How &quot;Sudden&quot; Was the Cambrian Explosion? Nick Matzke Misreads Stephen Meyer and the Paleontological Literature; <i>New Yorker</i> Recycles Misrepresentation - Evolution News & Views

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How "Sudden" Was the Cambrian Explosion? Nick Matzke Misreads Stephen Meyer and the Paleontological Literature; New Yorker Recycles Misrepresentation

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On June 19, the day after Darwin's Doubt was first available for purchase, Nick Matzke published a 9400-word "review" of the book in which it appears that he tried to anticipate many of Stephen Meyer's arguments. Unfortunately, he often either guessed wrong as to what Meyer would say or -- assuming he actually read the book as he claims -- misread many of Meyer's specific claims. As I showed in a previous response to Matzke, Matzke repeatedly misquoted Meyer, at one point claiming he referred to the Cambrian explosion as "instantaneous," when Meyer nowhere makes that claim. Indeed, Matzke faulted Meyer for not recognizing that the Cambrian explosion "was not really 'instantaneous' nor particularly 'sudden.'" Oddly, he also criticized Meyer for not recognizing that the Cambrian explosion "took at least 30 million years" -- despite expert opinion showing it was far shorter.

Since Matzke published his review, The New Yorker reviewed Meyer's book. Gareth Cook, the science writer who wrote the piece, relied heavily on Matzke's critical evaluation, even though Matzke is a graduate student and not an established Cambrian expert. Cook uncritically recycled Matzke's claim that the Cambrian explosion took "many tens of millions of years," even saying that the main problem with Darwin's Doubt is that Meyer failed to recognize this alleged fact.

DebatingDD.jpegSo, was Matzke right about the length of the Cambrian explosion? In fact, Matzke's preemptive -- or hastily written -- review not only misrepresented Meyer's view, it also misrepresented the length and character of the Cambrian explosion as numerous authoritative peer-reviewed scientific sources on the subject clearly show.

Before going on, let's briefly look first at what Meyer actually says. First, Meyer does not equate the Cambrian explosion with the entire radiation -- as most Cambrian experts also do not. By "radiation" here I mean the period of time in which all the new phyla, classes, orders that first arose during the Cambrian apparently did so. Instead, he equates the Cambrian explosion with the most explosive period of the Cambrian radiation (as most Cambrian experts do) in which the vast majority of the higher taxa arose. He asserts specifically that the re-dating of critical Cambrian strata in 1993 established that the strata documenting the first appearance of the majority of the Cambrian phyla and classes took place within a 10 million year period -- a period Meyer does equate with "the explosion of novel Cambrian animal forms." (pp. 71-72) As he describes it, "these studies [i.e., radiometric analyses of zircon crystals in Siberian rocks] also suggested that the explosion of novel Cambrian animal forms" took about 10 million years. (p. 71)

In affirming this, however, Meyer offers a nice discussion of how different scientists may judge the duration of the Cambrian explosion differently, depending upon how they choose to define it and how many separate events they decide to include. (See pp. 71-73.) Thus, Meyer notes that if paleontologists decide to include as part of the Cambrian explosion (a) the origin of the Ediacaran organisms in the late Precambrian, and (b) the small shelly fossils at the base of the Cambrian and (c) the main pulse of morphological innovation in the early Cambrian, and (d) subsequent diversification events right up until the end of the Cambrian period, they might claim that the Cambrian explosion lasted nearly 80 million years, as, for example, geologist Donald Prothero does (a point Meyer also notes in his book). Nick Matzke appears to include in the Cambrian explosion everything from the appearance of the small shelly fossils at the base of the Cambrian (541 million years ago) to the main pulse of morphological innovation (530-520 million years ago) to events in the late Cambrian (about 512-505 million years ago).

In any case, Meyer recognizes the conventional and somewhat subjective nature of attempts to define and delimit "the Cambrian explosion." He nevertheless accepts a 10-million-year duration of the explosion itself, in keeping with the common judgment of numerous Cambrian experts about the length of time in which the vast majority of new phyla and classes arose -- as I will document below. Yet, to circumvent issues of semantics and subjective definitions, Meyer focused his analysis on the problem of the origin of novel animal form, and, thus, the main or most explosive pulse of such "morphological innovation." This makes sense because the problem that Meyer ultimately addresses, and the problem that evolutionary biology must address, is that of building novel animal forms or body plans in the first place. Can the neo-Darwinian mechanism generate the amount of novel form and information that arises in the Cambrian period in the time allowed by the fossil record? By focusing his analysis on the main period of morphological innovation, Meyer defines clearly the most salient challenge posed to the adequacy of neo-Darwinian (and other evolutionary) mechanisms.

