Erwin and Valentine's <i>The Cambrian Explosion</i> Affirms Major Points in <i>Darwin's Doubt</i>: The Cambrian Enigma Is "Unresolved" - Evolution News & Views

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Erwin and Valentine's The Cambrian Explosion Affirms Major Points in Darwin's Doubt: The Cambrian Enigma Is "Unresolved"

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A while back at Jerry Coyne's blog, Why Evolution Is True, contributor Greg Mayer wrote a post encouraging readers to buy a new 2013 book about the Cambrian explosion if they "really want to learn something about this period in the history of life." He wasn't referring to Darwin's Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design by the "infamous Stephen Meyer," as Greg Mayer calls him.

Rather, Mayer suggests that people read a different book recently published by two paleontologists who are two of the leading mainstream scientific authorities on the Cambrian explosion, Douglas Erwin and James Valentine. The book is The Cambrian Explosion: The Construction of Animal Biodiversity (Roberts and Company, 2013). I ordered it as soon as I learned it was available. Having read it, I now wholeheartedly agree with Mayer that people should read The Cambrian Explosion. Anyone who reads the book will gain an appreciation of the magnitude of the explosion of biodiversity that appeared in the Cambrian, and also the size of the problem that it poses for evolutionary biology. This makes The Cambrian Explosion all the more worth reading, because as we'll see, they admit that from their vantage as evolutionary biologists, the Cambrian explosion is currently "unresolved."

As an initial compliment, I'd like to note that Erwin and Valentine's book contains many elegant and beautiful color photos, illustrations, and diagrams of Cambrian fossils and animals. You can some of these photos on the publisher's website. (I can only imagine what the art budget was!) It also offers probably the most comprehensive defense of current evolutionary thinking about the origin of animals in the Cambrian.

But there's something even more interesting about The Cambrian Explosion that makes it positively a must-read. Erwin and Valentine are not proponents of intelligent design. So obviously they're not going to agree with everything Stephen Meyer writes in Darwin's Doubt, especially when Meyer argues for intelligent design. But if you read their book carefully, you'll find that the authors articulate and affirm at least three core arguments that Stephen Meyer also makes in Darwin's Doubt. The introduction to their book, available free online here, includes clear statements of these points.

First, as the title suggests, The Cambrian Explosion acknowledges that the Cambrian explosion was a real event, and is not merely an artifact of an imperfect fossil record. They write:

[A] great variety and abundance of animal fossils appear in deposits dating from a geologically brief interval between about 530 to 520 Ma, early in the Cambrian period. During this time, nearly all the major living animal groups (phyla) that have skeletons first appeared as fossils (at least one appeared earlier). Surprisingly, a number of those localities have yielded fossils that preserve details of complex organs at the tissue level, such as eyes, guts, and appendages. In addition, several groups that were entirely soft-bodied and thus could be preserved only under unusual circumstances also first appear in those faunas. Because many of those fossils represent complex groups such as vertebrates (the subgroup of the phylum Chordata to which humans belong) and arthropods, it seems likely that all or nearly all the major phylum-level groups of living animals, including many small softbodied groups that we do not actually find as fossils, had appeared by the end of the early Cambrian. This geologically abrupt and spectacular record of early animal life is called the Cambrian explosion. (p. 5, emphases added)

They thus date the main pulse of the Cambrian explosion -- when "all or nearly all the major phylum-level groups of living animals" appeared -- to about 10 million years, consistent with the timescale given in Darwin's Doubt. After going through some objections to the claim that there really was an explosion, they conclude it was a real event:

Taken at face value, the geologically abrupt appearance of Cambrian faunas with exceptional preservation suggested the possibility that they represented a singular burst of evolution, but the processes and mechanisms were elusive. Although there is truth to some of the objections, they have not diminished the magnitude or importance of the explosion. ... Several lines of evidence are consistent with the reality of the Cambrian explosion. (p. 6, emphases added)

Second, as the book's subtitle suggests ("The Construction of Animal Biodiversity"), it correctly observes that explaining the Cambrian explosion requires explaining the origin of many diverse types of animal forms and body plans. Again, the authors write:

The subtitle of this book, The Construction of Animal Biodiversity, captures a second theme: the importance of building the networks that mediate the interactions. ... Increased genetic and developmental interactions were also critical to the formation of new animal body plans. By the time a branch of advanced sponges gave rise to more complex animals, their genomes comprised genes whose products could interact with regulatory elements in a coordinated network. Network interactions were critical to the spatial and temporal patterning of gene expression, to the formation of new cell types, and to the generation of a hierarchical morphology of tissues and organs. The evolving lineages could begin to adapt to different regions within the rich mosaic of conditions they encountered across the environmental landscape, diverging and specializing to diversify into an array of body forms. (pp. 8-9, emphasis added)

I'm not questioning whether they believe that animal body plans arose via unguided evolutionary processes. Obviously they do. What's important right here is that they recognize that explaining the Cambrian explosion requires explaining how the vast complexity and diversity of animal forms arose.

