The Eubacterial Flagellum in Context

Looking at the context of the bacterial flagellum gives us further insight into how the flagellum arose. Flagella are often thought of exclusively as swim-ming-motility organelles, yet they have a wide range of other functions. First and foremost is secretion. As we have seen, the flagellum secretes the subunits that form the hook and filament parts of the flagellum. But the flagellum also secretes nonflagellar proteins of importance to bacteria (Young et al. 1999).

The next function is adhesion. The flagellum attaches bacteria to surfaces; this is important for forming biofilms (Watnick et al. 2001), which allow cells to exploit resources on surfaces. Indeed, the ability of the flagellum to bind to cells is critical for pathogenic bacteria to attach to their host cells to attack them (Giron et al. 2002). Even nonmotile pathogenic bacteria express flagella that are crippled in terms of swimming (Andrade et al. 2002), presumably due to the role of flagella in adhesion and invasion of host cells. Importantly, flagella are central in organizing bacteria into a mass to produce a nonswimming form of motility called swarming (Kirov et al. 2002).

Dembski has said that the specification for the eubacterial flagellum is an outboard motor, but as we can see, the flagellum is, at the same time, a bilge pump and an anchor (to continue the nautical theme). If we view this organelle simply as an outboard motor, we have a distorted view of what it is and what it does.

When viewed as a swimming structure, the flagellum is IC. Remove the motor, and it stops functioning; remove the hook (universal joint), and it stops functioning; remove the filament, and it stops functioning (although in some bacteria removal of the filament results in weak motility). Viewing the flagellum as an outboard motor—and an IC motor at that—provides no insights into the origin or functioning of this structure.

But view it as a secretory structure, and it is not IC. Remove the filament, and it still works; remove the hook, and it still works; remove the motor, and it still works—not as well as with the motor, but it still works. But which, in Dembski's terms, is the original function? Secretion plays a crucial role in this organelle, and you can't make flagella without secretion, so secretion must be the original function.

This conclusion is backed up by the crucial role that secretion plays in other motility systems. Indeed, secretion is a common thread in all motility systems described so far. This is because one of the fundamental problems of a swimming system is how to get the structures that will be used as oars or propellers through the cell wall. Secretory systems, which are fundamental to the functioning of bacteria, have already solved this problem and would be needed to get the swimming structures across the cell wall. Therefore, it is understandable that evolution would build motility systems on top of existing secretory systems. Thus, Dembski's analysis is deeply flawed.

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