Jump to content

Talk:Cambrian explosion/Archive 1

Page contents not supported in other languages.
From Wikipedia, the free encyclopedia
Archive 1Archive 2Archive 3

"has provided some of the best insights into this period of dramatic evolutionary change and experimentation that laid the foundation for most major modern animal body plans."

Isn't this worded in a way that suggests evolution is planned and targeted? Which is highly disputable.
No, the wording is pretty typical.--Quadalpha 06:45, 14 January 2006 (UTC)

"The fish that appeared in the Cambrian was (sic) unlike any fish alive today." (sic)? was this copied from another source? the inline reference format is also rather... unwiki. pauli 03:32, 6 August 2005 (UTC)

Yes, this is within a quote from the source to whom I attributed it. Thus I retained the writer's grammar and marked it as ungrammatical using (sic) - Lizard1959

Football metaphor for phyla timeline

"Thsi would be as if one were pacing off the length of a football field (starting 4 bya), when between paces 78 and 79 all the different phyla suddenly sprang into existence." -- I don't think this American football analogy helps at all here. Could we find something more general? Daen 17:02, 22 November 2005 (UTC)

The 24-hour day analogy is often used, with a more universal image. --Wetman 23:59, 22 November 2005 (UTC)
I've made this change. Hope my math is correct. 24.207.32.170 23:36, 14 December 2005 (UTC)

Complexity theory

Stuart Kaufmann has a theory that aspects of the Cambrian explosion can be explained by the mathematics of complexity theory. See Sole, R, P Fernandez and S. Kauffman, (2003) Adaptive walks in a gene network model of morphogenesis: insights for the Cambrian explosion. Int. J. Dev. Biol. 47,685-93. -- Scribelrus 09:41, 19 February 2006 (UTC)

Nature article

An investigation in Nature [1] says that there are eleven inaccuracies in this article [2], although it does not specify what they are. I'm not knowledgeable enough to edit, but hopefully someone reading this is. 81.159.62.10 00:13, 15 December 2005 (UTC)

I realize that the article has substantially changed since the Nature review. However, we can't know for certain whether we've corrected the problems they identified, and for the time being (while we attempt to get more specific information from the Nature editors) it is safest to have an open admission of the potential problems. Realize that a lot of people (including journalists) are going to be visiting this page soon, looking for precisely such an admission. —Steven G. Johnson 03:08, 15 December 2005 (UTC)
Why am I reminded of those "What's wrong with this picture" pages in My Big Rainy-Day Fun Book? "Find seventeen mistakes in this picture" the editors challenged, and you drew a blue-crayon circle around the canary that was upside-down in its cage... --Wetman 00:10, 18 December 2005 (UTC)

Review text

I have restored the perhaps too rashly deleted text, which I wrote with care in order to replace the shopworn "The idea of an "explosion" of life in the Cambrian period is still being debated", with a brief overview of the changing perspective. I've enclosed it (temporarily) within the rubrics text under review. I have also restored the suppressed reference to Steven Jay Gould's standard popular account, literally a prizewinner though intensely resented in some cobwebby corners. Now, let us begin to edit, not merely delete. Breathless verbalisms like "This period of evolution is source to some of the most unusual fossils ever recovered" transmit little solid information. Let's discuss the Cambrian explosion with the reader in mind. The last paragraph, for instance, is a good summing-up, but it's not enough unless it has been prepared for by some details, which I've restored. --Wetman 03:43, 15 December 2005 (UTC)

Yeah, sorry about that, I started editting before you made those changes. I removed Gould because the only reference to it sounded like we were shilling for his book. If you have a better use for it, or just want to leave it in the references without a citation, I don't mind. Dragons flight 03:47, 15 December 2005 (UTC)

Eye development trigger theory

I've come across a theory that one of the causes of the Cambrian Explosion was the development of the eye at an early stage; this greatly enhanced the abilities of predators and prey and opened up a variety of niches for them.

I believe this is discussed in the book "In the Blink of an Eye" by Andrew Parker.