Third, and most importantly, Erwin and Valentine observe that standard neo-Darwinian mechanisms of repeated rounds of microevolution are not sufficient to explain the explosion of life in the Cambrian. They start by writing:

A third theme of this book is the tension between the nature of explanations for major evolutionary transitions in general and that of the Cambrian explosion in particular. (p. 9, emphasis added)

That's a good hint as to where they stand: the word "tension" is an artful way of saying that standard evolutionary mechanisms have a hard time accounting for the Cambrian explosion. They make this even more explicit later when they write:

As geologists, we view this tension as a debate over the extent to which uniformitarian explanations can be applied to understand the Cambrian explosion. Uniformitarianism is often described as the concept, most forcefully advocated by Charles Lyell in his Principles of Geology, that "the present is the key to the past" (Lyell 1830). Lyell argued that study of geological processes operating today provides the most scientific approach to understanding past geological events. Uniformitarianism has two components. Methodological uniformitarianism is simply the uncontroversial assumption that scientific laws are invariant through time and space. This concept is so fundamental to all sciences that it generally goes unremarked. Lyell, though, also made a further claim about substantive uniformitarianism: that the rates and processes of geological change have been invariant through time (Gould 1965). Few of Lyell's contemporaries agreed with him (Rudwick 2008). Today, geologists recognize that the rates of geological processes have varied considerably through the history of Earth and that many processes have operated in the past that may not be readily studied today.

The nature of appropriate explanations is particularly evident in the final theme of the book: the implications that the Cambrian explosion has for understanding evolution and, in particular, for the dichotomy between microevolution and macroevolution. If our theoretical notions do not explain the fossil patterns or are contradicted by them, the theory is either incorrect or is applicable only to special cases. Stephen Jay Gould employed the animals of the Burgess Shale and the early Cambrian radiation in his book Wonderful Life (Gould 1989) to advance his own view of evolutionary change. Gould argued persuasively for the importance of contingency -- dependence on preceding events--in the history of life. Many other evolutionary biologists have also addressed issues raised by these events. One important concern has been whether the microevolutionary patterns commonly studied in modern organisms by evolutionary biologists are sufficient to understand and explain the events of the Cambrian or whether evolutionary theory needs to be expanded to include a more diverse set of macroevolutionary processes. We strongly hold to the latter position. (pp. 9-10, emphases added)

I know that was a long passage, but read it carefully. What are they saying? They make it clear, especially in the last couple of sentences, that they think "microevolutionary processes" are not "sufficient to understand and explain the events of the Cambrian." Indeed, they later argue that microevolutionary processes are not sufficient to explain macroevolutionary ones, as they state: "the move from micro to macro forms a discontinuity." (p. 11)

This means that they don't believe "uniformitarian explanations can be applied to understand the Cambrian explosion." Why? Because evolutionary mechanisms we observe in the present day operate at rates that are too slow to explain what took place in the Cambrian period. They are careful not to put it in such plain terms, but that is the essence of their argument. But they do acknowledge that there was an "unusual period of evolutionary activity during the early and middle Cambrian." (p. 6) And this is also a major argument that Stephen Meyer makes in Darwin's Doubt.

And there are other statements in the book that admit key points that show how difficult it is to explain the Cambrian explosion through unguided evolutionary mechanisms. For example:

  • They acknowledge that something remarkable happened during the Cambrian period:
    Because the Cambrian explosion involved a significant number of separate lineages, achieving remarkable morphological breadth over millions of years, the Cambrian explosion can be considered an adaptive radiation only by stretching the term beyond all recognition. ... the scale of morphological divergence is wholly incommensurate with that seen in other adaptive radiations. (p. 341, emphasis added)
  • They argue that the Cambrian fauna evolved in a manner different from standard Darwinian processes, with few potential intermediates:
    [N]ovelty is rampant in the Ediacaran and Cambrian, but because so few intermediate species have been preserved, we are not able to assess whether these novelties are more apparent than real. The critical issue is the claim that evolutionary novelties may arise from different mechanisms than adaptive change ... Morphologic evolution is commonly depicted with lineages more or less gradually diverging from their common ancestor. New features arise along the evolving lineages ... Gould (1989, 38) characterized this pattern as the "cone of increasing diversity," but neither the Cambrian nor the living marine fauna display this pattern. (pp. 339-340)
  • They argue that the differences between the phyla that appear in the Cambrian make it difficult to imagine what the last common ancestor ("LCA") would have been:
    To be sure, all pairs of crown phyla had common ancestors; as far as we know, however, none of those bilaterian LCAs had features that would cause them to be diagnosed as members of living phyla, although that could be the case in a few instances. In other words, the morphological distances -- gaps -- between body plans of crown phyla were present when body fossils first appeared during the explosion and have been with us ever since. The morphological disparity is so great between most phyla that the homologous reference points or landmarks required for quantitative studies of morphology are absent. (p. 340)
    Stephen Meyer makes a very similar -- though more detailed argument -- in chapters 4 and 5 of Darwin's Doubt.
  • They admit that it is very difficult to understand the phylogenetic relationships of major animal groups:
    Although we would like to be able to predict that views of metazoan phylogenetic relationships will finally stabilize in the next few years, we must confess a certain degree of pessimism. Although we both feel quite strongly that the metazoan phylogeny presented in Chapter 4 and used throughout the book is right, the polytomies on the tree show that a number of issues remain unresolved. There has been ongoing debate about the topology of relationships at the base of the tree, including the paraphyly of sponges and the position of the ctenophores and acoels. The relationship of many of the lophotrochozoan clades remains very uncertain, and some of them may not even be monophyletic. There are also some groups of deuterostomes and ecdysozoans for which the phylogenetic placement continues to be debated. (pp. 336-337)

