Obviously, saying, “The best explanation is the one that best explains the facts or that best explains the most facts,” begs an important question. What does it mean to explain something well or best?
As it happens, historical scientists have developed criteria for deciding which explanation, among a group of competing possible hypotheses, provides the best explanation for some event in the remote past. The most important of these criteria is “causal adequacy.” As a condition of formulating a successful explanation, historical scientists must identify causes that are known to have the power to produce the kind of effect, feature, or event in question. In seeking to identify such causes, historical scientists evaluate hypotheses in light of their present knowledge of cause and effect. Causes that are known to produce the effect in question (or are thought capable of doing so) are judged to be better candidates than those that are not. For instance, a volcanic eruption provides a better explanation for an ash layer in the earth than an earthquake or a flood, because eruptions have been observed to produce ash layers, whereas earthquakes and floods have not.
One of the first historical scientists to develop the criterion of causal adequacy was the geologist Charles Lyell (1797–1875), who in turn influenced Charles Darwin. Darwin read Lyell’s magnum opus, The Principles of Geology, during his voyage on the HMS Beagle and employed Lyell’s principles of reasoning in On the Origin of Species. The subtitle of Lyell’s book summarized his central methodological principle: Being an Attempt to Explain the Former Changes of the Earth’s Surface, by Reference to Causes Now in Operation (1830–1833). Lyell argued that when scientists seek to explain events in the past, they should not invoke some unknown type of cause, the effects of which we have not observed. Instead, they should cite causes that are known from our uniform experience to have the power to produce the effect in question.22 Historical scientists should cite presently acting causes, that is, “causes now in operation.” This was the idea behind his uniformitarian method and its famous dictum: “The present is the key to the past.” According to Lyell, our present experience of cause and effect should guide our reasoning about the causes of past events. Darwin adopted this methodological principle as he sought to demonstrate that natural selection qualified as a vera causa, that is, a true, known, or actual cause of significant biological change.23 In other words, he sought to show that natural selection was “causally adequate” to produce the effects he was trying to explain.
The Only Known Cause
Both philosophers of science and leading historical scientists have emphasized causal adequacy as the key criterion by which competing hypotheses are judged. But philosophers of science have insisted that assessments of explanatory power lead to conclusive inferences only when there is just one known cause for the effect or evidence24 (see Fig. 17.3) in question. If there are many causes that can produce the same effect, then the presence of the effect does not definitively establish the cause. When scientists know of only one cause for a given effect, however, they can infer that cause and yet avoid the fallacy of affirming the consequent—the error of ignoring other possible causes with the power to produce the same effect.25 In that case, they can infer or detect a uniquely plausible past cause from the clues that are left behind.
This can happen in one of two ways. First, historical scientists might focus their investigation on a single fact (in isolation) for which only one cause happens to be known. In such a case, they can quickly and decisively infer the cause from the effect alone—without risk of affirming the consequent, because no other known cause produces the same effect. For example, because a volcanic eruption is the only known cause of a volcanic ash layer, the presence of such a layer at an archeological site strongly indicates the prior eruption of a volcano.
FIGURE 17.3
Schematic of the logical problem of retrodiction. Whether it is possible to reconstruct the past definitively or not depends upon whether there is a single cause or condition that gives rise to a present state or whether there are many possible past causes or conditions that give rise to a given present state. The diagram on the left portrays an information-destroying situation in which many past causes (or conditions) correspond to a given present state. The diagram on the right portrays an information-preserving situation in which only one past cause (or condition) corresponds to a present state. Adapted from Sober, Reconstructing the Past, 4.
In other cases where historical scientists encounter evidence for which there are many known causes, they will often broaden their investigation beyond an initial fact or set of facts. In such cases, they will use the strategy described above (as part of the method of multiple competing hypotheses), by looking for additional evidence until they find a piece for which there is only one known cause. They can then compare the explanatory power of the competing hypotheses. Using this strategy, historical scientists will choose the proposed cause with the demonstrated power to produce all the relevant evidence, including the new fact or piece of evidence for which there is only one known cause. For example, the discovery of the symmetrical pattern of ocean-floor magnetism on opposite sides of a mid-oceanic ridge allowed for a comparison of the explanatory power of the three hypotheses under consideration, leaving only seafloor spreading as a causally adequate explanation of all the relevant facts.
Such an approach often allows historical scientists to pick out a piece of evidence (from some combination of effects) for which there is only one known (or theoretically plausible) cause, thus making it possible to establish a past cause decisively. Though this strategy involves looking at a wider class of facts than the first strategy, the logical status of the inferences involved is the same. In each case, the presence of a fact (either standing on its own or in combination with other facts) for which only one cause is known allows historical scientists to make a definitive inference about the causal history in question without committing the fallacy of affirming the consequent. Logically, if a postulated cause is known to be a necessary condition or cause of a given event or effect, then historical scientists can validly infer that condition or cause from the presence of the effect. If it’s true that where there is smoke there is always first fire, then the presence of smoke wafting up over a distant mountain range decisively indicates the prior presence of a fire on the other side of the ridge.
