Darwin's Doubt

by Stephen C. Meyer

  Hemichordata: Shu et al., “Reinterpretation of Yunnanozoon as the Earliest Known Hemichordate”; Shu et al., “A New Species of Yunnanozoan with Implications for Deuterostome Evolution.”

  Hyolitha: Malinky and Skovsted, “Hyoliths and Small Shelly Fossils from the Lower Cambrian of NorthEast Greenland”; note that some authors consider Hyolitha to belong to phylum Mollusca, whereas others consider Hyolitha to represent an independent phylum.

  Lobopodia: Liu et al., “A Large Xenusiid Lobopod with Complex Appendages from the Lower Cambrian Chengjiang Lagerstätte”; Liu et al., “Origin, Diversification, and Relationships of Cambrian Lobopods”; Liu et al., “An Armoured Cambrian Lobopodian from China with Arthropod-Like Appendages”; Ou et al., “A Rare Onychophoran-Like Lobopodian from the Lower Cambrian Chengjiang Lagerstätte, Southwestern China, and Its Phylogenetic Implications.”

  Loricifera: Peel, “A Corset-Like Fossil from the Cambrian Sirius Passet Lagerstatte of North Greenland and Its Implications for Cycloneuralian Evolution.”

  Nematomorpha: Xianguang and Wen-guo, “Discovery of Chengjiang Fauna at Meishucun, Jinning, Yunnan.”

  Phoronida: Erwin et al., “The Cambrian Conundrum: Early Divergence and Later Ecological Success in the Early History of Animals.”

  Priapulida: Wills et al., “The Disparity of Priapulid, Archaeopriapulid and Palaeoscolecid Worms in the Light of New Data”; Hu et al., “A New Priapulid Assemblage from the Early Cambrian Guanshan Fossil Lagerstätte of SW China.”

  Sipuncula: Huang et al., “Early Cambrian Sipunculan Worms from Southwest China.” Some consider sipunculan worms to be a subgroup of the phylum Annelida based on phylogenomic analyses. See Struck et al., “Phylogenomic Analyses Unravel Annelid Evolution.”

  Tardigrada: Muller et al., “ ‘Orsten’ Type Phosphatized Soft-Integument Preservation and a New Record from the Middle Cambrian Kuonamka Formation in Siberia.”

  Vetulicolia: Shu, “On the Phylum Vetulicolia.”

  Nematoda: Erwin et al., “The Cambrian Conundrum: Early Divergence and Later Ecological Success in the Early History of Animals.”

  Nemertea(?): Schram, “Pseudocoelomates and a Nemertine from the Illinois Pennsylvanian.” Note that the presence of Nemertea in the fossil record is contested.

  Platyhelminthes: Poinar, “A Rhabdocoel Turbellarian (Platyhelminthes, Typhloplanoida) in Baltic Amber with a Review of Fossil and Sub-Fossil Platyhelminths.”

  Rotifera: Swadling et al., “Fossil Rotifers and the Early Colonization of an Antarctic Lake.”

  6. Erwin and Valentine, The Cambrian Explosion, 66–70.

  7. Hennig, Phylogenetic Classification.

  8. If anything, using a “rank-free” classification system may actually intensify the mystery of the Cambrian explosion. A single phylum may include many unique modes of organizing tissues, organs, and body parts, and these differences in organization may deserve to be recognized as different body plans as much as the differences that distinguish different phyla. As one proponent of the rank-free approach put it to me, “Why shouldn’t clams and squids [both of which belong to the single phylum Mollusca] be recognized as exemplifying unique body architectures every bit as much as trilobites and star fish [which belong to two different phyla, the arthropods and the echinoderms]?” In the traditional system, however, both clams and squids, and many other animals that exemplify equally pronounced differences in form within other phyla, will all fall within their respective individual phyla. For this reason, measuring the explosiveness of the Cambrian radiation solely by reference to the number of phyla that first appear in the Cambrian may actually minimize the severity of the problem, whereas dispensing with taxonomic ranking may actually tend to accentuate it.

  9. In addition, in 1999 paleontologists in southern China also found fossil remains of fish in the Cambrian period. Fish are vertebrates and members of the phylum chordata. Shu et al., “Lower Cambrian Vertebrates from South China,” 42–46; Shu et al., “Head and Backbone of the Early Cambrian Vertebrate Haikouichthys.”

  10. Quoted in Yochelson, Charles Doolittle Walcott, Paleontologist, 33.

  11. Gould, Wonderful Life, 49.

  12. Gould, Wonderful Life, 125–36; Budd, “The Morphology of Opabinia Regalis and the Reconstruction of the Arthropod Stem-Group,” 1–14.

