Yesterday, I posted A succinct case for Intelligent Design, which featured a 123-word critique of unguided mechanisms for evolution – in particular, neo-Darwinism – as an explanation for the genes, proteins and different kinds of body plans found in living things. The passage, which was taken from Dr. Stephen C. Meyer’s book, Darwin’s Doubt (Harper One, 2013), read as follows:
“This book has presented four separate scientific critiques demonstrating the inadequacy of the neo-Darwinian mechanism, the mechanism that Dawkins assumes can produce the appearance of design without intelligent guidance. It has shown that the neo-Darwinian mechanism fails to account for the origin of genetic information because: (1) it has no means of efficiently searching combinatorial sequence space for functional genes and proteins and, consequently, (2) it requires unrealistically long waiting times to generate even a single new gene or protein. It has also shown that the mechanism cannot produce new body plans because: (3) early acting mutations, the only kind capable of generating large-scale changes, are also invariably deleterious, and (4) genetic mutations cannot, in any case, generate the epigenetic information necessary to build a body plan.” (pp. 410-411)
I also quoted Dr. Meyer as pointing out that unguided processes could not account for the origin of life, since “explaining the origin of life requires – first and foremost – explaining the origin of the information or digital code present in DNA and RNA,” and contemporary naturalistic theories of life’s origin “fail to account for the origin of the genetic information necessary to produce the first selfreplicating organism.” I then posed a challenge to skeptical readers: how would you attempt to rebut Dr. Meyer’s case, in 200 words or less?
Various critics objected that my brief quote from Dr. Meyer failed to explain why Intelligent Design was a better alternative. However, a commenter named Mung helpfully supplied the following quote from Darwin’s Doubt:
…[E]ach of the features of the Cambrian animals and the Cambrian fossil record that constitute negative clues – clues that render neo-Darwinism and other materialistic theories inadequate as causal explanations – also happen to be features of systems known from experience to have arisen as the result of intelligent activity. In other words, standard materialistic evolutionary theories have failed to identify an adequate mechanism or cause for precisely those attributes of living forms that we know from experience only intelligence – conscious rational activity – is capable of producing. That suggests, in accord with the method of historical scientific reasoning elucidated in the previous chapter, the possibility of making a strong historical inference to intelligent design as the best explanation for the origin of those attributes. (p. 358)
Much to my astonishment, not one of the skeptics commenting on my thread took up my challenge, which was: how would you attempt to rebut Dr. Meyer’s case, in 200 words or less?
One commenter, Dr. Elizabeth Liddle, supplied a handy summary of Darwin’s evolutionary theory: “When self-replicators reproduce with heritable variance in reproductive success, variants that reproduce most successfully will become more prevalent”. She added that “we now know that variants can also become highly prevalent even if they do not contribute to reproductive success, and this actually makes Darwin’s mechanism even more successful, because not every variant needs to be reproductively more successful than the competition to stand a chance of propagating through the gene pool.” All well and good; but it completely fails to address my challenge. How, according to modern evolutionary theory, did the functional genes and proteins found in modern organisms evolve within the time available, and how did new body plans evolve, despite the observed inability of mutations to generate viable large-scale developmental changes, let alone epigenetic information?
Dr. Liddle, who is a psychologist but not a biologist, then launched an ad hominem attack on Dr. Meyer, declaring: “His understanding of evolutionary theory is weak, and actual evolutionary theory is a better alternative.” Barry Arrington then put up a post citing testimonials of well-credentialed biologists who praised Dr. Meyer’s book – a book which, I should add, was anonymously reviewed by two biologists and two paleontologists. No science book is altogether free from error, but we can safely assume that the likelihood of there being any scientific howlers in Darwin’s Doubt is negligible.
Dr. Liddle responded by citing a post of her own, over at The Skeptical Zone, in which she had previously exposed what she referred to as Meyer’s mistake. So I decided to read it. And after reading it, my verdict is: in all fairness, Dr. Liddle’s technical criticisms of three phylogenetic diagrams in Dr. Meyer’s book are valid ones; however, Dr. Liddle has a very poor grasp of the conundrum posed by the Cambrian explosion – much poorer, I might add, than Darwin’s was, in 1859.
Dr. Liddle’s error
Dr. Liddle reveals her faulty understanding of Dr. Meyer’s argument (and of the Cambrian explosion) when she writes (bolding is mine – VJT):
All branching events, in Darwin’s proposal, whether the resulting lineages end up as different phyla or merely different species, start in the same way, with two populations where there once was one, and a short morphological distance between them. It is perfectly true that the longer both lineages persist for, the greater the morphological distance will become. But that isn’t because they started different, or because the phyla come later. It’s because what we call phyla are groups of organisms with an early common ancestor, whose later descendents have evolved to form a group that has a large morphological distance from contemporary populations who descended from a different early common ancestor.
So when a phylum, or a class, or even a kingdom first diverges from a single population into two lineages, the “morphological distance” from the other lineage will be very short. We only call it a “phylum” because eventually, owning to separate evolution, that distance becomes very large.
In boxing circles, that’s what’s called leading with your chin. For my part, I’m no pugilist, unless one wishes to describe verbal sparring as boxing; but I can spot an incautious remark when I see one.
The simple point that Dr. Liddle fails to grasp is that the morphological distance between the various animal phyla hasn’t grown with time. It was just as big 520 million years ago as it is today. The really big morphological changes occurred right at the beginning, and the changes that occurred after that were specializations within each phylum which did not in any way increase the morphological distance between the various phyla. Arthropods and chordates were just as morphologically distinct 500 million years ago as they are now. Certainly, new classes of arthropods and chordates have appeared since then, but the changes that subsequently occurred in the body plans of various arthropod and chordate lineages are far more modest than the sharp differences we find between the different phyla. That is why the Cambrian explosion constitutes such a conundrum for paleontologists. And that is why Darwin felt he could only get round the conundrum by hypothesizing that the various phyla of animals had in fact diverged at a much earlier date, when (he believed) the morphological differences between them would have been much smaller.
