Though the paper was titled “A Merciful Death for the ‘Earliest Bilaterian,’ Vernanimalcula,” the authors were anything but merciful in wielding their arguments. Their article upbraided David J. Bottjer, the main paleontologist who has promoted the interpretation of Vernanimalcula as a bilaterian ancestor, for seeing what he wanted to see and disregarding the clear evidence of nonbiological mineralization. In a 2005 Scientific American article, Bottjer interpreted Vernanimalcula as the “oldest fossil animal with a bilaterian body plan yet discovered.” In that article, Bottjer claimed that Vernanimalcula confirmed the “suspicion that complex animals have a much deeper root in time” and “that the Cambrian was less of an explosion and more of a flowering of animal life.”51 After unequivocally rejecting Bottjer’s interpretation on the basis of their geochemical analysis, Bengtson and his coauthors rebuked Bottjer in rather personal terms:
It is likely that the fossils referred to [as] Vernanimalcula were interpreted as bilaterians because this was … the explicit quarry of its authors. If you know from the beginning not only what you are looking for, but what you are going to find, you will find it, whether or not it exists. As Richard Feynman (1974) famously remarked: “The first principle is that you must not fool yourself—and you are the easiest person to fool.” … Once you have fooled yourself you will fool other scientists.52
Bengtson and his colleagues insist that however much a few paleontologists such as Bottjer might have wanted to “heap” “evolutionary significance” on Vernanimalcula in order to relieve their cognitive dissonance about the Cambrian explosion, the evidence does not bear the weight of interpretation that had been placed upon it. Thus, they conclude Vernanimalcula should be “laid to a merciful rest,” since the interpretation of it had “taken on a life of its own, a life it never had.”53
A Deeper Problem
Though back in 2009, Jonathan Wells and I didn’t know about the most recent critical analysis of Vernanimalcula’s pretentions, we did know that many leading paleontologists had rejected attempts to identify Vernanimalcula as an animal form. Thus, during the question-and-answer period following the film, Jonathan Wells explained why these and other obscure and enigmatic Precambrian fossils (or imprints) failed to qualify as convincing precursors to any of the Cambrian animals, citing the work of leading authorities in paleontology. In each case, he noted that similarities between the Ediacaran forms and later Cambrian animals had proven superficial, because the Ediacaran forms lacked many key diagnostic features of specific Cambrian phyla.
At the same time, as I later reflected on the lecture, I recognized a deeper problem with attempts to resolve the mystery of the Cambrian explosion by pointing to a few Precambrian fossils. Many defenders of the Darwinian picture of the history of life seemed to assume that the discovery of any alleged Precambrian animal forms, however implausible as ancestors of specific Cambrian animals or however sparsely distributed in the vast sequences of Precambrian strata, would solve the mystery of the Cambrian explosion, especially if these forms exemplified some abstractly perceived commonality such as bilateral symmetry.
To see what’s wrong with this way of thinking, imagine an ambitious distance swimmer claiming that it would be possible to swim between California and Hawaii over a period of many months or years because of the small islands that provide way stations where he could eat, rest, and overnight at each stage along his marathon journey. But instead of showing that an archipelago dotting the route between California and Hawaii at reasonably accessible intervals actually exists, he points to a couple of barren atolls in the South Pacific far from the most likely course to Hawaii. Clearly, in that case, the claims of our intrepid hypothetical swimmer would not be credible. In a similar way, the claims of those who assert that a few isolated and anatomically enigmatic forms of life in the Precambrian solve the problem of the Cambrian explosion also lack credibility.
