Cambrian explosion

The Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex animals around Ma|530, as evidenced by the fossil record. [http://www.ucmp.berkeley.edu/cambrian/camb.html The Cambrian Period] ] [http://palaeo.gly.bris.ac.uk/Palaeofiles/Cambrian/timing/timing.html The Cambrian Explosion – Timing] ] This was accompanied by a major diversification of other organisms, including animals, phytoplankton, and calcimicrobes.cite journal
author = Butterfield, N.J.
year = 2001
title = Ecology and evolution of Cambrian plankton
journal = The Ecology of the Cambrian Radiation. Columbia University Press, New York
pages = 200–216
url = http://66.102.1.104/scholar?hl=en&lr=&ie=UTF-8&q=cache:9xeRu1SdF0QJ:www.earthscape.org/r3/ES14785/ch09.pdf+
accessdate = 2007-08-19 ] Before about Ma|580 , most organisms were simple, composed of individual cells occasionally organised 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) and the diversity of life began to resemble today’s.cite journal
author = Bambach, R.K.
coauthors = Bush, A.M., Erwin, D.H.
year = 2007
title = Autecology and the filling of Ecospace: Key metazoan radiations
journal = Palæontology
volume = 50
issue = 1
pages = 1–22
doi = 10.1111/j.1475-4983.2006.00611.x]

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,cite book
author = Buckland, W.
year = 1841
title = Geology and Mineralogy Considered with Reference to Natural Theology
publisher = Lea & Blanchard ] and Charles Darwin saw it as one of the main objections that could be made against his theory of evolution by natural selection.cite book
title=On the Origin of Species by Natural Selection
author=Darwin, C
authorlink=Charles Darwin
year=1859
pages=315–316
publisher=Murray, London, United Kingdom
isbn=1602061440
oclc=176630493]

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 significance

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. 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.cite book
title=On the Origin of Species by Natural Selection
author=Darwin, C
authorlink=Charles Darwin
year=1859
pages=315–316
publisher=Murray, London, United Kingdom
isbn=1602061440
oclc=176630493]

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. [cite journal|authorlink=Charles Walcott| title=Cambrian Geology and Paleontology|journal=Smithsonian Miscellaneous Collections|volume=57|pages=14|author=Walcott, C.D.|year=1914]

More recently it was discovered that the history of life on earth goes back at least Ma|3550|million years: 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 Ma|1400, in China and Montana. Rocks dating from Ma|565|543 contain fossils of the Ediacara biota, organisms so large that they must have been multi-celled, but very unlike any modern organism. P. E. Cloud argued in 1948 that there was a period of "eruptive" evolution in the Early Cambrian, [ cite journal | author=Cloud, P.E. | year=1948 |title=Some problems and patterns of evolution exemplified by fossil invertebrates | journal=Evolution | volume=2 | issue=4 | pages=322–350 | url=http://www.jstor.org/pss/2405523 | accessdate=2008-07-17 | doi=10.2307/2405523 ] but as recently as the 1970s there was no sign of how the "relatively" modern-looking organisms of the Middle and Late Cambrian arose.

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 as but different from any living animals.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.] cite book | title=The Burgess Shale | author=Whittington, H.B. | authorlink=Harry Whittington | coauthors = Geological Survey of Canada | year=1985 | publisher=Yale University Press | isbn=0660119013 | oclc=15630217 ] 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",cite book
title=Wonderful Life: The Burgess Shale and the Nature of History
author=Gould, S.J.
authorlink=Steven Jay Gould
year=1989
publisher=W.W. Norton & Company
isbn=0393027058
oclc=185746546 187362116 18983518 231046931] 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.citation | contribution=Early skeletal fossils
author = Bengtson, S. | editor=Lipps, J.H., and Waggoner, B.M. | title=Neoproterozoic- Cambrian Biological Revolutions
year = 2004
journal = Palentological Society Papers
volume = 10
pages = 67–78
url = http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf | accessdate=2008-07-18 ]

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.cite journal
author = McNamara, K.J.
title = Dating the Origin of Animals
journal = Science
volume = 274
number= 5295
pages = 1993–1997
date = 20 December 1996
doi = 10.1126/science.274.5295.1993f] cite journal
author = Awramik, S.M.
title = Precambrian columnar stromatolite diversity: Reflection of metazoan appearance
journal = Science
volume = 174
number=4011
pages = 825–827
date = 19 November 1971
doi=10.1126/science.174.4011.825
year = 1971
pmid = 17759393] 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.

Types of evidence

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

Dating the Cambrian

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. [e.g. cite journal
title=Recent radiometric dating of some Cambrian rocks in southern Australia: relevance to the Cambrian time scale
author=Jago, J.B.
coauthors = Haines, P.W.
journal=Revista Española de Paleontología
pages=115–22
year=1998]

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.e.g. Cite journal
last = Gehling
first = James
last2 = Jensen
first2 = Sören
last3 = Droser
first3 = Mary
last4 = Myrow
first4 = Paul
last5 = Narbonne
first5 = Guy
title = Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland
journal = Geological Magazine
volume = 138
issue = 2
pages = 213–218
month = March
year = 2001
url = http://www.journals.cambridge.org/action/displayAbstract?fromPage=online&aid=74669
doi = 10.1017/S001675680100509X]

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

Body fossils

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.cite journal |author=Benton MJ, Wills MA, Hitchin R |title=Quality of the fossil record through time |journal=Nature |volume=403 |issue=6769 |pages=534–7 |year=2000 |pmid=10676959 |doi=10.1038/35000558

: Non-technical [http://palaeo.gly.bris.ac.uk/cladestrat/news.html summary] ] 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.cite journal | author = Butterfield , N.J. | year =2003 | title = Exceptional Fossil Preservation and the Cambrian Explosion | journal = Integrative and Comparative Biology | volume = 43 | issue = 1 | pages = 166–177 | doi = 10.1093/icb/43.1.166 | url=http://icb.oxfordjournals.org/cgi/content/abstract/43/1/166 | accessdate=2008-06-28 ] 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. cite book | author=Cowen, R. | title=History of Life | publisher=Blackwell Science | isbn=0931292387 | year=2002 | oclc=53325609 ]

