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The bacterial flagellar motor: brilliant evolution or intelligent design?

The complexity of nature's most impressive swimmer leads some to mistakenly believe it was designed, but it is proof of evolution at work, writes Matt Baker.

In terms of speed and agility, flagella-powered bacteria would leave Olympic swimmers for dead. They swim hundreds of body lengths in a second, and can change direction in a fraction of that time.

The source of this incredible mobility is the microscopic equivalent of an outboard motor — the bacterial flagellar motor. At one millionth of the size of a grain of sand, this motor rotates up to five times faster than a Formula1 engine, spinning the whip-like flagella and driving the bacterium forward. Most remarkably, the motor builds itself by assembling its constituent parts. It's one of the pinnacles of evolution, and, from a nanotechnology standpoint, it routinely outperforms our latest man-made nanomachines.

It is perhaps not surprising then that such complexity and technology has been hijacked for use as proof, via intelligent design, of the existence of a creator.

Intelligent design is a theory advocated by the new-wave of creationists that are primarily located in the US. It holds that some aspects of life are so complex that they cannot have evolved through a series of steps via natural selection, and therefore must have been designed in one go.

A central tenet of this theory is the notion of 'irreducible complexity'. This asserts that some biological machines — like the flagellar motor — must be the product of design, because if you were to remove one or two components from the motor it would not function properly, or at all. The logic being, this motor was designed as a whole construction — it didn't evolve through a series of steps, so the individual parts of the motor would serve no purpose on their own.

So the creationist argument relies on us finding no evidence of individual parts of the motor having a role outside of bacterial flagella.

Luckily, individual components of the bacterial flagellar motor have indeed been found elsewhere. And they work. So the motor is 'reducible', and certainly not 'irreducibly complex'.

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The flagellar motor: complex, but 'reducible'

Proof of the flagellar motor's 'reducibility' — that it's component parts can function elsewhere — comes in the form of the injectisome; another fabulous molecular machine found in bacteria. This needle-like complex is used by disease-causing bacteria to punch holes in the host's target cells.

The protein machinery used to assemble the proteins that make up the punching needle is identical to that used to assemble the 'propeller' part of the flagellar motor — the filament and hook of the motor. In addition, nine core proteins of the flagellar motor share common ancestry with injectisome proteins — the genes that code for them are so similar they have clearly come from the same genetic ancestor.

In fact, the flagellar motor contains a wealth of other evidence pointing not to intelligent design, but to its evolutionary origins.

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Evidence for evolution, not creation

Bacteria swim in many different ways, and the motors that drive their swimming are widely varied, implying an adaptive response to an environment — a hallmark of evolution. So while the flagellar motor of freshwater Salmonella is powered by protons (hydrogen ions, H+), motors of other bacteria that live in salt water environments, like Vibrio alginolyticus are powered by sodium ions (Na+) from the salty environs.

There's also considerable variety in propeller shape across different bacterial species — propellers can be straight or curly, left- or right-handed, and more or less rigid. In fact, genetic sequencing of the proteins that make up the propellers has shown that there must be thousands of different bacterial flagellar systems.

In the aftermath of the first legal challenges to curriculum requirements to teach intelligent design, evolutionary biologists Mark Pallen and Nicholas Matzke wrote "either there were thousands or millions of individual creation events ... or one has to accept that the highly diverse contemporary flagellar systems have evolved from a common ancestor".

Recent work on flagellar motors in species other than bacteria, such as single-celled archaea, show they also swim by a rotary motor, but one that is completely unrelated to the bacterial motors. The archaeal motors sometimes use a completely different power source (ATP hydrolysis), and their propeller grows from the base, instead of from the tip. This indicates that convergent evolution has taken place: two completely separate evolutionary paths have converged towards rotary powered swimming.

While all this may seem relatively harmless, the intelligent design movement is well funded, slickly presented, and actively challenges educational curricula in many countries. It is a dangerously well-articulated distraction from the large body of evidence supporting evolutionary theory.

Scientists studying the flagellar motor can contribute by demonstrating that it is clearly not a smoking gun that proves intelligent design.

Typically, intelligent design proponents persevere despite this evidence. They simply adjust their goal posts by selecting other systems to act as poster boys for irreducible complexity. It is difficult to respond to these movable challenges. But as we learn more about the origins of these and other complex systems, we can at least reduce the number of available candidates used to prop up the theory of intelligent design.

And we most certainly will learn more about the flagellar motor — some of the most exciting work explaining exactly how it has arisen is yet to be done. It still has much to contribute, not only to our understanding of the wonders of natural technology, but also to teach us how we can adapt its natural technology to make bespoke bionanotechnology in the next era.

About the author: Dr Matt Baker was awarded a John Monash Scholarship to complete a DPhil in biological physics at Oxford University studying the effects of low temperature on the mechanism of the bacterial flagellar motor. He returned to Australia in 2013 and is currently based at the Victor Chang Cardiac Research Institute where he uses DNA nanostructures to modify bacterial flagellar motors to understand how they have arisen. Matt was one of RN's top 5 under 40 scientists in 2015.



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