To establish the length of the most explosive period of innovation within the Cambrian explosion itself, Meyer cites the work of MIT geochronologist Samuel Bowring and his colleagues as well the work of another group led by Smithsonian paleontologist Douglas Erwin. The Bowring-led study showed that (in their words) "the main period of exponential diversification" within the Cambrian lasted "only 5-6 million years" (emphasis added). Meyer explains:

An analysis by MIT geochronologist Samuel Bowring has shown that the main pulse of Cambrian morphological innovation occurred in a sedimentary sequence spanning no more than 6 million years. Yet during this time representatives of at least sixteen completely novel phyla and about thirty classes first appeared in the rock record. In a more recent paper using a slightly different dating scheme, Douglas Erwin and colleagues similarly show that thirteen new phyla appear in a roughly 6-million-year window. (p. 73)

To see why Meyer made these claims, take a look first at the following figure that Bowring and his colleagues included in their definitive 1993 article, published in the journal Science. They use radiometric methods to date the different stages of the Cambrian period, including the crucial Tommotian and Atdabanian stages in which the greatest number of new animal phyla and classes arise. Note that the so-called Manykaian stage of the Cambrian period lasts about 10-14 million years. Note also that the main pulse of morphological innovation didn't begin during this stage but rather during the Tommotian and Atdabanian -- a period that they describe as taking between "5 to 10 million years," and in a more detailed passage as taking about 5-6 million years.

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From Samuel A. Bowring, John P. Grotzinger, Clark E. Isachsen, Andrew H. Knoll, Shane M. Pelechaty, Peter Kolosov, "Calibrating Rates of Early Cambrian Evolution," Science, Vol. 261 (September 3, 1993): 1293-1298. Reprinted with permission from AAAS.

In the figure above, the Tommotian and Atdabanian stages of the Cambrian period together span only about 5 million years, starting at about 530 and ending about 525 million years ago. Bowring's figure also depicts the total number of classes and orders present at any given time during the Cambrian period. The biggest increases in morphological innovation occur during the Tommotian and Atdabanian stages. Indeed, during this period the number of known orders nearly quadruples. Moreover, Bowring and his colleagues also make clear that this period corresponds to the main pulse of Cambrian morphological innovation as measured by the number of new phyla and classes that first appear. They note that, while a few groups of animals do arise in the earliest Manykaian stage of the Cambrian, the most rapid period of "exponential increase of diversification," corresponding to the Tommotian and Atdabanian stages, "lasted only 5 to 6 m.y." They explain:

[T]he initial (Manykaian) interval of slow diversification followed the ediacaran faunal epoch by no more than 20 million years (m.y.) and lasted approximately 14 m.y. In contrast, if we accept the age of 525 Ma for the Atdabanian-Botomian boundary, then the Tommotian-Atdabanian period of exponential increase of diversification lasted only 5 to 6 m.y. In any event it is unlikely to have exceeded 10 m.y. Numbers of phyla, classes, orders, families, and genera all reached or approached their Cambrian peaks during the short Tommotian-Atdabanian interval. For phyla and classes, most of the diversity known for the Phanerozoic [the eon of time since the Cambrian] as a whole differentiated by the end of the Atdabanian. (emphasis added)

Meyer also cites a 2011 paper in Chapter 3 (page 73) by Douglas Erwin and several colleagues. Although Erwin et al. use slightly different starting and ending dates and different names for the stages of the Cambrian period, they too estimate that that the most explosion stage took about 5-6 million years. Indeed, the supplemental documentation to their article shows 13 or 14 new phyla arising during "Stage 3" of the Cambrian period, a stage that corresponds to a narrow 5-6 million year window (see figure 3 in their article), just as Meyer wrote in Darwin's Doubt.

Erwin and his colleagues note that "most paleontologists favor a near literal reading of the fossil record, supporting a rapid (~25-million-year) evolutionary divergence of most animal clades near the base of the Cambrian" -- a duration a bit shorter than but close to the "at least 30 million years" given by Matzke. But here the authors are talking about not only the most explosive stage (Stage 3) or stages (Stages 2 and 3) of the Cambrian, but also Stage 1, which they and most experts usually exclude from "the Cambrian explosion."

