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In physics, "almost everything is already discovered, and all that remains is to fill a few unimportant holes." (See Jolly.) Therefore, on Physics SE, people are veering off into different directions: biology, for example.

Thus, it happens that a question about bicycles generates some discussion about evolution in biology and animals with wheels.

Three explanations are offered for the apparent lack of wheely animals (also on Wikipedia, where, by the way, most Physics SE questions are answered perfectly).

  1. Evolutionary constraints: "[A] complex structure or system will not evolve if its incomplete form provides no benefit to an organism."

  2. Developmental and anatomical constraints.

  3. Wheels have significant disadvantages (e.g., when not on roads).

Now, I suggest that all three can be "solved".

  1. With time.

  2. With a symbiotic relationship between a wheel-like animal and a "driver"-like animal, although this gets awfully close to a "driver"-animal to jump onto an actual (man-made) wheel. (So, perhaps, you can suggest a better loophole around this constraint.)

  3. Roads are presumably not the only ecological niche where animals with wheels could thrive. I'm thinking of frozen lakes, although there skates would be better than wheels.

What, therefore, is the explanation for there not being any wheeled animals? Please consider, in your answer, the counterfactual: What assumption of yours would be falsified once a wheely animal is discovered?

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Wheels are possible on the molecular level — bacterial flagella are rotating cores inside a molecular motor, but wheels larger than the flagellum have not really been found.

enter image description here

Defining a wheel as a freely rotating joint that can rotate indefinitely in one direction, a single animal with a wheel is an improbable* development that would require a single animal have two separate parts (axle/wheel and body).

[*read as: pretty much impossible]

It's hard to imagine how such a thing could evolve. A wheel and axle would need to be made of living tissue, otherwise it would be vulnerable to wear and tear. Wheels also have problems going over uneven terrain, which is really all terrain animals live in. It's difficult to imagine what sort of selection conditions would be strong enough to push animals away from legs.

If you include driver-vehicle symbionts where the 'car' and 'wheel' are actually two animals, then they have evolved. Parasites can have all sorts of symbiotic control over their victims including as means of transport. The Jewel Wasp is one which is the most suggestive of what you may be thinking. The wasp stings its victim (a cockroach) in the thorax to immobilize the animal and then again just behind its head. After this, the wasp can ride the beetle, steering it by holding its antennae back to its nest where the roach is immobilized to feed the wasp larvae there.

(see section "Pet cockaroaches" in this reference.)

As to the three schools of thought you added to the question, I would probably rather say there were two strong arguments against. The first is whether there is an evolutionary path to wheels (argument 1 in your question), which I doubt. Given even a large amount of evolutionary time you will not see a naked human being able to fly on their own power. Too many structural characteristics of the body plan have been made to all be reversed so that wings or other means of aerial conveyance will show up. The same can be said for wheels when the body plans have fins/legs/and wings already.

Argument 3, which I also tend to agree with, is perhaps more convincing. By the time a pair of animals makes a symbiotic relationship to do this, or a single macroscopic animal evolves wheels, they will literally develop legs and walk away. When life came onto the land this happened, and since then it's happened several times. It's sort of like saying that the random movement of water molecules might line up to run a stream uphill. There's just such a strong path downwards, that the statistical chances of you seeing it happen are nil.

This is a hypothetical case, but arguing this in a convincing way I think you would need to lay out: a) an environment whose conditions created enough of a selective advantage for wheels to evolve over legs or other similar adaptations we already see. Perhaps based on the energy efficiency of wheels; b) some sort of physiological model for the wheels that convey a reasonable lifestyle for the wheel.

There are lots of questions that would need to be satisfied in our thought experiment. Here are some: "the symbiotic wheel would be spinning constantly; if it died the driver creature would be completely defenseless"; "if the ground were bumpy, all these wheeled animals would get eaten"; "the wheel symbiont — how would it eat while its spinning all the time? Only fed by the driver? Even symbionts such as barnacles or lampreys on the flanks of sharks still have their own ability to feed."

For many similar questions the same sort of discussion ensues where there are many disadvantages which outweigh advantages for animals. e.g. "why are all the flying animals and fish and plants even more similar to airplanes than helicopters?"

Sorry if I seem negative, but way back in grad school I actually did go over some of these angles.


UPDATE: First Gear found in a Living Creature. A European plant-hopper insect with one of the largest accelerations known in biology has been found to have gears! (There's a movie on the article page. )

The little bug has gears in its exoskeleton that synchronize its two jumping legs. Once again selection surprises.

The gears themselves are an oddity. With gear teeth shaped like cresting waves, they look nothing like what you'd find in your car or in a fancy watch. There could be two reasons for this. Through a mathematical oddity, there is a limitless number of ways to design intermeshing gears. So, either nature evolved one solution at random, or, as Gregory Sutton, coauthor of the paper and insect researcher at the University of Bristol, suspects, the shape of the issus's gear is particularly apt for the job it does. It's built for "high precision and speed in one direction," he says.

The gears do not rotate 360 degrees, but appear on the surface of two joints to synchronize them as they wind up like a circular spring. The gear itself is not living tissue, so the bug solves the problem of regenerating the gear by growing a new set when it molts (i.e. gears that continually regenerate and heal are still unknown). It also does not keep its gears throughout its lifecycle. So the arguments here still stand; the exception still supports the rule.

