Darwin’s Breakdown
Irreducible Complexity & Design at the Foundation of Life
by Michael Behe
The topic of evolution is both fascinating and vexing. Fascinating, because it involves questions of who we are, how we got here, and how we relate to the world around us. Vexing, because it provokes arguments and political battles. The battles are all the more frustrating when fought by polarized factions with preset agendas such as, on the one hand, defending a literal interpretation of the Bible, or, on the other, keeping any hint of transcendence out of the public schools.
Many people, discouraged by the bickering, are tempted to ignore the topic. For a Christian, this is a mistake. In his letter to the Pontifical Academy of Sciences several years ago ( L’Osservatore Romano, Oct. 30, 1996), Pope John Paul II wrote that evolution is “an essential subject which deeply interests the church, since revelation, for its part, contains teaching concerning the nature and origins of man.” An informed Christian, engaged with the world, should have a basic understanding of the facets of evolution that are of particular importance to the faith.
In his letter the pope made several statements that seemed on the surface to conflict. He acknowledged that evolution is “more than a hypothesis” and that “this theory has been progressively accepted by researchers, following a series of discoveries in various fields of knowledge.” He also noted, however, that “rather than the theory of evolution, we should speak of several theories of evolution.” In part the several theories have to do with “the different explanations advanced for the mechanism of evolution.” How can evolution be so well-supported that it is more than a hypothesis and yet there still be uncertainty about its mechanism?
The difficulty arises because the word evolution can be used in different senses, and equivocation can easily confuse people. In one sense evolution just means common descent—that living creatures are all related to a common ancestor. It is in this sense, I think, that the pope meant that evolution is well-supported. Common descent is a possible explanation for the similarities among species, and modern science has shown many similarities, especially at the molecular level, that were unknown to earlier scientists. Explaining similarities, however, is the easy part. You just have to say that some features remained the same. Explaining the many profound differences between organisms is the hard part.
In another sense evolution is sometimes used to mean Darwin’s particular theory. Darwin proposed the theory of natural selection to account for the differences between organisms. Darwin saw that there was variation in all species, and he reasoned that animals whose random variation gave them an edge in the struggle to survive would tend to leave more offspring than others. If the variations were inherited, then the characteristics of the species would change over time. In other words, Darwin proposed a mechanism to drive evolution. Although it clearly can explain relatively small changes, the sufficiency of Darwin’s mechanism to account for larger, more complex changes in organisms remains in question. Pope John Paul II apparently had in mind the doubts about Darwin’s mechanism when he referred to “several theories of evolution.”
The Professor and the Cardinal
Why do such arcana matter? To many Christians, the problem with Darwin’s theory is in the single word “random.” Ever since the theory was first proposed, persons antagonistic to the Church, including some prominent scientists, have aggressively asserted that the randomness is not merely epistemic; it is ontological. In other words, they claim that science sees no purpose in living things because there is no purpose, and therefore there is no God. For example, the Oxford zoologist Richard Dawkins has remarked (Science 1997, vol. 277, p. 892) that “The universe we observe has precisely the properties we should expect if there is at bottom no design, no purpose, no evil and no good, nothing but pointless indifference.” Clearly such assertions go well beyond the domain of science. Nonetheless, because science has considerable authority in our culture, scientists are accorded great respect even when they are dispensing bad philosophy.
Although in his letter the pope did not address the randomness assumed by Darwin’s mechanism, his close advisor, Cardinal Joseph Ratzinger, did discuss it in a little book entitled In the Beginning: A Catholic Understanding of the Story of Creation and the Fall (Eerdmans, 1986). There he wrote:
Let us go directly to the question of evolution and its mechanisms. Microbiology and biochemistry have brought revolutionary insights here. . . . [W]e must have the audacity to say that the great projects of the living creation are not the products of chance and error. . . . [They] point to a creating Reason and show us a creating Intelligence, and they do so more luminously and radiantly today than ever before (pp. 54–56).
Let us note three things about the cardinal’s argument. First, he is claiming that life is the result of intelligent purpose, not ontologically random events. Second, he bases this claim on physical evidence (i.e., the great projects of the living creation that point to a creating Reason), not on scriptural or theological arguments. Finally, he implies that biochemistry, the study of the molecular basis of life, provides particularly strong support for this view. He has a formidable point.
