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Word of the Day: Irreducible Complexity

A novel about Christian apologetics and atheismMicrobiologist 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.1

Let’s look at a popular example, the remarkable bacterial flagellum. Built of several dozen different proteins, 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 good agenda-less 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? Remove any protein from the flagellum and it doesn’t function. So if one step back in time from the working flagellum was 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.

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 one 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.

There is much more to say about why the idea of irreducible complexity has not won over the science of biology, including attacks on how good an example the flagellum is of irreducible complexity, but that is a tangent for this post. For more on this topic, check out the links below.

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.

1 Michael J. Behe, Darwin’s Black Box (Touchstone, 1996), p. 39.

Photo credit: harrymoon

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