Editing Evolution of flagella (section)

==Archaeal flagellum==
{{Further|Archaellum}}
The recently elucidated archaeal flagellum, or ''[[archaellum]]'', is [[Analogy (biology)|analogous]]—but not [[Sequence homology|homologous]]—to the bacterial one. In addition to no sequence similarity being detected between the genes of the two systems, the archaeal flagellum appears to grow at the base rather than the tip, and is about 15 [[nanometer]]s (nm) in diameter rather than 20.

Sequence comparison indicates that the archaeal flagellum is homologous to bacterial [[Type IV pili]], filamentous structures outside the cell.<ref>{{Cite journal|doi=10.1139/m94-011 |vauthors=Faguy DM, Jarrell KF, Kuzio J, Kalmokoff ML |title=Molecular analysis of archael flagellins: similarity to the type IV pilin-transport superfamily widespread in bacteria |journal=Canadian Journal of Microbiology |volume=40 |issue=1 |pages=67–71 |date=January 1994 |pmid=7908603}}</ref> Pilus retraction provides enables a different form of bacterial motility called "twitching" or "social gliding" which allows bacterial cells to crawl along a surface, They are assembled through the [[Type II secretion system]]. They can also promote swimming, but no species of bacteria is known to use its Type IV pili for both swimming and crawling.