Darwin Said It Best
In 1996 I wrote Darwin’s Black Box: The Biochemical Challenge to Evolution (Free Press), whose main point Cardinal Ratzinger anticipated ten years earlier. “Black box” is a term used in science for a machine or system that does interesting things, but whose inner workings are unknown. To Darwin and other nineteenth-century scientists, the cell was a black box. Darwin advanced his theory in an age when the fundamental mechanisms of life were completely obscure.
Modern science has found that, far from the simple “protoplasm” that many nineteenth-century scientists believed, the cell contains ultra-sophisticated molecular machines. The assumption that the basis of life is simple has turned out to be the polar opposite of the case. Now that modern science has unveiled the surprising complexity of molecular life, how can we decide if Darwin’s theory can account for it? It turns out that Darwin himself set the standard. In his Origin of Species, he acknowledged that:
If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.
But what type of biological system could not be formed by “numerous, successive, slight modifications”? A system that is irreducibly complex. Irreducible complexity is just a fancy phrase I use to mean a single system that is composed of several interacting parts, where the removal of any one of the parts causes the system to cease functioning.
Figure 1
Let’s consider an everyday example of irreducible complexity: the humble mousetrap. The mousetraps that my family uses consist of a number of parts (Figure 1). There are: (1) a flat wooden platform to act as a base; (2) a metal hammer, which does the actual job of crushing the little mouse; (3) a spring with extended ends to press against the platform and the hammer when the trap is charged; (4) a sensitive catch that releases when slight pressure is applied; and (5) a metal bar that connects to the catch and holds the hammer back when the trap is charged. Now you can’t catch a mouse with just a platform, add a spring and catch a few more mice, add a holding bar and catch a few more. All the pieces of the mousetrap have to be in place before you catch any mice. Therefore the mousetrap is irreducibly complex.
An irreducibly complex system cannot be produced directly by numerous, successive, slight modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional. An irreducibly complex biological system, if there is such a thing, would be a powerful challenge to Darwinian evolution. Since natural selection can only choose systems that are already working, then if a biological system cannot be produced gradually it would have to arise as an integrated unit for natural selection to have anything to act on.
Let me add a word of caution. Demonstration that a system is irreducibly complex is not a proof that there is absolutely no gradual route to its production. Although an irreducibly complex system can’t be produced directly, one can’t definitively rule out the possibility of an indirect, circuitous route. However, as the complexity of an interacting system increases, the likelihood of such an indirect route drops precipitously. And as the number of unexplained, irreducibly complex biological systems increases, our confidence that Darwin’s criterion of failure has been met skyrockets toward the maximum that science allows.
The Cilium
Mousetraps are one thing, biochemical systems are another. So we must ask, are any biochemical systems irreducibly complex? Yes, it turns out that many are. A good example is the cilium. Cilia are hairlike structures on the surfaces of many animal and lower plant cells that can move fluid over the cell’s surface or “row” single cells through a fluid. In human beings, for example, cells lining the respiratory tract each have about 200 cilia that beat in synchrony to sweep mucus towards the throat for elimination.
What is the structure of a cilium? A cilium consists of a bundle of fibers called an axoneme. An axoneme contains a ring of 9 double “microtubules” surrounding two central single microtubules. Each outer doublet consists of a ring of 13 filaments fused to an assembly of 10 filaments. The filaments of the microtubules are composed of two proteins called alpha and beta tubulin. The 11 microtubules forming an axoneme are held together by three types of connectors: outer doublets are joined to the central microtubules by radial spokes; adjacent outer doublets are joined to each other by linkers of a highly elastic protein called nexin; and the central microtubules are joined by a connecting bridge. Finally, every doublet bears two arms, an inner arm and an outer arm, both containing a protein called dynein.
Although even this seems complex, a brief description can’t do justice to the full complexity of the cilium, which has been shown by biochemical analysis to contain about 200 separate kinds of protein parts.
Figure 2
But how does a cilium work? Experiments have shown that ciliary motion results from the chemically powered “walking” of the dynein arms on one microtubule up a second microtubule so that the two microtubules slide past each other (Figure 2). The protein crosslinks between microtubules in a cilium prevent neighboring microtubules from sliding past each other by more than a short distance. These crosslinks, therefore, convert the dynein-powered sliding motion to a bending motion of the entire axoneme.
