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The Cambrian explosion
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The Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex animals around 530 million years ago, as evidenced by the fossil record.[1][2] This was accompanied by a major diversification of other organisms, including animals, phytoplankton, and calcimicrobes.[3] Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80 million years the rate of evolution accelerated by an order of magnitude (as defined in terms of the extinction and origination rate of species[4]) and the diversity of life began to resemble today’s.[5]

The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the mid 19th century,[6] and Charles Darwin saw it as one of the main objections that could be made against his theory of evolution by natural selection.[7]

The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid evolution; and what it would imply about the origin and evolution of animals. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures left in Cambrian rocks.

History and significanceEdit

Main article: Evolutionary history of life

Geologists as long ago as Buckland (1784–1856) realised that a dramatic step-change in the fossil record occurred around the base of what we now call the Cambrian.[6] Charles Darwin considered this sudden appearance of many animal groups with few or no antecedents to be the greatest single objection to his theory of evolution: indeed, he devoted a substantial chapter of The Origin of Species to this problem.[7]

American palæontologist Charles Walcott proposed that an interval of time, the “Lipalian”, was not represented in the fossil record or did not preserve fossils, and that the ancestors of the Cambrian animals evolved during this time.[8]

More recently it was discovered that the history of life on earth goes back at least 3,450 million years[9]: rocks of that age at Warrawoona in Australia contain fossils of stromatolites, stubby pillars that are formed by colonies of micro-organisms. Fossils (Grypania) of more complex eukaryotic cells, from which all animals, plants and fungi are built, have been found in rocks from 1,400 million years ago, in China and Montana. Rocks dating from 565 to 543 million years ago contain fossils of the Ediacara biota, organisms so large that they must have been multi-celled, but very unlike any modern organism.[10] P. E. Cloud argued in 1948 that there was a period of "eruptive" evolution in the Early Cambrian,[11] but as recently as the 1970s there was no sign of how the relatively modern-looking organisms of the Middle and Late Cambrian arose.[10]

Opabinia BW2

Opabinia made the largest single contribution to modern interest in the Cambrian explosion.

The intense modern interest in this "Cambrian explosion" was sparked by the work of Harry B. Whittington and colleagues, who in the 1970s re-analysed many fossils from the Burgess Shale (see below) and concluded that several were complex but different from any living animals.[12][13] The most common organism, Marrella, was clearly an arthropod, but not a member of any known arthropod class. Organisms such as the five-eyed Opabinia and spiny slug-like Wiwaxia were so different from anything else known that Whittington's team assumed they must represent different phyla, only distantly related to anything known today. Stephen Jay Gould’s popular 1989 account of this work, Wonderful Life,[14] brought the matter into the public eye and raised questions about what the explosion represented. While differing significantly in details, both Whittington and Gould proposed that all modern animal phyla had appeared rather suddenly. This view was influenced by the theory of punctuated equilibrium, which Eldredge and Gould developed in the early 1970s and which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change.[15]

But other analyses, some more recent and some dating back to the 1970s, argue that complex animals similar to modern types evolved well before the start of the Cambrian.[16][17][18] There has also been intense debate whether there was a genuine "explosion" of modern forms in the Cambrian and, to the extent that there was, how it happened and why it happened then.[19]

Types of evidenceEdit

Deducing the events of half a billion years ago is difficult, and evidence comes from biological and chemical signatures in rocks.

Dating the CambrianEdit

Accurate absolute radiometric dates for much of the Cambrian, obtained by detailed analysis of radioactive elements contained within rocks, have only rather recently become available, and for only a few regions.[20]

Relative dating (A was before B) is often sufficient for studying processes of evolution, but this too has been difficult, because of the problems involved in matching up rocks of the same age across different continents.[21]

Therefore dates or descriptions of sequences of events should be regarded with some caution until better data become available.

Body fossilsEdit

Fossils of organisms' bodies are usually the most informative type of evidence. Fossilisation is a rare event, and most fossils are destroyed by erosion or metamorphism before they can be observed. Hence the fossil record is very incomplete, increasingly so, further back in time. Despite this, they are often adequate to illustrate the broader patterns of life's history.[22] There are also biases in the fossil record: different environments are more favourable to the preservation of different types of organism or parts of organisms.[23] Further, only the parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. Since most animal species are soft-bodied, they decay before they can become fossilised. As a result, although there are 30-plus phyla of living animals, two-thirds have never been found as fossils.[10]

File:Marella200x155.png

The Cambrian fossil record includes an unusually high number of lagerstätten, which preserve soft tissues. These allow palæontologists to examine the internal anatomy of animals which in other sediments are only represented by shells, spines, claws, etc – if they are preserved at all. The most significant Cambrian lagerstätten are the early Cambrian Maotianshan shale beds of Chengjiang (Yunnan, China) and Sirius Passet (Greenland);[24] the middle Cambrian Burgess Shale (British Columbia, Canada)[25]; and the late Cambrian Orsten (Sweden) fossil beds.

