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).
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 skeptically 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).
(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
(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.
(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.