To appreciate another aspect of this problem, let’s revisit the claims about Vernanimalcula as a possible ancestor of all the bilaterian phyla. On the one hand, for such a form to qualify as the ancestor common to a large number of specific phyla (such as the many bilaterian phyla that arise in the Cambrian), it must exhibit the basic bilaterian characteristics such as bilateral symmetry and what is called “triploblasty,” the presence of three distinct tissue layers (endoderm, mesoderm, and ectoderm). At the same time, a viable candidate for the role of common ancestor cannot by definition manifest any of the differentiating characteristics that distinguish the individual Cambrian phyla and their respective body plans from each other. For example, any bilaterian that manifests the characteristic exoskeleton of, say, an arthropod cannot also qualify as a plausible ancestor of a chordate, because chordates have internal skeletons or notochords. The logic of these distinct body designs precludes sharing both anatomical characteristics. For this reason, any hypothetical bilaterian common ancestor could only have existed as a kind of lowest anatomical common denominator, or what evolutionary biologists call a “ground plan,” having only those few features that are common to all of the animal forms that allegedly evolved from it.
But this creates a dilemma. If a fossilized form is simple enough to qualify as the common ancestor of later highly differentiated bilaterian phyla, then it will necessarily lack most of the important distinguishing anatomical features of those specific phyla. That means that all the interesting anatomical novelties that differentiate one phylum from another must arise along the separate lineages branching out from the alleged common ancestor well after its origin in the fossil record. Heads, jointed limbs, compound eyes, guts, anuses, antennae, notochords, stereoms, lophophores (a tentacled feeding organ), and numerous other distinguishing characteristics of many different animals must come later on many distinct lines of descent. Yet the gradual evolutionary origin of these characteristics is not documented in the Precambrian fossil record. These characteristics do not appear until they arise suddenly in the Cambrian explosion.
For this reason, indistinct fossils such as Vernanimalcula—even if we take them as representing a common ancestor of many bilaterians—document little of the Darwinian story of the history of animal life. Hugely significant gaps in the fossil record would still remain, because the Precambrian fossil record simply does not document the gradual emergence of the crucial distinguishing characteristics of the Cambrian animals. The important anatomical novelties that define the individual Cambrian phyla as well as their first clear representatives arise as suddenly as ever.
To say that a form such as Vernanimalcula, or any of the other relatively indistinct Ediacaran forms, solves the problem of the missing Precambrian fossil record would be a bit like saying that a metal cylinder demonstrates all the steps involved in the construction of a toaster, automobile, submarine, or jet airplane simply because all these technological objects utilize “metal enclosures.” It’s true that each of these complex systems uses metal enclosures, but the presence of an enclosed metal surface is only a necessary, and not nearly a sufficient, condition of the origin of these various technological systems. Similarly, finding a simple but otherwise unadorned bilaterally symmetric form of life would hardly solve the problem of fossil discontinuity, because it would not by itself document the emergence of the unique characteristics of the individual bilaterian animals.
This paradox is well known to paleontologists who work on the Cambrian radiation. Charles Marshall and James Valentine, for instance, describe the difficulty of attempting to characterize an “undiagnostic” group, by which they mean a possible ancestral “stem” group that lacks the specialized characteristics of its presumptive evolutionary progeny. They write:
When trying to unravel the origins of the animal phyla … the hardest to examine is the phase between the actual cladogenic origin of a phylum and the time that it acquired its first phylum-specific characteristic(s). Even if we have fossils from this phase in a phylum’s history, we will not be able to prove their kinships at the level of phyla.54
/> Thus, even if Vernanimalcula, or some other fossil form, were simple enough and animal-like enough to qualify as a so-called ur (or original) form of animal life, it would paradoxically, for just that reason, be incapable of establishing itself as an unequivocal common ancestor of some specific Cambrian phylum.
And there’s no relief in the other direction. If an alleged ancestral form manifests the distinguishing features of one of the specific Cambrian phyla—if, for example, Vernanimalcula or some other isolated form presented a convincing set of distinctively arthropod or chordate or echinoderm characteristics, then the very presence of those features would necessarily preclude the possibility of that specific animal form representing the common ancestor of all other Cambrian forms. The more an animal form manifests the characteristics of one phylum or group within the phylum, the less plausible it becomes as the ancestor of all the other animal phyla.