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);cite journal
author = Morris, S.C. | year = 1979 | title = The Burgess Shale (Middle Cambrian) Fauna | journal = Annual Review of Ecology and Systematics
volume = 10 | issue = 1 | pages = 327–349
doi = 10.1146/annurev.es.10.110179.001551 ] the middle Cambrian Burgess Shale (British Columbia, Canada)cite journal
author = Yochelson, E.L.
year = 1996
title = Discovery, Collection, and Description of the Middle Cambrian Burgess Shale Biota by Charles Doolittle Walcott
journal = Proceedings of the American Philosophical Society
volume = 140 | issue = 4 | pages = 469–545
url = http://links.jstor.org/sici?sici=0003–049X(199612)140%3A4%3C469%3ADCADOT%3E2.0.CO%3B2–8
accessdate = 2007-04-24] ; 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.citation
author = Butterfield, N.J.
year = 2001
chapter = Ecology and evolution of Cambrian plankton
title = The Ecology of the Cambrian Radiation
publisher = Columbia University Press, New York
pages = 200–216
url = http://66.102.1.104/scholar?hl=en&lr=&ie=UTF-8&q=cache:9xeRu1SdF0QJ:www.earthscape.org/r3/ES14785/ch09.pdf
accessdate = 2007-08-19 ] 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.cite journal
author = Signor, P.W. |year= 1982
title = Sampling bias, gradual extinction patterns and catastrophes in the fossil record
journal = Geological implications of impacts of large asteroids and comets on the earth
id = A 84–25651 10–42
location= Boulder, CO |publisher = Geological Society of America
pages = 291–296 | url = http://www.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8425675AH | accessdate = 2008-01-07]

Trace fossils

consist mainly of tracks and burrows on and under what was then the seabed.

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.e.g. cite journal
author = Seilacher, A.
year = 1994
title = How valid is Cruziana Stratigraphy?
journal = International Journal of Earth Sciences
volume = 83
issue = 4
pages = 752–758
url = http://www.springerlink.com/index/WP279834395100KH.pdf
accessdate = 2007-09-09
format=PDF] Whilst exact assignment of trace fossils to their makers is generally impossible, traces may provide the earliest physical evidence of the appearance of moderately complex animals (comparable to earthworms).

Geochemical observations

Several chemical markers indicate a drastic change in the environment around the start of the Cambrian. The markers are consistent with a mass extinction,cite journal
author = Amthor, J.E.
coauthors = Grotzinger, J.P., Schroder, S., Bowring, S.A., Ramezani, J., Martin, M.W., Matter, A.
year = 2003
title = Extinction of" Cloudina "and "Namacalathus" at the Precambrian-Cambrian boundary in Oman
journal = Geology
volume = 31
issue = 5
pages = 431–434
doi = 10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2 ] or with a massive warming resulting from the release of methane ice.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. 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 techniques

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. Characters 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 acestor 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, [ cite journal
author=Hug, L.A., and Roger, A.J.
title=The Impact of Fossils and Taxon Sampling on Ancient Molecular Dating Analyses
journal=Molecular Biology and Evolution | year=2007 | volume=24 | issue=8 | pages=889–1897
doi=10.1093/molbev/msm115 ] and estimates produced by different techniques vary by a factor of two.cite journal
doi = 10.1073/pnas.0503660102 | pmid = 15983372 | author = Peterson, Kevin J., and Butterfield, N.J.
journal = Proceedings of the National Academy of Sciences | volume = 102 | issue = 27
pages = 9547 | year = 2005
title = Origin of the Eumetazoa: Testing ecological predictions of molecular clocks against the Proterozoic fossil record]

Explanation of a few scientific terms

A phylum is the highest level in the Linnean system for classifying animals. Phyla can be thought of as groupings of animals based on general body plan. [cite book | last = Valentine
first = James W. | year = 2004 | title = On the Origin of Phyla | publisher = University Of Chicago Press | location = Chicago | isbn = 0226845486 | pages = 7"Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. usually organisms were grouped according to their morphological similarities as perceived by those early workers, and those groups were then grouped according to their similarities, and so on, to form a hierarchy."] Despite the seemingly different "external" appearances of organisms, they are classified into phyla based on their "internal" and developmental organizations. [cite book
last = Parker
first = Andrew
year = 2003
title = In the blink of an eye: How vision kick-started the big bang of evolution
publisher = Free Press
location = Sydney
isbn = 0743257332
pages = 1–4"The job of an evolutionary biologist is to make sense of the conflicting diversity of form – there is not always a relationship between internal and external parts. Early in the history of the subject, it became obvious that internal organisations were generally more important to the higher classification of animals than are external shapes. The internal organisation puts general restrictions on how an animal can exchange gases, obtain nutrients and reproduce."] For example, despite their obvious differences, spiders and barnacles both belong to the phylum Arthropoda; but earthworms and tapeworms, although similar in shape, belong to different phyla.

