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The Crucible of Creation: The Burgess Shale and the Rise of Animals

de Simon Conway Morris

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1952106,832 (3.78)12
'tells a great story and manages to be informative at all levels. Conway Morris has a collector's eye for the sort of entertaining yet informative snippets that keep readers on their toes.' New Scientist Located in the west of Canada, the Burgess Shale contains a unique collection of fossil remains, and has become an icon for those studying the history of life. This remarkable book takes us on a fresh journey back in time through the Burgess Shale and its astonishing collection of pre-Cambriancreatures. In an entertaining and readable style, Simon Conway Morris paints a vivid picture of the critical period which saw the diversification of all the major animal groups, and takes a controversial stance on current evolutionary theories that is sure to provoke much interest and debate. 'It is less bleak in its assessment of life on earth and it is spiritually uplifting, rather than dry and mechanistic as some would have us believe' THES R 'The centerpiece of The Crucible of Creation is a description, authoritative and readable, of the animals themselves. New York Times Book Review… (mais)
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Wonderful Life by Stephen Jay Gould compared with Crucible of Creation by Simon Conway Morris

Successful science populizers always seem to evoke sour grapes among their colleagues. (Not Isaac Asimov, for some reason – perhaps because he dropped his career as a scientist once he became a popular writer). Carl Sagan wasn’t popular among the Cornell faculty, although I knew one of his grad students who said he was a decent guy. When I mentioned Zahi Hawass’ name to my Egyptian guides, their immediate response was “He’s an idiot!”. I had one close encounter with Stephen Jay Gould, at a reception after a talk he gave at the Paleontological Research Institute; I didn’t see anything out of the ordinary, but my companion latter commented that Gould had an enormous ego. I knew one of Gould’s grad students, too, who had nothing against him (she did say he was big fan of acrostic puzzles, which is a plus in my book).

Gould certainly did raise hackles in the paleontological community; perhaps partially because of his success as a popularizer but also perhaps because was not exactly gentle in handling some of his colleagues – particularly Richard Dawkins and E. O. Wilson. That makes this pair of books especially ironic, since Gould goes out of his way to praise Morris for his work on the Burgess Shale fauna, and Morris turns around and jumps on Gould with both feet.

The Burgess Shale fauna was discovered by American paleontologist and scientific administrator Charles Walcott in 1909. (Ironically, even though the locality is in British Columbia, the bulk of collected material is in the Smithsonian; to “preserve” the site, Parks Canada will not allow any further collection, which means all remaining fossils will weather away into silt). The Burgess Shale is one of those rare fossil sites known as Konservat-Lagerstätten, where unusual conditions preserve soft body parts. When first discovered, it was the earliest such site known, dating from the Middle Cambrian. Walcott never got around to thoroughly studying the specimens he’d collected; his cursory examination “shoehorned” (Gould’s term) things into existing taxonomic groups. Later reappraisal in the 1970s and 1980s – by Harry Whittington, Derek Briggs, and Simon Conway Morris, all at Cambridge – showed that while there are familiar forms such as trilobites and brachiopods, much of the Burgess fauna is (Gould’s term) “weird wonders” – animals that didn’t fit easily into conventional taxonomic schemes. Some initially had no discernible affinities with any modern animal – the poster child for this group was Hallucigenia sparsa.

Others, while clearly (for example) arthropods, didn’t fit the characteristics of any modern arthropod group; they weren’t uniramians (insects and myriapods), crustaceans, chelicerates, or trilobites (those are Gould’s major groups; the high level classification of arthropods has been rearranged since, based on molecular genetics – several times, in fact). Gould goes great lengths to show the effort that went into the Cambridge studies; the book is liberally illustrated with camera lucida drawings of squished weird thingees.

Gould takes the weirdness of the Burgess Shale Fauna and runs with it. His main themes are disparity and contingency. “Disparity” is divergence in body plans, and it’s Gould’s contention that it was at a maximum in the Cambrian and has been decreasing ever since (“Disparity” is not to be confused with “diversity”, which is the number of species; there are, for example, a hundred or so species of fruit flies which means they are highly diverse – but they all look more or less the same, meaning they have low disparity). “Contingency” is the idea that the history of life is high dependent on chance – one tiny thing lives or dies in the Cambrian and 550 million years later the whole biosphere looks completely difference. Gould uses the metaphor of “rewinding the tape” and cites the film It’s a Wonderful Life as an example of how a small change can make a difference – and also as the source for his title.