And that is the dilemma in a nutshell. Highly differentiated and complex Precambrian forms by themselves could not have been ancestors common to all the Cambrian phyla; whereas undifferentiated forms simple enough to have been ancestral to all the Cambrian phyla leave no evidence, by themselves, of the gradual emergence of the complex anatomical novelties that define the Cambrian animals. Either way—whether the few alleged Precambrian ancestors are viewed as simple and relatively undifferentiated or complex and highly differentiated—the fossil record, given its otherwise pervasive pattern of discontinuity, does not establish the gradual evolution of numerous anatomical and morphological novelties. Instead, only a true series of transitional intermediates in which the fossil record documents both the existence of an original animal form and the gradual appearance of the key distinguishing anatomical features and novelties (and the Cambrian animals themselves) would remedy this deficiency. And yet that is precisely what the Precambrian fossil record has failed to document.
As Graham Budd and Sören Jensen state, “The known [Precambrian/Cambrian] fossil record has not been misunderstood, and there are no convincing bilaterian candidates known from the fossil record until just before the beginning of the Cambrian (c. 543 Ma), even though there are plentiful sediments older than this that should reveal them.”55 Thus they conclude, “The expected Darwinian pattern of a deep fossil history of the bilaterians, potentially showing their gradual development, stretching hundreds of millions of years into the Precambrian, has singularly failed to materialize.”56
Dilemma on Display
During the question-and-answer session that followed the screening of Darwin’s Dilemma, none of the University of Oklahoma Ph.D. students or science faculty who attended the museum-sponsored lecture challenged my colleague Jonathan Wells when he explained why leading paleontologists do not think the exotic Precambrian forms cited in the lecture were ancestors of Cambrian forms. This nonreaction seemed a little odd given the stress the museum’s own expert had laid on these claims, and given that he had made these claims rather emphatically in the same building to many of the same people just three hours earlier.
On our flight out of town the next day, Jonathan Wells told me something that cast our experience there in an even odder light. He’d had a chance to walk around the Sam Noble Science Museum after the lecture and before our event. He discovered that the museum has a display that vividly illustrates the severity of what we called Darwin’s dilemma. Wells recorded some of his observations on his return to Seattle. One part of the account of what he saw while touring the museum’s own display about the Cambrian explosion is worth quoting at length:
[The display] seemed factually accurate for the most part, emphasizing (among other things) that many of the Cambrian explosion fossils were soft-bodied—which puts the lie to the common explanation that their precursors are absent from the fossil record because they lacked hard parts. The exhibit also made it clear that the Ediacaran fossils went extinct at the end of the pre-Cambrian, so (with a few possible exceptions) they could not have been ancestral to the Cambrian phyla. One particular panel in the exhibit caught my attention. It showed over a dozen of the Cambrian phyla at the top of a branching tree with a single trunk, but none of the branch points corresponded to a real living thing. Instead, the branch points were artificial technical categories such as “Ecdysozoa,” “Lophotrochozoa,” “Deuterostomia,” and “Bilateria.” The artificiality of the branch points emphasized that the branching-tree pattern imposed on the fossil evidence was itself an artificial construct.
So, after all the controversy, it turned out that the museum that had sponsored the lecture denying Darwin’s Cambrian dilemma itself has an excellent display indicating that the expected ancestral forms of the Cambrian animals—the very ones that Darwin hoped to find a hundred and fifty years ago—are still missing from the Precambrian fossil record. But then why would the museum sponsor the presentation that it did? It’s hard to say, I suppose, but I’ve seen this dynamic before in discussions about Darwinian evolution. Evolutionary biologists will acknowledge problems to each other in scientific settings that they will deny or minimize in public, lest they aid and abet the dread “creationists” and others they see as advancing the cause of unreason. Perhaps just our presence on campus raising questions about contemporary Darwinism made them feel defensive on behalf of “science.” It’s an understandable, if ironic, human reaction, of course, but one that in the end deprives the public of access to what scientists actually know. It also perpetuates the impression of evolutionary biology as a science that has settled all the important questions at just the time when many new and exciting questions—about the origin of animal form, for example—are coming to the fore.
