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Rotating locomotion in living systems – the use of wheels and propellers by organisms – has long been pondered among biologists and writers of speculative fiction. Rolling and wheeled creatures have appeared in the legends of many cultures. While other human technologies, like wings and lenses, have common analogues in the natural world, and several species are able to roll, structures that propel by rotating relative to a fixed body are represented only by the corkscrew-like bacterial flagella. Macroscopic organisms have apparently never evolved wheels, and this is attributed to two main factors: limitations of evolutionary and developmental biology, and disadvantages of wheels, when compared with limbs, in many natural environments. Wheels, beyond the molecular scale, may not be within the reach of natural evolution, and may be infeasible to grow and maintain with biological processes. Compared with limbs, they are often less energy-efficient, less versatile, and less capable of traversing or avoiding obstacles. These environment-specific disadvantages of wheels also explain why some historical civilizations abandoned them. (Full article...)
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