I have always been amazed by the magnificent variety of life on Earth. And exactly for this reason, there is one question that has always intrigued me: why no single living being moves using wheels or propellers? It may be that the answer is in the mechanics. Wheels and propellers (as gears and nuts) require bodies with disjoint parts, kind of tricky to produce. But, from everything evolution was able to come upon, I do not doubt for one minute that nature could have created beings with excellent gear systems.
The nonexistence of wheels in nature is probably not due to a biomechanical impediment, but due to random circumstances in our evolutionary history. It is, however, strange that even in the crazy Precambrian hippie times, no propellers swam around.
Anyway, for each phylum, the definition of core properties of the bodies, as their symmetry or existing disjoint parts was made half a billion years ago. Since then, we changed, but always within limits. Limbs can specialize into arms or wings, but we cannot grow new limbs. Borrowing Dawkins’ metaphor, in evolution you can slowly climb the mount, but if you try to jump to another one, you die.
Even without wheels, life on Earth is amazing. It spawns the most incredible shapes, spreading over 11 orders of magnitude in size, from the smallest virus to a giant fungus in Oregon. If such variety exists in our world, think of how it could just be beyond it? And this is something that bothers me with the extraterrestrials that inhabit my TV: with few variations, they are all ridiculously similar to humans. ETs, as the gods, are made in our image, after our likeness.
It could be that we, humans, and occasional ETs have a common ancestor, which is rather improbable to say the least. (By the way, Star Trek has played this card to explain why all intelligent beings in the Galaxy differ only by their forehead wrinkles.)
It could also be that our body plans are especially appropriate to host intelligent life. Thus, wherever intelligent life appeared in the universe, it would look like us. Although I would not bet two cents on this “evolutionary attractor” hypothesis, it is true that features like wings and eyes appeared independently more than once on Earth, showing that certain solutions are really pretty much efficient.
Well, back to Earth, in our planet it is mandatory: mammals and reptiles have four limbs. They may be in the form of legs, arms, wings or fins, but they will always count to four. If I were asked to guess, I would tell that angels are mammals as much as dragons are reptiles. For this reason they must have in addition to a pair of legs either a pair of arms or a pair of wings, never both together. Do you need more reasons to not believe in either of these creatures?
Four limbs, no wheels… these are limits in our planet. Out there, the limits are only the laws of physics. And it is quite funny to think that there may be worlds where cats with wheels chase birds with propellers.