5
The Genes Tell the Story?
Reconstructing the history of life has a lot in common with detective work. Neither detectives nor evolutionary biologists can directly observe the events in the past that interest them most. Detectives typically did not see the crime occur. Evolutionary biologists did not witness the origin of animals or other groups of organisms. Yet this limitation doesn’t mean either group of investigators lacks the evidence to determine with some confidence what happened. Detectives and evolutionary biologists as well as many other historical scientists—paleontologists, geologists, archeologists, cosmologists, and forensic scientists—do this regularly, based on careful inference from the clues or evidence left behind.
Many evolutionary biologists have commented on the forensic nature of their work. Here’s how Richard Dawkins puts it: “I have used the metaphor of a detective, coming on the scene of the crime after it is all over and reconstructing from the surviving clues what must have happened.”1
Perhaps the most obvious surviving traces of ancient life are fossils. But as evolutionary biologists and paleontologists have come to realize that the Precambrian fossil record has not furnished the confirmation that Darwin hoped it would, many have looked to other kinds of clues to establish the gradual emergence of Cambrian animal life from a common ancestor.
In this effort, contemporary evolutionary biologists have followed the example of Darwin himself. Though Darwin argued that a general progression of simpler to more complex forms of life in the fossil record meshed nicely with his theory, he was acutely aware that the discontinuity of the fossil record, particularly as evidenced in Precambrian and Cambrian strata, did not. This is why he emphasized other types of evidence to establish his theory of universal common descent.
In a famous chapter in On the Origin of Species titled “The Mutual Affinities of Organic Beings,” Darwin made his case not on the basis of the fossil evidence, but on the basis of similar anatomical structures in many distinct organisms. He noted, for example, that the forelimbs of frogs, horses, bats, humans, and many other vertebrates exhibited a common five-digit (“pentadactyl”) structure or organization (see Fig. 5.1). To explain such “homologies,” as he called them, Darwin posited a vertebrate ancestor that possessed pentadactyl limbs in rudimentary form. As a menagerie of modern vertebrates evolved from this common ancesto
r, each retained in its own way the basic pentadactyl mode of organization. For Darwin, his theory of descent with modification from a common ancestor explained these similarities better than the received view of many older nineteenth-century biologists such as Louis Agassiz or Richard Owen, both of whom thought homologies reflected the common design plan of a creative intelligence.
FIGURE 5.1
The common five-digit pattern of the pentadactyl limb as manifested in four modern animals. Copyright Jody F. Sjogren. Used with permission.
In reconstructing the evolutionary history of life, most evolutionary biologists today emphasize the importance of homology. They assume that similarities in anatomy and in the sequences of information-bearing biomacromolecules such as DNA, RNA, and protein point strongly to a common ancestor.2 They also assume that the degree of difference in such cases is on average proportional to the time elapsed since the divergence from a common ancestor. The greater the difference in the common feature or molecular sequence, the farther back the ancestor from which the feature or sequence arose.
Evolutionary biologists have used this approach to try to discern the evolutionary history of the Cambrian animals. If the Precambrian fossil record refuses to disclose the secrets of Precambrian evolution, so goes the thinking, perhaps the study of comparative anatomy and molecular homologies will. Given the well-established problem with the fossil evidence, many evolutionary biologists now particularly emphasize the importance of clues from molecular genetics. As evolutionary biologist Jerry Coyne, of the University of Chicago, notes, “Now we have a powerful, new, and independent way to establish ancestry: we can look directly at the genes themselves. By sequencing the DNA of various species and measuring how similar these sequences are, we can reconstruct their evolutionary relationships.”3
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