Historical Inference and Intelligent Design
What does all this have to do with the Cambrian explosion?
Quite a lot. In my investigation of the historical scientific method, I found that whether they always realize it or not, historical scientists typically use the method of inference to the best explanation. They make abductive inferences about past causes from present clues, evidence, or effects. This later suggested to me that if there were features of the Cambrian explosion or the Cambrian animals that would be “expected as a matter of course” if an intelligent designer had played a role in that event, then it was at least possible to formulate the hypothesis of intelligent design as a historical (abductive) scientific inference. An advocate of intelligent design could reason in a standard historical scientific way:
MAJOR PREMISE: If intelligent design played a role in the Cambrian explosion, then feature (X) known to be produced by intelligent activity would be expected as a matter of course.
MINOR PREMISE:Feature (X) is observed in the Cambrian explosion of animal life.
CONCLUSION: Hence, there is reason to suspect that an intelligent cause played a role in the Cambrian explosion.
Of course, a historical scientist would only be justified in making such an abductive inference to the past activity of an intelligent cause if “feature X” is evident in the Cambrian explosion and if intelligent design is known to produce “feature X.” Moreover, just because the Cambrian explosion may exhibit some feature or features for which intelligent design is a known cause does not mean that intelligent design was necessarily the actual cause (or the best explanation) of those features. Only if the Cambrian event and animals exhi
bit features for which intelligent design is the only known cause may a historical scientist make a decisive inference to a past intelligent cause.
We are left with two crucial questions. Are there in fact such features present in the record of the Cambrian explosion or in the animals that arise in it—features that are known from our experience to be produced by intelligent causes such that they would justify making a tentative abductive inference to intelligent design? Are there also perhaps features of the Cambrian event that are known from our experience to be produced by intelligent causes, and only intelligent causes, justifying a more definitive inference to past intelligent activity as the best explanation for the relevant evidence? Might “the butler” have done it after all?
18
Signs of Design in the Cambrian Explosion
Well-crafted mystery novels, like real-world crime investigations, unfold with a distinctive logic. There is a death to be explained and, at the start, an indefinitely large universe of possible causes. That universe can be made smaller, narrowing to the one true cause, as more and more clues come to light. Those clues typically come in two forms: positive evidence, or indicators of what likely happened (e.g., .38 caliber shell casings on the ground and bullet wounds in a body) and negative evidence, or indicators of what could not have happened.
Let’s say that the local sheriff who discovered the body of the estate owner (from my illustration in the previous chapter), did so as he was making his rounds on a dirt road that makes a close approach to the beach at the end of the estate where the owner died. And let’s say that, as a result, the sheriff just happened to find the body soon after the murder had taken place. Let’s suppose, further, that the sheriff had the presence of mind to immediately measure the temperature of the body only to find that the victim was still warm, indeed almost as warm as a living person. Clearly, in this situation, the sheriff would conclude that the victim had just died. At that point in the investigation, a physical regularity would govern the sheriff’s reasoning, one that tells a lot about who didn’t commit the murder. Following death, the human body cools to the surrounding temperature at a known rate. So, making allowances for vehicular transport, whoever committed the murder could not have gone beyond a certain distance from the remote estate at the time the body was found.
These facts would immediately provide a rock-solid alibi to the vast majority of humanity, anyone located safely outside that radius when the body was discovered. Of course, calling this information a negative clue is really only a convention of naming. “Negative” and “positive” refer to how we conceive of the implications of a fact, but not to the fact itself: the evidence, after all, is what it is. Even so, facts both exclude and allow competing possible hypotheses. As they accumulate, they typically paint a picture, a profile, of the actual cause of the event in need of explanation. Thus, when we say “the body temperature of the deceased rules out the 7 billion people who were well beyond the radius set by the cooling rate,” we could equally well have said, “the body temperature implicates some person within 30 miles of the estate when the sheriff arrived,” a population of possible suspects much smaller than when we started.
As I have described the many attempts to explain the scientific enigma motivating this book, that mystery has, in one sense, progressively deepened. As more and more attempts to explain the Cambrian explosion of animal life have failed, the evidence that these various competing theories fail to explain may be considered a set of negative clues—evidence that effectively precludes certain possible causes or explanations. I’ve already explained why the received version of evolutionary theory, neo-Darwinism, fails to account for the explosion of information and form in the Cambrian period. I’ve also examined more recent evolutionary theories and shown why they too fail to explain key aspects of the evidence. To this point, then, much of the evidence has returned a negative verdict. It has told us a lot about what, in all probability, did not cause the Cambrian explosion. But, as in our hypothetical murder case, an accumulating body of evidence that makes one set of explanations less and less plausible may also begin to paint a picture of an alternative cause and the true explanation.