Wonderful Life is full of incorrect information. It’s not that Gould makes errors of fact, but rather that facts change rapidly in science. The most egregious error isn’t his fault; the original hardback edition used a painting of the Burgess Shale fauna by famous paleontological artist Charles Knight for the cover painting. When the paperback was reissued after Gould’s death, the publishers must have thought the original cover was too dull and changed it to a dramatic view of a pair of pterygotid eurypterids attacking a school of anaspid fish. The catch is that neither group shows up until the Silurian while the book is illustrating the Middle Cambrian; rather like having a squadron of F-16s at the Battle of Hastings. Other errors illustrate the advance of science; Gould comments that the metazoans are polyphyletic, based on molecular genetics studies; these turned out to be incorrect. He taps lungfish for the origin of tetrapods while more recent work suggests crossopterygians. The original reconstruction of Hallucigenia cited by Gould turned out to be upside down (and possibly backwards; it’s still not clear which end is the front). In his later chapters on contingency, Gould cites the Eocene bird Diatryma as a predator when suggesting that if things had gone a little differently the dominant life form in the Cenozoic could have been birds rather than mammals; Diatryma is now believed to be a herbivore. A little later, the South American phorusrhacids are given as an example of a South American species that was doing quite well until placental mammals arrived; as it happens phorusrhacids continued to do well, moving northward along with opossums, armadillos and sloths; their fossils are found in Texas and Florida. (Although Gould doesn’t mention it, they’re also possibly an example of a violation of an evolutionary “law” – the idea that once a feature is lost it never reappears; phorusrhacids appear to have begun to change their wings back into arms).

That brings us to Simon Conway Morris and his book on the Burgess Shale, which is both a general discussion of the fauna and a direct refutation of Gould (Morris apologizes in his introduction, noting that Gould had nothing but praise for the Cambridge groups work on the Burgess Shale animals). Morris, alas, is not as good a writer as Gould, although he seems to adopt some of Gould’s mannerisms – references to classical literature and namedropping. He also uses an unfortunate device to explain Burgess Shale ecology – a time-travelling submarine – to the extent that one of the illustrations shows a sampling device scooping up Middle Cambrian priapulids. This ends up being annoying rather than illustrative. Gould has much better illustrations; he uses camera lucida drawings, which show the critical parts of the animals while emphasizing that the fossils aren’t perfect; Morris tends to go with color reconstructions – which perhaps make the animals look better understood than they actually are – or photographs, which often don’t show much more than an incomprehensible blob. Finally, Morris does a little straw-man erection, accusing Gould of abandoning Darwinian evolution. Gould was known, of course, for minimizing the importance of adaptation compared to the influence of random events, and also for suggesting that there might some sort of unknown “forces” at work in macroevolution. However, Gould doesn’t invoke either of these ideas – at least not overtly – in Wonderful Life.

In the end, though, despite a less comfortable writing style and distracting time travelers, it’s Morris that’s right and Gould that’s wrong (in my opinion, at least). Morris has the advantage of being in on the discovery of additional “Burgess Shale type” localities, particularly Sirius Passet in north Greenland and Chengjiang in China. There are now “Burgess Shale-like” faunal assemblages known from Poland, China, additional sites in Canada, Greenland, and several locations in the USA; in fact, there’s some suggestion that preservation conditions (possibly due to clay mineralogy) were different in the Cambrian. Specimens from the additional sites elucidate a lot of the “weird wonders”; many (including Hallucigenia) are shown to belong to a group called “lobopods”, which Morris convincingly groups with the arthropods. Morris also has a second advantage: he’s a lot more specialized than Gould; while Gould was known for “big picture” works, Morris’ bibliography is heavy on Cambrian invertebrate paleontology and includes some fairly obscure publications (many in Chinese).

Once he gets to work, Morris does a good job going after Gould’s main points. He takes Gould to task over “disparity”, noting that it never really gets defined. Gould’s argument that there was more “disparity” in the Cambrian is based on arthropod taxonomy; the modern concept of main arthropod groups (below the phylum but above the class) is based on the type of appendages and where they attach on the body. Many of the Burgess Shale arthropods don’t fit into this scheme, and in Gould’s view that makes them more “disparate”. Morris counters with cladistics. Cladistic phylogenies work based on bifurcations; a stem divided into two branches. Although cladistic diagrams have “stem”, “crown” and “sister” groups, there really isn’t anything corresponding to classical phyletic taxonomic groups above the species. The supposed increased disparity in the Cambrian is just an artifact of the way humans formerly interpreted evolution. In fact, Morris provides a cladogram that incorporates all the Burgess Shale arthropod fauna and the main modern groups. (I note Gould was never comfortable with cladistics; Morris isn’t either, citing the problems of convergence and character selection – but he’s more comfortable than Gould).

