Darwin’s Dangerous Idea: Evolution and the Meanings of Life
by Daniel C. Dennett
Simon and Schuster, 586 pp., $30.00
As an evolutionary biologist, I am used to being misunderstood by philosophers. Even my favorite philosopher, Karl Popper, although he later repented, argued for many years that evolution theory is metaphysics rather than science. It is therefore a pleasure to meet a philosopher who understands what Darwinism is about, and approves of it.
Dennett goes well beyond biology. He sees Darwinism as a corrosive acid, capable of dissolving our earlier belief and forcing a reconsideration of much of sociology and philosophy. Although modestly written, this is not a modest book. Dennett argues that, if we understand Darwin’s dangerous idea, we are forced to reject or modify much of our current intellectual baggage—for example, the ideas of Stephen Jay Gould, Noam Chomsky, Jerry Fodor, John Searle, E.O. Wilson, and Roger Penrose. As it happens, he is not the first to see how farreaching are the effects of Darwin’s idea. Darwin himself wrote, “He who understands baboon would do more toward metaphysics than Locke.” It is a remarkable fact that these words were written in a private notebook when Darwin was still seeking a theory of evolution.
Dennett’s central thesis is that evolution by natural selection is an algorithmic process. An algorithm is defined in the OED as “a procedure or set of rules for calculation and problem-solving.” The rules must be so simple and precise that it does not matter whether they are carried out by a machine or an intelligent agent; the results will be the same. He emphasizes three features of an algorithmic process. First, “substrate neutrality”: arithmetic can be performed with pencil and paper, a calculator made of gear wheels or transistors, or even, as was hilariously demonstrated at an open day at my son’s school, jets of water. It is the logic that matters, not the material substrate. Second, mindlessness: each step in the process is so simple that it can be carried out by an idiot or a cogwhell. Third, guaranteed results: whatever it is that an algorithm does, it does every time (although, as Dennett emphasizes, an algorithm can incorporate random processes, and so can generate unpredictable results).
How can it be that such a mindless process can generate such wonderful results? In particular, how can it have produced us? Before Darwin, it was the accepted opinion of both philosophers and biologists that the complex adaptations of living things implied an intelligent designer. The essence of Darwin’s dangerous idea is that adaptations can arise by natural selection, without need of intelligence: that is, they can be the products of an algorithmic process. Dennett repeatedly uses the analogy of “cranes” and “skyhooks.” These are both devices for lifting things—in evolution, for generating increasingly complex designs—but of very different kinds. A crane is a structure or process which is itself the product of the natural selection of replicating entities, but which, once it has arisen, makes it possible for still more complex structures to evolve.
Two examples will make the point clearer. The first populations of replicating entities lacked sex: that is, there was no way in which different replicators could unite to form a new individual. Once sex did arise, it greatly accelerated the evolutionary process. Sex is, in Dennett’s terminology, a crane. Sex did not arise because it would accelerate evolution in the future: natural selection does not have foresight. Indeed, there is still controversy among evolutionary biologists about how and why sex did originate, although I think that a plausible account is now possible.
To give a second example of a crane, many animals learn by experience. The brain is as much a product of natural selection as the liver or the kidney. In the simplest animals it serves mainly to generate fixed responses to external stimuli. Once, however, the connectivity of the neurons in an individual can be modified by experience, an animal will alter its behavior in a useful way as a result of that experience. It is also possible to program a computer so that the strength of the internal connections alter with experience; such computers are surprisingly good at learning. As B.F. Skinner pointed out, trial-and-error learning is an exact analogue of evolution by natural selection. Dennett agrees with Skinner about this, although rejecting much else that he said. The important point is that the brain is, in Dennett’s words, a crane. It evolved by natural selection, but, once evolved, it made the evolution of further complexity possible.
Dennett’s view of evolution, then, is one of cranes building cranes building cranes, each new crane arising by an essentially mindless process of selection. I fully agree with this view. Indeed, my recent book with Eörs Szathmáry, The Major Transitions in Evolution,1 is an account of this succession of cranes, starting with the origin of the first replicators and the genetic code, and ending (so far) with the origin of human language. The problem is to explain how each new crane arose by a process of selection, without miracles, or “skyhooks.” Skyhooks are a stark contrast. They are mysterious lifting devices, whose origin cannot be explained. It is Dennett’s thesis that we must eschew skyhooks and make do with cranes.
