The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,83

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 motor, from DNA and other biochemical evidence. This turns out not to be especially difficult. Many details are still provisional, as is all science, but the story is now sufficiently complete to disprove the contention that the motor exhibits a type of complexity that rules out all evolutionary explanations. Agreed, that doesn’t prove that the current evolutionary explanation is correct. That must be confirmed, or denied, by further scientific investigations. But it’s quite different from asking whether, in principle, any such explanation can exist.

The most fully developed synthesis of these proposals, put together by Nicholas Matzke, starts with a general-purpose pore. This evolves into a pore with more specific functions. At this early stage, the structure is not a motor, but it already has a very useful, entirely different, function: it can transport molecules out of the cell. In fact, it is recognisable as a primitive version of so-called Type III Export Apparatus, which exists in modern bacteria, and DNA sequences support this. Further changes, in which the pore’s function is successively improved, or changed by exaptation, provide an entirely plausible evolutionary route to the bacterial motor, increasingly supported by DNA evidence.fn6

Yes, if you take away enough parts of the bacterial motor, then it might not be a very good motor any more. But evolution didn’t know it was supposed to be making a motor.

So ‘design’ isn’t what it is often thought to be, even for human technology, let alone biology. Each innovative step may be driven by human intentions, but what works, and what passes on to later technology, evolves. To some extent, cars evolved from horse-drawn carriages, and a ballpoint pen is the lineal descendant of a quill made from a feather. We can legitimately compare these developments to mammals evolving from a Devonian fish that came out of the water onto land, or to our little middle-ear bones being the lineal descendants of bony gill structures in that fish.

Evolution is not efficient. It throws an awful lot of things away. Innumerable land vertebrate species have gone extinct. Similarly, most human designs don’t work. From the enormous number on offer, only a few develop into sophisticated structure/function niches. We are all bound by tradition, as well as by functional constraints that require any new development to fulfil the same functions as its ancestor. There’s a classic example: Apollo rockets were moved to their launching-pads on rails that were much too close together for stability, because the gauge of America’s railways came from mine railways that were two horses wide. So the Moon project was jeopardised by horse’s asses.

To be specific, let’s think about better mousetraps. Mousetrap evolution is a process, not just a succession of models; it branches into the future. The pattern that has a metal bar coming down and (one hopes) breaking the mouse’s neck, has expanded into dozens of different models, some computer-controlled. Those that