  • They admit that molecular clock studies have had great difficulty in explaining the timing of the divergence of various animals groups:
    In general, however, the nodes dated by molecular clock techniques are significantly older than nodes judged from the fossil record, commonly as much as one-third older, and some, surely erroneous, have been literally 1 billion years older. (p. 74)
  • They acknowledge that the evolution of key characters of animals remain unresolved, such as the transition of supposedly primitive animals like sponges to more complex ones:

    The pathway from sponges to eumetazoans is the most enigmatic of any evolutionary transition in metazoans. This transition occurred during the Cryogenian, almost contemporaneously with the diversification of sponges. Many biologists concerned with metazoan phylogeny have been convinced that "ontogeny recapitulates phylogeny" and have therefore assumed that the planktonic larval stages of invertebrate phyla represented their ancestral forms. The benthic nature of sponges and the paraphyly of the major clades demonstrate that planktonic stages could not have been ancestral to eumetazoans. Further, there are no living intermediates between sponges and eumetazoans, with the possible exception of placotozoans, and no obvious hints from the fossil record. (p. 324)
  • They admit that another important transition in the origin of animals is also unresolved:
    The greatest conundrum about the origin of eumetazoans -- more specifically, cnidarians -- is, however the evolution of the cnidocyst, the defining structure of the clade. (p. 324)
  • They admit that simplistic hypotheses that rely solely upon increased oxygen in the water, or climate changes, are insufficient to explain the Cambrian explosion:
    Simultaneous changes in so many different metazoan clades during the latest Ediacaran and early Cambrian, as well as associated changes in the plankton (chap. 6), point to either a pronounced environmental shift or ecological feedback. Thus, it is hardly surprising that increased concentrations of oxygen in shallow marine waters to levels that would sustain larger and more active animals have figured in several hypotheses, with evolutionary processes responding to this environmental opportunity. Even if historically true, as seems plausible, such an event hardly "explains" the biological diversification. (p. 319)

Many of these points are made in some or more detail by Stephen Meyer in Darwin's Doubt. But probably the most striking statement by Erwin and Valentine comes when they concede that they lack resolved evolutionary explanations for how the diversity of the Cambrian animals arose, and why these basic body plans haven't changed since that time:

The patterns of disparity observed during the Cambrian pose two unresolved questions. First, what evolutionary process produced the gaps between the morphologies of major clades? Second, why have the morphological boundaries of these body plans remained relatively stable over the past half a billion years? (p. 330, emphasis added)

Don't miss the importance of this quote: two of the leading scientists who study the Cambrian explosion just stated that the processes that produced the diverse body plans in the Cambrian are "unresolved." This is exactly why the journal Science, when recently reviewing Erwin and Valentine's book, stated:
The Ediacaran and Cambrian periods witnessed a phase of morphological innovation in animal evolution unrivaled in metazoan history, yet the proximate causes of this body plan revolution remain decidedly murky. The grand puzzle of the Cambrian explosion surely must rank as one of the most important outstanding mysteries in evolutionary biology.

(Christopher J. Lowe, "What Led to Metazoa's Big Bang?," Science, Vol. 340: 1170-1171 (June 7, 2013) (emphasis added).)
It seems Greg Mayer was right about one thing: readers of the Why Evolution is True blog will learn something important from reading The Cambrian Explosion: They will learn that some of the central arguments in Darwin's Doubt are correct. This adds to the growing chorus of recent scientific publications which affirm some of Meyer's core arguments.

Now the fact that Erwin and Valentine acknowledge these key points in no way makes The Cambrian Explosion a substitute for Stephen Meyer's book. Darwin's Doubt is (in my view) the most current and credible critique of neo-Darwinism available today, explaining the fundamental problems that hamper Darwinian theory as it attempts to explain the Cambrian explosion. Meyer shows how the key problems facing neo-Darwinism are not simply being imagined by proponents of intelligent design, but are stated clearly in peer-reviewed research by scientists totally unaffiliated with the ID community. The recognition and analysis of these problems has a distinguished academic pedigree, going back decades in the writings and discussions of very mainstream theorists. Moreover, Meyer does something else that Erwin and Valentine don't do: he describes (and critiques) the many post-Darwinian theories being proposed as alternatives to Darwinism by the growing number of evolutionary biologists who have become disillusioned with the neo-Darwinian account.

Nonetheless, I'm pleased that Jerry Coyne's blog is recommending Erwin and Valentine's The Cambrian Explosion. Unlike Greg Mayer, however, I am not interested in discouraging people from reading books. Quite to the contrary, if the topic of the Cambrian explosion really interests you, I suggest you pick up copies of both The Cambrian Explosion and Darwin's Doubt.