Perhaps somebody more familiar than myself with the theory could add a description and references for it?

Wonderful Life

An article onm the Cambrian explosion that does not mention [[Stephen Jay Gould's 1989 Wonderful Life: , winner of The Aventis Prizes for Science Books in 1991, would be a caprice, perhaps just that of a couple of Wikipedia editors. Gould wrote about evolution so clearly, so publicly, so successfully, that he continues to be resented in some corners. --20:35, 17 December 2005 (UTC)

Problem

One of the problems with this article is that specific facts have not been referenced. Also, why are we going against NPOV and saying that Gould's book is the best source for anything? I'm not saying it's not: however, as we all know, we don't take a stance on such matters. May I suggest we reword this to merely say that Gould wrote a book that is accessible to the layman? - 211.30.173.113 04:20, 18 December 2005 (UTC)

If it's not the best, just name its two competitors, and we'll mention all three! Or perhaps there are other issues at work. If so, speak up! out with them! --Wetman 04:33, 18 December 2005 (UTC)

OK, how about The Crucible of Creation : The Burgess Shale and the Rise of Animals by Simon Conway-Morris? Or The Fossils of the Burgess Shale by Briggs, Erwin, and Collier? And, of course, who could forget Burgess Shale by H. Whittington?

Saying that Gould's book is "the best" is a value judgement and therefore POV. Unless you have some external reference for this claim? Otherwise it's just an opinion. I'm not saying Gould's book is bad, just that describing this book as the best possible work in the field in not in accordance with Wikipedia's NPOV policy. I propose the simple change that the book be described as "a general account for the layman. eaolson 05:06, 18 December 2005 (UTC)

New effort?

I've attempted a rather comprehensive re-write of the CE article trying to bring into account rather more recent perspectives that have emerged from recent research on the subject. this is by no means (AT ALL!) perfect, and needs references just for a start. However, I hope it forms the basis of a useful discussion

Graham Budd


--Grahbudd 17:59, 21 December 2005 (UTC)

Hi Graham. I don't know what happened re: your effort to improve the article, but what you've written below seems a huge improvement on the current article. Just for starters it introduces the idea that the "explosion" may not be real at all, and perhaps some sort of taphonomic artifact, etc. - the current article seems to omit this detail, along with many others. Anyway, I'm all for you replacing the article with your text. Regarding references, as I'm sure you've aware, the current issue of Phil. Trans. seems to be some sort of special issue on life around this time. There's even a review of the explosion itself [3] that might be handy on this count. Anyway, I really just wanted to try to get things rolling again re: your efforts. Best regards, --Plumbago 09:24, 1 June 2006 (UTC)

I've now removed the text here, as it is on the main page. Grahbudd 13:16, 2 September 2006 (UTC)


Nature errors to correct

The results of what exactly Nature suggested should be corrected is out... italicize each bullet point once you make the correction. -- user:zanimum

  • “Prior to the discovery of the Burgess Shale ….”. Absolutely wrong! The existing fossil record clearly demonstrated the existence of triploblasts, e.g. trilobites.
  • Diploblasts are much more complex, and to write “every cell …[is]… in contact with its watery mineral-rich environment” is basically wrong.

Diploblastic/triploblastic; actually refers to germ layers, not “layers” in adult animal.

  • Diploblasts have “internal organs” e.g. gonads.
  • Evidence for Ediacaran triploblasts was available long before discovery of phosphatized embryos.
  • Ediacaran fauna is known to span c. 565-540 Ma, i.e. much more than 10 Ma before Precambrian-Cambrian boundary.“
  • “Sexual reproduction” almost certainly evolved long before “Snowball Earth”.
  • Ediacaran faunas are no older than c. 570 Ma; no convincing trace fossils occur at 600 Ma (or earlier)
  • Cloudina is misspelt.
  • Cloudina is effectively Ediacaran in age, and is not know to extend into the Tommotian.
  • Cambrian fish “unlike any fish alive today”: highly misleading.