  13. Darwin, On the Origin of Species, 307.

  14. Ward, On Methuselah’s Trail, 29–30.

  15. Darwin’s prediction is, of course, not a prediction in the narrow sense of forecasting a future event or process. But historical scientists regularly speak of predictions in the sense of expectations about what will be revealed about the past if and when the relevant body of evidence is uncovered.

  16. Great Canadian Parks, Yoho National Park, www.greatcanadianparks.com/bcolumbia/yohonpk/page3.htm (accessed October 23, 2012).

  17. Dawkins, Unweaving the Rainbow, 201.

  18. Darwin, On the Origin of Species, 120.

  19. Darwin, On the Origin of Species, 125.

  20. Lewin, “A Lopsided Look at Evolution,” 292.

  21. Erwin, Valentine, and Sepkoski, “A Comparative Study of Diversification Events,” 1183. See also Erwin et al., “The Cambrian Conundrum: Early Divergence and Later Ecological Success in the Early History of Animals,” 1091–97; Bowring et al., “Calibrating Rates of Early Cambrian Evolution,” 1293–98.

  22. Hennig, Phylogenetic classification, 219. Erwin and Valentine in their 2013 book, The Cambrian Explosion, reaffirm the remarkable morphological disparity present from the beginning of the Cambrian period despite low species diversity. They note that since they first brought attention to the top-down pattern of Cambrian disparity preceding diversity using classical Linnaean categories in 1987, paleontologists have developed measures of disparity within the phylogenetic classification system—measures that reaffirm the same pattern within a rank-free classification system (217).

  23. For a more technical description of the processes behind the Burgess Shale fossil formation, see Briggs, Erwin, and Collier, The Fossils of the Burgess Shale, 21–32; Conway Morris, The Crucible of Creation, 106–107.

  24. Gould, Wonderful Life, 274–75.

  25. Walcott, “Cambrian Geology and Paleontology II,” 15.

  26. Gould, Wonderful Life, 108.

  27. Gould, Wonderful Life, 273.

  Chapter 3: Soft Bodies and Hard Facts

  1. Nash, “When Life Exploded,” 66–74.

  2. Today fossils from the Burgess can be viewed in Canada at the Royal Ontario Museum in Toronto, the Tyrell Museum in Drumheller, Alberta, and at a smaller exhibition close to the Burgess Shale in Golden, British Columbia.

  3. Briggs, Erwin, and Collier, Fossils of the Burgess Shale.

  4. Desmond Collins, Misadventures in the Burgess Shale, 952–53.

  5. Some paleontologists have also gone so far as to argue that the Cambrian explosion is nothing more than an artifact of classification, and therefore does not require explanation. Budd and Jensen, for example, argue that the problem of the Cambrian explosion resolves itself if one keeps in mind the cladistic distinction between “stem” and “crown” groups. Since crown groups arise whenever new characters are added to simpler, more ancestral stem groups during the evolutionary process, new phyla will inevitably arise once a stem group has arisen. Thus, for Budd and Jensen, what requires explanation is not the crown groups corresponding to the new phyla, but the earlier, less derived, stem groups that presumably arose deep in the Precambrian. Yet since these earlier stem groups are by definition less derived, explaining them will be, in their view, considerably easier than explaining the origin of the Cambrian animals de novo. In any case, for Budd and Jensen, the explosion of new phyla in the Cambrian does not require explanation. As they put it, “given that the early branching points of major clades is an inevitable result of clade diversification, the alleged phenomenon of phyla appearing early and remaining morphologically static does not seem to require particular explanation.” [Budd and Jensen, �
��A Critical Reappraisal of the Fossil Record of the Bilaterian Phyla,” 253.] Nevertheless, Budd and Jensen’s attempt to explain away the Cambrian explosion begs crucial questions. Granted, as new characters are added to existing forms, novel morphology and greater morphological disparity will likely result. But what causes new characters to arise? And how does the biological information necessary to produce new characters originate? (See Chapters 9–16.) Budd and Jensen do not specify. Nor can they say how derived the ancestral forms are likely to have been, and what processes might have been sufficient to produce them. Instead they simply assume the sufficiency of some unspecified evolutionary mechanisms. Yet, as I show in Chapters 10–16, this assumption is now problematic. In any case, Budd and Jensen do not explain what causes the origin of biological form and information in the Cambrian.

  6. See Cloud, “The Ship That Digs Holes in the Sea,” 108. See also a history of offshore drilling on the website of the National Ocean Industries Association, http://www.noia.org/website/article.asp?id=123 (accessed July 8, 2011).

