The Scientific Controversy Over the Cambrian Explosion
Posted by ssbg on August 16, 2006
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Darwin called his theory “descent with modification.” The phrase reflected
Darwin’s belief that all organisms are modified descendants of a common ancestor that
lived in the distant past. The only illustration in Darwin’s book The Origin of Species
shows the “tree of life” pattern one would expect to find in the fossil record if his theory
were true. The common ancestor would come first, as a single species at the base of the
tree. Minor differences among individuals would appear first, and these differences
would eventually increase until one species had become two or more. Differences among
species would then grow until some species became so different they would be classed as
separate genera; genera would diverge to become separate families, families would
diverge to become separate orders, and so on. Eventually differences would become so
great that where there had originally been one major division or “phylum,” there would
now be two. Today there are several dozen animal phyla. The major ones include the
nematodes (roundworms), annelids (earthworms and leeches), mollusks (clams and
snails), arthropods (lobsters and insects), echinoderms (starfish and sea urchins) and
chordates (fishes and mammals).
If Darwin’s theory were true, then a long accumulation of minor differences must
have preceded the major differences we now see among the animal phyla. As Darwin
himself wrote, before the different phyla appeared there must have been “vast periods”
during which “the world swarmed with living creatures” (Excerpt A, p. 83). In the fossil
record, however, most of the major animal phyla appear fully formed at the beginning of
the geological period known as the Cambrian, with no fossil evidence that they branched
off from a common ancestor. Darwin was aware of this, acknowledging in The Origin ofSpecies that “several of the main divisions of the animal kingdom suddenly appear in the
lowest known fossiliferous rocks.” He called this a “serious” problem which “at present
must remain inexplicable; and may be truly urged as a valid argument against the views
here entertained” (Excerpt A, pp. 82, 85).
(A) Charles Darwin, The Origin of Species, Sixth Edition (New York: D,
Appleton, 1890), Chapter X.
Darwin remained convinced that his theory was true, however. He speculated that
ancestors of the different phyla had not been found because the fossil record was
imperfect. If, as it seemed, rocks before the Cambrian had been deformed by heat and
pressure, or eroded away, then fossil ancestors might never be found. He acknowledged,
though, that he really had “no satisfactory answer” to the problem (Excerpt A, p. 84).
Subsequent fossil collecting, however, has yielded many fossils of organisms that
lived before the Cambrian. Fossil beds in Canada (the Burgess shale) and China (the
Chengjiang fauna) have also yielded much richer collections of Cambrian fossils than
were available to Darwin and his contemporaries. Reviewing the evidence in 1991,
Berkeley paleontologist James Valentine and his colleagues noted: “During the past 40
years, rocks older than what had now been considered to be the base of the Cambrian
have indeed yielded fossils that now permit much more detailed assessments of early
metazoan [i.e., multicellular animal] evolution” (Excerpt B, p. 280). Valentine and his
colleagues found that “it has not proven possible to trace transitions” between the phyla,
and the evidence points to a Cambrian “explosion” that “was even more abrupt and
extensive than previously envisioned” (Excerpt B, pp. 281, 294). The authors concluded
that “the metazoan explosion is real; it is too big to be masked by flaws in the fossil
record” (Excerpt B, p. 318).
Some scientists have suggested that fossil ancestors for the animal phyla are
missing not because the rocks have been deformed or eroded, but because animals before
the Cambrian lacked hard parts, and thus never fossilized in the first place. According to
this hypothesis, the Cambrian explosion merely represents the sudden appearance of
shells and skeletons in animal that had evolved long before. The fossil evidence,
however, does not support this hypothesis. First, as Harvard paleontologist Stephen Jay
Gould and Cambridge paleontologist Simon Conway Morris have pointed out, the
majority of Cambrian explosion fossils are soft-bodied (Stephen Jay Gould, Wonderful
Life [New York: Norton, 1989]; Simon Conway Morris, The Crucible of Creation
[Oxford: Oxford University Press, 1998). Second, the fossil evidence points to the
appearance of many new body plans in the Cambrian, not just the acquisition of hard
parts by existing phyla. According to Berkeley paleontologist James Valentine, the
Cambrian explosion “involved far more major animal groups than just the durably
skeletonized living phyla.” It was “new kinds of organisms, and not old lineages newly
donning skeleton-armor, that appeared” (Excerpt C, p. 533). Valentine concluded: “the
record that we have is not very supportive of models that posit a long period of the
evolution of metazoan phyla” before the Cambrian (Excerpt C, p. 547).