Comments by Philip Chalmers

The Cambrian Explosion is a tough subject because making sense of it needs contributions from so many different disciplines. So the first problem is to define the minimum academic level expected of the article's readers in the relevant subjects- e.g. senior high school, undergrad or postgrad?

The basic definition needs to be both shorter and clearer, for example: The "Cambrian Explosion" refers to the geologically sudden appearance in the fossil record of complex multi-cellular macroscopic organisms between roughly 542 and 530 million years ago (Mya). ((IMO the sentences about diversification of microscopic organisms, about geochemical perturbations and about the 3 key questions are not part of the definition, and are more relevant to the subsequent analysis.))

For readers at the less advanced academic levels, the article should then explain why the CE is important: Darwin recognised it as the largest single difficulty with his theory of evolution; Creationists have therefore seized on it as a weakness in the theory of evolution; the CE provides a severe test of many theories and methodologies in biology and paleontology. And then introduce the key questions: The CE has generated a great deal of interest and controversy among scientists, particularly: i) is the “explosion” real?; ii) what does it tell us about the origin and evolution of animals? and iii) what were its causes?

Then the history much as above ("Geologists as long ago as Buckland (1784-1856) realised ... Gould’s popular account of this work, Wonderful Life, published in 1989.)

I'd prefer the section "Dating the Cambrian" to be written in less obviously academic style.

"Dating the Cambrian" would also be a good time to introduce an overview diagram, perhaps a simplified version of the one at http://www.pnas.org/cgi/content/full/97/9/4426, to pull together: the dates; names of eras, periods and ages; names of significant fossil faunas; and hypothetical family tree of multi-celled animals. I suggest the diagram should open in a new window so that readers can refer to it without losing their place in the text. It might also be useful to distinguish between "conventional" and "exceptional" finds, e.g. by a different background colour.

"Trace fossils" should explain the term at the start of the text, e.g. "Trace fossils (tracks and burrows)". I suggest the paragraph should have 3 distinct parts: (a) Pre-Cambrian trace fossils are 2-dimensional (the animals apparently did not penetrate much below the suface of the sea-bed), but even the earliest Cambrian trace fossils show that some animals burrowed vertically to (?depth) - “widening of the behavioural repertoire” (Conway Morris 1989); (b) Trace fossils often appear in earlier rocks than body fossils which show the kind of equipment that would be needed for such behaviour; (c) but matching trace fossils with even the most general kinds of body fossils is an uncertain business.

The article needs to explain "Conventional record" - the fact that fossils generally are very rare (especially since we believe the early multi-celled animals were soft-bodied) and finds usually represent only one species, so they tell us little about the ecosystem and it's hard to tell whether the organism was advanced / typical / primitive for its time and environment. That's why the exceptional lagerstatten are so important.

I think "Exceptional record" should first explain why these sites are exceptional: the very fine-grained rocks preserve very small details; they also preserve both soft-bodied organisms and soft parts of hard-shelled animals (mainly internal, e.g. digestive and reproductive organs). Then point out that the Cambrian has more than its share of "exceptional" sites, although no-one's really sure why. Then additional separate paragraphs summarising the contents and implications main "exceptional" Cambrian finds in chronlogical sequence - Chengjiang (? first genuine vertebrates earlier than previously thought) and Sirius Passet, Burgess Shale (actually discovered first) and Orsten.

If the section "Geochemistry" is to be kept, it needs to explain the direction and significance of each of the 3 variations in isotope ratios.

The article probably needs an additional "Data sources" section about molecular biology, explaining its reasoning, assumptions and limitations.

I suggest dividing "Is the explosion real?" into distinct paragraphs or sub-sections, preferably in the order "body fossils", "trace fossils", "molecular biology" (i.e. from most to least certain evidence), theoretical considerations (e.g distinction between “crown” and “stem” groups) and finally conclusions. Under "body fossils" I'd be inclined to give most attention to Chengjiang because it sharpens the issues by the earliest known examples of present-day lineages such as fish, brachiopods and arthropods.

The 2nd paragraph of "What caused the Cambrian Explosion?" needs to split into (groups of) paragraphs, each with a clear theme.

