The Bacterial Flagellum
Molecular Machines Museum
bacterial flagellum is an example of what Michael Behe describes as an
irreducibly complex system. In his book, Darwin's Black Box, he explains
that such irreducibly complex systems could not have arisen by a gradual
step-by-step Darwinian process.
Because the bacterial flagellum is necessarily
composed of at least three parts -- a paddle,a rotor, and a motor -- it
is irreducibly complex. Gradual evolution of the flagellum, like the cilium,
therefore faces mammoth hurdles. (p.72)
summarizes the structure of the bacterial flagellum in these terms:
boast a marvelous swimming device, the flagellum, which has no counterpart
in more complex cells. In 1973 it was discovered that some bacteria
swim by rotating their flagella. So the bacterial flagellum acts as
a rotary propellor -- in contrast to the cilium, which acts more like
of a flagellum is quite different from that of a cilium. The flagellum
is a long, hairlike filament embedded in the cell membrane. The external
filament consists of a single type of protein, called "flagellin." The
flagellin filament is the paddle surface that contacts the the liquid
during swimming. At the end of the flagellin filament near the surface
of the cell, there is a bulge in the thickness of the flagellum. It
is here that the filament attaches to the rotor drive. The attachment
material is comprised of something called "hook protein." The filament
of a bacterial flagellum, unlike a cilium, contains no motor protein;
if it is broken off, the filament just floats stiffly in the water.
Therefore the motor that rotates the filament-propellor must be located
somewhere else. Experiments have demonstrated that it is located at
the base of the flagellum, where electron microscopy shows several ring
structures occur. The rotary nature of the flagellum has clear, unavoidable
consequences ... (pp. 70-72)
consequences Behe refers to are inferred by the nature of its irreducibly
complex components, the discovery of which undermines a Darwinian explanation
of origins. Behe concludes:
In summary, as
biochemists have begun to examine apparently simple structures like
cilia and flagella, they have discovered staggering complexity, with
dozens or even hundreds of precisely tailored parts. It is very likely
that many of the parts we have not considered here are required for
any cilium to function in a cell. As the number of equired parts increases,
the difficulty of gradually putting the system together skyrockets,
and the likelihood of indirect scenarios plummets. Darwin looks more
and more forlorn. New research on the roles of the auxiliary proteins
cannot simplify the irreducibly complex syetem The intransigence of
the problem cannot be alleviated; it will only get worse. Darwinian
theory has given no explanation for the cilium or flagellum. The overwhelming
complexity of the swimming systems push us to think it may never give
an explanation. (p. 73)
concludes that such irreducibly complex systems were ultimately the result
of intelligent design. (It should be pointed out that Behe has no objections
to the concept of universal common ancestry. His objections to evolution
are limited to the rejection of the neo-Darwinian mechanism as a sufficient
explanation for the origin of all biological systems.)
Copyright © 1998 Michael J. Behe. All rights reserved.
International copyright secured.
File Date: 6.10.98
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