Morris’ response to Gould’s “contingency” segues nicely with his critique of “disparity”. There are a lot of “body plans” – potential morphospace occupants – that just won’t work. If you are going to swim fast in the ocean, you’re going to end up looking like a squid or a fish or an ichthyosaur or a cetacean or a penguin; the physical world will eliminate some of the possible body plans an organism can have and thus reduce “disparity”, and “rewinding the tape” won’t change the physical constraints. Thus even if you time travel back to the Cambrian and obliterate some lineage, something else will step up to the plate. Gould’s ideas on contingency may have been influenced by his minimization of the role of adaptation; perhaps it might not be as important as the most ultra-adaptationists make it, but an organism still has to live in its environment and there’s only so many ways you can do that (Morris actually cites some science fiction “alternate histories” here, specifically Kingsley Amis’ The Alteration and Keith Robert’s Pavane; I wonder if he has encountered Harry Turtledove).

There are a number of other works and the Burgess Shale and similar fauna; these are the only two I know of that get into evolutionary philosophy. Sadly, it’s excruciatingly difficult to visit the classic sites; the Chengjiang locality is probably the easiest – although you have to get to China first. You can go to Walcott’s site in Canada, but only under escort and actually collecting anything would involve a character-building experience with the authorities, even if you are an accredited professional. Sirius Passet can only be done by a major Arctic expedition. The American sites are possibilities, although they don’t have the faunal richness of the others. The nearest to me would be the Wheeler Shale in Millard County, Utah; probably as close as you can get to the Middle of Nowhere in the continental US. I’m getting a little old to spend days hammering rocks apart in the desert, but having my own Hallucigenia or Anomalocaris would make it worthwhile. ( )
  setnahkt | Jan 1, 2018 |
Dit is het tweede boek dat ik lees over de Burgess Shale, de vindplaats van 70000 fossielen in een kleistenen laag hoog in de Canadese Rocky Mountains. 530 miljoen jaar geleden maakte deze laag deel uit van een onderzeese klif vlak voor de kust van een continent bestaand uit wat nu Noord-Amerika en Groenland is, toen 90 graden naar rechts gedraaid en in horizontale richting uitgestrekt langs de evenaar. De dieren die nu als fossielen liggen opgeslagen in musea in Noord-Amerika leefden aan de voet van die klif op een diepte van 40 meter. Hun perfecte bewaring -tot in de kleinste details- danken ze waarschijnlijk aan het feit dat ze in een modderverschuiving werden gevangen.

De Burgess Shale werd ontdekt in 1909 door Charles Walcott, maar het was pas met de studie van de fossielen door een team uit Cambridge in de jaren zeventig en tachtig van de vorige eeuw dat hun betekenis duidelijk werd. Simon Conway Morris, de auteur van dit boek, was één van de deelnemers aan dit team.

Het belangrijkste deel van The Crucible of Creation bestaat uit een beschrijving van de meest opmerkelijke dieren die in de Burgess Shale zijn teruggevonden. Wezens van enkele millimeters tot 1 meter lang, vaak met een dusdanig vreemd uiterlijk en schijnbaar afwijkend bouwplan dat paleontoloog Stephen Jay Gould ze als ‘weird monsters’ bestempelde. Conway Morris plaatst ze in hun natuurlijke habitat en laat zien hoe het mogelijk is aan de hand van de fossielen niet alleen hun uiterlijk maar ook hun levenswijze te reconstrueren.

Het boek bevat twee uitsmijters. Ten eerste een discussie over de zogenaamde Cambrische explosie, de plotse toename van levensvormen in het vroege Cambrium zo’n 580 miljoen jaar geleden, waarvan de fossielen in de Burgess Shale de weerslag zijn. Conway Morris bespreekt of het hier om een echte toename in diversiteit gaat of over een artefact door een betere bewaring van fossielen uit deze periode en schaart zich achter de algemeen aanvaarde conclusie dat het een reëele gebeurtenis betreft. Controversiëler, Conway Morris ziet de motor achter deze plotse versnelling in evolutionaire verandering vooral in het ontstaan van de eerste roofdieren. Hun aanwezigheid leidt tot een hogere selectiedruk en dus tot een grotere nood tot adaptatie.

