July 29, 2004 | David F. Coppedge

Cambrian Explosion Explained, or Explained Away?

James Valentine, an authority on early fossils, has just published a new 600-page book on the Cambrian explosion with the Darwinesque title, On the Origin of Phyla (U. of Chicago Press, 2004).  Stefan Bengtson (Swedish Museum of Natural History) reviewed it in the July 29 issue of Nature.1  He points out that “Darwin wisely called his best-known work On The Origin of Species; the origin of phyla is an even stickier problem, and Valentine deserves credit for tackling it at such breadth.”.  He is not sure, however, that Valentine succeeded in explaining “one of the most significant revolutions in the history of life, the Cambrian explosion.”
    One complaint is that in all those pages Valentine said little about the ecology or physical environment in which the “explosion of body plans” took place.  Also, Bengtson is not convinced that the usual explanation is meaningful that a phylum is simply a clade (category) of all animals that diverged from a common ancestor when two body plans diverged in the remote past; “This avoids the question of how body plans arise and whether there may be others not represented by living forms,” he chides.  Worse, Valentine fell into a logical trap, he feels:

Defining a body plan isn’t easy, however.  Valentine’s definition, for example, is dangerously circular: “an assemblage of morphological features shared among members of a phylum-level group”.  What does that mean, except that when we define a phylum we also define its body plan, or vice versa?  Valentine proposes to define the origin of a phylum by the acquisition of a key apomorphy – a unique derived trait.  This may be more subjective and less convenient than letting the total (stem and crown) group or the crown group define the phylum, but it gives due priority to biological significance over methodological convenience.  After all, we want to know how different kinds of organism evolve by natural selection, and how they interact with each other and with the environment.  They do that with their phenotypes, not their pedigrees.

Bengtson also considers the suggestion that body plans represented “more or less the total number of possible solutions to the problem of being an animal, or whether there were numerous other possibilities that came into being but became extinct because of bad luck or bad design.”  (The evidence shows a decrease in body plans after the explosion due to extinction, not a gradual rise in diversity.)  But is this just explaining away the evidence?

The pattern of diminishing evolutionary novelty subsequent to this event, he says, may have been due less to developmental constraints than to a saturation effect (candidates for new adaptive radiations were already available among existing body plans).  He also believes that the Cambrian explosion produced a lot more homoplasies (similar characters with independent origins) than most phylogenetic analyses suggest – in my view an extremely important point that calls for much more careful character evaluation than is commonly done.  He is clearly not impressed, then, by some recent attempts to use fossils to bridge gaps between phyla.

If the reader is left wondering how the body plans arose in the first place, the final paragraph of this book review may not be all that satisfying.  How could environmental changes generate the information necessary to produce fins, eyes, jointed limbs, propulsion mechanisms, and so much more that is evidenced in the Cambrian fossils?

Valentine seems most happy with intrinsic biological mechanisms for the rapid appearance of phyla.  Large parts of the book deal with developmental prerequisites (such as cell-type numbers and gene regulation) for the event.  Ecological interactions, such as predation, are given more cursory treatment.  As for the physical environment, he merely concludes, somewhat apologetically, that although physical environmental factors were “supremely important”, he does not see any evidence that extraordinary environmental events were causally connected with the Cambrian explosion.  Given that extraordinary environmental events did indeed occur shortly before the explosion, I would give the jury just a little more time to ponder the question.  But first I would make sure they had read this magnificent book.

So how did the body plans arise in a geological blink of an eye?  This question was apparently not on the agenda.
    Next day in Science,2 R. Andrew Cameron also reviewed Valentine’s book.  This review praised and criticized different things.  Cameron first dismisses the analogy “explosion,” primarily because he claims that molecular studies put the origin of the phyla farther back into the precambrian; consequently, he claims, it was “neither an explosion nor did it happen in the Cambrian,” although he does agree that the Chengjian fossils display “representatives of almost all major groups of animals” (see 07/20/2004 headline).  He mentions the possibility that ancestors were soft-bodied and small, resulting in a poor fossil record; “Perhaps the conditions of the Cambrian environment allowed the rapid appearance of hard skeletal parts, greatly favored fossilization, or both.”  But then he mentions the discovery of “fossil pre-Cambrian embryos from the Doushantuo Formation of southwest China, estimated to be 40 to 55 million years older than the base of the Cambrian,” so being soft and small did not hinder these specimens from becoming fossilized.
    Cameron understands the problem of the Cambrian explosion, and claims it is more of a problem now than in Darwin’s day:

The question of when and how higher taxonomic groups like phyla evolved differs markedly from the one Darwin addressed 145 years ago in The Origin of Species.  It is not simply different in scale but also in quality.  Although it is somewhat easier to see how changes in single genes can lead to differences among species that render some more capable of surviving in particular environments, it is more difficult to account for the many changes that lead to entirely different bodyplans as a simple accumulation of single-gene effects.  For example, marine stickleback fishes possess bony plates and spines that presumably prevent predation, while their freshwater relatives show a loss of this armor through changes that can be attributed to a single gene [see 06/18/2004 headline].  However, entire organ systems or embryonic germ layers, features that distinguish higher taxa, can be explained in terms of the gene regulatory networks whose architecture is hardwired into the genome.

So the question for the origin of phyla is how did these hardwired gene regulatory networks arise?  Cameron claims that Valentine “does not incorporate a molecular model in his final synthesis,” so he offers one himself: major changes might arise through changes in regulatory genes like transcription factors.  Can he give us an example?  “For instance, a morphogenetic program may evolve with relatively minimal changes to establish a new spatial domain of expression for a cell-differentiation program, and the resultant animal has a new body part.”  He does not elaborate.
    Cameron praises the first two sections of the book that discuss the origins of the phyla, descriptions of the phyla, and the fossil record.  The third section grapples with the evolution of the phyla.  This section is lacking, the reviewer thinks: “The pictures he delineates here reveal correlations uniting different levels of biological organization, but absent are firm statements about causal mechanisms from which predictions could be made.
    Cameron leaves us with one more concern.  “In view of the volatility of the ideas and the controversy that still exist in this particular area of evolutionary biology, one might argue that it is too early to explain the causes of the origin of phyla.  But as Valentine aptly points out, the time will never be exactly right: there are always more information to incorporate and more ideas to organize.”
    Incidentally, Nature also reported discovery of an arthropod fossil that pushes its group, the Euthycarcinoids, back 50 million years into the Cambrian.  “Despite its antiquity and marine occurrence,” they admit with surprise, “the Cambrian species demonstrates that morphological details were conserved in the transition to fresh water.”

1Stefan Bengtson, “The body-plan explosion,” Nature 430, 506 (29 July 2004); doi:10.1038/430506a.
2R. Andrew Cameron, “Evolution: Hunting for Origins,” Science, Vol 305, Issue 5684, 613-614, 30 July 2004, [DOI: 10.1126/science.1100684].
3Vaccari et al., “Cambrian origins and affinities of an enigmatic fossil group of arthropods,” Nature 430, 554 – 557 (29 July 2004); doi:10.1038/nature02705.

Satisfied?  Apparently in 600 pages, Valentine did not answer the most basic and fundamental question, how did all this biological complexity emerge in a short time?  Pounding the earth with meteors and tidal waves and volcanoes won’t do it.  Invoking a new predator won’t create an elaborate escape mechanism in the prey; it might just mean the predator will eat everything and then starve.  Cameron’s folklore is simplistic: a regulatory gene mutates and presto!  A new body part!  Can duplicating some protuberance generate an eye?  Come on.
    Let’s parse Cameron’s carefully-worded closing lines.  He said, “In view of the volatility of the ideas and the controversy that still exist in this particular area of evolutionary biology, one might argue that it is too early to explain the causes of the origin of phyla.”  Cameron, like Valentine is well aware of the pounding the Darwin Party is getting from the Intelligent Design Movement with the Cambrian Explosion hammer (see The Cambrian Explosion: Biology’s Big Bang by Meyer, Ross, Nelson and Chien (12/01/2003) available online at the Discovery Institute).  The Cambrian Explosion is only controversial because the Darwinians have no answer, and the creationists and ID proponents know it.  So he’s worried that Valentine’s new book is going to provide even more ammo to the enemy.  His coded message to the Darwin Party can be translated, “What do you think you are doing, Jim, letting the creationists know we’re up a creek?  Better to say nothing than to advertise our weaknesses!”
    Does anyone see in either of these reviews any real, logical explanation for the explosive appearance of trilobites, worms, jellyfish, corals, and vertebrates, with any evidence to back it up?  Each of these organisms is composed of irreducibly complex cells, and organs made up of irreducibly complex parts.  When they first appear in the fossil record, they are already fully formed and operating.  If the world’s expert can’t explain this after 30 years of thinking about it, then maybe there isn’t an explanation – from a Darwinian viewpoint.  This requires some fresh blood from thinkers not wedded to a dying, outmoded, falsified model that is on the verge of extinction.  Any takers?

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