Passage Summary: Bacteria use a tail-like motor to swim, but it only works if it's complete. Because of this, the argument claims that an ancestor with only a partial motor wouldn't have been helped by it at all.

Conclusion: An early ancestor of bacteria possessing only a few parts of a flagellum would not have gained any survival benefit from those parts.

Reasoning: The flagellum is a complex structure that cannot function as a means of propulsion unless all of its many parts are present and working together.

Analysis: This argument relies on a very narrow view of biological utility. It assumes that the *only* way these specific parts could help a bacterium survive is by acting as a flagellum for swimming. To be a necessary assumption, we need to find a statement that rules out other possibilities—specifically, that those few parts couldn't have served some other beneficial purpose. If those parts helped the ancestor in a different way (like sensing the environment), the conclusion that they provided 'no survival advantage' would fall apart.