(B) James W. Valentine et al., “The Biological Explosion at the Precambrian-
Cambrian Boundary,” Evolutionary Biology 25 (1991): 279-356.
(C) James W. Valentine, “The Macroevolution of Phyla,” pp. 525-553 in Jere
H. Lipps & Philip W. Signor (editors), Origin and Early Evolution of the
Metazoa (New York: Plenum Press, 1992).
Recent studies have also emphasized the abruptness of the Cambrian explosion.
After reviewing the geological dating of rocks near the Precambrian-Cambrian boundary,
Bowring and his colleagues reported in 1993 that the Cambrian explosion of animal
phyla was “unlikely to have exceeded 10 million years” (Excerpt D, p. 1297). As
Valentine, Jablonski and Erwin pointed out in 1999, this is “less than 2% of the time from
the base of the Cambrian to the present day” (Excerpt E, p. 852). Since the time from the
Cambrian to the present is only about one seventh of the time since the origin of life on
Earth, this means the Cambrian explosion was geologically very abrupt, indeed.
According to Valentine, Jablonski and Erwin, extensive new data “do not muffle the
explosion, which continues to stand out as a major feature in early metazoan history”
(Excerpt E, p. 851).
(D) Samuel A. Bowring et al., “Calibrating Rates of Early Cambrian
Evolution,” Science 261 (1993): 1293-1298.
(E) James W. Valentine, David Jablonski & Douglas H. Erwin, “Fossils,
molecules and embryos: new perspectives on the Cambrian explosion,”
Development 126 (1999): 851-859.
What significance does the Cambrian explosion have for evaluating Darwin’s
theory that all animals are modified descendants of a common ancestor? As we have
seen, Darwin himself considered it a serious problem (Excerpt A). Although Darwin’s
theory predicts that animal evolution should proceed from the “bottom up,” with the
largest differences emerging last, James Valentine and his colleagues wrote in 1991 that
the pattern of the Cambrian explosion “creates the impression that metazoan evolution
has by and large proceeded from the ‘top down’ ” (Excerpt B, p. 294). Harry Whittington,
an expert on the Cambrian fossils from the Burgess shale, wrote in 1985: “It may well be
that metazoan animals arose independently in different areas. I look sceptically upon
diagrams that show the branching diversity of animal life through time, and come down
at the base to a single kind of animal” (Excerpt F, p. 131). Evolutionary biologist Jeffrey
Levinton, though convinced of the common ancestry of animals, acknowledged in 1992
that the Cambrian explosion — “life’s big bang,” as he called it — remains “evolutionary
biology’s deepest paradox” (Excerpt G, p. 84). Although “the body plans that evolved in
the Cambrian by and large served as the blueprints for those seen today,” Levinton saw
“no reason to think that the rate of evolution was ever slower or faster than it is now. Yet
that conclusion still leaves unanswered the paradox posed by the Cambrian explosion and
the mysterious persistence of those ancient body plans” (Excerpt G, pp. 84, 90). In 1999,
University of California biologist Malcolm Gordon wrote: “Recent research results make
it seem improbable that there could have been single basal forms for many of the highest
categories of evolutionary differentiation (kingdoms, phyla, classes)” (Excerpt H, p. 331).
Gordon concluded: “The traditional version of the theory of common descent apparently
does not apply to kingdoms [i.e., plants, animals, fungi, bacteria] as presently recognized.
It probably does not apply to many, if not all, phyla, and possibly also not to many
classes within the phyla” (Excerpt H, p. 335).
(F) Harry B. Whittington, The Burgess Shale (New Haven, CT: Yale
University Press, 1985).
(G) Jeffrey S. Levinton, “The Big Bang of Animal Evolution,” Scientific
American 267 (November, 1992): 84-91.
(H) Malcolm S. Gordon, “The Concept of Monophyly: A Speculative Essay,”
Biology and Philosophy 14 (1999): 331-348.
So the Cambrian explosion is real, and for some biologists it is at least
paradoxical and mysterious from the perspective of Darwin’s theory. For other
biologists, it actually constitutes evidence against Darwin’s hypothesis that all animals
evolved from a single common ancestor. Yet some scientists continue to defend
Darwin’s theory by arguing that the Cambrian explosion is perfectly consistent with it.
