The flagellum, which bacteria use to swim, requires many parts before it can propel a bacterium at all. Therefore, an...

SamA on October 3, 2019

Hello @farnoushsalimian,

Premise: The flagellum, which bacteria use to swim, requires many parts before it can propel a bacterium.

Conclusion: An ancestor of bacteria had only a few of these parts would gain no survival advantage from them.

Before looking at the answer choices, I see a problem with this argument. Just because the ancestor couldn't use the flagellum to swim, does that mean that it provided no survival advantage whatsoever? What if the flagellum has a secondary purpose as well? At this point, I don't believe that the conclusion can be properly drawn. The necessary assumption that we are looking for will be something that fixes the argument. Let's go through the answer choices.

A. The argument doesn't require that the few flagellum parts are a disadvantage. We only need them to provide no survival advantage. This assumption takes it too far.

B. This fixes the hole in the argument that I described above, which makes it the correct answer. It eliminates the possibility of the flagellum's secondary purpose. The ability to swim is the only advantage it could have provided. It didn't, therefore it provided no advantage.

C. The passage only says "many" parts are required. We don't know if all are required, but the argument doesn't depend on that possibility.

D. If we assume this, then it defeats the argument altogether. The conclusion is about an ancestor that had only a few flagellum parts.

E. Maybe some of the bacteria's ancestors had another way to swim? This still does not eliminate a secondary advantage of the flagellum, so our argument does not require it.