While lagerstätten preserve far more than the conventional fossil record, they are far from complete. Because lagerstätten are restricted to a narrow range of environments (where soft-bodied organisms can be preserved very quickly, e.g. by mudslides), most animals are probably not represented; further, the exceptional conditions that create lagerstätten probably do not represent normal living conditions.[26] In addition, the known Cambrian lagerstätten are rare and difficult to date, while Precambrian lagerstätten have yet to be studied in detail.

The sparseness of the fossil record means that organisms usually exist long before they are found in the fossil record – this is known as the Signor-Lipps effect.[27]

Trace fossilsEdit

File:CambrianRusophycus.jpg

Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces) and marks left by feeding.[28][29] Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily-fossilized hard parts, and which reflects organisms' behaviour. Also many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them.[30] Whilst exact assignment of trace fossils to their makers is generally impossible, traces may for example provide the earliest physical evidence of the appearance of moderately complex animals (comparable to earthworms).[29]

Geochemical observationsEdit

Main article: Early Cambrian geochemical fluctuations

Several chemical markers indicate a drastic change in the environment around the start of the Cambrian. The markers are consistent with a mass extinction,[31][32] or with a massive warming resulting from the release of methane ice. [19] Such changes may reflect a cause of the Cambrian explosion, although they may also have resulted from an increased level of biological activity – a possible result of the explosion.[19] Despite these uncertainties, the geochemical evidence helps by making scientists focus on theories that are consistent with at least one of the likely environmental changes.

Phylogenetic techniquesEdit

Cladistics is a technique for working out the “family tree” of a set of organisms. It works by the logic that, if groups B and C have more similarities to each other than either has to group A, then B and C are more closely related to each other than either is to A. Characteristics which are compared may be anatomical, such as the presence of a notochord, or molecular, by comparing sequences of DNA or protein. The result of a successful analysis is a hierarchy of clades – groups whose members are believed to share a common ancestor. The cladistic technique is sometimes fallible, as some features, such as wings or camera eyes, evolved more than once, convergently – this must be taken into account in analyses.

From the relationships, it may be possible to constrain the date that lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved more than X million years ago.

It is also possible to estimate how long ago two living clades diverged – i.e. approximately how long ago their last common ancestor must have lived – by assuming that DNA mutations accumulate at a constant rate. These "molecular clocks", however, are fallible, and provide only a very approximate timing: they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved,[33] and estimates produced by different techniques vary by a factor of two.[34] However, the clocks can give an indication of branching rate, and when combined with the constraints of the fossil record, recent clocks suggest a sustained period of diversification through the Ediacaran and Cambrian.[35]

