Evolution of flagella
The evolution of flagella is of great interest to biologists because the three known varieties of flagella – (eukaryotic, bacterial, and archaeal) each represent a sophisticated cellular structure that requires the interaction of many different systems. Eukaryotic flagellum There are two competing groups of models for the evolutionary origin of the eukaryotic flagellum (referred to as cilium below to distinguish it from its bacterial counterpart). Recent studies on the microtubule organizing center suggest that the most recent ancestor of all eukaryotes already had a complex flagellar apparatus. Endogenous, autogenous and direct filiation models These models argue that cilia developed from pre-existing components of the eukaryotic cytoskeleton (which has tubulin and dynein also used for other functions) as an extension of the mitotic spindle apparatus. The connection can still be seen, first in the various early-branching single-celled eukaryotes that have a microtubule basal body, where microtubules on one end form a spindle-like cone around the nucleus, while microtubules on the other end point away from the cell and form the cilium. A further connection is that the centriole, involved in the formation of the mitotic spindle in many (but not all) eukaryotes, is homologous to the cilium, and in many cases