Annotated image | caption=Stem groups.|

annotations =A phylum is not a fundamental division of nature, such as the difference between electrons and protons. It is simply a very high-level grouping in a classification system created to describe all currently living organisms. This system is imperfect, even for modern animals: different books quote different numbers of phyla, mainly because they disagree about the classification of a huge number of worm-like species. As it is based on living organisms, it accommodates extinct organisms poorly, if at all. citation | author=Jefferies, R.P.S. | date=1979 | title=The origin of chordates — a methodological essay | editor=House, M.R., | collection=The origin of major invertebrate groups | pages=443–477 | publisher=Academic Press | location=London summarised in cite journal | author=Budd, G.E. | title=The Cambrian Fossil Record and the Origin of the Phyla | journal=Integrative and Comparative Biology | volume=43 | issue=1 | pages=157–165 | doi=10.1093/icb/43.1.157 | url=http://icb.oxfordjournals.org/cgi/content/full/43/1/157 | year=2003 ]

The concept of stem groups was introduced to cover evolutionary "aunts" and "cousins" of living groups. A crown group is a group of closely-related living animals plus their last common ancestor plus all its descendants. A stem group is offshoots from members of the lineage earlier than the last common ancestor of the crown group; it is a "relative" concept, for example tardigrades are living animals which form a crown group in their own right, but Budd (1996) regarded them also as being a stem group relative to the arthropods.cite journal
author=Craske, A.J., and Jefferies, R.P.S. | date=1989
title=A new mitrate from the Upper Ordovician of Norway, and a new approach to subdividing a plesion
journal=Palaeontology | volune=32 | pages=69–99 ] Triploblastic means consisting of 3 layers, which are formed in the embryo, quite early in the animal's development from a single-celled egg to a larva or juvenile form. The innermost layer forms the digestive tract (gut); the outermost forms skin; and the middle one forms muscles and all the internal organs except the digestive system. Most types of living animal are triploblastic – the best-known exceptions are Porifera (sponges) and Cnidaria (jellyfish, sea anemones, etc.).

The bilaterians are animals which have right and left sides at some point in their life history. This implies that they have top and bottom surfaces and, importantly, distinct front and back ends. All known bilaterian animals are triploblastic, and all known triploblastic animals are bilaterian. Living Echinoderms (starfish, sea urchins, sea cucumbers, etc.) "look" radially symmetrical (like wheels) rather than bilaterian, but their larvae exhibit bilateral symmetry and some of the earliest echinoderms may have been bilaterally symmetrical. [ cite journal | title=Paired gill slits in a fossil with a calcite skeleton | journal=Nature | issue=417 | pages=841–844 | month=June | year=2002 | doi=10.1038/nature00805 | url=http://www.nature.com/nature/journal/v417/n6891/abs/nature00805.html | author=Dominguez, P., Jacobson, A.G., Jefferies, R.P.S. | volume=417 ] Porifera and Cnidaria are radially symmetrical, non-bilaterian and non-triploblastic..

Coelomate means having a body cavity (coelom) which contains the internal organs. Most of the phyla featured in the debate about the Cambrian explosion are coelomates: arthropods, annelid worms, molluscs, echinoderms and chordates – the non-coelomate priapulids are an important exception. All known coelomate animals are triploblastic bilaterians, but some triploblastic bilaterian animals do not have a coelom – for example flatworms, whose organs are surrounded by unspecialized tissues.

Precambrian life

Our understanding of the Cambrian explosion relies upon knowing what was there beforehand – did the event herald the sudden appearance of a wide range of animals and behaviours, or did such things exist beforehand?

Evidence of animals around Ma|1000

:"For further information, see Acritarch and Stromatolite

Changes in the abundance and diversity of some types of fossil have been interpreted as evidence for "attacks" by animals or other organisms. Stromatolites, stubby pillars built by colonies of microorganisms, are a major constituent of the fossil record from about Ma|2700, but their abundance and diversity declined steeply after about Ma|1250. This decline has been attributed to disruption by grazing and burrowing animals.cite journal
author = McNamara, K.J.
title = Dating the Origin of Animals
journal = Science
volume = 274
number= 5295
pages = 1993–1997
date = 20 December 1996
url=http://www.sciencemag.org/cgi/content/full/274/5295/1993f
accessdate=2008-06-28
doi = 10.1126/science.274.5295.1993f
year = 1996] cite journal
author = Awramik, S.M.
title = Precambrian columnar stromatolite diversity: Reflection of metazoan appearance
journal = Science
volume = 174
number=4011
pages = 825–827
date = 19 November 1971
url = http://www.sciencemag.org/cgi/content/abstract/174/4011/825
accessdate=2007-12-01
doi=10.1126/science.174.4011.825
year = 1971
format = abstract
pmid = 17759393] Citation
author=Bengtson, S.
date=2002
contribution=Origins and early evolution of predation
title=The fossil record of predation. The Paleontological Society Papers 8
editors=Kowalewski, M., and Kelley, P.H.
pages=289– 317
publisher=The Paleontological Society
url=http://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf
accessdate=2007-12-01
format = Free full text]

Precambrian marine diversity was dominated by small fossils known as acritarchs. This term describes almost any small organic walled fossil – from the egg cases of small metazoans to resting cysts of many different kinds of green algae. After appearing around Ma|2000, acritarchs underwent a boom around Ma|1000, increasing in abundance, diversity, size, complexity of shape and especially size and number of spines. Their increasingly spiny forms in the last 1 billion years may indicate an increased need for defence against predation. Other groups of small organisms from the Neoproterozoic era also show signs of anti-predator defenses. Citation
author=Bengtson, S.
date=2002
contribution=Origins and early evolution of predation
title=The fossil record of predation. The Paleontological Society Papers 8
editors=Kowalewski, M., and Kelley, P.H.
pages=289– 317
publisher=The Paleontological Society
url=http://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf
format = Free full text] A consideration of taxon longevity appears to support an increase in predation pressure around this time,Cite journal
title = Predation defeats competition on the seafloor
author = Stanley
year = 2008
journal = Paleobiology
volume = 34
pages = 1
url = http://paleobiol.geoscienceworld.org/cgi/content/extract/34/1/1
format = extract
doi = 10.1666/07026.1] However, in general, the rate of evolution in the Precambrian was very slow, with many cyanobacterial species persisting unchanged for billions of years.

If these predatory organisms really were metazoans, this means that Cambrian animals didn't appear "from no-where" at the base of the Cambrian – predecessors had existed for hundreds of millions of years.