  7. Schuchert and Dunbar, Textbook of Geology, Part II, Historical Geology, 72–76, 125–30; Stokes, Essentials of Earth History: An Introduction to Historical Geology, 162–64; Zumberge, Elements of Geology, 62–67, 214–15; Dunbar, Historical Geology, 13–15, 129–33.

  8. Müller et al., “Digital Isochrons of the World’s Ocean Floor,” 3212.

  9. Walcott, “Cambrian Geology and Paleontology II,” 2–4.

  10. Gould, Wonderful Life, 275.

  11. Some have even suggested that the transitional intermediate forms leading to the Cambrian animals only existed in the larval stage. See Davidson, Peterson, and Cameron, “Origin of Bilaterian Body Plans,” 1319.

  12. Wray, Levinton, and Shapiro, “Molecular Evidence for Deep Pre-Cambrian Divergences Among Metazoan Phyla.” For other recent expressions of this version of the artifact hypothesis, see Simpson, Fossils and the History of Life, 72–74; Ward, Out of Thin Air, 5; Eldredge, The Triumph of Evolution and the Failure of Creationism, 46; and Schirber, “Skeletons in the Pre-Cambrian Closet.” Though contemporary paleontologists commonly attribute the absence of Precambrian ancestral forms to their alleged lack of hard parts or appreciable size, earlier geologists and paleontologists have also employed this version of the artifact hypothesis. For example, in 1941 Charles Schuchert and Carl Dunbar stated: “We may infer, therefore, that life probably was abundant in the seas of Cryptozoic time and especially during the Proterozoic, but was of a low order and doubtless small and soft-tissued, so that there was little chance for actual preservation of fossils” (Textbook of Geology, Part II, 124). And as early as 1894 W. K. Brooks asserted: “the zoological features of the Lower Cambrian are of such a character as to indicate that it is a decided and unmistakable approximation to the primitive fauna of the bottom, beyond which life was represented only by minute and simple surface animals not likely to be preserved as fossils” (“The Origin of the Oldest Fossils and the Discovery of the Bottom of the Ocean,” 360–61).

  13. Marshall, “Explaining the Cambrian ‘Explosion’ of Animals,” 357, 372. For an authoritative refutation of this version of the artifact hypothesis, see Conway Morris, The Crucible of Creation, 140–44; Conway Morris, “Darwin’s Dilemma: The Realities of the Cambrian ‘Explosion’,” 1069–83.

  14. Schopf and Packer, “Early Archean (3.3-Billion to 3.5-Billion-Year-Old) Microfossils from Warrawoona Group, Australia,” 70; Schopf, “Microfossils of the Early Archean Apex Chert.”

  15. Schopf and Packer, “Early Archean (3.3-Billion- to 3.5-Billion-Year-Old) Microfossils from Warrawoona Group, Australia,” 70; Hoffmann et al., “Origin of 3.45 Ga Coniform Stromatolites in Warrawoona Group, Western Australia.”

  16. Jan Bergström states: “Animals such as arthropods and brachiopods cannot exist without hard parts. The absence of remains of skeletons and shells in the Precambrian therefore proves that the phyla came into being with the Cambrian, not before, even if the lineages leading to the phyla were separate before the Cambrian” (“Ideas on Early Animal Evolution,” 464).

  17. Valentine and Erwin, “Interpreting Great Developmental Experiments.”

  18. Valentine, “Fossil Record of the Origin of Bauplan and Its Implications,” especially 215.

  19. Chen and Zhou, “Biology of the Chengjiang Fauna,” 21.

  20. Chen and Zhou, “Biology of the Chengjiang Fauna,” 21. Or as Valentine explains, “the interpretation of the explosion as an artifact of the evolution of durable skeletons has got it backward: the skeletons are artifacts, more or less literally, of the evolutionary explosion.” Valentine, On the Origin of Phyla, 181.

  21. Ivantsov, “A New Reconstruction of Kimberella, a Problematic Metazoan,” 3.

  22. Edgecombe, “Arthropod Structure and Development,” 74–75.

  23. Frederick Schram, The Crustacea.

  24. Simpson, Fossils and the History of Life, 73. Indeed, an exoskeleton is far more than a mere covering for the soft parts of, say, a chelicerate or crustacean, because it provides the sites for the attachment of the muscles and various other tissues. Further, the limbs (including the mouthparts and in some instances certain reproductive components) are encased in exoskeletal elements that can articulate, allowing the arthropod to move, feed, and mate. An exterior skeleton of any shrimp, for example, also has interior projections that comprise its endophragmal system, which provides support for the animal’s internal musculature and organs. At the same time, the skeleton of any arthropod is a product of, and in turn regulates, its metabolism and physiology. In order for the first members of Fuxianhuia or Marrella to have grown (and possibly metamorphosed during their development), they would have had to have successively secreted a new skeleton beneath the old one; to have shed the used exoskeletons; and to have hardened each new exoskeleton. This tight functional integration suggests the implausibility of evolutionary models that envision the Arthropod exoskeleton arising late as a kind of accretion to an already integrated system of soft parts.