Now, let us consider what this implies. What components are needed for a cilium to work? Ciliary motion certainly requires microtubules; otherwise, there would be no strands to slide. Additionally we require a motor, or else the microtubules of the cilium would lie stiff and motionless. Furthermore, we require linkers to tug on neighboring strands, converting the sliding motion into a bending motion, and preventing the structure from falling apart. All of these parts are required to perform one function: ciliary motion. Just as a mousetrap does not work unless all of its constituent parts are present, ciliary motion simply does not exist in the absence of microtubules, connectors, and motors. Therefore, we can conclude that the cilium is irreducibly complex—an enormous monkey wrench thrown into its presumed gradual, Darwinian evolution.
A Complex Delivery System
Another example of irreducible complexity is the system that targets proteins for delivery to subcellular compartments. The eukaryotic cell contains a number of subcellular compartments for specialized tasks, like rooms in a house. These include lysosomes for digestion, Golgi vesicles for export, and others. Unfortunately, the machinery for making proteins is outside these compartments, so how do the proteins that perform tasks in subcellular compartments find their way to their destination? It turns out that proteins that will wind up in subcellular compartments contain a special amino acid sequence near the beginning called a “signal sequence.” As the proteins are being synthesized, a complex molecular assemblage called the signal recognition particle or SRP, binds to the signal sequence. This causes synthesis of the protein to halt temporarily. During the pause in protein synthesis the SRP binds the trans-membrane SRP receptor, which causes protein synthesis to resume and which allows passage of the protein into the interior of the endoplasmic reticulum (ER). As the protein passes into the ER the signal sequence is cut off.
Figure 3
For many proteins the ER is just a way station on their travels to their final destinations (Figure 3). Proteins that will end up in a lysosome are enzymatically “tagged” with a carbohydrate residue called mannose-6-phosphate while still in the ER. An area of the ER membrane then begins to concentrate several proteins; one protein, clathrin, forms a sort of geodesic dome called a coated vesicle, which buds off from the ER. In the dome there is also a receptor protein that binds to both the clathrin and to the mannose-6-phosphate group of the protein that is being transported. The coated vesicle then leaves the ER, travels through the cytoplasm, and binds to the lysosome through another specific receptor protein. Finally, in a maneuver involving several more proteins, the vesicle fuses with the lysosome and the protein is at its destination.
During its travels our protein interacted with dozens of macromolecules to achieve one purpose: its arrival in the lysosome. Virtually all components of the transport system are necessary for the system to operate, and therefore the system is irreducible. The consequences of a gap in the transport chain can be seen in the hereditary defect known as I-cell disease. It results from a deficiency of the enzyme that places the mannose-6-phosphate on proteins to be targeted to the lysosomes. I-cell disease is characterized by progressive retardation, skeletal deformities, and early death.
Detection of Design
Many other examples of irreducibly complex cellular systems could be cited. In the face of such unexpectedly sophisticated machinery, what have scientists said? A number of prominent scientists have come to doubt that natural selection can account for them. The shortcomings of Darwinian explanations have been noted by Stuart Kauffman, Lynn Margulis, Brian Goodwin, James Shapiro, and others. Each of them has proposed an alternative to Darwinism that appeals to some sort of blind, general law. To date, however, none of the alternatives has had much success.
While I agree with these scientists that many biochemical systems can’t be explained by natural selection, I differ in the alternative I offer. I argue that the systems show strong evidence of design—purposeful, intentional design by an intelligent agent.
How do we detect design? The criteria for deciding that something has been designed have recently been outlined with mathematical and philosophical rigor by William Dembski in The Design Inference (Cambridge University Press, 1998). Here I would like to give a simple, intuitive criterion for suspecting design in discrete physical systems. In these cases design is most easily apprehended when a number of separate, interacting components are ordered in such a way as to accomplish a function beyond the individual components. To illustrate, think of the Far Side cartoon by Gary Larson in which an exploring team is going through a jungle. The lead explorer is pulled up and left dangling from a tree by a vine wrapped around his foot, and has been skewered by wooden spikes aimed precisely at the position in the air where the vines pulled him. A companion turns to another and confides, “That’s why I never walk in front.”