Think I’m making this up? Allow me to quote a few experts. (Bolding is mine – VJT.)
The fossil record suggests that the major pulse of diversification of phyla occurs before that of classes, classes before that of orders, and orders before families. This is not to say that each higher taxon originated before species (each phylum, class, or order contained at least one species, genus, family, etc. upon appearance), but the higher taxa do not seem to have diverged through an accumulation of lower taxa.
Erwin, D., Valentine, J., and Sepkoski, J. (1988). “A Comparative Study of Diversification Events.” Evolution, vol. 41, p. 1183.
Described recently as “the most important evolutionary event during the entire history of the Metazoa,” the Cambrian explosion established virtually all the major animal body forms — Bauplane or phyla — that would exist thereafter, including many that were ‘weeded out’ and became extinct. Compared with the 30 or so extant phyla, some people estimate that the Cambrian explosion may have generated as many as 100. The evolutionary innovation of the Precambrian/Cambrian boundary had clearly been extremely broad: “unprecedented and unsurpassed,” as James Valentine of the University of California, Santa Barbara, recently put it.
(Lewin, Roger; “A Lopsided Look at Evolution,” Science, 241:201, 1988.)
This disquieting discovery led Lewin to muse aloud:
“Why, in subsequent periods of great evolutionary activity when countless species, genera, and families arose, have there been no new animal body plans produced, no new phyla?”
And here’s a quote from Valentine et al., to cap it all:
Taxa recognized as orders during the (Precambrian-Cambrian) transition chiefly appear without connection to an ancestral clade via a fossil intermediate. This situation is in fact true of most invertebrate orders during the remaining Phanerozoic as well. There are no chains of taxa leading gradually from an ancestral condition to the new ordinal body type. Orders thus appear as rather distinctive subdivisions of classes rather than as being segments in some sort of morphological continuum.
Valentine, J.W., Awramik, S.M., Signor, P.W., and Sadler, P.M. (1991) “The Biological Explosion at the Precambrian-Cambrian Boundary.” Evolutionary Biology, Vol. 25, Max K. Hecht, editor, Plenum Press, New York and London, p.284.
In their most recent book, The Cambrian Explosion: The Construction of Animal Biodiversity (Roberts and Company, 2013), Douglas Erwin and Jim Valentine freely acknowledge that the stark differences between the phyla that appear over a 10-million-year interval during the Cambrian period make it difficult to even imagine what the last common ancestor (“LCA”) would have looked like:
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)
If highly respected experts in the field acknowledge the stark differences between the various animal phyla from their very first appearance, and if these same experts are genuinely perplexed as to why no new phyla have appeared since the Cambrian, then we can be sure that Dr. Liddle’s breezy assertion that the reason why no new phyla have appeared since then is that not enough time has elapsed rests on a fundamental misunderstanding of how evolution works, at the morphological level. Dr. Liddle evidently believes that the morphological differences between taxa are a simple function of time: groups which diverged a mere 10 million years ago might be classified as different genera, while groups which diverged 100 million years ago would probably be classified into different classes, and groups that diverged 500 million years ago would be classed as different phyla. As she writes: “We only call it a ‘phylum’ because eventually, owning to separate evolution, that distance becomes very large.” For Liddle, the statement that any two animal phyla diverged at least 500 million years ago is trivially true: if they had diverged more recently, we wouldn’t call them phyla, but classes, orders, families, genera or species, depending on the time when they diverged.
Now, if fossils were classified into different taxa purely on the basis of the (mostly random) changes that have accumulated in their genomes over millions of years, then Dr. Liddle would be correct. But that’s not how we classify fossils, because we don’t have their genomes. DNA has a half-life of just 521 years. When classifying fossils into different phyla, scientists have no choice but to go by their morphological characteristics. What Dr. Liddle overlooks is that even if most genetic changes accumulate at a slow and relatively steady pace, it doesn’t follow that morphological changes do. Nor does it follow that epigenetic changes accumulate in this way.
The sudden appearance of new animal body plans during a narrow window comprising a mere 1/1,000 of the Earth’s geological history is a non-trivial fact, when contemporary evolutionary biologists continue to find deeply puzzling. Charles Darwin did too, for he wrote:
“I cannot doubt that all the Silurian trilobites have descended from some one crustacean, which must have lived long before the Silurian age….Consequently, if my theory be true, it is indisputable that before the lowest Silurian strata was deposited, long periods elapsed, as long as, or probably longer than, the whole interval from the Silurian to the present day…..The case must at present remain inexplicable; and may be truly urged as a valid argument against the views here entertained.”
The Origin of Species. 1859. London: John Murray. 1st edition, pp. 306 – 308.
(I should point out that at the time when Darwin wrote, the strata that we now call Cambrian were classified as belonging to the Silurian period.)
Darwin ascribed the Cambrian explosion to imperfections in the fossil record. Today, we know better. Darwin was an intellectually honest scientist; his Origin of Species contains several chapters devoted to rebutting the scientific difficulties in connection with his theory. One wonders what Darwin would have concluded, if he had known then what we know now.
I hope that fair-minded readers will conclude that Dr. Liddle has fundamentally misunderstood the argument Dr. Stephen Meyer was making in his book, Darwin’s Doubt.