The Science of Discworld IV

evolved. Nature is an opportunist. A technological example of exaptation is the use of disc-recorded sound for music. Edison originally developed the phonograph for a more serious purpose, to record for posterity the last words of famous men and the historical speeches of politicians. He greatly deplored its use for frivolities like music, but accepted payments gracefully, nevertheless.
    Exaptation is one of the less obvious tricks that evolution has up its sleeve, and is often the solution to evolutionary puzzles, in which a particular function can occur only when several interrelated structures apparently have to appear simultaneously, but none of them can perform that function on its own. Although it’s tempting to deduce that such structures can’t evolve at all, they can if exaptations occur. Then the structures concerned initially perform different functions.
    A classic instance is the bacterial flagellum, a structure that proponents of intelligent design argue cannot possibly have evolved by any conceivable route. The flagellum allows some bacteria to move of their own volition. Its most important component is a tiny molecular motor, which causes the flagellum to rotate, much as the motor of a boat turns the propeller. The bacterial motor fn5 is made from a large number of different protein molecules. Until recently, evolutionary biologists could offer no convincing explanation of the origin of such a complex structure by natural selection.
    In 1978 Robert MacNab wrote: ‘One can only marvel at the intricacy, in a simple bacterium, of the total motor and sensory system … What advantage could derive … from a “preflagellum” [meaning asubset of its components], and yet what is the probability of “simultaneous” development?’ In 1996 Michael Behe, a biochemist and leading proponent of intelligent design, repeated MacNab’s worries in
Darwin’s Black Box
, together with several similar evolutionary puzzles. He concluded that while many, indeed most, features of living creatures have evolved, some cannot possibly have done so because they are irreducibly complex: if you remove any component, they cease to function.
    It’s a genuine puzzle, but before invoking some unspecified genie-of-the-lamp, without independent evidence that it exists, we ought to make sure that conventional evolutionary processes definitely can’t hack it. Intelligent design doesn’t just argue that some specific evolutionary route is wrong: it claims a proof that
in principle
no such route can exist. If you’re going to invoke a general principle of this kind to assert the existence of a supernatural being or a highly advanced cosmic designer, you need to close any loopholes in your logic. Otherwise your entire philosophy will be built on sand, whatever actually happened. The Book of Genesis could be true in every detail, but your supposed proof would still be nonsense if its logic were defective.
    In response to intelligent design, biochemists have taken a closer look at the proteins in the bacterial motor and the associated genes. The most prominent components of these motors are rings of proteins, which are very common in evolution. What use is a ring? It has a hole. Holes are amazingly useful to a bacterium or a cell, because they can function as pores or sockets. Pores let in molecules from the outside world, or expel molecules into the outside world. Different-sized pores deal with different-sized molecules. That’s something that natural selection can work on: a mutation in the DNA that codes for the protein can lead to one with a similar, but slightly different, shape or size. As soon as a pore does a useful job, evolution can find a pore that is better at doing that job, if there is one.
    Sockets allow bacteria or cells to attach new structures, either inside or outside the cell membrane. Many different molecules can fit into the same socket, and again, evolution has plenty of opportunities to work with. What began as a pore can become a socket if something happens to fit into it. When the two modules come together, their function may change. Exaptation demolishes irreducible complexity as an obstacle to evolution. You don’t even have to prove exactly how a given structure evolved, because irreducible complexity allegedly rules out not just the actual route, but
any conceivable one
.
    So let’s conceive.
    A number of biologists have attempted to deduce a plausible or likely evolutionary route to a bacterial