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More on Small Shelly Fossils and the Length of the Cambrian Explosion: A Concluding Response to Charles Marshall

ScienceCover-1.gifIn my previous replies to Charles Marshall's review in Science of Darwin's Doubt, I've responded to his critiques of the main argument of the book -- in particular, his claims that (a) the Cambrian explosion would not have required a significant increase in new genetic information and (b) his claim that my positive argument for intelligent design, based on the need for an increase in genetic (and other forms of biological) information, represents a purely negative "god-of-the-gaps" argument.

In this concluding response, I will address two other substantive, but minor, criticisms that Marshall makes of Darwin's Doubt. In his September 20 review in Science, Marshall claims that the book fails to discuss the small shelly fossils that arise at the base of the Cambrian period, and thereby exaggerates the brevity of the Cambrian explosion,1 treating it as 10-million-year event, rather than a roughly 25-million-year one as Marshall sometimes (but not always) does.2

The first of these two claims is false. Darwin's Doubt does discuss the small shelly fossils on page 425 in the following paragraph:

The Cambrian period 543 mya is marked by the appearance of small shelly fossils consisting of tubes, cones, and possibly spines and scales of larger animals. These fossils, together with trace fossils, gradually become more abundant and diverse as one moves upward in the earliest Cambrian strata (the Manykaian Stage, 543-530 mya).3

Of course, Marshall in his review implies that Darwin's Doubt should have treated the first appearance of the small shelly fossils as part of the Cambrian explosion. The main pulse of morphological innovation that many Cambrian paleontologists designate as the explosion took place between 530 and 520 million years ago. Marshall faults Darwin's Doubt for failing to include the first appearance of the small shelly fossils beginning 12-13 million years earlier (543-542 million years ago) as the beginning of the explosion, a decision that would imply a 22-23 million year event, rather than a 10 million year event.

Readers should note that Casey Luskin has already extensively rebutted the claim that Darwin's Doubt exaggerates the brevity of the Cambrian explosion. I have done the same in my reply to John Farrell published in National Review.4 As I explain in my response to Farrell, Darwin's Doubt "affirms the widely-accepted figure among Cambrian paleontologists of about 10 million years for the main pulse of morphological innovation in the Cambrian period that paleontologists typically designate as 'the explosion.'" Luskin, in his article, also documents that this figure is widely accepted among many Cambrian experts, including Valentine and Erwin, whom Marshall cites affirmatively in his review.5

Moreover, as Luskin shows in a new article, Marshall himself, like many other Cambrian experts, does not regard the small shelly fossils as obviously ancestral to most of the animals that arise in the main explosive period of the Cambrian radiation. In one 2006 paper he depicts them as (apparently) disconnected from the later more significant pulses of morphological innovation.6 In fact, Marshall notes repeatedly that the small shelly fossils are "largely problematic" and "hard to diagnose even at the phylum level."7 Moreover, in a technical article published in 2010, Marshall specifically excludes the small shelly fossils from the ten million year "geologically abrupt appearance of fossils representing quite disparate body plans" that he and co-author James Valentine designate "as the Cambrian explosion."8

In any case, treating the first appearance of the small shelly fossils as the beginning of the Cambrian explosion does little to explain the main pulse of the morphological innovation that occurs later during the 10-million-year window that paleontologists commonly designate as "the explosion." As I acknowledge in Darwin's Doubt, it is entirely possible to assign a different duration to the "Cambrian explosion" depending upon how many separate paleontological events scientists choose to include within that designation. Nevertheless, quibbling of that sort reduces the debate to one of semantics. The key question is not how many different events should be included within the designation "Cambrian explosion." Nor is it about the total amount of time that some arbitrarily designated series of separate paleontological events covers. Instead, the key question is what caused the discontinuous appearance of morphological novelty within specific, and measurably narrow, windows of geological time -- whatever we choose to call them. Thus, Darwin's Doubt focuses on the crucial Tommotian and Atdabanian stages of the Cambrian explosion -- where 13-16 new animal phyla arose within a 5-6 million year window -- as a defining challenge to the efficacy of the neo-Darwinian mechanism.9 Marshall doesn't explain how the origin of the small shelly fossils diminishes the problem of the origin of the morphological novelty within that window of time.