ReferencesEdit

  1. ^ The Cambrian Period
  2. ^ The Cambrian Explosion – Timing
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  5. ^ Bambach, R.K.; Bush, A.M., Erwin, D.H. (2007). "Autecology and the filling of Ecospace: Key metazoan radiations". Palæontology 50 (1): 1–22. doi:10.1111/j.1475-4983.2006.00611.x. 
  6. ^ a b Buckland, W. (1841). Geology and Mineralogy Considered with Reference to Natural Theology. Lea & Blanchard. 
  7. ^ a b Darwin, C (1859). On the Origin of Species by Natural Selection. Murray, London, United Kingdom. pp. 315–316. ISBN 1602061440. OCLC 176630493. 
  8. ^ Walcott, C.D. (1914). "Cambrian Geology and Paleontology". Smithsonian Miscellaneous Collections 57: 14. 
  9. ^ Holland, Heinrich D (3 1 1997). "Evidence for life on earth more than 3850 million years ago". Science 275: 38. doi:10.1126/science.275.5296.38. 
  10. ^ a b c Cowen, R. (2002). History of Life. Blackwell Science. ISBN 0931292387. OCLC 53325609. 
  11. ^ Cloud, P.E. (1948). "Some problems and patterns of evolution exemplified by fossil invertebrates". Evolution 2 (4): 322–350. doi:10.2307/2405523. http://www.jstor.org/pss/2405523. Retrieved on 2008-07-17. 
  12. ^ Whittington, H. B. (1979). Early arthropods, their appendages and relationships. In M. R. House (Ed.), The origin of major invertebrate groups (pp. 253–268). The Systematics Association Special Volume, 12. London: Academic Press.
  13. ^ Whittington, H.B.; Geological Survey of Canada (1985). The Burgess Shale. Yale University Press. ISBN 0660119013. OCLC 15630217. 
  14. ^ Gould, S.J. (1989). Wonderful Life: The Burgess Shale and the Nature of History. W.W. Norton & Company. ISBN 0393027058. OCLC 185746546. 
  15. ^ Bengtson, S. (2004). Early skeletal fossils. in Lipps, J.H., and Waggoner, B.M.. "Neoproterozoic- Cambrian Biological Revolutions" (PDF). Palentological Society Papers 10: 67–78. http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf. Retrieved on 2008-07-18. 
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  17. ^ Awramik, S.M. (19 November 1971). "Precambrian columnar stromatolite diversity: Reflection of metazoan appearance". Science 174 (4011): 825–827. doi:10.1126/science.174.4011.825. PMID 17759393. 
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  20. ^ e.g. Jago, J.B.; Haines, P.W. (1998). "Recent radiometric dating of some Cambrian rocks in southern Australia: relevance to the Cambrian time scale". Revista Española de Paleontología: 115–22. 
  21. ^ e.g. Gehling, James; Jensen, Sören; Droser, Mary; Myrow, Paul; Narbonne, Guy (March 2001). "Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland". Geological Magazine 138 (2): 213–218. doi:10.1017/S001675680100509X. http://www.journals.cambridge.org/action/displayAbstract?fromPage=online&aid=74669. 
  22. ^ Benton MJ, Wills MA, Hitchin R (2000). "Quality of the fossil record through time". Nature 403 (6769): 534–7. doi:10.1038/35000558. PMID 10676959. 
    Non-technical summary
  23. ^ Butterfield , N.J. (2003). "Exceptional Fossil Preservation and the Cambrian Explosion". Integrative and Comparative Biology 43 (1): 166–177. doi:10.1093/icb/43.1.166. http://icb.oxfordjournals.org/cgi/content/abstract/43/1/166. Retrieved on 2008-06-28. 
  24. ^ Morris, S.C. (1979). "The Burgess Shale (Middle Cambrian) Fauna". Annual Review of Ecology and Systematics 10 (1): 327–349. doi:10.1146/annurev.es.10.110179.001551. 
  25. ^ Yochelson, E.L. (1996). "Discovery, Collection, and Description of the Middle Cambrian Burgess Shale Biota by Charles Doolittle Walcott". Proceedings of the American Philosophical Society 140 (4): 469–545. http://links.jstor.org/sici?sici=0003–049X(199612)140%3A4%3C469%3ADCADOT%3E2.0.CO%3B2–8. Retrieved on 2007-04-24. 
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  27. ^ Signor, P.W. (1982). "Sampling bias, gradual extinction patterns and catastrophes in the fossil record". Geological implications of impacts of large asteroids and comets on the earth (Boulder, CO: Geological Society of America): 291–296. A 84–25651 10–42. http://www.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8425675AH. Retrieved on 2008-01-07. 
  28. ^ "What is paleontology?". University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/faq.php#paleo. Retrieved on 2008-09-18. 
  29. ^ a b Fedonkin, M.A., Gehling, J.G., Grey, K., Narbonne, G.M., Vickers-Rich, P. (2007). The Rise of Animals: Evolution and Diversification of the Kingdom Animalia. JHU Press. pp. 213–216. ISBN 0801886791. http://books.google.co.uk/books?id=OFKG6SmPNuUC&pg=PA213&lpg=PA213&dq=trace+fossil+complex+animal&source=web&ots=gxRn0yEcyh&sig=0GXGIpXruR6rHt27iOCJf08QcTw&hl=en&sa=X&oi=book_result&resnum=2&ct=result#PPA216,M1. Retrieved on 2008-11-14. 
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  32. ^ Amthor, J.E.; Grotzinger, J.P., Schroder, S., Bowring, S.A., Ramezani, J., Martin, M.W., Matter, A. (2003). "Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman". Geology 31 (5): 431–434. doi:10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2. 
  33. ^ Hug, L.A., and Roger, A.J. (2007). "The Impact of Fossils and Taxon Sampling on Ancient Molecular Dating Analyses". Molecular Biology and Evolution 24 (8): 889–1897. doi:10.1093/molbev/msm115. 
  34. ^ Peterson, Kevin J., and Butterfield, N.J. (2005). "Origin of the Eumetazoa: Testing ecological predictions of molecular clocks against the Proterozoic fossil record". Proceedings of the National Academy of Sciences 102 (27): 9547. doi:10.1073/pnas.0503660102. PMID 15983372. 
  35. ^ Peterson, Kevin J. (2008). "The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records". Philosophical Transactions of the Royal Society B: Biological Sciences 363: 1435. doi:10.1098/rstb.2007.2233. 


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