Fossils of the Doushantuo formation

The Ma|580|million year old cite journal
title=U-Pb Ages from the Neoproterozoic Doushantuo Formation, China
journal=Science
date=1 April 2005
issue=5718
pages=95–98
doi=10.1126/science.1107765
url=http://www.sciencemag.org/cgi/content/abstract/308/5718/95
author=Condon, D., Zhu, M., Bowring, S., Wang, W., Yang, A., and Jin, Y.
year=2005
volume=308
format=abstract
pmid=15731406] Doushantuo formation harbours microscopic fossils which may represent early bilaterians. Some have been described as animal embryos and eggs, although some of these may represent the remains of giant bacteria.:citation
author = Xiao, S., Zhang, Y. & Knoll, A. H.
title = Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite
journal = Nature
volume = 391
pages = 553–558
doi = 10.1038/35318
year = 1998 :cite journal
title=Cellular and Subcellular Structure of Neoproterozoic Animal Embryos
journal = Science
volume =314
pages = 291–294
doi=10.1126/science.1133129
year=2006
author=Hagadorn, J. W.:cite journal
title = Evidence of giant sulphur bacteria in Neoproterozoic phosphorites
journal = Nature
volume = 445
pages = 198–201
doi = 10.1038/nature05457
year = 2007
author = Bailey, Jake V.] Another fossil, "Vernanimalcula", has been interpreted as a coelomate bilaterian,cite journal
author = Chen, J.Y.
coauthors = Bottjer, D.J.; Oliveri, P.; Dornbos, S.Q.; Gao, F.; Ruffins, S.; Chi, H.; Li, C.W.; Davidson, E.H.
date = 2004-07-09
title = Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian
journal = Science
volume = 305
issue = 5681
pages = 218–222
doi = 10.1126/science.1099213
year = 2004
pmid = 15178752] but may simply be an infilled bubble.cite journal
author = Bengtson, S.
coauthors = Budd, G.
year = 2004
title = Comment on ‘‘Small bilaterian fossils from 40 to 55 million years before the Cambrian’’
journal = Science
volume = 306
pages = 1291a
doi = 10.1126/science.1101338
pmid = 15550644]

These fossils form the earliest hard-and-fast evidence of animals, as opposed to other predators.cite journal
author = Philip, C. J.; Neil, J.; John, A.; ,
year = 2006
title = Synchrotron X-ray tomographic microscopy of fossil embryos
journal = Nature
volume = 442
issue = 7103
pages = 680
doi = 10.1038/nature04890]

Burrows

The traces of organisms moving on and directly underneath the microbial mats that covered the Ediacaran sea floor are preserved from the Ediacaran period, about Ma|565. They were probably made by organisms resembling earthworms in shape, size, and how they moved. The burrow-makers have never been found preserved, but because they would need a head and a tail, the burrowers probably had bilateral symmetry – which would in all probability make them bilaterian animals. [citation
author = Fedonkin, M.A.
year = 1992
chapter = Vendian faunas and the early evolution of Metazoa
title = Origin and early evolution of the Metazoa
location = New York
publisher = Springer
editors = Lipps, J., and Signor, P. W.
pages = 87–129
url = http://books.google.co.uk/books?id=gUQMKiJOj64C&pg=PP1&ots=BkpdtmDml1&sig=ap0OD3JXuSkTVhJTSqQbT5uC2P8
isbn = 0306440679
accessdate = 2007-03-08
oclc = 231467647 25873335 ] They fed above the sediment surface, but were forced to burrow to avoid predators.The Rise and Fall of the Ediacaran Biota|Dzik, J|The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian–Cambrian transition|405|414|30]

Around the start of the Cambrian (about Ma|542) many new types of traces first appear, including well-known vertical burrows such as "Diplocraterion" and "Skolithos", and traces normally attributed to arthropods, such as "Cruziana" and "Rusophycus". The vertical burrows indicate that worm-like animals acquired new behaviours, and possibly new physical capabilities. Some Cambrian trace fossils indicate that their makers possessed hard exoskeletons, although there were not necessarily mineralised.cite journal| title=The Proterozoic and Earliest Cambrian Trace Fossil Record; Patterns, Problems and Perspectives| author=Jensen, S.| journal=Integrative and Comparative Biology| doi = 10.1093/icb/43.1.219| year=2003| volume=43| pages=219]

Burrows provide firm evidence of complex organisms; they are also much more readily preserved than body fossils, to the extent that the absence of trace fossils has been used to imply the genuine absence of large, motile bottom-dwelling organisms.Fact|try Budd|date=June 2008 They provide a further line of evidence to show that the Cambrian explosion represents a real diversification, and is not a preservational artefact.Indeed, as burrowing became established, it allowed an explosion of its own, for as burrowers disturbed the sea floor, they aerated it, mixing oxygen into the toxic muds. This made the bottom sediments more hospitable, and allowed a wider range of organisms to inhabit them – creating new niches and the scope for higher diversity.cite journal | author = Seilacher, Adolf | coauthors = Luis A. Buatoisb, M. Gabriela Mángano | date = 2005-10-07 | title = Trace fossils in the Ediacaran–Cambrian transition: Behavioral diversification, ecological turnover and environmental shift | journal = Palaeogeography, Palaeoclimatology, Palaeoecology | volume = 227 | issue = 4 | pages = 323–356 | doi = 10.1016/j.palaeo.2005.06.003 | year = 2005 ]

