the god of the tiny gaps
Darwin's Black Box: The Biochemical Challenge to Evolution
by Michael J. Behe, Free Press, $25
Reviewed by Andrew Pomiankowski
Darwin knew all about the problem of "organs of extreme perfection and complication". He devoted a brilliant chapter to them in On the Origin of Species. Gradual evolution of a complex organ such as the human eye seemed to be impossible in the 19th century. But Darwin showed that many modern animals have eyes that are much simpler in structure. He gave a plausible account of how complex eyes gradually evolved from simple light-sensitive pigmented cells, through a series of functional intermediates. His explanation of complexity is one of his great triumphs.
Limited by the science of his day, Darwin could not delve much beneath the anatomical structures visible under a light microscope. Since the 1950s a deeper understanding of the molecular basis of life has been possible fuelled by increasing knowledge of the workings of DNA, molecular biology and better instrumentation. Is anything different within the Lilliputian workings of the cell? Behe's claim is that biologists have been quietly overlooking the true difficulties posed by our newer understanding of the molecular basis of life.
Take, for example, the structure of the cilium. A cilium is a hair-like whip used by primitive unicellular organisms for swimming. In more advanced organisms, cilia have a variety of uses, from causing liquids to move over stationary cells to helping propel sperm. Electron microscopy has uncovered the common structure of the cilium, a ring of microtubules that run along its length. Each microtubule is made up of tubulin molecules, stacked into rods which are joined together by dyenin motor proteins and nexin linker proteins. These three molecules work together to cause the paddle-like motion of the cilium.
Isolated cilia still beat when provided with energy. If dyenin is removed, however, the cilium becomes stiff and inert; it cannot move without its motor. But if you remove the nexin protein, something rather unexpected happens. The rod-like tubulin stacks slide past each other and the whole cilium unravels. Nexin linkers are needed to convert the sliding force of the tubulin stacks into the bending motion of the cilium.
Behe describes the cilium as irreducibly complex: each of its three molecular parts is essential to its function, and if any one is removed, it fails. This leads to Behe's bigger claim that irreducible complexity appears throughout the biochemistry of the cell--in blood clotting, the movement of molecules across cell membranes, the immune system, and in fact almost everywhere biochemists have looked in detail. What at first glance look like simple systems turn out on closer inspection to be highly interdependent complexes that lose all function once one part of the whole is removed.
If Behe is right, a direct gradual evolutionary route--such as that of the complex eye--seems unlikely. The final function cannot have dictated the evolution of the parts. In this case, the standard Darwinian explanation is to show that the intermediate stages had other functions. For example, our jaw was once a leg. And this looks like an entirely plausible explanation for cilia. Microtubules are used in a variety of structural roles in the cell, and motor proteins use microtubules to move themselves and other molecules around the cell. It seems a small step to link pre-existing microtubules and motor proteins to produce a bending rod-like structure and a proto-cilium. But Behe despairs of such guesses. He dismisses them as the mere ramblings of a fertile imagination. What, he asks, were the critical events and how exactly did they happen?
At this point I find myself partly agreeing with Behe. If biochemistry is to fall within evolutionary biology, we need detailed case histories. Behe's trawl of the scientific literature on cilia found only three major attempts to understand their evolution. Each is interesting, covering issues such as the possible origin of cilia as independent symbiotic bacteria, the use of cilia in phototaxis and mechanical difficulties in the evolution of cilia.
Evolutionary thinking is pushing at the frontiers of knowledge. But the general lack of interest in biochemical evolution that Behe reveals is typical. Only in the area of DNA and protein sequence analysis has evolution been taken seriously. Why haven't biochemists tried harder to understand how specific complex systems evolved?
I think there are two reasons that explain the dearth in understanding. The first is the sheer difficulty of the problem. Consider cilia, for instance. Cilia are found throughout the unicellular and multicellular eukaryotes--cells with nuclei containing genetic material carried on chromosomes. So their origin must have been very early, around the time eukaryotes became distinct from the bacteria, whose genetic material is distributed between plasmids and nuclei. Deducing the causes of evolutionary events so long ago is never going to be easy. Cilia might be related to the bacterial flagellum, a similar whip-like propulsion devise. But flagella have a very different construction and rotate to help bacteria to swim, rather than show the paddling motion of cilia.
And the comparative biochemistry of cilia is also almost nonexistent. Nothing is known about variation in cilia design across different groups of organisms, although it undoubtedly exists. Nobody has attempted to relate this variation to differences in function or to reveal hints of the proto-cilium. Similar problems bedevil many attempts to uncover biochemical evolution.
But there is a bigger problem. Most biochemists have only a meagre understanding of, or interest in, evolution. As Behe points out, for the thousand- plus scholarly articles on the biochemistry of cilia, he could find only a handful that seriously addressed evolution. This indifference is universal. Pick up any biochemistry textbook, and you will find perhaps two or three references to evolution. Turn to one of these and you will be lucky to find anything better than "evolution selects the fittest molecules for their biological function".
Behe is good at exposing the paucity of evolutionary thought in the field of biochemistry. But in Darwin's Black Box, he reveals that he is also part of the problem, falling back on the old, limp idea of "design". He takes irreducible complexity as a statement of fact, rather than an admission of ignorance, claiming that the "purposeful arrangement" of biochemical parts must be the result of an intelligent designer. So what we have here is just the latest, and no doubt not the last, attempt to put God back into nature. But it is an old blind alley. To understand molecular design, we need a biochemical account of evolution.
So I think that Darwin's Black Box is a missed opportunity. You can read it to tell you what is wrong with biochemistry. Behe is also very good at making biochemistry easy to understand. But don't be fooled by his claim that molecular systems are irreducibly complex, or that a supernatural designer is needed. Biochemistry is yet another area of biology still awaiting its Darwinian revolution.
ANDREW POMIANKOWSKI is a Royal Society research fellow at the department of biology, University College London.
© Copyright New Scientist, IPC Magazines Limited 1996