Comment by Cyclical | Hacker News Reader

Comment by Cyclical | Hacker News Reader

The reason one does not see freely rotating bodies such as wheels or propellers in nature comes down to perhaps the most fundamental aspect of biological systems: they're alive. Because they're alive, resources need to be supplied to them and waste needs to be removed. Propagating nutrients across a freely rotating interface can be an intractable problem, and this is not even considering the question of _how_ one could induce free and unbounded rotation in a biological joint. Think of how muscles work, with discrete attachment points and it becomes clear that free rotation isn't possible with muscles. The only method of inducing rotation I can come up with off the top of my head is some sort of turbine design, with rotation being driven by fluid flow.

The notable exception to the absence of free rotation in nature is the flagellum, which solves the nutrient supply issue by being so small and self-contained that it doesn't need nutrients, just the presence of an ion concentration gradient.

Maple seeds spin like an autogyro propeller. This is possible because they are no longer attached to the tree. An aeronautical engineer once told me that a single blade propeller is theoretically most efficient by some metric. It would seem the trees have found that solution.

Great point! I think a valuable thing to understand is that wheels and propellers are very obviously more efficient than legs and wings, respectively. There's a reason ornithopters are at most a curiosity outside of fiction (looking at you Dune!)

Maple seeds are able to take advantage of the efficiency gains of revolute motion by having their entire body rotate rather than using a static body with a freely-moving prop.

Here is an idea: make the rotating part dead (like mollusk shell), and use muscles to push a paddle to add impulse to rotation, like pedaling a bicycle. Should be doable.

Dead things have an unfortunate tendency to break, and cannot self-repair.

It doesn’t have to last forever, only until it reproduces.

But how easy is it for that to evolve?

Not sure if any creature has evolved ball bearings…

That part is easy, actually: make the bearing surface alive, and regenerate it upon wear. It might exude some lubricant too while we’re at it, should not be hard to evolve.

If you can excuse the overt sexuality of his work, Piers Anthony depicted an interesting alien race of beings with a biological ball-bearing method of propulsion in one of the Cluster series books - I forget the title.

I too loved "A Subtle Knife" :)

https://hisdarkmaterials.fandom.com/wiki/Mulefa

> The most notable feature of the mulefa was their use of seed pods as wheels. Once a zalif reached puberty, they were able to fit their spurs into the disc-shaped pods, and propelled themselves along "roads" of solidified lava with their side legs.

This is definitely a possibly solution to the problem of keeping your propeller/wheel alive, but does present some problems in its own right. As another commenter stated, dead objects like teeth, hair, nails, claws, and scales tend to break and once broken cannot be repaired without entirely replacing the object. As well, growing a structure with a complex form like a propeller requires providing nutrients and removing waste from the structure while the cellular scaffolding is forming. Thus we are still faced with the issue of nutrient transfer, though I can imagine a multistage process in which the structure is initially non-rotating and then the connective tissue atrophies to free the now-nonliving rigid body.

Reply on news.ycombinator.com