Ediacaran organisms

At the start of the Ediacaran period, much of the acritarch fauna, which had remained relatively unchanged for hundreds of millions of years, became extinct, to be replaced with a range of new, larger species which would prove far more ephemeral. This radiation, the first in the fossil record, is followed soon after by an array of unfamiliar, large, fossils dubbed the Ediacara biota, [ cite journal | title=The Avalon Explosion: Evolution of Ediacara Morphospace
month=January | year=2008 | volume=319| issue=5859 |pages=81–84
doi=10.1126/science.1150279
url=http://www.sciencemag.org/cgi/content/abstract/319/5859/81
author=Shen, B., Dong, L., Xiao, S. and Kowalewski, M.
journal=Science | format=abstract | pmid=18174439] which flourished for 40 million years until the start of the Cambrian.cite journal | author = Grazhdankin | year = 2004 | doi = 10.1666/0094-8373(2004)030<0203:PODITE>2.0.CO;2 | issn = 0094–8373 | volume = 30 | pages = 203 | title = Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution | journal = Paleobiology ] Most of this "Ediacara biota" were at least a few centimeters long, significantly larger than any earlier fossils. The organisms form three distinct assemblages, increasing in size and complexity as time progresses. cite journal | author=Erwin, D.H. | title=The origin of bodyplans
journal=American Zoologist | month=June | volume=39 | issue=3
pages=617–629 | doi=10.1093/icb/39.3.617
url=http://findarticles.com/p/articles/mi_qa3746/is_199906/ai_n8843690/pg_1
year=1999 | format=free full text ]

Many of these organisms were quite unlike anything that appeared before or since, resembling discs, mud-filled bags, or quilted mattresses – one palæontologist proposed that the strangest organisms should be classified as a separate kingdom, Vendozoa.cite journal
author = Seilacher, A.
year = 1992
title = Vendobionta and Psammocorallia: lost constructions of Precambrian evolution
journal = Journal of the Geological Society, London
volume = 149
issue = 4
pages = 607–613
url = http://jgs.lyellcollection.org/cgi/content/abstract/149/4/607
doi = 10.1144/gsjgs.149.4.0607
issn = 0016–7649
accessdate = 2007-06-21
format = abstract ]

At least some may have been early forms of the phyla at the heart of the "Cambrian explosion" debate, having been interpreted as early molluscs ("Kimberella"),cite journal
author = Martin, M.W.
coauthors = Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L.
year = 2000
date = 2000-05-05
title = Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution
journal = Science
volume = 288
issue = 5467
pages = 841
doi = 10.1126/science.288.5467.841
url = http://www.scienceonline.org/cgi/content/abstract/288/5467/841
accessdate = 2007-05-10
format = abstract
pmid = 10797002] cite journal
author =Fedonkin, M. A.
coauthors =Waggoner, B.
title =The late Precambrian fossil Kimberella is a mollusc-like bilaterian organism
journal =Nature
volume =388
pages =868–871
year =1997
url =http://www.nature.com/nature/journal/v388/n6645/abs/388868a0.html
doi = 10.1038/42242
format =abstract ] echinoderms ("Arkarua"); [cite journal | author=Mooi, R. and Bruno, D. | title=Evolution within a bizarre phylum: Homologies of the first echinoderms | journal=American Zoologist | volume=38 | pages=965–974 | year=1999 | url=http://icb.oxfordjournals.org/cgi/reprint/38/6/965.pdf] and arthropods ("Spriggina", [cite journal | author =McMenamin, M.A.S | title ="Spriggina" is a trilobitoid ecdysozoan | journal =Abstracts with Programs | publisher=Geological Society of America | volume =35 | issue=6 | pages =105 | year =2003 | url=http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm | format =abstract] "Parvancorina" [ cite journal | title= A "Parvancorina"-like arthropod from the Cambrian of South China | journal=Historical Biology: A Journal of Paleobiology | volume=18 | issue=1 | month=March | pages=33–45 | author=Lin, J-P., Gon, S.M., Gehling, J.G., Babcock, L.E., Zhao, Y-L., Zhang, X-L,, Hu, S-X., Yuan, J-L., Yu, M-Y., and Peng, J. | doi= 10.1080/08912960500508689 | year= 2006 ] ). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans.cite journal
author = Butterfield, N.J.
year = 2006
title = Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale
journal = Bioessays
volume = 28
issue = 12
pages = 1161–6
doi = 10.1002/bies.20507
accessdate = ] However there seems little doubt that "Kimberella" was at least a triploblastic bilaterian animal. These organisms are central to the debate about how abrupt the Cambrian explosion was. If some were early members of the animal phyla seen today, the "explosion" looks a lot less sudden than if all these organisms represent an unrelated "experiment", and were replaced by the animal kingdom fairly soon thereafter (40M years is "soon" by evolutionary and geological standards).

Fossils of "Cloudina" and "Sinotubulites" have been found in sediments formed near the end of the Ediacaran period. Although they are as hard to classify as most other Ediacaran organisms, they are important in two other ways. First, they are the earliest known calcifying organisms (organisms that built shells out of calcium carbonate).citation
author = Miller, A.J.
year = 2004
title = A Revised Morphology of Cloudina with Ecological and Phylogenetic Implications
url = http://ajm.pioneeringprojects.org/files/CloudinaPaper_Final.pdf
accessdate = 2007-04-24 ] cite journal
author = Conway Morris, S.
coauthors = Mattes, B.W., Chen, M.
year = 1990
title = The early skeletal organism" Cloudina": new occurrences from Oman and possibly China
journal = American Journal of Science
volume = 290
pages = 245–260
accessdate = ] cite journal
author = Grant, SW
year = 1990
title = Shell structure and distribution of" Cloudina", a potential index fossil for the terminal Proterozoic
journal = American Journal of Science
issue = 290-A
pages = 261–294
accessdate = ] Even more striking is the fact that "Cloudina" specimens show evidence of borings by predators, while "Sinotubulites" fossils found in the same locations do not. This suggests that: there were predators that were sufficiently advanced to penetrate shells; these predators found "Cloudina" a more inviting target than "Sinotubulites".cite journal
date = 17 July 1992
title = Predatorial Borings in Late Precambrian Mineralized Exoskeletons
journal = Science
volume = 257
issue = 5068
pages = 367
doi = 10.1126/science.257.5068.367
accessdate =
author = Bengtson, S. and Zhao, Y.
pmid = 17832833 ] A possible "arms race" between predators and prey is one of the most promising components of theories that attempt to explain the Cambrian explosion.

