THREE years before his death in 1805, English philosopher William Paley proposed a now-famous thought experiment. Imagine discovering a watch on the heath: how would you explain its intricate arrangement of parts, its clear design for a purpose? Naturally, you’d conclude that it was built by a watchmaker, not blown together by chance. By analogy, Paley argued, the natural world is full of designed complexity which must therefore also have a creator: God.

Had Paley been in a position to know about it, he would no doubt have considered a remarkable little device called the bacterial flagellum to be an excellent example of designed complexity. With its intricate arrangement of interconnecting parts, the flagellum looks no less designed than a watch, and would surely have had Paley reaching for the existence of its “maker”.

Modern biology, of course, has no need for omniscient designers. Evolution – Richard Dawkins’s blind watchmaker – is all that is needed to explain the origin of complexity in nature. Even so, latter-day Paleys continue to search for evidence of design in the living world. The bacterial flagellum has become their cause célèbre – and a focal point in science’s ongoing struggle against unreason.

The bacterial flagellum is one of the most complex and elegant pieces of biological machinery known. It is the bacterial world’s outboard motor, rotating at high speeds to propel bacteria through their watery environments. It is made up of about 40 proteins that self-assemble into three basic modules – the basal body, hook and filament (see Diagram).

The flagellum is complex, but is it too complex to have evolved through natural selection? Until recently it was surprisingly hard …