The Crucible of Creation: The Burgess Shale and the Rise of Animals
by Simon Conway Morris
Oxford University Press, Oxford, England, 1998, 242 pages
ISBN 0-19-850256-7, $30.00 (US).
Reviewed by Steve Leslie
Earth Sciences at the University of Arkansas at Little Rock, 2801 South University, Little Rock, Arkansas 72204-1099, USA.
After devoting more than a quarter century to the study of the Burgess Shale biota, Simon Conway Morris has summarized his vision of life during the Middle Cambrian in The Crucible of Creation. This well-organized text is thoroughly footnoted and referenced, and contains a helpful glossary. Conway Morris provides the reader with an informative, basic discussion of the origins of life and animals on Earth. He then gives a colorful description of Charles Walcott's discovery of the Burgess Shale, and describes the more recent discoveries of Burgess Shale-type biotas in Greenland and China. The literary vehicle of time-travel transports the reader to the Cambrian seas, where we are given a first-hand view of the remarkable diversity of life during the Middle Cambrian. The final chapters of the book are devoted to the implications that the Burgess Shale and Burgess Shale-type biotas have for evolutionary thought.
Conway Morris reveals his view of what the fossils of the Burgess Shale tell us about evolution and he draws quite different conclusions than does Stephen Jay Gould, whose Wonderful Life (Gould 1989) drew extensively on Conway Morris' Burgess Shale work. The preface states: "One of my purposes in this book is to discuss the basis upon which Gould builds many of his conclusions, especially concerning the roles of historical contingency and the evolutionary explanations for the apparently remarkable range of morphological types we see in the Cambrian…As will be clear from this book I believe, however, that a different message can be read from the Burgess Shale than that promoted by Gould." Conway Morris visits this topic from different perspectives throughout the book. He criticizes the idea that contingency plays a central role in evolution in an effort to persuade the reader that all that glitters is not Gould. Unfortunately, this theme is articulated in a way that gives the reader the idea that Conway Morris' dispute with Gould is as personal as it is intellectual. So pervasive is the perception that Gould, rather than his ideas, is wrong that he equates Gould by name to contingency throughout the book from the Preface through the last footnote in the last chapter. Conway Morris’ zeal in this matter is the books main detraction, and serves to distract the reader from his truly interesting ideas. If the reader can get past these distractions, they will find that "The Crucible of Creation" contains some very enjoyable passages and thought provoking ideas.
Chapter one, Imprint of evolution, begins with a description of basic principles of evolutionary theory, and provides interesting discussion of some controversies in the details of evolutionary mechanisms. Conway Morris asserts that a highly reductionist view of evolution is "not so much wrong, as simply seriously incomplete." In an effective analogy between evolution and an oil painting, Conway Morris explains (p. 9) what he views as the shortcomings of a reductionist view of evolution. The reductionist viewpoint "has explained the nature and range of pigments; how extraordinary azure colour was obtained, what effect cobalt has, and so on. But the description is quite unable to account for the picture itself."
Conway Morris sets the stage for much of the discussion that follows by stating that the role of contingency in evolution is flawed. Contingency is defined in the glossary of The Crucible of Creation as "the notion that the present-day world arises as a result of chance events in the past, some of which were in themselves trivial and at the time seemingly of minor importance." Conway Morris suggests that convergence in evolution infers that there are not an unlimited number of biological possibilities, as Gould (1989) implied, but that the forms of life are restricted and channeled. Contingency in his view is inevitable, but unremarkable. This debate of contingency sensu Gould versus constraint sensu Conway Morris is focused back to a provocative and popularized paragraph from Gould's (1989, p. 289) book Wonderful Life: "Finally if you will accept my argument that contingency is not only resolvable and important, but also fascinating in a special sort of way, then the Burgess not only reverses our general ideas about the source of pattern -- it also fills us with a new kind of amazement… at the fact that humans ever evolved at all. We came this close (put your thumb about a millimeter away from your index finger), thousands and thousands of times, to erasure by the veering of history down another sensible channel. Replay the tape a million times from a Burgess beginning, and I doubt that anything like Homo sapiens would ever evolve again. It is, indeed, a wonderful life." Conway Morris asserts that the important question is not "Would a particular lineage evolve again?", but "Would a particular property, in his example consciousness, arise again?" This question is answered by suggesting that with the reality of convergence, Gould's metaphorical tape of life can be run as many times as we like and in principle intelligence (consciousness?) will surely emerge. Here, and in other chapters, convergence as presented by Conway Morris is taken nearly to the point of having a deterministic quality. The first chapter closes with an unanticipated spiritual overtone (p. 14): "We might be a product of the biosphere, but it is one with which we are charged to exercise stewardship. We might do better to accept our intelligence as a gift, and it may be a mistake to imagine that we shall not be called to account." The questions "Who has charged us?" "Who has given us the gift?" and "To whom we are accountable?" are left to the reader's discretion.