Cambrian life

mall shelly fauna

Fossils known as “small shelly fauna” have been found in many parts on the world, and date from just before the Cambrian to about 10 million years after the start of the Cambrian (the Nemakit-Daldynian and Tommotian ages; see timeline). These are a very mixed collection of fossils: spines, sclerites (armor plates), tubes, archeocyathids (sponge-like animals) and small shells very like those of brachiopods and snail-like molluscs – but all tiny, mostly 1 to 2 mm long.cite journal
author = Matthews, S.C.
coauthors = Missarzhevsky, V.V.
date = 1975-06-01
title = Small shelly fossils of late Precambrian and early Cambrian age: a review of recent work
journal = Journal of Geological Society
volume = 131
issue = 3
pages = 289
doi = 10.1144/gsjgs.131.3.0289
year = 1975 ]

While small, these fossils are far more common than complete fossils of the organisms that produced them; crucially, they cover the window from the start of the Cambrian to the first lagerstatten: a period of time that is otherwise lacking in fossils. Hence they supplement the conventional fossil record, and allow the fossil ranges of many groups to be extended.

Early Cambrian trilobites and echinoderms

The earliest Cambrian trilobite fossils are about 530 million years old, but the class was already quite diverse and worldwide, suggesting that they had been around for quite some time.cite journal
author =Lieberman, BS
title =Testing the Darwinian Legacy of the Cambrian Radiation Using Trilobite Phylogeny and Biogeography
journal =Journal of Paleontology
volume =73
issue =2
year =1999
url=http://jpaleontol.geoscienceworld.org/cgi/content/abstract/73/2/176
format =abstract]

The earliest generally-accepted echinoderms appeared at about the same time; unlike modern echinoderms, these early Cambrian echinoderms were not all radially symmetrical.cite journal
author =Dornbos, S.Q. and Bottjer, D.J.
title =Evolutionary paleoecology of the earliest echinoderms: Helicoplacoids and the Cambrian substrate revolution
journal =Geology
volume =28
issue =9
pages =839–842
year =2000
url =http://geology.geoscienceworld.org/cgi/content/full/28/9/839
doi =10.1130/0091-7613(2000)28<839:EPOTEE>2.0.CO;2]

These provide firm data points for the "end" of the explosion, or at least indications that the crown groups of modern phyla were represented.

Burgess shale type faunas

The Burgess shale and similar lagerstatten preserve the soft parts of organisms, which provides a wealth of data to aid in the classification of enigmatic fossils. It often preserved complete specimens of organisms only otherwise known from dispersed parts, such as loose scales or isolated mouthparts. Further, the majority of organisms and taxa in these horizons are entirely soft bodied – hence absent from the rest of the fossil record.cite journal
title = Exceptional Fossil Preservation and the Cambrian Explosion
year = 2003
journal = Integrative and Comparative Biology
volume = 43
issue = 1
pages = 166–177
doi = 10.1093/icb/43.1.166
author = Butterfield, Nicholas J.] Since a large part of the ecosystem is preserved, the ecology of the community can also be tentatively reconstructed.Verify source|Caron 2006 needs incorporating – see talk page|date=July 2008However, the assemblages may represent a "museum": a deep water ecosystem that is evolutionarily "behind" the rapidly diversifying faunas of shallower waters.cite journal
author = Conway Morris, Simon
year = 2008
title = A Redescription of a Rare Chordate, Metaspriggina Walcotti Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada
journal = Journal of Paleontology
volume = 82
pages = 424
doi = 10.1666/06-130.1]

Because the lagerstatten provide a mode and quality of preservation that's virtually absent outside of the Cambrian, lots of organisms appear completely different to anything known from the conventional fossil record. This led early workers in the field to attempt to shoehorn the organisms into extant phyla; the shortcomings of this approach led them to erect a multitude of new phyla to accommodate all the oddballs. It has since been realised that most oddballs diverged from lineages before they established the phyla we know todayClarifyme|date=August 2008 – slightly different designs, which were fated to perish rather than flourish into phyla, as their cousin lineages did.

The preservational mode is rare in the preceding Ediacaran period, but those assemblages known show no trace of animal life – perhaps implying a genuine absence of macroscopic metazoans.cite journal
title = A Reassessment of the Neoproterozoic Miaohe Carbonaceous Biota in South China
author = Xiao, Shuhai; M. Steiner; A. H. Knoll.
year = 2002
journal = Journal of Paleontology
volume = 76
pages = 345–374
doi = 10.1666/0022-3360(2002)076<0347:MCCIAT>2.0.CO;2]

Early Cambrian crustaceans

Crustaceans, one of the three great modern groups of arthropods, are very rare throughout the Cambrian. Convincing crustaceans were once thought to be common in Burgess shale-type biotas, but none of these individuals can be shown to fall into the crown group of "true crustaceans".cite journal
url = http://www.nature.com/nature/journal/vaop/ncurrent/full/nature06724.html
doi = 10.1038/nature06724
author = Harvey, T.H.; Butterfield, N.J.
journal = Nature
year = 2008
title = Sophisticated particle-feeding in a large Early Cambrian crustacean
volume = 452
pages = 868
issue = 7189
accessdate = 2008-06-11] The Cambrian record of crown group crustaceans comes from microfossils. The Swedish Orsten horizons contain later Cambrian crustacea, but only organisms smaller than 2 mm are preserved. This restricts the data set to juveniles and miniaturised adults.

A more informative data source is the organic microfossils of the Mount Cap formation, Canada. This late Early Cambrian assemblage (Ma|510|515) consists of microscopic fragments of arthropods' cuticle, which is left behind when the rock is dissolved with a strong acid. The diversity of this assemblage is similar to that of modern crustacean faunas. Most interestingly, analysis of fragments of feeding machinery found in the formation shows that it was adapted to feed in a very precise and refined fashion. This contrasts with most other early Cambrian arthropods, which fed messily by shovelling anything they could get their feeding appendages on into their mouths. This sophisticated and specialised feeding machinery belonged to a large (~30 cm) [PalAss2007|author=Harvey, T.P.H.] organism, and would have provided great potential for diversification: specialised feeding apparatus allows a number of different approaches to feeding and development, and creates a number of different approaches to avoid being eaten

Early Ordovician radiation

After a mass extinction at the Cambrian-Ordovician boundary, another radiation occurred, which established the taxa which would dominate the Palaeozoic..

