What's the Most Likely Explanation?

Here's a five minute video showing just a few of the basic details of how a bacterial flagellum works and how it is constructed. Keep in mind that what you're watching is an animation that barely scratches the surface of the complexity of these microscopic organelles. Also keep in mind that the basic philosophical/scientific controversy today is over whether such structures - and the myriad of other structures in the cell - developed as a consequence of blind, impersonal forces and processes or, alternatively, whether they are the product of an intelligent, purposeful agent:

Suppose you were completely neutral on the question of what could have produced this. Suppose that you had no metaphysical inclinations either for or against theism or naturalism. Which of the two would you think to be the most likely explanation for such a structure? Would you think that such functional complexity is more or less likely to have arisen by chance mutations and natural selection or as a result of intelligent engineering by a mind?