Indeed, Erwin, writing more recently with James Valentine in their new book The Cambrian Explosion, dates the Cambrian explosion to "a geologically brief interval between about 530 to 520 Ma" (emphasis added):

[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 soft-bodied 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. (The Cambrian Explosion, p. 5, emphases added)

Like many Cambrian experts, Erwin and Valentine focus their analysis on that part of the Cambrian radiation in which the greatest amount of morphological innovation arises -- and define "the Cambrian explosion" accordingly. They believe that nearly the full breadth of Cambrian diversity arose in less than ten million years, writing: "the basic structure of Phanerozoic ecosystems had been achieved within at most 10 million years after the onset of bilaterian diversification." (The Cambrian Explosion, p. 226, emphasis added) Moreover, many other Cambrian experts focus on precisely this period of the origin of maximum morphological novelty in their discussion (and definition) of the Cambrian explosion. They define the Cambrian explosion as an event that encompassed about (or even less than) 10 million years just as Meyer does, not one that took "at least 30 million years" as Matzke claimed. For example:

  • Prominent paleontologist Robert Carroll stated in Trends in Ecology and Evolution that the Cambrian explosion took less than ten million years:

    "The most conspicuous event in metazoan evolution was the dramatic origin of major new structures and body plans documented by the Cambrian explosion. Until 530 million years ago, multicellular animals consisted primarily of simple, soft-bodied forms, most of which have been identified from the fossil record as cnidarians and sponges. Then, within less then 10 million years, almost all of the advanced phyla appeared, including echinoderms, chordates, annelids, brachiopods, molluscs and a host of arthropods. The extreme speed of anatomical change and adaptive radiation during this brief time period requires explanations that go beyond those proposed for the evolution of species within the modern biota." (Robert L. Carroll, "Towards a new evolutionary synthesis," Trends in Ecology and Evolution, Vol. 15: 27-32 (January, 2000), emphasis added.)
  • An article in the journal Development by Erwin, Valentine and David Jablonski explains that:

    "The Cambrian explosion is named for the geologically sudden appearance of numerous metazoan body plans (many of living phyla) between about 530 and 520 million years ago, only 1.7% of the duration of the fossil record of animals." (James W. Valentine, David Jablonski and Douglas H. Erwin, "Fossils, molecules and embryos: new perspectives on the Cambrian explosion," Development, Vol. 126: 851-859 (1999), emphases added.)
  • Another article in a major evolution journal states that "recent geological investigations suggest that the Cambrian explosion may have occurred within a period of only 5-10 million years." (Michael A. Bell, "Origin of the metazoan phyla: Cambrian explosion or proterozoic slow burn," Trends in Ecology and Evolution, Vol. 12: 1-2 (January 1, 1997), emphasis added.)
  • A paper in BioEssays states: "Because of the sudden appearance of a near complete diversity of animal body plans in the fossil record around 530- 520 million years ago, this diversification is commonly referred to as the 'Cambrian explosion'." (Tanya Vavouri and Ben Lehner, "Conserved noncoding elements and the evolution of animal body plans," BioEssays, Vol. 31: 727-735 (2009), emphasis added.)
  • Another paper by the eminent biologist Susumu Ohno states, "this Cambrian explosion, during which nearly all the extant animal phyla have emerged, was of an astonishingly short duration, lasting only 6-10 million years." (Susumu Ohno, "The notion of the Cambrian pananimalia genome," Proceedings of the National Academy of Sciences, USA, Vol. 93: 8475-8478 (August, 1996), emphasis added.)
  • A paper by Andrew R. Parker of the Department of Zoology at the Natural History Museum in London states: "The Cambrian explosion, or Big Bang in animal evolution, was the most dramatic event in the history of life on Earth. During this blink of an eye in such history, most phyla found today evolved their first hard parts and distinct shapes at the same time. In other words, it is the event where animals suddenly took on very different appearances, in the form they exist today. The event itself, however, occupied only a small part of the Cambrian period, somewhere between 520 and 515 Ma. Prior to this, there were only three animal phyla with the type of external shapes they still possess today. Yet in a geological instant later there were at least several more -- and perhaps most -- of the phyla known today." (Andrew R. Parker, "On the origin of optics," Optics & Laser Technology, Vol. 43: 323-329 (2011), emphasis added.)
  • Even a 2007 paper in Journal of College Science Teaching, authored by zoologist Thomas Gregg (who criticizes intelligent design in the article) states: "The Cambrian explosion is the appearance of several dozen fossilized species with different body plans over a period of 5-15 million years during the Cambrian period." (Thomas Gregg, "Intelligent Design: Jonathan Wells and the Tree of Life," Journal of College Science Teaching (July/August, 2007), emphasis added.)

