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Microbiologist Michael Behe coined the term “irreducible complexity” to describe a system in which every part is mandatory. Here is his definition:

By irreducibly complex I mean a single system composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning. (Michael J. Behe, Darwin’s Black Box [Touchstone, 1996], p. 39)

Let’s look at a popular example, the remarkable bacterial flagellum. Built of several dozen different protein types, this tiny motor with a whip-like appendage can propel a bacterium 60 cell lengths per second. Compare this to the cheetah, the fastest land animal, which sprints at 25 body lengths per second. (Here’s a video showing the structure of the flagellum.)

The irreducible complexity claim is this: imagine turning the clock of evolution back. Which protein was the last to be put in place? Pick any of them—the resulting one-protein-less motor wouldn’t work. So if one step back in time from a working flagellum is something useless, no matter which protein you remove, why would evolution have created this thing? Evolution doesn’t spend effort slowly building elaborate nonfunctioning appendages on the remote chance that with a few more mutations over 100,000 generations it might get lucky and create something useful. But Intelligent Design comes to the rescue by postulating a Designer that put everything together all at once.

A parallel example

We can topple this thinking by considering an arch. Which was the last stone to be put in place in an arch? If you try to turn the clock back by removing the central keystone, the arch falls. So that couldn’t have been last. But try removing any stone from the arch and the same thing happens. This makes the arch irreducibly complex, using this Intelligent Design thinking, with a Designer levitating the stones into place all at once as the only explanation.

But of course this is nonsense. If you imagine watching a movie of the building of an arch played backwards, the first change you’d see was not a stone removed but the last piece of scaffolding put into place. Then the remainder of the scaffolding to support the stones, then the stones removed one at a time, and then the scaffolding removed.

In the same way, the step that preceded the bacterial flagellum might have been the removal of an unnecessary piece of scaffolding.

What is “the” bacterial flagellum?

Another problem with the irreducible complexity idea is that it imagines a single, beautiful design. In fact, there are many varieties, just as you’d expect with a messy process like evolution. Look in the other domains of life (Archaea and our domain of Eukarya) and you find even more variety.


The bacterial flagellum might have evolved from the Type III secretory system, a needle-like structure used to detect and infect other cells. For example, the bacterium that causes bubonic plague uses this mechanism to inject toxins. Look to other domains, and other predecessors are possible. These plausible stepping stones show how the flagellum might have evolved.

For a layman like me, the bottom line is that this decades-old argument has had plenty of time to be evaluated, and it hasn’t convinced biologists. Evolution stands.

Science may well have unanswered questions regarding the origin of the flagellum, but “I don’t know” is no reason to invent a Designer. And you can be sure that once the origin of the bacterial flagellum is sufficiently well understood, this argument will be discarded like a used tissue and some other complex feature of biology (and there’s always something) will be seized upon by the Intelligent Design advocate as the wooden stake that will finally destroy the monster that is evolution.

If the past is any indication, our ID friend will have a very long wait.

The question I get asked by religious people all the time is,
without God, what’s to stop me from raping all I want?
And my answer is: I do rape all I want.
And the amount I want is zero.
And I do murder all I want,
and the amount I want is zero.
The fact that these people think
that if they didn’t have this person watching over them
that they would go on killing, raping rampages
is the most self-damning thing I can imagine.
— Penn Jillette

(This is an update of a post that originally appeared 1/28/12.)

Photo credit: adair broughton

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CROSS EXAMINED After graduating from MIT, Bob Seidensticker designed digital hardware, and he is a co-contributor to 14 software patents. For more than a decade, he has explored the debate between Christianity...