  25. Brocks et al., “Archean Molecular Fossils and the Early Rise of Eukaryotes.”

  26. Conway Morris, The Crucible of Creation, 47–48; Gould, “The Disparity of the Burgess Shale Arthropod Fauna and the Limits of Cladistic Analysis.”

  27. Wiwaxia is considered soft-bodied, but it does have harder scales and spines. See Conway Morris, The Crucible of Creation, 97–98.

  28. Valentine, “The Macroevolution of Phyla,” sec. 3.2, “Soft-Bodied Body Fossils,” 529–31.

  29. Conway Morris, The Crucible of Creation, 82.

  30. Conway Morris, The Crucible of Creation, 76, 99.

  31. Conway Morris, The Crucible of Creation, 68, 73, 74; “Burgess Shale Faunas and the Cambrian Explosion.”

  32. Conway Morris, The Crucible of Creation, 107.

  33. Conway Morris, The Crucible of Creation, 92, 184.

  34. Conway Morris, “Burgess Shale Faunas and the Cambrian Explosion.”

  35. Conway Morris, The Crucible of Creation, 103; Conway Morris, “Burgess Shale Faunas and the Cambrian Explosion.”

  36. A recent scientific paper reinterprets Nectocaris as a cephalopod mollusk, though it also acknowledges the problems long associated with the definitive classification of this animal. See Smith and Caron, “Primitive Soft-Bodied Cephalopods from the Cambrian.”

  37. Conway Morris, The Crucible of Creation, 140.

  38. Hou et al., The Cambrian Fossils of Chengjiang, China, 10.

  39. Hou et al., The Cambrian Fossils of Chengjiang, China, 10, 12.

  40. Hou et al., The Cambrian Fossils of Chengjiang, China, 13.

  41. Hou et al., The Cambrian Fossils of Chengjiang, China, 10, 12.

  42. Hou et al., The Cambrian Fossils of Chengjiang, China, 23.

  43. Bergström and Hou, “Chengjiang Arthropods and Their Bearing on Early Arthropod Evolution,” 152.

  44. Burgess Shale fossils from the middle Cambrian (515 million years ago) confirm that many of these fully soft-bodied
Cambrian organisms were long-lived and geographically widespread.

  45. Chen et al., “Weng’an Biota”; Chien et al., “SEM Observation of Precambrian Sponge Embryos from Southern China.”

  46. Chen et al., The Chengjiang Biota: A Unique Window of the Cambrian Explosion. This book is currently available only in the Chinese language. The translated English version is being completed by Paul K. Chien, of the University of San Francisco.

  47. Chen et al., “Weng’an Biota”; Chien et al., “SEM Observation of Precambrian Sponge Embryos from Southern China.”

  48. For other alternative interpretations, see Huldtgren et al., “Fossilized Nuclei and Germination Structures Identify Ediacaran ‘Animal Embryos’ as Encysting Protists,” 1696–99; Xiao et al., “Comment on ‘Fossilized Nuclei and Germination Structures Identify Ediacaran ‘Animal Embryos’ as Encysting Protists’,” 1169; Huldtgren et al., “Response to Comment on ‘Fossilized Nuclei and Germination Structures Identify Ediacaran ‘Animal Embryos’ as Encysting Protists’,” 1169.

  49. Erwin and Valentine, The Cambrian Explosion, 778.

  50. Chien et al., “SEM Observation of Precambrian Sponge Embryos from Southern China.” Sponges are assumed by most evolutionary biologists to represent a side branch, not a node on evolutionary tree of life leading to the Cambrian phyla. Thus, sponges are not regarded as plausible transitional intermediates between Precambrian and Cambrian forms (nor are they regarded as ancestral to other Cambrian animals).

  51. Some have challenged the interpretation of these Precambrian microfossils as embryos, arguing that they are, instead, large microorganisms. For example, Therese Huldtgren and colleagues have argued that these fossils “have features incompatible with multicellular metazoan embryos” and that “the developmental pattern is [more] comparable with nonmetazoan holozoans,” a group that includes one-celled protozoans. [Huldtgren et al., “Fossilized Nuclei and Germination Structures Identify Ediacaran ‘Animal Embryos’ as Encysting Protists,” 1696–99.]