Now, every person who sees the cartoon knows immediately that the trap was designed. But how does one know that? How does the audience apprehend that the trap was designed? One can tell that the trap was designed because of the way the parts interact with great specificity to perform a function. Like the mousetrap in Figure 1, no one would mistake the cartoon trap for an accidental arrangement of parts.
I argue that many biochemical systems were designed by an intelligent agent. Our apprehension of the design of the cilium or intracellular transport rests on the same principles as our apprehension of the design of the jungle trap: the ordering of separate components to achieve an identifiable function that depends sharply on the components.
A Deliberate Resistance
In all the reviews of my book that I am aware of, no one claims that the biochemical systems I describe have already been explained by science. James Shreeve, reviewing the book for the New York Times says, “Mr. Behe may be right that given our current state of knowledge, good old Darwinian evolution cannot explain the origin of blood clotting or cellular transport.” In the National Review microbiologist James Shapiro of the University of Chicago writes, “There are no detailed Darwinian accounts for the evolution of any fundamental biochemical or cellular system, only a variety of wishful speculations.”
In Nature University of Chicago evolutionary biologist Jerry Coyne, although very unfriendly to the concept of intelligent design, states, “There is no doubt that the pathways described by Behe are dauntingly complex, and their evolution will be hard to unravel. . . . We may forever be unable to envisage the first proto-pathways.” In New Scientist Andrew Pomiankowski writes, “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.’” So apparently everyone at least agrees that complex biochemical systems have yet to be explained.
However, none of the reviewers who are biologists agrees with the conclusion of intelligent design. Shreeve worries, “Shouldn’t we leave something for our children and grandchildren to puzzle out besides which systems in the cell are intelligently designed and which are not? Because something is beyond our understanding today does not mean it will be beyond theirs.” Shapiro laments, “Sadly, despite its valuable critique of an all-too-often unchallenged orthodoxy, Darwin’s Black Box fails to capture the true excitement of contemporary biology because it is fighting the battles of the past rather than seeing the vision of the future.” Coyne sniffs, “It is not valid, however, to assume that, because one man cannot imagine such pathways, they could not have existed.” And Pomiankowski discerns, “So what we have here is just the latest, and no doubt not the last, attempt to put God back into nature.”
It is clear from the quotations, I think, that the reviewers are not rejecting design because there is scientific evidence against it, or because it violates some principle of logic. Rather, I believe they find design unacceptable because they are uncomfortable with the theological ramifications of the theory. In his essay to the Pontifical Academy of Science, Pope John Paul II noted that a theory of evolution has two parts, the mechanism and the philosophy attached to that mechanism. Putting it like that, however, makes it sound as if any philosophy can be mixed and matched with any mechanism. But the situation is not really that clear-cut. While Catholics and many other Christians could accommodate the mechanism of Darwin to their theology (with the reservation that the course of evolution is not truly random, but foreordained by God), materialists require something like Darwinism because, ultimately, materialism says that life and intelligence had to arise unaided from brute matter.
A theory of intelligent design, however, holds implicitly that there is a designer capable of planning and executing the phenomenal intricacies of life on earth. Although there are, at least in theory, some exotic candidates for the role of designer that might be compatible with materialist philosophy (such as space aliens or time travelers), few people will be convinced by these, and will conclude that the designer is beyond nature. Many scientists are unable or unwilling to accept such a designer because that goes against their prior commitment to materialism, or at least to a functional materialism in the course of their work.
Nonetheless, I remain optimistic that the scientific community will eventually accept intelligent design, even if the acceptance is discreet and muted. The reason for optimism is the advance of science itself, which almost every day discovers new intricacies in nature, fresh reasons for recognizing the design inherent to life and the universe.
Michael Behe, Ph.D. (Biochemistry, University of Pennsylvania), is Associate Professor in the Department of Biological Sciences at Lehigh University in Pennsylvania, and the author of Darwin’s Black Box: The Biochemical Challenge to Evolution (Free Press). His biochemical research has been funded by the National Institutes of Health and the National Science Foundation. He is a member of the Biophysical Society and the American Society for Molecular Biology and Biochemistry. This article is adapted from essays published in Ethics and Medics and Creation and Evolution: The Proceedings of the October 1997 ITEST Workshop.
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