the flagellum is supplied with nutrients and oxygen by diffusion of these from the main body in a liquid state. However, as an organism gets bigger, diffusion of these nutrients is not enough to keep it alive. Animals generally supply their body with oxygen and nutrients through the blood, which is transported around the body in small vessels (arteries and veins). A rotating element cannot be sustained in this way – all the vessels would tangle as the element rotates. There are no suitable couplings that allow fluids to pass from the body to the rotating element. Although maybe the pycnogonids, which have their organs in their legs, display a viable route towards evolution of rotating limbs.
Legs, wings and fins are easy to evolve
In comparison, legs, wings and fins are really easy to evolve. Moving anything backwards and forwards, be it a limb or appendage or the whole body, generates instability in movement that generates some level of propulsion. Moving from this to a hydrodynamically efficient fin, or an aerodynamically efficient wing is easy. There are studies of sea butterflies (pterobranchs) showing their transition from flapping their fins to row in water when they are small to flying through the water as they get larger, without rearranging the neural control or muscular systems too drastically. This is exactly why we have seen fins, flippers, wings and so on evolve over and over in different groups.
Wings, fins and legs are better…
Simply put, in terms of efficiency, endurance and energy use, the way animals have done it is better. At Animal Dynamics, we have run propellers and wings of the same size driven by the same motor and found that the wings are 20-30% more efficient in terms of power to thrust than equivalent sized propellers driven with the same input power. The reason for this is simple; for anything sweeping through a fluid, the speed goes linearly from the hinge to the tip in a wing and from the centre to the outer edge in a propeller. For vehicles of the same overall size, this distance is longer in wings and so they are more efficient. It’s basic geometry.
…so why did we build propellers and wheels?
Well, because they are mechanically simple. It is easy to build a rotary joint and gears with the tools we have available to us. There are still a few occasions where rotational propulsion is better which is when absolute top speeds are needed. Like a jet engine with one moving part. Cars can perhaps go faster than things on legs, but we have to flatten the world to make this happen, which is a fundamental mistake for both the environment and for economical reasons.
Only now are we on the cusp of being able to mechanically build legs, wings and fins. This is mostly due to advances in processors and battery weight, fuelled by the mobile phone industry, as well as advances in motor efficiency. At Animal Dynamics, we are developing a flapping-wing propulsion micro-drone, Skeeter, which flies with flapping wings. This mode of flight increases the efficiency of energy input to power, which increases endurance.
Find out more about our skeeter project here
Written by: Zoe Griffiths, Jonny Page, Adrian Thomas)