One of these is Alan Gishlick of the National Center for Science Education, a group that
opposes any criticisms of Darwinian evolution in biology classrooms. In written
comments submitted to the Texas State Board of Education at their textbook-adoption
hearing July 9, 2003, Gishlick criticized a book by biologist Jonathan Wells, Icons ofEvolution (Washington, DC: Regnery Publishing, 2000). In his comments, Gishlick
wrote that the Cambrian explosion actually occurred “over a 15-20 million year period”
and that “the ‘top-down’ appearance of body plans is, contrary to Wells, compatible with
the predictions of evolution” (Excerpt I, p. 15). Gishlick’s claim about the duration of the
Cambrian explosion is at odds, however, with the published views of James Valentine
and his colleagues (Excerpt B, p. 279; Excerpt E, pp. 851-853) and Samuel Bowring and
his colleagues (Excerpt D). Furthermore, if by “evolution” Gishlick means “Darwinian
evolution,” then his claim that a “top-down” pattern is consistent with evolution conflicts
with the published views of Harry Whittington (Excerpt F) and Malcolm Gordon
(Excerpt H). Clearly, Gishlick’s disagreements are not just with Wells.
Gishlick also argued that the major differences between animal phyla are not so
major after all. He wrote: “The most primitive living chordate Amphioxus is very similarto the Cambrian fossil chordate Pikia [sic — actually Pikaia]. Both are basically worms
with a stiff rod (the notochord) in them. The amount of change between a worm and a
worm with a stiff rod is relatively small, but the presence of a notochord is a major ‘bodyplan’
distinction of a chordate. Further, it is just another small step from a worm with a
stiff rod to a worm with a stiff rod and a head (e.g., Haikouella; Chen et al., 1999) or aworm with a segmented stiff rod (vertebrae), a head and fin folds (e.g., Haikouichthyes;
Shu et al., 1999). Finally add a fusiform body, fin differentation, and scales; the result is
something resembling a ‘fish’ ” (Excerpt I, p. 15). Yet Gishlick’s fanciful scenario ignores
most of what biologists know about worms and chordates. There are several fundamental
anatomical differences between worms and chordates, which can be found in any good
biology textbook; possession of a notochord is only one of them. If chordates were
simply worms with a stiff rod, they might not even be classed as a separate phylum.
Furthermore, from an evolutionary perspective worms and chordates are not closely
related. In standard evolutionary trees (such as the ones reproduced here from the Sixth
Edition of Campbell & Reece’s Biology), chordates (green arrow at top of pp. 636 & 640
in Excerpt J) are considered closer to echinoderms (starfish and sea urchins) than they are
to any of the worm phyla (two of which are indicated by the pink and orange arrows at
the top of the same diagrams in Excerpt J). Gishlick cites two scientific articles to
support his argument: The first points out that the most primitive chordates might have
rudimentary brains and thus be closer to chordates with heads than previously thought,
but it doesn’t not address the problem of how the first chordate originated (Excerpt K, p.
522). The second article actually contradicts Gishlick’s suggestion that once a worm
posses a stiff rod it could easily evolve into a vertebrate. According to Shu and his
colleagues, “the derivation of the first vertebrates from the cephalochordates [i.e., more
primitive chordates] must have entailed a major reorganization of the body” (Excerpt L,
p. 46). Once again, Gishlick’s disagreements are not just with Wells.
(I) Alan Gishlick, “Comments on the Discovery Institute’s ‘Analysis of the
Treatment of Evolution in Biology Textbooks’,” submitted to the Texas
Education Agency in connection with their July 9, 2003 public hearing on
(J) Neil A. Campbell & Jane B. Reece, Biology, Sixth Edition (San Francisco:
Benjamin Cummings, 2002).
(K) J.-Y. Chen, Di-ying Huang & Chia-Wei Li, “An early Cambrian
craniate-like chordate,” Nature 402 (1999): 518-522.
(L) D.-G. Shu et al., “Lower Cambrian vertebrates from South China,”
Nature 402 (1999): 42-46.
Since the abruptness and extensiveness of the Cambrian explosion are so well
documented, there is no excuse for a biology textbook to deal with the animal fossil
record without even mentioning it. Furthermore, since some biologists maintain that the
Cambrian explosion presents a challenge — or at least a “paradox” — for one of the
fundamental tenets of Darwin’s theory, any biology textbook that doesn’t discuss that
challenge fails to provide students with the resources to think critically about the most
widely taught scientific explanation for evolution.