Evolution of the flagellum

They said it couldn't be done. Now scientists are filling in the picture of how the flagellum – an "irreducibly complex" system – evolved.

The ongoing wars over the teaching of evolution (particularly in the US) have elevated into public consciousness an unlikely topic: molecular microbiology. Modern day creationists, rebranding themselves as ‘Intelligent Design theorists’, have made sweeping, not to say unfounded, claims about the limits on the power of evolution on the basis of a tiny nanomachine called the bacterial flagellum.

This complex structure, made of about 40 interacting proteins, is essentially an outboard motor that powers bacteria through their watery environment. At the heart of the flagellum is a rotary motor that drives a long, whip-like tail, which propels the bacterium as it spins round. It is a magnificent work of molecular engineering (see below).

For ID theorists, it is more than just awe-inspiring in its complexity and elegance. To them, it speaks of intelligent design. For tactical reasons the nature of this designer is often left unspecified. Yet the context of these claims makes it clear that a notion of a creator, of the kind found in Judaeo-Christian cosmologies, is lurking behind the scenes.

In this week’s New Scientist, I have a feature on what has been found, and what remains unclear, in flagellar research. Scientists do not claim to have wrapped up the story on flagellum evolution. But what is more interesting is the way recent scientific debates about the flagellum highlight the intellectual bankruptcy of ID theory. If you took the ID case seriously, you’d say “OK, the flagellum is irreducibly complex and could not have evolved – done.” You might then move onto the next difficult issue in evolutionary biology, and say the same.

The scientists I spoke with, by contrast, have a rather different epistemological approach. Yes, the evolution of complex molecular machines poses difficult questions, but that’s what makes them interesting and rewarding to study. And it’s not that evolutionists just want to club together to shout, “Look, the flagellum evolved – job done!”. They want to get some real explanatory purchase on the problem.

This concern with actually working out the details inevitably throws up different ideas, which other scientists then critically evaluate. Analyses are criticised, hypotheses scrutinised and conclusions questioned. This is the sign of healthy science in action; it leads to real insights and refined understanding. In short, the evolutionary approach is a genuinely testable theory, and a viable research programme. Falling back on ID, on the other hand, reveals an intellectual lack of nerve. Where evolutionary biologists face up to the mysteries the universe presents, and are prepared to put in the hard work required to crack them, IDists give up on trying to reach any sort of understanding whatsoever.