Several months ago, I published an article here exploring the breath-taking complexity and elegance of bacterial flagellar assembly.

Since that time, I have been directed to the following stunning animation of flagellar assembly. Though it barely scratches the surface of this intricate process, this video will give you a taste of just how fantastically sophisticated flagellar biosynthesis and function really are.

One of my favorite resources on flagellar assembly is the book Pili and Flagella: Current Research and Future Trends, edited by Ken Jarrell. The most interesting chapter in that book was contributed by Shin-Ichi Aizawa and is entitled, “What is Essential for Flagellar Assembly?”

On page 91, Aizawa provides one of the most revealing passages in the book:

Since the flagellum is so well designed and beautifully constructed by an ordered assembly pathway, even I, who am not a creationist, get an awe-inspiring feeling from its “divine” beauty (Pallen and Matzke, 2006). However, if the flagellum has evolved from a primitive form, where are the remnants of its ancestor? Why don’t we see any intermediate or simpler forms of flagella than what they are today? How was it possible that the flagella have evolved without leaving traces in history?

Yet he then goes on to say just a few pages (95-96) later,

All flagellar genes seem to have evolved under strong selective pressure for efficiency of assembly. After all, the first one who can build her flagellum and swim to the food passes on her DNA. Flagella are beautiful, ion-powered nanomachines that run with so-called 100% efficiency and their assembly pathway has been streamlined by evolution to minimize the time of the assembly process. The flagellum has acquired its beauty by evolving such a sophisticated, efficient machine.

Pretty much everyone is agreed that such marvels of engineering give the overwhelming appearance of having been designed. Indeed, computational cell biologist Kathryn Appelgate (an ardent Darwinist, whose writing often appears on the Biologos website) wrote an essay last year entitled “Bacterial Flagellum: Appearances Can Be Deceiving.” She noted:

The resemblance is so striking, we find it difficult to resist extending the analogy to how the flagellum originated. We know that all outboard motors are designed by intelligent engineers; the parts are carefully crafted to work together for an intended purpose. The bacterial flagellum also has many well-matched components. Together they perform the same job as the outboard motor — swimming. Since the flagellum wasn’t designed by human engineers, it seems only reasonable to infer that it was designed by Someone Else.

Of course Applegate goes on to claim (unconvincingly) that the flagellum was not in fact intelligently designed. But the legitimate question has been raised as to how such a system arose: Is this system actually designed? Or is the design merely apparent? Well, since we all — at least most of us — are in agreement that the flagellum looks like a designed system, it would be unwise to rule out that proposition a priori. By closing off one possible answer, we potentially limit ourselves to a set of false choices.

Since all of the purported naturalistic “explanations” of this system fail, and since we have positive reason (i.e., our uniform and repeated experience of cause and effect) to suspect that this system might really be the product of intelligent causality, it seems to stand to reason that the design inference be regarded as a scientifically plausible point of view.

The flagellum is a system that has been regarded by many as irreducibly complex — that is to say, it requires a minimal cohort of its parts in order to retain functional utility. For example, without a protein called FliK, both the ability to switch and export filament and the hook-length control are completely lost. Likewise, in the absence of the cap protein FliD, the flagellin monomers are lost from the cell.

Since there is essentially no evidence that a system as complex and sophisticated as flagellar assembly could have arisen by virtue of a mutation/selection mechanism, how can we be so sure that it did evolve in that fashion? If the flagellum gives the overwhelming appearance of having been designed by an intelligent agent, are we not justified in inferring design until a more compelling candidate explanation (which better explains the data) is offered?