A new phylum, the Bryozoa, is first observed after this Ordovician radiation; the total number of orders doubled, and families tripled,cite journal
author = Droser, Mary L.; Finnegan, Seth
year = 2003
title = The Ordovician Radiation: A Follow-up to the Cambrian Explosion?
journal = Integrative and Comparative Biology
volume = 43
issue = 1
pages = 178
doi = 10.1093/icb/43.1.178] increasing marine diversity to levels typical of the Palaeozoic,cite journal
author=Marshall, C.R.
title=Explaining the Cambrian “Explosion” of Animals
journal=Annu. Rev. Earth Planet. Sci.
volume=34
pages=355–384
doi=10.1146/annurev.earth.33.031504.103001
url=http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.earth.33.031504.103001?journalCode=earth
year=2006
format=abstract] and disparity to levels approximately equivalent to today's.

How real was the explosion?

The fossil record as Darwin knew it seemed to suggest that the major metazoan groups appeared in a few million years of the early to mid-Cambrian, and even in the 1980s this still appeared to be the case.

However, evidence of Precambrian metazoa is gradually accumulating. If the Ediacaran "Kimberella" was a mollusc-like protostome (one of the two main groups of coelomates), the protostome and deuterostome lineages must have split significantly before Ma|550 (deuterostomes are the other main group of coelomates). Even if it is not a protostome, it is widely accepted as a bilaterian. Since fossils of rather modern-looking Cnidarians (jellyfish-like organisms) have been found in the Doushantuo lagerstätte, the Cnidarian and bilaterian lineages must have diverged well over Ma|580. cite journal | title=The last common bilaterian ancestor | author=Erwin, D.H., and Davidson, E.H. | journal=Development |volume=129 | pages=3021–3032 | year=2002 | url=http://dev.biologists.org/cgi/content/full/129/13/3021 | accessdate=2008-07-10 ]

Trace fossils and predatory borings in "Cloudina" shells provide further evidence of Ediacaran animals. Some fossils from the Doushantuo formation have been interpreted as embryos and one ("Vernanimalcula") as a bilaterian coelomate, although these interpretations are not universally accepted. cite journal
year=2004
title=Response to Comment on “Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian”
doi = 10.1126/science.1102328
accessdate=
author=Chen, J.Y., Oliveri, P., Davidson, E. and Bottjer, D.J.
journal=Science
volume=306
pages=1291b] Earlier still, predatory pressure has acted on stromatolites and acritarchs since around ma|1250.

The presence of Precambrian animals somewhat dampens the "bang" of the explosion: not only was the appearance of animals gradual, but their evolutionary radiation ("diversification") may also not have been as rapid as once thought. Indeed, statistical analysis shows that the Cambrian explosion was no faster than any of the other radiations in animals' history.ref|pulse|4

There is little doubt that disparity – that is, the range of different organism "designs" or "ways of life" – rose sharply in the early Cambrian.cite journal | author = Bambach | year = 2007 | doi = 10.1111/j.1475-4983.2006.00611.x | title = Autecology And The Filling Of Ecospace: Key Metazoan Radiations | journal = Palaeontology | volume = 50 | pages = 1 ] However recent research has overthrown the once-popular idea that disparity was exceptionally high throughout the Cambrian, before subsequently decreasing. [cite journal | author = Erwin, D.H.| year = 2007| doi = 10.1111/j.1475-4983.2006.00614.x| title = Disparity: Morphological Pattern And Developmental Context| journal = Palaeontology| volume = 50| pages = 57 ] In fact, disparity remains relatively low throughout the Cambrian, with modern levels of disparity only attained after the early Ordovician radiation.

The diversity of many Cambrian assemblages is similar to today's. [citation|doi=10.1666/0094-8373(2003)029<0349:MDOCAA>2.0.CO;2|issn=0094-8373|year=2003|volume=29|pages=349|title=Morphological diversity of Carboniferous arthropods and insights on disparity patterns through the Phanerozoic|author=Lofgren, Andrea Stockmeyer|journal=Paleobiology]

Possible causes of the “explosion”

Despite the evidence that moderately complex animals (triploblastic bilaterians) existed before and possibly long before the start of the Cambrian, it seems that the pace of evolution was exceptionally fast in the early Cambrian. Possible explanations for this fall into three broad categories: environmental, developmental, and ecological changes. Any explanation must explain the timing and magnitude of the explosion. It is also possible that the "explosion" requires no special explanation.

Changes in the environment

Increase in oxygen levels

Earth’s earliest atmosphere contained no free oxygen; the oxygen that animals breathe today, both in the air and dissolved in water, is the product of billions of years of photosynthesis. As a general trend, the concentration of oxygen in the atmosphere has risen gradually over about the last 2.5 billion years.