In case you didn't notice, none of these authorities are saying the Cambrian explosion "took at least 30 million years."

Matzke does cite one paper when attempting to justify his claim that the Cambrian explosion "took at least 30 million years, and was not really 'instantaneous' nor particularly 'sudden.'" But that source -- a 2005 paper in Paleobiology by Kevin J. Peterson, Mark A. McPeek, and David A. D. Evans -- does not place exact numbers on the timescale of the Cambrian explosion, so it doesn't help Matzke's case much. Indeed, a close analysis of the figure Matzke posts from that paper shows that it too reveals a rapid pulse of diversification in the mid-early Cambrian. Moreover, two of those three authors directly contradicted Matzke's thesis about the length of the Cambrian explosion in a paper in BioEssays, published four years later:

Part of the intrigue with the Cambrian explosion is that numerous animal phyla with very distinct body plans arrive on the scene in a geological blink of the eye, with little or no warning of what is to come in rocks that predate this interval of time. The abruptness of the transition between the "Precambrian" and the Cambrian was apparent right at the outset of our science with the publication of Murchison's The Silurian System, a treatise that paradoxically set forth the research agenda for numerous paleontologists -- in addition to serving as perennial fodder for creationists. The reasoning is simple -- as explained on an intelligent-design t-shirt.
Fact: Forty phyla of complex animals suddenly appear in
the fossil record, no forerunners, no transitional forms
leading to them; "a major mystery," a "challenge." The Theory
of Evolution -- exploded again (idofcourse.com).
Although we would dispute the numbers, and aside from the last line, there is not much here that we would disagree with. Indeed, many of Darwin's contemporaries shared these sentiments, and we assume -- if Victorian fashion dictated -- that they would have worn this same t-shirt with pride.

(Kevin J. Peterson, Michael R. Dietrich and Mark A. McPeek, "MicroRNAs and metazoan macroevolution: insights into canalization, complexity, and the Cambrian explosion," BioEssays, Vol. 31: 736-747 (2009) (emphases added).)
Matzke appears unaware of what the very authorities he cites have said about the length of the Cambria explosion.

Indeed, unquestionably, many senior Cambrian paleontologists and other established Cambrian experts contradict Matzke's claim about the length of the Cambrian explosion. Of course, Matzke is free to define the Cambrian explosion in whatever idiosyncratic way he chooses. However by defining it as a series of separate events in the fossil record spanning "at least 30 million years'" he not only introduces confusion around a term with a relatively stable meaning in paleontology, he diverts attention from the crucial problem of explaining the most explosive appearance of evolutionary and morphological novelty that "the Cambrian explosion" has commonly been used to describe. In any case, Matzke's attack on Meyer on this point is entirely unjustified.

Was the Cambrian Explosion "Sudden"?
But what about Matzke's claim that Meyer should not have referred to the event as geologically "sudden"? We have already seen that Valentine, Jablonski, and Erwin called the Cambrian explosion "geologically sudden." As it turns out, many other authors in the technical literature have used that exact terminology to describe the Cambrian explosion:

  • "Nobody seriously doubts that the sudden appearance in the fossil record of numerous marine animal groups of both familiar and enigmatic type close to the base of the Cambrian reflects one of the important events in the history of the biosphere." (R.A. Fortey, D.E.G. Briggs, M.A. Wills "The Cambrian evolutionary cexplosion': decoupling cladogenesis from morphological disparity," Biological Journal of the Linnean Society, Vol. 57: 13-33 (1996), emphasis added.)
  • "Beautifully preserved organisms from the Lower Cambrian Maotianshan Shale in central Yunnan, southern China, document the sudden appearance of diverse metazoan body plans at phylum or subphylum levels, which were either short-lived or have continued to the present day." (J.Y. Chen, "The sudden appearance of diverse animal body plans during the Cambrian explosion," International Journal of Developmental Biology, Vol. 53: 733-51 (2009), emphases added.)
  • "...the sudden expansion in phyla of the Cambrian explosion" (Lynn Helena Caporale, "Putting together the pieces: evolutionary mechanisms at work within genomes," BioEssays, Vol. 31: 700-702 (2009), emphasis added.)
  • A college-level invertebrate biology textbook states: "Most of the animal phyla that are represented in the fossil record first appear, "fully formed" and identifiable as to their phylum, in the Cambrian .... The fossil record is therefore of no help with respect to understanding the origin and early diversification of the various animal phyla..." (R. S. K. Barnes, P. Calow, P. J. W. Olive, D. W. Golding, and J. I. Spicer, The Invertebrates: A New Synthesis, 3rd ed. (Malden, MA: Blackwell Scientific Publications, 2001), pp. 9-10, emphasis added.)
  • "...the sudden appearance of a near complete diversity of animal body plans in the fossil record around 530-520 million years ago" (T. Vavouri and B. Lehner, "Conserved noncoding elements and the evolution of animal body plans," BioEssays, Vol. 31: 727-735 (July 31, 2009), emphasis added.)
  • "...the profound morphological gaps among the major groups, set against the background of sudden appearances in the fossil record of many novel taxa and the absence of easily recognizable transitional forms" (Richard K. Grosberg, "Out on a Limb: Arthropod Origins," Science, Vol. 250: 632-633 (November 2, 1990), emphasis added.)
  • "Darwin recognized that the sudden appearance of animal fossils in the Cambrian posed a problem for his theory of natural selection. ... Recent geochronological studies have reinforced the impression of a 'big bang of animal evolution' by narrowing the temporal window of apparent divergences to just a few million years." (Gregory A. Wray, Jeffrey S. Levinton, Leo H. Shapiro, "Molecular Evidence for Deep Precambrian Divergences," Science, Vol. 74: 568-573 (October 25, 1996), emphasis added.)
  • "The apparently sudden origin of animal phyla has contributed to the view that phyla represent a fundamental level of organization." (Lindell Bromham, "What can DNA Tell us About the Cambrian Explosion?," Integrative and Comparative Biology, Vol. 43: 148-156 (2003), emphasis added.)
  • "The fossil record of metazoa shows a sudden expansion at around 550-530 million years ago." (Science, Vol. 288: 929 (May 12, 2000), emphasis added.)
  • "This paucity of metazoan fossils in the strata of Earth is broken by the sudden appearance of highly developed metazoan fossils in the Cambrian, a pattern colloquially referred to as the Cambrian evolutionary 'explosion'." (Christopher W. Wheat and Niklas Wahlberg, "Phylogenomic Insights into the Cambrian Explosion, the Colonization of Land and the Evolution of Flight in Arthropoda," Systematic Biology, Vol. 62: 93-109 (2013), emphasis added.)
  • "[T]he fossil record displays the sudden appearance of intracellular detail and the 32 phyla." (Michael A. Crawford, C. Leigh Broadhurst, Martin Guest, Atulya Nagar, Yiqun Wang, Kebreab Ghebremeskel, Walter F. Schmidt, "A quantumtheory for the irreplaceable role of docosahexaenoic acid in neural cell signalling throughout evolution," Prostaglandins, Leukotrienes and Essential Fatty Acids, Vol. 88: 5-13 (2013), emphasis added.)
  • "The Cambrian explosion in animal evolution during which all the diverse body plans appear to have emerged almost in a geological instant is a highly publicized enigma." (Eugene V. Koonin, "The Biological Big Bang model for the major transitions in evolution," Biology Direct, Vol. 2: 21 (2007), emphasis added.)
  • "At the beginning of the Cambrian, however, life took a sudden turn toward the complex. In a few million years -- the equivalent of a geological instant -- an ark's worth of sophisticated body types filled the seas. This biological burst, dubbed the Cambrian explosion, produced the first skeletons and hard shells, antennae and legs, joints and jaws. It set the evolutionary stage for all that followed by giving rise to most of the major phyla known on Earth today. Even our own chordate ancestors got their start during this long-past era." (Richard Monastersky, Science News, Vol. 146 (9) (August 27, 1994), emphases added.)

So, again, Matzke's claims stand at odds with the technical literature in a field he purports to represent. At the very least, Meyer seems fully justified in calling the Cambrian explosion "sudden" because so may other authorities use that same term--authorities I suspect Nick Matzke never told The New Yorker about. It's too bad The New Yorker, once legendary for meticulous fact-checking, didn't dig a little deeper but instead relied on Matzke's claims, which have turned out to be incorrect.

There is a concluding irony in all this. As Meyer shows in Chapter 10 and Chapter 12 of Darwin's Doubt, the extreme rarity of genes and proteins in sequence space means that even thirty million years is not nearly enough time to give the neo-Darwinian mechanism a realistic opportunity to generate a new gene or protein -- let alone a new form of animal life. Further, as he shows in Chapter 12, the calculated waiting times using the standard principles of population genetics for the occurrence of just a few (three or more) coordinated mutations vastly exceed 30 million years. In his review, Matzke summarily dismissed these arguments, neither engaging nor rebutting them, as other articles here at ENV have shown, and will do so in more detail.


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