PS if you wish, you can contact me via the email form on my web site, www.benefit-from-it.com

Objective observations on the Cambrian "explosion"

At some point we have to address the fact that both creationists and proponenents of "intelligent design" claim the Cambrian Explosion, over and over, as supporting their respective positions. I don't think it does, for several reasons that I think should be stressed in this article--and these are objective observations, not interpretations of the data. First, molecular estimates aside there is ample fossil evidence of metazoan life before the Cambrian, with concrete evidence for at one modern phylum (Porifera) and pretty good evidence for at least a couple more (almost certainly Cnidaria, and possibly arthropods and some worm-like phyla). Second, at least one phylum (Bryozoa) makes its first appearance in the fossil record well after the Cambrian explosion, and several other modern phyla have no fossil record at all, so it's impossible to say when they originated (although it's quite likely they date to the Cambrian). But even though most of the phyla appeared in the Cambrian Explosion, in most cases their earliest representatives are nothing like any modern representatives of the same phyla (to the point that in many cases it's difficult to say with certainty just what phylum several Cambrian organisms belong in). The assertion that "all phyla originated in the Cambrian explosion" conveniently overlooks the fact that a huge amount of evolutionary novelty appeared within those phyla well after the Cambrian explosion, and many well-known modern groups only appeared much later, e.g., insects and virtually all vertebrates. Also, the phyla are frequently represented as appearing not only suddenly but completely unique and different from each other, whereas there is clear evidence that some phyla are more closely related to each other than to other phyla (e.g., the deuterostome phyla). In fact these acknowledged relationships have at times resulted in certain phyla being split or lumped--that is, the definition of "phylum" is not as objective as we would all like to think--and if they are split you could indeed make the argument that several phyla did not appear until later. The "body plan" definition of phyla is a highly subjective one.

Finally, the focus on the appearance of animal phyla in the Cambrian explosion overlooks plants, especially land plants, which have played an important part in the subsequent evolution of life; vascular plants appear well after the Cambrian, and flowering plants even later than that. Discussions of the "Cambrian Explosion" tend to overemphasize those things that did appear (i.e., the earliest representatives of most metazoan phyla) while overlooking those things that did not appear (e.g., classes and other major groups within the phyla, plants, etc.)

There are many, many good websites dealing with the Cambrian Explosion and they should be linked here. One excellent non-web reference is the book "On the Origin of Phyla" by James W. Valentine, which while not specifically about the Cambrian Explosion, has a wealth of information about the definitions, origins, relationships, and evolution of the phyla, all of which are pertinent to the Cambrian Explosion. I would strongly recommend that anybody who intends to work on this article read this book first. MrDarwin 16:46, 7 January 2006 (UTC)

To do: mention tech. only once

The opening section seems to apologize for the slow progress of technology about a half-dozen times. It should note ONCE that advances in technology have changed the picture, give a quick review of the old picture and then stick to the facts as currently understood. -- Pinktulip 14:07, 25 January 2006 (UTC)

Chengjiang fauna?

The early Cambrian fossils coming out of Chengjiang, China merit a mention as they complement the Burgess Shale fossils nicely, but are about 10 million years older. They certainly help give a more complete picture of the "Cambrian Explosion". See Hou et al. 2004, "The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life" and also the website http://www.fossilmuseum.net/Fossil_Sites/Chengjiang.htm for an overview. There's a little bit of information already in the article Maotianshan shales, which could be linked. MrDarwin 20:52, 26 January 2006 (UTC)


This Article Needs Work

This is truly one of the worst wikipedia articles I have read. People who look up the topic "cambrian explosion" want to learn about it, not read an article that is 90% devoted to calling the whole thing a misnomer that has come about because of a lack of technology. This article is SO one sided. There have been literally thousands of authors who have written about the explosive development of life during the cambrian period. And this articles has only the view of a handfull of cellular ichnologists who want to voice thier opinions and give no stage to other side of this issue. This article seriously needs work.

