Now, let us sit back, review the workings of the cilium, and consider what it implies. Cilia are composed of at least a half dozen proteins: alpha-tubulin, beta-tubulin, dynein, nexin, spoke protein, and a central bridge protein. These combine to perform one task, ciliary motion, and all of these proteins must be present for the cilium to function. If the tubulins are absent, then there are no filaments to slide; if the dynein is missing, then the cilium re- mains rigid and motionless; if nexin or the other connecting proteins are missing, then the axoneme falls apart when the filaments slide.

What we see in the cilium, then, is not just profound complexity, but it is also irreducible complexity on the molecular scale. Recall that by “irre- ducible complexity” we mean an apparatus that requires several distinct components for the whole to work. My mousetrap must have a base, ham- mer, spring, catch, and holding bar, all working together, in order to func- tion. Similarly, the cilium, as it is constituted, must have the sliding filaments, connecting proteins, and motor proteins for function to occur. In the absence of any one of those components, the apparatus is useless.

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The components of cilia are single molecules. This means that there are no more black boxes to invoke; the complexity of the cilium is final, funda- mental. And just as scientists, when they began to learn the complexities of the cell, realized how silly it was to think that life arose spontaneously in a

Figure 20.2 Schematic drawing of part of a cilium. The power stroke of the motor protein dynein, attached to one microtubule, against subfiber B of a neighboring microtubule causes the fibers to slide past each other. The flexible linker protein, nexin, converts the sliding motion to a bending motion.