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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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There is no one single explanation for such questions, we can only speculate (with well-reasoned arguments, I hope) about this. –  Mad Scientist Apr 16 '13 at 18:16
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You may enjoy reading the Uplift Saga books by David Brin (quite an accomplished astrophysicist himself). They contain some of my favorite aliens in sci-fi, including the wheeled G'Kek! –  terdon Apr 17 '13 at 18:53
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There's a nice video from VSauce about that topic, explained for the average Biology Joe like me: youtube.com/watch?v=sAGEOKAG0zw Michael answers various questions of this type on his channel, very interesting and entertaining. –  owzim May 8 '13 at 21:28
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His Dark Materials gives a good answer to this (spoiler: it has wheeled animals) –  Konrad Rudolph May 9 '13 at 13:40
    
@KonradRudolph it does? Damn, I should read it again, I don't remember them. –  terdon Sep 13 '13 at 3:02
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3 Answers

up vote 36 down vote accepted

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

A single animal with a wheel is an improbable* development that would require a single animal have two separable 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 you added to the question, I would probably rather say I felt there were two strong arguments against. The first is whether there is an evolutionary path to wheels, which I doubt (argument 1 in your question). I really don't think there is. Given enough 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 grow 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 water might cooperate 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 seem to be lots of questions like "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."

A similar question — why are all the flying animals and fish and plants even more similar to airplanes than helicopters? Same sort of discussion ensues.

For all these scenarios I see a lot of disadvantages and not a lot of advantages.

Sorry if I seem negative, but when I was a grad student I actually did think about this, so I'm familiar with the issues somewhat.

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 suprises.

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 issus 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.

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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. –  Glen The Udderboat 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
    
1) So, "this" refers to the flagellum? 2) Also, I shifted my speculations to exoskeletons turning into wheels. :) –  Glen The Udderboat Apr 17 '13 at 18:32
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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 6:40
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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 at 16:00
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In your question, you mention frozen lakes and rightly say that skates would be far easier than wheels, but you could have used salt flats as an example.

I think the reason there are no actual wheels is a mixture of ontogenetic and phylogentic barriers, maintenance (nutrition), lubrication and control (nerve connections). There would have to be severe and sustained evolutionary incentives to go from something simple, cheap and extant (legs) to something way more difficult and complex.

However, there are several species of plants and animals that use rolling as a form of locomotion. Tumbleweeds are a good example of plants, and for animals, there are spiders, caterpillars and salamanders.

It only takes a small evolutionary development to take something like this from a passive motion (rolling down hill to escape predators) to active (perhaps creating waves of flexion to roll like... ahem, caterpillar tracks).

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Thanks. 1) Why "severe"? Since, if there is no functional intermediate, I presume it has to happen in one jump, only making it (much) less probable (?). I don't see how such chances would improve by the severity of the "incentives". 2) I was thinking along the lines of a some sort of an exoskeleton, that, in time, separates, but does remain pierced through the animal. (Is that unthinkable?) It could have the form of two wheels and an axle. :) –  Glen The Udderboat Apr 17 '13 at 15:13
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Something as complicated as wheels (if even possible with multicellular animals) could never happen as a result of a single, lucky mutation. The term 'exaptation' is where evolution takes an existing functional unit, and starts using it for a different purpose. It takes many generations so the evolutionary pressure would have to be sustained. It would have to be severe to be more beneficial than just sticking with legs. –  DaleyPaley Apr 17 '13 at 22:00
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Shigeta's molecular answer is spot on. However, at the large scale level I think the key problem with a biological wheel needs to be spelled out clearly: how does an organism with separate parts maintain these parts? Let's suppose that an organism evolves a fully rotational joint, how do they then provide nutrients to the tissue on the other side? If they don't provide nutrients how does self-repair in this tissue continue without nutrients?

Mind you, I'm not sure that the biological problems with a wheel are the real answer. I'm not sure how a wheel evolves. What's the functional intermediate? How does a semi-connected wheel out-perform a limb? Remember than evolution is canalised by what has gone before.

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Thanks. I take it that your tentative suggestion is that the anatomical barrier is (at least among) the critical one(s), right? –  Glen The Udderboat Apr 17 '13 at 18:20
    
Um, I not sure which argument is strongest. I think I favour the evolutionary angle, myself. Both are though, I think, sufficient answers but the evolutionary one probably better reflects the historical reality of matter. Wheels didn't evolve because there was no beneficial intermediate for them to pass through. –  Jack Aidley Apr 17 '13 at 19:46
    
OK. So, hypothetically, if a wheeled animal was discovered tomorrow, this would make you doubt the theory of evolution? :) Or would you conclude that there must have been some beneficial intermediate? Or what? Show your cards! :) –  Glen The Udderboat Apr 17 '13 at 19:57
    
Equally I'd conclude there must be some anatomic way it works too. –  Jack Aidley Apr 17 '13 at 20:36
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I mean that finding an animal with wheels would demonstrate both that it is anatomically possible and that it is evolutionarily possible. I consider both to be good arguments as to why we'll never find such a thing but I suspect the reality is that the space of possible organisms that might be anatomically possible has never actually been explored because there's no evolutionarily advantageous pathways that leads there. –  Jack Aidley Apr 18 '13 at 13:48
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