Agreed. And does anyone feel the sentence "The Cambrian explosion continues to fascinate the non-specialist" is remarkably condescending? NeighborTotoro 22:47, 27 August 2006 (UTC)
Seconded. As it happens, Graham Budd (I've forgotten their userid) wrote an alternative version, which you can see above. It's much, much better than what we've got at the moment - not surprising as Graham's a researcher in this area. Anyway, you might want to look that up and down to see what you think. It's not perfect either, but it's a step forward IMHO. Cheers, --Plumbago 09:51, 28 August 2006 (UTC)


Text of old article

I have decided to take the plunge, and am entirely replacing the old text of the article, which I reproduce here. I have read some of the helpful comments above and tried to make some adjustments to my text. I've included a time-scale, and will try to upload other images as time permits. Of course, everone else is free too as well!

Graham


The Cambrian explosion refers to the geologically sudden appearance of complex multi-cellular macroscopic organisms between roughly 542 and 530 mya. This period marks a sharp transition in the fossil record with the appearance of the earliest members of many metazoan phyla (multicellular animals) including the first known vertebrate (Myllokunmingia). The "explosive" appearance of this adaptive radiation results both from rapid evolutionary change and the limits of previous technology to appreciate microfossils which formed the foundation of the fossil record before this time.

From the modern point of view, the apparently explosive radiation from obscure beginnings was partly an artefact of disregarding microfossils, which were scarcely detectable with 19th-century technology, and concentrating solely on the hard-shelled macrofossils that defined the phyla well established by 19th-century biologists, all of which were multicellular animals. Apparently abruptly, many kinds of fossils appearing in the Burgess Shale were seen showing obvious skeletal body features, whereas the traces of the "small shelly fauna" of Cambrian beginnings were ignored because of difficulties with their analysis.

With time, advanced microscopy has gradually revealed the range of earlier microfossils. Prior to the discovery in 1909 of the Burgess Shale — incompletely published at the time and largely forced into existing categories as "precursors" — no fossilizations of early soft-bodied organisms had been published, and the vast reach of undiscovered earlier life was consigned to an enormous space of time — the "Pre-Cambrian" of old-fashioned schoolbooks.

More recent microfossil finds have shown "Pre-Cambrian" life consisting of more than single-celled organisms or simple diploblastic fauna. In 1994, phosphatized triploblastic embryos were discovered in rocks from southern China (Xiao et al. 1998). Evidence for Ediacaran triploblasts was available long before this discovery.


Causes of the Cambrian explosion

There is no universally accepted cause, and the matter is the subject of ongoing debate within the scientific community. A wide range of biological and geological factors have been proposed as possible triggers for the explosion. These range from ecological competition, Hox genes, introduction of the eye, and the breakup of Rodinia. One highly speculative idea is that some new but unknown parasite transferred significant genes between species so that innovation was quickly shared between species. Recently scientists have suggested major climatic changes, including a near-global glaciation, may have played a role.

The Cambrian explosion may have been precipitated by several environmental changes occurring in and just before this period. First the Varangian glaciation gave rise to a Snowball Earth in which all, or nearly all, of the oceans were covered entirely with ice. This was followed by a deglaciation and rapid global warming just before the beginning of the explosion itself. In modern Arctic environments, single-celled organisms often form mats on the underside of ice sheets in order to maximize their exposure to sunlight. It is possible that adaptations useful to the maintenance of such colonies also assisted in the formation of the first triploblastic animals estimated to be 570 Ma in age (Xiao et al. 1998). In addition, the Snowball Earth environment would have given rise to relatively few ecological niches, so the subsequent deglaciation and global warming may have provided an impetus for rapid evolution to fill many new environments.

Rising levels of photosynthetic atmospheric oxygen produced by mats of photosynthetic bacteria and algae during the Ediacaran may have played a role in the emergence of large metazoans that required oxygen for respiration. Recent work has linked this increase in atmospheric oxygen to changes in global soil weathering patterns following the Cryogenian and the possible emergence of a primitive terrestrial biota (Kennedy et al. 2006).

