For example, rotary nanomachines used by cells for propulsion demonstrate exaptation of rotary mechanisms ( Beeby et al., 2020). Nevertheless, it appears that at the molecular scale new functions also result from exaptation of existing underlying mechanisms instead of radical genesis of fully formed new machines. How such processes operate at the molecular scale remains less well understood. Lightweight feathered wings, for example, existed in nascent forms before exaptation for flight ( Gould and Vrba, 1982). Gould and Vrba coined the phrase exaptation to define this process, distinguishing it from adaptation, in which a pre-existing feature evolves to become more beneficial in its current role. In many cases, evolution of new functions in such macroscopic structures involved co-option of a pre-existing feature for the new role. How do molecular machines evolve new roles? Macroscopic structures such as wings or eyes have evolved from simpler limbs or photoreceptors. How Does Rotary Motion Evolve in Molecular Machines? Here we review what is known about how the archaellum got its rotation, what clues exist, and what more is needed to address this question. Nevertheless, determining exactly how the archaellum got its rotation remains frustratingly elusive. Satisfyingly, the archaellum is one of many members of the large type IV filament superfamily, which includes pili, secretion systems, and adhesins, relationships that promise clues as to how the rotating archaellum evolved from a non-rotary ancestor. It features a long helical propeller attached to a cell envelope-embedded rotary motor. The archaellum, an archaeal analog of the bacterial flagellum, is one of the simplest rotary motors. Particularly captivating is the evolution of rotation in molecular machines, as it evokes familiar machines that we have made ourselves. How new functions evolve fascinates many evolutionary biologists.
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