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Comment by xyzzyz | Hacker News Reader

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.

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

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

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.

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.

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.

But how easy is it for that to evolve?

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