Bacterial flagella and eukaryotic cilia are irreducibly complex,
Since nonfunctional intermediates cannot be preserved by natural
selection, these systems can only be explained by intelligent design.
Behe, Michael J. 1996. Darwin's Black Box, New York: The Free Press, pp.
- This is an example of argument from incredulity,
irreducible complexity can evolve naturally.
Many of the
proteins in the bacterial flagellum or eukaryotic cilium are similar to
each other or to proteins for other functions. Their origins can
easily be explained by a series of gene duplication events followed by
modification and/or co-option, proceeding gradually through
intermediate systems different from and simpler than the final
One plausible path for the evolution of flagella goes through the
following basic stages (keep in mind that this is a summary, and
that each major co-option event would be followed by long periods of
gradual optimization of function):
- A passive, nonspecific pore evolves into a more specific passive
pore by addition of gating protein(s). Passive transport
converts to active transport by addition of an ATPase that couples
ATP hydrolysis to improved export capability. This complex forms
a primitive type-III export system.
- The type-III export system is converted to a type-III secretion
system (T3SS) by addition of outer membrane pore proteins (secretin
and secretin chaperone) from the type-II secretion system. These
eventually form the P- and L-rings, respectively, of modern
flagella. The modern type-III secretory system forms a structure
strikingly similar to the rod and ring structure of the flagellum
(Hueck 1998; Blocker et al. 2003).
- The T3SS secretes several proteins, one of which is an adhesin (a
protein that sticks the cell to other cells or to a substrate).
Polymerization of this adhesin forms a primitive pilus, an
extension that gives the cell improved adhesive capability. After
the evolution of the T3SS pilus, the pilus diversifies for various
more specialized tasks by duplication and subfunctionalization of
the pilus proteins (pilins).
- An ion pump complex with another function in the cell fortuitously
becomes associated with the base of the secretion system structure,
converting the pilus into a primitive protoflagellum. The initial
function of the protoflagellum is improved dispersal. Homologs of
the motor proteins MotA and MotB are known to function in diverse
prokaryotes independent of the flagellum.
- The binding of a signal transduction protein to the base of the
secretion system regulates the speed of rotation depending on the
metabolic health of the cell. This imposes a drift toward
favorable regions and away from nutrient-poor regions, such as
those found in overcrowded habitats. This is the beginning of
- Numerous improvements follow the origin of the crudely functioning
flagellum. Notably, many of the different axial proteins (rod,
hook, linkers, filament, caps) originate by duplication and
subfunctionalization of pilins or the primitive flagellar axial
structure. These proteins end up forming the axial protein family.
The eukaryotic cilium (also called the eukaryotic flagellum or
undulipodium) is fundamentally different from the bacterial flagellum.
It probably originated as an outgrowth of the mitotic spindle in a
primitive eukaryote (both structures make use of sliding microtubules
and dyneins). Cavalier-Smith (1987; 2002) has discussed the origin of
these systems on several occasions.
- The bacterial flagellum is not even irreducible. Some bacterial
flagella function without the L- and P-rings. In experiments with
various bacteria, some components (e.g. FliH, FliD (cap), and the
muramidase domain of FlgJ) have been found helpful but not absolutely
essential (Matzke 2003). One third of the 497 amino acids of flagellin
have been cut out without harming its function (Kuwajima 1988).
Furthermore, many bacteria have additional proteins that are required
for their own flagella but that are not required in the "standard"
well-studied flagellum found in E. coli. Different bacteria have
different numbers of flagellar proteins (in Helicobacter pylori, for
example, only thirty-three proteins are necessary to produce a working
flagellum), so Behe's favorite example of irreducibility seems actually
to exhibit quite a bit of variability in terms of numbers of required
parts (Ussery 1999).
Eukaryotic cilia are made by more than 200 distinct proteins, but even
here irreducibility is illusive. Behe (1996) implied and Denton
(1986, 108) claimed explicitly that the common 9+2 tubulin
structure of cilia could not be substantially simplified. Yet
functional 3+0 cilia, lacking many microtubules as well as some of the
dynein linkers, are known to exist (Miller 2003, 2004).
- Eubacterial flagella, archebacterial flagella, and cilia use entirely
different designs for the same function. That is to be expected if
they evolved separately, but it makes no sense if they were the work of
the same designer.
Matzke, N. J. 2003. Evolution in (brownian) space:
a model for the origin of the bacterial flagellum.
(see also 'Background to "Evolution in (Brownian) space"',
Dunkelberg, Pete. 2003. Irreducible complexity demystified
Musgrave, Ian. 2000. Evolution of the bacterial flagella.
- Blocker, Ariel, Kaoru Komoriya, and Shin-Ichi Aizawa. 2003. Type III
secretion systems and bacterial flagella: Insights into their function
from structural similarities. Proceedings of the National Academy of
Science USA 100(6): 3027-3030.
- Cavalier-Smith, T. 1987. The origin of eukaryote and
archaebacterial cells. Annals of the New York Academy
of Sciences 503: 17-54.
- Cavalier-Smith, T. 2002. The phagotrophic origin of eukaryotes and
phylogenetic classification of Protozoa. International Journal of
Systematic and Evolutionary Microbiology 52: 297-354.
- Denton, M. 1986. Evolution: A Theory in Crisis.
Bethesda, MD: Adler & Adler.
- Hueck, C. J. 1998. Type III protein secretion systems in bacterial
pathogens of animals and plants. Microbiology and Molecular Biology
Reviews 62: 379-433.
- Kuwajima, G. 1988. Construction of a minimum-size functional
flagellin of Escherichia coli. Journal of Bacteriology 170:
- Matzke, N. J. 2003. (see above)
- Miller, K. 2003. Answering the biochemical argument from design.
in: Manson, N. (Ed.), God and design: the teleological argument and
modern science, Routledge, London, pp. 292-307.
- Miller, K. 2004. The flagellum unspun. In Debating Design: from
Darwin to DNA, 81-97, eds. Dembski, W., and M. Ruse, New York:
Cambridge University Press.
- Ussery, D. 1999. (see below)
Ussery, David. 1999. A biochemist's response to "The biochemical
challenge to evolution". Bios 70: 40-45.
created 2001-2-17, modified 2003-12-15