Firstly, why might wheels be a useful adaptation? Well, they could provide an efficiency and simplicity of motion in some circumstances - I can certainly imagine animals using wide wheels to trundle across the soft sediments of the ocean floor for example (much like the wire-wheeled lunar rovers from Apollos 15-17). However, legs and fins generally work pretty well, with wheels really coming into their own on straight, smooth, hard surfaces. These are not common in nature, though humans produce plenty of them - and hence plenty of wheels. So, a lack of evolutionary advantage might be one reason why natural wheels are not widely seen.
Secondly, what do I even mean by a wheel? Here, I am only considering something that has an axle or bearing. There are plenty of organisms that roll - the South American pebble toad Oreophrynella nigra that tumbles down slopes to avoid predation, the wide variety of tumbleweed plants (and the rarer 'tumblefruits' such as Physaria) that disperse seeds as they roll with the wind, and the puffballs of the genus Bovista that are also blown around and so disperse their spores more widely. Ocean currents roll the coral Porites lutea across the sea floor and the small stomatopod mantis shrimp Nannosquilla decemspinosa can curl up and roll slowly like a wheel if stranded on a shallow damp sandy shore, thus returning to the sea. These are all interesting in their own right, and there are other examples, but none of them are wheels.
In fact, there don't appear to be any organisms that roll along on wheels in the way that humans' various vehicles do. As mentioned above, there may simply be no evolutionary pressure to produce a wheel, but there are also developmental constraints. For example, to have a wheel in a multicellular organism is tricky because, to be able to rotate freely, the wheel needs to be detached from the rest of the organism. If this is the case, how could it maintain a blood supply, neural connections and so on? Two options come to mind:
1. The wheel could be made of 'dead' material secreted by the organism, such as carapace material. This could grow as a toroid (doughnut-shaped) swelling on a limb/axle and gradually separate by thinning near the limb. This could produce a passive wheel on an axle much like a wood-turner produces a freely movable (but not removable) ring from a single piece of wood.
2. The wheel could be alive but self-contained. If a ring of cells developed as above and then detached, to be an effectively autonomous wheel, it would have to have its own energy supply (photosynthesis, chemosynthesis?) and so on.
Neither of these options have been discovered in nature, though this does not mean they never will - my feeling is that the lack of need is more likely to prevent wheels evolving than developmental problems. So far, I have not differentiated between passive and active wheels i.e. whether they simply roll like a cart (reducing the friction that would be caused by dragging) or are actively rotated by an energy source. Active wheels are developmentally even more problematic as a torque needs to be applied - in animals, motive force is produced by muscles, but this would not work on wheels as they need to be freely rotating. However, in bacteria, the problems of producing motive force, overcoming inertia and so on have been solved. In fact, the only example discovered so far of a true biological wheel (an active one that produces continuous propulsive torque around a fixed structure), is the bacterial flagellum, the a propeller-like thread used for locomotion. Where the flagellum enters the cell membrane, there is a motor protein that works like a rotary engine, powered the flow of hydrogen ions (i.e. protons) across the bacterial cell membrane down a concentration gradient created by a proton pump. A similar system using a sodium ion pump exists in the genus Vibrio.
|The structure of the flagellar base showing cutaway details of the 'motor'. Thanks go to Mariana Ruiz Villarreal for putting this and other diagrams in the public domain.|
At an even smaller scale, the enzyme ATP synthase (which is involved in energy storage and transfer within cells) is somewhat similar to bacterial flagellar motors and is likely to be an example of modular evolution i.e. where two separate structures or sub-units (which evolved and previously functioned separately) become joined or associated, and in doing so gain a new function.
So, although true wheels have not been discovered in multicellular organisms, and both developmental and utility constraints make their evolution highly unlikely, maybe impossible, there are ways that wheels might be used in nature:
1. Through symbiosis, joining two otherwise unrelated structures/organisms in order to get round the developmental problems preventing direct evolution of wheels. This could be instinctive (imagine an extension of dung-ball rolling by dung-beetles) and is an idea which has been explored in fiction, e.g. in the Amber Spyglass (Philip Pullman, 2000). In this book, an alien race known as the Mulefa use large, round seed pods as wheels. They put these on sideways-oriented claws (which act as axles) on two of their legs, using the other two legs to push themselves along. The symbiotic aspect occurs because the trees that produce the seed pods depend on the rolling action under the weight of the Mulefa to break open the pods and allow the seeds to disperse and germinate. A number of other science fiction novels consider biological wheel use in a variety of ways, but Pullman's is probably my favourite so far, though other examples include David Brin's Brightness Reef (1995) and Infinity's Shore (1996), and Wheelers (2000), co-authored by Ian Stewart and Jack Cohen (who happen to be a couple of Terry Pratchett's collaborators if you like a bit of nerd-trivia).
2. Through tool use. Humans do this, using wheels widely - could other species do the same, even if with less technological sophistication? I'm just waiting to see corvids start rolling past...
OK, I think that's enough speculation for one day - if anyone out there does know of other examples of 'bio-wheels', I love to hear about them, so feel free to add a comment.