The second chapter, "Setting the scene," is a basic overview of life through deep time. Conway Morris provides a brief, yet sufficient, overview of life from its first appearance, through the rise of Ediacaran "animals," to the Cambrian explosion. The idea that the Ediacaran biota truly represent animals, and how they fit within known phyla, is an area of considerable debate. The idea, developed by Adolph Seilacher, that the Ediacaran biota represent a separate evolutionary lineage has met with, albeit skeptical, approval in recent years. Conway Morris, however, points to Ediacaran-like fossils from the Burgess Shale and asserts that based on similarities of these potential Ediacaran holdovers, that Seilacher’s "hypothesis needs to be largely scrapped."
Chapter 3, The discovery of the Burgess Shale, does not simply include the standard "who found what and when" aspects of the history of Burgess Shale study. Although the chronology of the study is part of this chapter, Conway Morris describes through personal experience the way that knowledge about Burgess Shale organisms has advanced though study and restudy, and the process of hypothesis formulation-publication-hypothesis reformulation. These lessons are told though a subchapter entitled "Two mistakes." The first mistake is the story of how he recognized that a fossil first described as Canadia sparsa by Walcott was actually something quite different. After careful preparation and study, Conway Morris published a revised description of this truly strange-looking fossil and called it Hallucigenia. Later, based on meticulous preparation and better preserved material, Conway Morris’ interpretation of Hallucigenia was shown to be upside-down and probably backwards by Lars Ramsköld and Hou Xianguang (Ramsköld and Hou 1991; Ramsköld 1992). The second mistake is the "piecing together" of Anomalocaris, which was originally identified as two separate fossils. The part that we now know as an anterior limb was originally thought to be the abdomen of a primitive crustacean, and the mouth-parts were originally thought to be a primitive jellyfish. The story of "Two mistakes" is a brief, well-organized, explanation that illustrates how paleontology works.
Perhaps the most compelling aspect of the book is Chapter 4, Journey to the Burgess Shale. Conway Morris uses the concept of a time machine of the imagination, in much the same way that Carl Sagan in his popular Public Television series Cosmos used the concept of a spaceship of the imagination, to take us on a trip to observe the Middle Cambrian Burgess Shale biota in vivo. We are able to view the complex communities of Burgess Shale organisms in the way that they live in Conway Morris' mind. The seemingly lifeless black and white photos of various "mud-dwellers," "mud-stickers," "strollers, walkers, and crawlers," and "swimmers and floaters" are brought to life by Conway Morris' vivid narrative of an imaginary encounter, complete with color plates depicting the Middle Cambrian ecology. The reader is aboard a submersible with a team of scientists to view the ecology of the Burgess Shale biota and, on occasion, to capture and dissect the animals. This chapter allows us to see Burgess Shale organisms and ecosystems as they are seen though the eyes of one of the most prolific Burgess Shale paleontologists. Truly an interesting journey!
Chapter 5, The search for new Burgess Shales, documents the amazing number of Burgess Shale-type localities that have been discovered from the Lower and Middle Cambrian. The relatively recent discoveries of the Sirius Passet biota in Greenland and the Chengjiang biota, Yunnan Province, China are discussed in general terms to provide a foundation and context for the final chapters.
In Chapters 6, 7, and 8, Conway Morris describes what he believes are some of the significant ways that study of the Burgess Shale and Burgess Shale-type deposits add to knowledge of evolution of life on Earth. This three-chapter section challenges not only Gould's contingency theory, but also the idea put forth by Gould (1989) that morphological disparity has decreased since the Cambrian. Central to Gould's (1989) argument for an "inverted cone of life" is that the animals of the Burgess Shale are of such strange body plan that they must represent extinct phyla. To make his point that this is not necessarily true, Conway Morris uses evidence linking Anomalocaris to the arthropods, while Wiwaxia and the halkieriids are shown in evolutionary relationships with molluscs, annelids, and brachiopods. The point of his argument is that these "weird" animals may be understood best as early or "transitional" stages in the development of known phyla, rather than representatives of extinct phyla (Figure 1). It is simpler to see them as early or "transitional" stages than as phyla now gone. That stated, Conway Morris uses examples from the trilobite literature, and then arthropods as a whole to illustrate that disparity has not decreased, but has instead increased through geologic time. Specifically, Conway Morris’ argument is that there was rapid increase in morphological disparity during the Early and Middle Cambrian, and that disparity has increased throughout geologic time in a step-like (dare I say punctuated?) manner (Figure 2).
Simon Conway Morris’ The Crucible of Creation boldly challenges the ideas set forth in Wonderful Life in a thought-provoking manner that is sure to be the basis for many interesting discussions. The Crucible of Creation summarizes many of the major advances in the study of the rise of animal life as evidenced by the spectacular fossils of Early and Middle Cambrian Burgess Shale-type biotas. Anyone interested in natural history will find this book both informative and enjoyable.
Copyright: Palaeontologia Electronica, 15 March 1999