Two really interesting and related papers from about a year ago showing existence of a toolkit of proteins (they call it CORE, but it’s previously known as set of proteins making up the T3SS export apparatus) that is shared by both flagella and the deliciously named injectisome. It’s deeply conserved but just as interesting is the fact that it also supports nanotube formation, which implies ancient, broad, conserved functions in molecular traffic between bacterial cells.
Does this “explain” the flagellum? That question is incoherent, which is to be expected from a “question” raised by the ID movement. What this means, IMO, is that the flagellum (more accurately, flagella) is one specialized reuse of a core machinery that can be repurposed extensively and effectively. That is the theme of the Trends in Genetics review posted above, and of Nick Matzke’s writing all those years ago.
Pathogenic E. coli Extracts Nutrients from Infected Host Cells Utilizing Injectisome Components