Profile of the Suspect
Long before detectives know the actual identity of a suspect, they will often compose a profile of the person they are seeking. One leading paleontologist has used this strategy to begin to draw a bead on the cause responsible for the Cambrian explosion. Douglas Erwin has dedicated his career to solving the problem of the origin of animal body plans (see Fig. 18.1). Trained at the University of California by James Valentine, another Cambrian expert, Erwin has worked closely over the past decade with Eric Davidson, whom we first met in Chapter 13, trying to determine what happened to cause dozens of novel body plans to appear—and appear rapidly—in the Cambrian period.
Both Erwin and Davidson have now ruled out standard neo-Darwinian theory—vehemently in Davidson’s case. He says that the standard theory “gives rise to lethal errors.”1 But Erwin and Davidson go further. They have assembled what is, in essence, a clue sheet—a list of key evidences that must be explained. Using that list, they have begun to sketch, at least in outline, a profile of the cause behind the Cambrian explosion.
On the positive side of the ledger, they conclude that this cause must have several attributes in order to explain key facts about the fossil record as well as what it takes to build animals. In particular, the cause must be capable of generating a top-down pattern of appearance; it must be capable of generating new biological form relatively rapidly; and it must be capable of constructing, not merely modifying, complex integrated genetic circuits (specifically, the developmental gene regulatory networks discussed in Chapter 13).
On the negative side, Davidson and Erwin rule out both observed microevolutionary processes and postulated macroevolutionary mechanisms (such as punctuated equilibrium and species selection) as explanations for the origin of the key features of the Cambrian explosion. They insist that the requirements for constructing animal body plans de novo “cannot be accommodated by microevolutionary [or] macroevolutionary theory.”2
FIGURE 18.1
Douglas Erwin. Courtesy UPHOTO/Cornell University.
In Chapter 13, I discussed their reason for coming to this conclusion: developmental gene regulatory networks, once in place, cannot be perturbed (or mutated) without “catastrophic”3 consequences to the developing animal. Thus, fundamentally new gene regulatory networks (dGRNs) cannot evolve gradually from preexisting dGRNs, if those evolutionary changes require perturbing the deepest nodes of the earlier dGRNs. Yet building new dGRNs capable of producing new animals requires precisely such fundamental alterations in preexisting dGRNs. But then how would new regulatory networks ever arise? Davidson and Erwin insist that no current theory of evolution explains the origin of these systems. Thus, they conclude that the cause of the Cambrian explosion is not described by any currently proposed theory of microor macroevolution.
In saying this, Erwin emphasizes the uniqueness of the innovations that occurred in the Cambrian explosion. He explains: “Unlike later events, the most significant developmental events of the Cambrian radiation involved the proliferation of cell types, developmental hierarchies and epigenetic cascades.”4 Consequently, he concludes, “The crucial difference between the developmental events of the Cambrian and subsequent events is that the former involved the establishment of these developmental patterns, not their modification.”5 For this reason, Erwin denies that the central event of the Cambrian explosion—the origin of novel body plans—has any parallel to currently observed biological processes. Rather, he insists that the events of the past were fundamentally different—that profound asymmetries exist between evolution then, and evolution now.6 Thus, he amplifies his denial of the sufficiency of current evolutionary theory by adding one additional attribute, albeit a negative one, to his portrait of “the suspect”: the cause responsible for generating the new animal forms, whatever it was, must have been u
nlike any observed biological process operating in actual living populations today.
Profiling a Cause
All of this raises an obvious question. Could the negative clues that increasingly disconfirm materialistic evolutionary theories also be positive indicators of a different kind of cause—perhaps even an intelligent cause?
By sketching the profile of the kind of cause needed to explain the origin of animal life, Davidson, Erwin, and many other evolutionary biologists may have, inadvertently, rendered the idea of intelligent design a bit less inconceivable. To see why, let’s quickly review Erwin and Davidson’s profile of the suspect. They have concluded that the cause of the origin of the new animal forms in the Cambrian explosion must be capable of:
generating new form rapidly
generating a top-down pattern of appearance
constructing, not merely modifying, complex integrated circuits
They have also concluded that this cause is:
not described by any currently proposed theory of microor macroevolution
unlike any observed biological process operating in actual living populations today
Erwin and Davidson, no friends of intelligent design, have sketched a partial profile of an adequate cause as befits their particular interest in the importance of gene regulatory networks (Davidson) and fossil discontinuity (Erwin). But other evolutionary biologists have contributed to this picture as well. Simon Conway Morris marvels at “the uncanny ability of evolution to navigate to the appropriate solution through immense ‘hyperspaces’ of biological possibility.”7 As a result, he argues that evolution may in some way be “channeled” toward propitious functional and/or structural end points—without specifying any known evolutionary mechanism that can so direct evolution to such end points.8 James Shapiro proposes a mechanism of evolutionary change that relies on preprogrammed adaptive capacity—without explaining where such preprogramming comes from.9 Several of the new evolutionary theories discussed in the previous chapters presuppose, but do not explain, the existence of both genetic and epigenetic forms of information, highlighting the need for a cause capable of generating such information in the first place.
Darwin's Doubt Page 39