Additional Note: In his book "the God Delusion" (Chapter 4 somewhere) Richard Dawkins muses that the flagellar motor is the only example of a freely rotating axle that he knows of, and that a wheeled animal might be a true example of 'irreducibly complexity' in biology... but the fact that there is no such example is probably to the point.

shigeta
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  • Thanks! (Is there a word missing or something else wrong in the 2nd paragraph? I can't understand the sentence properly.) Should I take your answer as follows: Wheeled animals must have two separable parts; that is unlikely, because it hasn't happened. –  Apr 17 '13 at 18:01
  • I did an edit for clarity. It is just a theory as they say, but i don't think its really possible for a single animal to be made of two separable and rejoinable parts such as would be required to have an axle of a wheel. Nothing even remotely like this has ever occurred in biology as far as I can see. That's not at all to say that we have seen everything that biology can do. I'll try to formulate a specific principle it violates, but really two living independent parts are two creatures... symbionts don't really live this way. – shigeta Apr 17 '13 at 18:26
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  • So, "this" refers to the flagellum? 2) Also, I shifted my speculations to exoskeletons turning into wheels. :)
  • –  Apr 17 '13 at 18:32
  • @Gugg you're right its a sentence fragment caused by my sporadic dementia. I have outlined a response to the exoskeleton idea; legs have a lot going for them. Its always possible... but it just might never show up. – shigeta Apr 17 '13 at 19:31
  • OK, great! Now, onto my request in the question. Suppose a wheeled animal would be discovered tomorrow (and you wouldn't be given further information than that). What assumption would you drop first/doubt most? –  Apr 17 '13 at 20:03
  • I guess that would be my assumption that the animal somehow managed to nourish a living wheel/axle system and not be separate from the rest of the animal. the exoskeleton is not a bad idea. Another possibility is that somehow that legs / wings are better forms of locomotion than wheels for all environments that animals currently live in. – shigeta Apr 17 '13 at 20:41
  • Thanks. That seems to put you (primarily) in the anatomical camp 2. (And secondarily in camp 3.) –  Apr 17 '13 at 20:56
  • You are getting more clear with your argument for, but there is still a ways to go -I added some response to my answer here. Its a good exercise to think this through. Good one. – shigeta Apr 18 '13 at 03:15
  • I think I would argue that there are serious issues in all three categories here. Maybe the anatomical arguments are less important than the evolutionary constraints. I guess that would be #1 for me since the anatomical argument is based on the idea that the anatomy of wheels are just too many adaptations away from what we have now. The inefficiencies of wheels make that evolutionary distance even greater. – shigeta Apr 23 '13 at 18:47
  • Many thanks for your thoughts on this! Quite helpful to a layman. –  Apr 23 '13 at 19:01
  • you're not so much a layman anymore ! :) good question. – shigeta Apr 23 '13 at 20:37
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    Heh, I just saw the geared animal article and came back to this question to make a comment about it. But shigeta beat me to it! :) – Alexander D. Scouras Sep 15 '13 at 06:40
  • here's a +1 anyway :) – shigeta Sep 15 '13 at 23:40
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    Having two symbiotic creatures, one wheel and one body would be interesting. A bit like a wheelbarrow but with back legs that kick to propel the creature forward, resting on the front wheel creature. It is hard to imagine it being a practical or survivable. – Beo Apr 10 '14 at 15:55
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    I guess there is also nothing preventing two or more creatures being gestated simultaneously in one parent forming separate wheel(s) and body offspring, except for it being overly complicated not seeming to be very survivable. – Beo Apr 10 '14 at 16:00
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    Very good answer. One additional thought: The "wheel symbiont" and the "body symbiont" must also have a mechanism to transfer torque between them (an "engine") to actually propel the whole thing forward, otherwise they could only roll downhills. This makes it anatomically even more complicated and more difficult to see an evolutionary pathway. – Stephan Matthiesen Sep 22 '15 at 09:59
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    @Beo Your wheelbarrow (front wheel, back legs) is an interesting idea. But if the wheel gets stuck, it would have to be able to also stand up on its legs to pull the wheel out again. Unless it lives on a hard smooth surface, it would probably have to get up and walk a lot, so what's the advantage of having the wheel? – Stephan Matthiesen Sep 22 '15 at 10:46
  • There are co-gestated symbionts of course - but they are usually symbiotic microorgansms. Id not be surprised to find a parasite that co-gestated... Parasites are not symbionts of course. For some reason animals have a hard time living together completely symbiotically. – shigeta Sep 24 '15 at 15:10
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    @shigeta - such a great answer! Wish you were still answering questions here. :) – anongoodnurse May 30 '22 at 13:07
  • thanks! it was a great way to work on my writing :) at one time i had 20% of the content here (!) I am still on occasionally but so many good people on the site now :) I am still looking for questions that i can really do a special service for. – shigeta Jul 26 '22 at 15:25
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    Dawkins also reviews reasons large animals would be unlikely to have wheels in The Ancestor's Tale. Fortunately, they don't apply to gears in Issus coleoptratus. – J.G. Apr 12 '23 at 21:44