In the Blink of an Eye (2004) postulates that the Cambrian explosion was caused by the evolution of the eye, which caused intense diversification because predators could better locate prey, and vice versa.

Diversification

Of the 20 or so metazoan phyla with extensive fossil records, at least 11 first appeared in the Cambrian. Of the remainder, one is known from the Precambrian and the other eight from the Phanerozoic eon (Collins 1994). An additional 12 soft-bodied phyla have poorly defined fossil records, many of which are conjectured to be Cambrian in origin.

Though this period is definitely of special significance in terms of rapid diversification and the emergence of new forms, some of that significance is likely to be overstated by the focus on macroscopic forms in the ways phyla are observed and defined. Molecular evidence suggests that at least six animal phyla had established themselves as distinct evolutionary paths during the Precambrian (Wang et al. 1999).

The large variety of fossil life forms found in the Burgess Shale, the much younger Maotianshan shales of China, and more than three dozen other Cambrian sites on every continent except Antarctica leads many researchers to doubt that 10 to 15 Ma is sufficiently long to explain the great diversity exhibited in the Cambrian fossil record. An emerging view is that the Cambrian explosion is the macroscopic conclusion to a prolonged period of evolution begun ca. 30 Ma earlier with the innovation of multicellular organisms.

The Cambrian explosion continues to fascinate the non-specialist. Stephen Jay Gould's Wonderful Life (1989) provides the best general account for the layman of the paleontological analysis of the Burgess Shale; the book was the 1991 winner of The Aventis Prizes for Science Books.

--Grahbudd 12:21, 2 September 2006 (UTC)

Vernanimalcula

This article, before it goes to hard copy, should take into consideration the discovery of a microscopic bilateral metazoan, Vernanimalcula in the Doushantuo Formation, which with other work of the 1990s suggests that the Cambrian explosion is an apparent explosion— of large metazoans with easily fossilizable hard parts. --Wetman 05:23, 9 September 2006 (UTC)

Vernanimalcula Pt 2

Hi, I've added a slightly longer reference to V. in the Proterozoic predecessors section. I must admit that I am strongly of the opinion that this fossils is nothing more than a diagenetically overgrown blob with no affinities to the bilaterians at all. Indeed, I published a response in Science with one of my colleagues to push this view when the original paper came out. Despite the molecular date data (which in itself hardly gives a consistent picture, although it all tends to place the protostome-deuterostome split before the Cambrian), and the various stuff out of Duoshantuo, AND changing views of Ediacaran fossils. it is still hard to put one's hand on one's heart and say: "THIS is a Precambrian bilaterian!" without an act of faith being involved. Still, I hope I have managed to maintain some sort of NPOV....Grahbudd 15:30, 13 September 2006 (UTC)

Hydrogen peroxide factor

Sunlight-created hydrogen peroxide could have been a factor. Geobiologists Solve "Catch-22 Problem" Concerning the Rise of Atmospheric Oxygen (SEWilco 05:35, 10 December 2006 (UTC))

This is talking about a much much earlier period. Dragons flight 06:12, 10 December 2006 (UTC)
Wrong snowball. Oops. (SEWilco 16:19, 10 December 2006 (UTC))

A deleted ext. reference

The ext. reference

was deleted by User:Woohookitty. It reads like a perfectly mainstream, though rather basic, explication of the Cambrian explosion and the kinds of evidence available, presented by Jeffrey S. Levinton, Ph.D., State University of New York, and including a useful series of responsible webite links. Is there a factor here that we ignore? This note is copied at User talk:Woohookitty. --(Wetman 11:54, 14 January 2007 (UTC)

User:Woohookitty responded at User talk:Wetman "I removed that link when I thought he was a spammer. If you want to reinstate it, be my guest." A look at the deleted contributions from new User:Scilit reveals over a hundred deleted links at science articles in the last two days. "Be my guest" is therefore something of a challenge. --Wetman 12:16, 14 January 2007 (UTC)
Retrieved from "https://en.wikipedia.org/w/index.php?title=Talk:Cambrian_explosion/Archive_1&oldid=935189377"