In het laatste deel gaat de schrijver in de clinch met Stephen Jay Gould, die ongeveer tien jaar eerder met Wonderful Life een boek over het zelfde onderwerp publiceerde. Goulds stelling in dat boek was dat de diversiteit in het Cambrium veel groter moet zijn geweest dan nu en dat de vele ‘weird monsters’ in de Shale staan voor een groot aantal onbekende phyla. Slechts enkele hiervan zouden stand houden tot in onze tijd. Volgens Gould was er dus een maximale ontplooing van levensvormen, het is te zeggen een maximale diversiteit in het Cambrium, gevolgd door een gestage verarming in diversiteit met het verstrijken van de tijd, waarbij overleven of niet overleven van een bepaalde evolutionaire tak meer te maken had met toeval dan met eigen verdienste. Conway Morris heeft twee overtuigende argumenten tegen deze fel omstreden visie van Gould. Ten eerste hebben nieuwe vondsten in Groenland en China duidelijk gemaakt dat Gould de vreemdheid van de dieren in de Burgess Shale overschat heeft en dat het niet om volledig andere en ongekende levensvormen gaat. De meeste van hen vinden, bij nader onderzoek, mooi hun plek in de gekende classificaties en blijken geen op zichzelf staande phyla maar primitieve exponenten van geleedpotigen (arthropoden) en weekdieren (molluscen) te zijn. Ten tweede is er het één en ander aan te merken op de grote rol die Gould ziet weggelegd voor het toeval in de evolutie. Om kort te zijn, volgens Gould zouden we, als we de evolutionaire film zouden terugdraaien en daarna weer afspelen tot in onze tijd, telkens totaal verschillende en onvoorspelbare uitkomsten krijgen, afhankelijk van een bijna oneindig aantal toevallige factoren. Conway Moris is van mening dat evolutie wel degelijk een min of meer voorspelbaar proces is omdat het aantal richtingen waarin ze kan plaatsvinden beperkt is. Deze beperking komt voort uit de omgeving waarin evolutionaire veranderingen zich voordoen. Er zijn immers maar een aantal efficiënte manieren waarop een wezen zich door water kan voortbewegen, kan vliegen, zich kan voortplanten etctetera. Dit leidt tot het goed gekende fenomeen van convergentie, namelijk dat verschillende evolutionaire takken onafhankelijk van elkaar dezelfde eigenschappen verwerven (vleugels, vinnen enzovoorts). Het herafspelen van de evolutionaire film geeft volgens Conway Morris dus niet een oneindig aantal mogelijke en totaal verschillende werelden, maar uitkomsten die in grote lijnen op elkaar lijken. De rol van toeval is daarom niet zo groot en beperkt zich tot de details.

Conway Morris brengt hiermee een stevige correctie op het verhaal van Gould en The Crucible of Creation is dan ook verplichte kost voor iedereen die Goulds Wonderful Life gelezen heeft. Hij heeft misschien niet de flair van Gould en kan geregeld wat zurig uit de hoek komen, bijvoorbeeld wanneer hij een verband legt tussen Goulds vermeende libertijnse opvattingen en zijn bovenbeschreven visie op evolutie. Desondanks, moest ik één boek kiezen over de Burgess Shale, dan is het dat van Simon Conway Morris. ( )
  BartGr. | Oct 1, 2011 |
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'tells a great story and manages to be informative at all levels. Conway Morris has a collector's eye for the sort of entertaining yet informative snippets that keep readers on their toes.' New Scientist Located in the west of Canada, the Burgess Shale contains a unique collection of fossil remains, and has become an icon for those studying the history of life. This remarkable book takes us on a fresh journey back in time through the Burgess Shale and its astonishing collection of pre-Cambriancreatures. In an entertaining and readable style, Simon Conway Morris paints a vivid picture of the critical period which saw the diversification of all the major animal groups, and takes a controversial stance on current evolutionary theories that is sure to provoke much interest and debate. 'It is less bleak in its assessment of life on earth and it is spiritually uplifting, rather than dry and mechanistic as some would have us believe' THES R 'The centerpiece of The Crucible of Creation is a description, authoritative and readable, of the animals themselves. New York Times Book Review

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