In addition to the Darwinian notion of natural selection, there are two other sources of Dennett’s view of the world. The first is a set of ideas that includes computing science, artificial intelligence, and cognitive science: it is from here that he acquired his conviction that algorithmic processes can generate mind-like activities. For example, computers, following simple rules, can design railway timetables or play chess, tasks once thought to require the operation of the human mind. The second is the gene’s-eye view of evolution pioneered by G.C. Williams and Richard Dawkins. According to this view, evolution is a necessary consequence of the existence of replicating entities; in biology, those entities are genes, but the principle holds for any kind of replicators. I have thought for some time that Dawkins and Williams have made a more fruitful contribution to philosophy than most philosophers, and I am pleased to see this opinion so generously recognized.
Dennett uses one other general idea which, perhaps because of my personal history, I find particularly appealing. This is the idea of “reverse engineering.” Usually, engineers start with a function they wish to perform, and design a structure to do the job. Biologists often find themselves confronted by a structure and ask themselves what function it was selected to perform. Harvey’s discovery that the heart is a pump was an early triumph of such reverse engineering.
As it happens, engineers occasionally do reverse engineering. In 1945, when I was working in an aircraft design office, the Royal Aircraft Establishment at Farnborough put on an exhibition of recently captured German equipment. A friend and I spent two fascinating days looking at the equipment and asking ourselves, “Why did they do that?” Both the V1 flying bomb and the V2 rocket were on show, and we spent some time trying to figure out how they worked. I seem to remember that the V2 rocket had a gyroscope puzzlingly connected to the fuel supply to the motors; surely, one would think, it should be connected to the guidance system. I leave it as an exercise to any readers who fancy themselves as reverse engineers to work out why, if my memory is correct, it was connected to the fuel supply. Since I became a biologist, I have spent most of my time asking questions like that.
Of course, when thinking about the V2 rocket I was thinking about a product of human design, whereas, a few years later, when I was thinking about the shapes of mammalian teeth, I was asking why mammals were better at chewing, and so left more descendants. But this difference had no effect on the way I thought about the two problems. Indeed, I have become increasingly convinced that there is no way of telling the difference between an evolved organism and an artifact designed by an intelligent being. Thus imagine that the first spacemen to land on Mars are met by an object which appears to have sense organs (eyes, ears) and organs of locomotion (legs, wings). How will they know whether it is an evolved organism, or a robot designed by an evolved organism? Only, I think, by finding out where it came from, and perhaps not even then.
Dennett suggests that criticisms of the neo-Darwinist synthesis come, in the main, from those who are reluctant to believe that they are the product of an algorithmic process and who lust after skyhooks. First among these, he suggests, is Stephen Jay Gould. Gould occupies a rather curious position, particularly on his side of the Atlantic. Because of the excellence of his essays, he has come to be seen by non-biologists as the preeminent evolutionary theorist. In contrast, the evolutionary biologists with whom I have discussed his work tend to see him as a man whose ideas are so confused as to be hardly worth bothering with, but as one who should not be publicly criticized because he is at least on our side against the creationists. All this would not matter, were it not that he is giving non-biologists a largely false picture of the state of evolutionary theory.
There are, Dennett suggests, three main aspects of Gould’s thought which reveal a wish to escape from Darwin’s algorithmic grip. The first is his critique, with Richard Lewontin, of the “adaptationist paradigm.” I have some responsibility for this critique. As organizer of a symposium in London on adaptation, I invited Lewontin, as a well-known critic of naive adaptationist arguments, to contribute. Lewontin, for reasons that, as an exaircraft engineer, I well understand, dislikes flying, and suggested that he write a joint paper with Gould, which Gould would present. The result was the now-famous paper entitled “Spandrels of San Marco.” Its thesis is that many structures in the animal world are not adapted for any function, but, like the spandrels of San Marco, are accidental and unselected consequences of something else. Further, they argued, many adaptive explanations are “Just So Stories,” unsupported by evidence.
By and large, I think their paper had a healthy effect. There are plenty of bad adaptive stories: we can all laugh at the suggestion that flamingos are pink because it camouflages them against the sunset. Their critique forced us to clean up our act and to provide evidence for our stories. But adaptationism remains the core of biological thinking. Confronted with feathers, or eyes, or ribosomes, we cannot not ask what they are for. It would be no more plausible to suppose that they are accidental and non-selected byproducts of something else than it would be to suppose that the gyroscope in the V2 rocket was connected as it was because some German fitter made a mistake.
W.H. Freeman, 1995.↩
W.H. Freeman, 1995.↩