Moreover, as I discussed in a previous response to Marshall (and in Chapters 10 and 12 of Darwin's Doubt), even a duration of 25 million years would not appreciably diminish the problem facing contemporary evolutionary theory. In the first place, 25 million years would not provide enough opportunities for the mutation/selection process to search more than a tiny fraction of the relevant sequence spaces necessary to produce even a single new gene or functional protein.10 Second, the calculated waiting times required to evolve multi-mutation features also suggest that even pushing the beginning of Cambrian explosion back to the first appearance of the small shelly fossils, as Marshall suggests we should, does not provide enough time for many complex biological features to evolve.11 Marshall does not attempt to refute these experimentally based quantitative arguments. Consequently, it's hard to see how my decision not to make more of these enigmatic small shelly fossils in any way undermines the main arguments of Darwin's Doubt.

REFERENCES:

(1) Specifically, Marshall charges: "Meyer completely omits mention of the Early Cambrian small shelly fossils and misunderstands the nuances of molecular phylogenetics, both of which cause him to exaggerate the apparent suddenness of the Cambrian explosion." Charles R. Marshall, "When Prior Belief Trumps Scholarship," Science, 341 (September 20, 2013): 1344.

(2) In fact, Marshall sometimes treats the Cambrian explosion as a ten million year event that does not include the first appearance of the small shelly fossils. As Marshall co-writes with James Valentine: "By the beginning of the Cambrian Period, near 543 million years ago, a few kinds of 'small shelly' fossils are found, <2mm in largest dimension. The small shellys rose to a peak in abundance and diversity during the period from 530 to 520 million years ago, when representatives of living phyla are found among them. During that same period, a chiefly larger-bodied invertebrate fauna of up to a dozen phyla, and including many soft-bodied forms, is also first represented by fossils. This geologically abrupt appearance of fossils representing quite disparate bodyplans of many living metazoan phyla is termed the Cambrian explosion..." Charles R. Marshall and James W. Valentine, "The Importance of Preadapted Genomes in the Origin of the Animal Bodyplans and the Cambrian Explosion," Evolution 64-5 (2010): 1189-1201.

(3) Darwin's Doubt, p.425.

(4) National Review, Letters Section, September 30, 2013.

(5) See Casey Luskin, "How 'Sudden' Was the Cambrian Explosion? Nick Matzke Misreads Stephen Meyer and the Paleontological Literature; New Yorker Recycles Misrepresentation," Evolution News & Views, July 16, 2003 at http://www.evolutionnews.org/2013/07/how_sudden_was_074511.html

(6) See Figure 1, Charles R. Marshall, "Explaining the Cambrian 'Explosion' of Animals," Annual Reviews of Earth and Planetary Sciences 34 (2006): 355-384. (See Figure 1).

(7) Charles R. Marshall, "Explaining the Cambrian 'Explosion' of Animals," Annual Reviews of Earth and Planetary Sciences 34 (2006): 355-384.

(8) As Marshall writes: "By the beginning of the Cambrian Period, near 543 million years ago, a few kinds of 'small shelly' fossils are found, <2mm in largest dimension. The small shellys rose to a peak in abundance and diversity during the period from 530 to 520 million years ago, when representatives of living phyla are found among them. During that same period, a chiefly larger-bodied invertebrate fauna of up to a dozen phyla, and including many soft-bodied forms, is also first represented by fossils. This geologically abrupt appearance of fossils representing quite disparate bodyplans of many living metazoan phyla is termed the Cambrian explosion..." Charles R. Marshall and James W. Valentine, "The Importance of Preadapted Genomes in the Origin of the Animal Bodyplans and the Cambrian Explosion," Evolution 64-5 (2010): 1189-1201.

(9) http://www.evolutionnews.org/2013/10/to_build_new_an077541.html

(10) See Darwin's Doubt, Chapter 10.

(11) See Darwin's Doubt, Chapter 12.