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That’s simple-minded nonsense.’ So instead, they ask what use half a bacterial flagellum is, and thereby repeat the identical error in a different context.
    We owe this example to Michael Behe, a biochemist who was baffled by the complexity of bacterial flagella. These are the ‘tails’ that bacteria use to move around, tiny ‘screws’ like a ship’s propeller, driven by a rotary molecular motor. Some forty proteins are involved in making such a motor, and if you miss any of them out, it won’t work. In his 1996 Darwin’s Black Box , Behe claimed that the only possible way to make a flagellum was to encode the whole structure, in advance, in bacterial DNA. This code could not have evolved from anything simpler, because the flagellum is ‘irreducibly complex’.An organ or biochemical system is said to be irreducibly complex if removing any of its parts causes it to fail. Behe deduced that no irreducibly complex system can evolve. The example of the bacterial flagellum quickly became a cornerstone of the intelligent design movement, and Behe’s principle of irreducible complexity was promoted as an unavoidable barrier to the evolution of complex structures and functions.
    There are several excellent books that debate intelligent design: we’ve mentioned two earlier in a footnote. It’s fair to say that the antis are winning the debate hands down – even in books edited by the pros, such as Debating Design . Perhaps the biggest problem for the pros is that Behe’s fundamental concept of ‘irreducible complexity’ has fatal flaws. With his definition, the deduction that an irreducibly complex system cannot evolve is valid only if evolution always consists of adding new parts. If that were the case, then the logic is clear. Suppose we have an irreducibly complex system, and suppose that there is an evolutionary route leading to it. Focus on the final step, where the last part is added. Then whatever came before must have been a failure, so it couldn’t have existed. This is absurd: end of story.
    However, evolution need not merely add identifiable components, like a factory-worker assembling a machine. It can also remove them – like a builder using scaffolding and then taking it down once it’s done its job. Or the entire structure can evolve in parallel. Either possibility allows an irreducibly complex system to evolve, because the next to last step no longer has to start from a system that lacks that final, vital piece. Instead, it can start from a system with an extra piece, and remove it. Or add two vital pieces simultaneously. Nothing in Behe’s definition of irreducible complexity prohibits either of these.
    Moreover, ‘fail’ is a slippery concept: a watch that lacks hands is a failure at telling the time, but you can still use it to detonate a time-bomb, or hang it on a string to make a plumb-line. Organs and biochemical systems often change their functions as they evolve, as we’ve just seen in the context of the eye. No satisfactory definitionof ‘irreducible complexity’ – one that really does constitute a barrier to evolution – has yet been suggested.
    According to Kenneth Miller in Debating Design : ‘the great irony of the flagellum’s increasing acceptance as an icon of the anti-evolutionist movement is the fact that research had demolished its status as an example of irreducible complexity almost at the very moment it was first proclaimed’. Removing parts from the flagellum do not cause it to ‘fail’. The base of the bacterial motor is remarkably similar to a system that bacteria use to attack other bacteria, the ‘type III secretory system’. So here we have the basis of an entirely sensible and plausible evolutionary route to the flagellum, in which protein components do get added on. When you remove them again, you don’t get a working flagellum – but you do get a working secretory system. The bacterial method of propulsion may well have evolved from an attack