Shortage of oxygen might well have prevented the rise of large, complex animals. The amount of oxygen an animal can absorb is largely determined by the area of its oxygen-absorbing surfaces (lungs and gills in the most complex animals; the skin in less complex ones); but the amount needed is determined by its volume, which grows faster than the oxygen-absorbing area if an animal’s size increases equally in all directions. An increase in the concentration of oxygen in air or water would increase the size to which an organism could grow without its tissues becoming starved of oxygen. However, members of the Ediacara biota reached metres in length; clearly oxygen did not limit "their" growth.e.g. cite journal
author = Knoll, A.H.
coauthors = Carroll, S.B.
date = 1999-06-25
title = Early Animal Evolution: Emerging Views from Comparative Biology and Geology
journal = Science
volume = 284
issue = 5423
pages = 2129
doi = 10.1126/science.284.5423.2129
year = 1999
pmid = 10381872 ] Other metabolic functions may have been inhibited by lack of oxygen, for example the construction of tissue such as collagen, required for the construction of complex structures,cite journal
author = Towe, K.M.
date = 1970-04-01
title = Oxygen-Collagen Priority and the Early Metazoan Fossil Record
journal = Proceedings of the National Academy of Sciences
volume = 65
issue = 4
pages = 781–788
doi = 10.1073/pnas.65.4.781
url=http://www.pnas.org/cgi/content/abstract/65/4/781
year = 1970
format = abstract
pmid = 5266150 ] or to form molecules for the construction of a hard exoskeleton.cite journal
doi = 10.1089/ast.2005.5.415
author = Catling, D.C.; Glein, C.R.; Zahnle, K.J.; McKay, C.P.
journal = Astrobiology
volume = 5
issue = 3
pages = 415–438
year = 2005
title = Why O₂ Is Required by Complex Life on Habitable Planets and the Concept of Planetary "Oxygenation Time"
accessdate = ]

nowball Earths

In the late Neoproterozoic (extending into the early Ediacaran period), the Earth suffered massive glaciations in which most of its surface was covered by ice. This may have caused a mass extinction, creating a genetic bottleneck; the resulting diversification may have given rise to the Ediacara biota, which appears soon after the last "Snowball Earth" episode. cite journal
title=A Neoproterozoic Snowball Earth
journal=Science
date=28 August 1998
volume=281
issue=5381
pages=1342–1346
doi=10.1126/science.281.5381.1342
url=http://www.sciencemag.org/cgi/content/abstract/281/5381/1342
author=Hoffman, P.F., Kaufman, A.J., Halverson, G.P., and Schrag, D.P.
year=1998
format=abstract
pmid=9721097] However, the snowball episodes occurred a long time before the start of the Cambrian, and it is hard to see how so much diversity could have been caused by even a series of bottlenecks; the cold periods may even have "delayed" the evolution of large size.

Developmental Explanations

A range of theories are based on the concept that minor modifications to animals' development as they grow from embryo to adult may have been able to cause very large changes in the final adult form. The hox genes, for example, control which organs individual regions of an embryo will develop into. For instance, if a certain hox gene is expressed, a region will develop into a limb; if a different hox gene is expressed in that region (a minor change), it could develop into an eye instead (a phenotypically major change).

Such a system allows a large range of disparity to appear from a limited set of genes, but such theories linking this with the explosion struggle to explain why the origin of such a development system should by itself lead to increased diversity or disparity. Evidence of Precambrian metazoans combines with molecular datacite journal | author = Renaud de Rosa, Jennifer K. Grenier5,2, Tatiana Andreeva3, Charles E. Cook, André Adoutte1, Michael Akam4, Sean B. Carroll2 and Guillaume Balavoine | year = 1999 | doi = 10.1038/21631 | journal = Nature | volume = 399 | pages = 772|title=Hox genes in brachiopods and priapulids and protostome evolution] to show that much of the genetic architecture that could feasibly have played a role in the explosion was already well established by the Cambrian.

Ecological Explanations

These focus on the interactions between different types of organism. Some of these hypotheses deal with changes in the food chain; some suggest arms races between predators and prey, and others focus on the more general mechanisms of coevolution. Such theories are well suited to explaining why there was a rapid increase in both disparity and diversity, but they must explain why the "explosion" happened when it did.

End-Ediacaran mass extinction

Evidence for such an extinction includes the disappearance from the fossil record of the Ediacara biota and shelly fossils such as "Cloudina", and the accompanying perturbation in the d13c|link record.Mass extinctions are often followed by adaptive radiations as existing clades expand to occupy the ecospace emptied by the extinction. However, once the dust had settled, overall disparity and diversity returned to the pre-extinction level in each of the Phanerozoic extinctions.

Evolution of eyes

Parker has proposed that predator-prey relationships changed dramatically after eyesight evolved. Prior to that time hunting and evading were both close-range affairs – smell, vibration, and touch were the only senses used. When predators could see their prey from a distance, new defensive strategies were needed. Armor, spines, and similar defenses may also have evolved in response to vision. [ cite book | author=Parker, Andrew | title=In the Blink of an Eye |publisher= Perseus Books |location=Cambridge, Massachusetts, USA | year=2003 | isbn=0738206075 | oclc=52074044 54977231 ] Nevertheless many scientists doubt that vision could have caused the explosion. Eyes may well have evolved long before the start of the Cambrian.cite journal | author = McCall | year = 2006 | doi = 10.1016/j.earscirev.2005.08.004 | title = The Vendian (Ediacaran) in the geological record: Enigmas in geology's prelude to the Cambrian explosion | journal = Earth-Science Reviews | volume = 77 | pages = 1 ] It is also difficult to understand why the evolution of eyesight would have caused an explosion, since other senses such as smell and pressure detection can detect things further away than they can be seen under the sea, but the appearance of these other senses apparently did not cause an evolutionary explosion.

Arms races between predators and prey

The ability to avoid or recover from predation often makes the difference between life and death, and is therefore one of the strongest components of natural selection. The pressure to adapt is stronger on the prey than on the predator: if the predator fails to win a contest, it loses its lunch; if the prey is the loser, it loses its life. cite journal
title=Arms races between and within species
volume=205
issue=1161
pages=489–511
date=September 21 1979
url=http://links.jstor.org/sici?sici=0080–4649(19790921)205%3A1161%3C489%3AARBAWS%3E2.0.CO%3B2–1
author=Dawkins, R. and Krebs, R.J.
day=21
doi=10.1098/rspb.1979.0081
journal=Proceedings of the Royal Society: Biological Sciences Series B ]

But there is evidence that predation was rife long before the start of the Cambrian, for example in the increasingly spiny forms of acritarchs, the holes drilled in "Cloudina" shells, and traces of burrowing to avoid predators. Hence it is unlikely that the "appearance" of predation was the trigger for the Cambrian "explosion", although it may well have exhibited a strong influence on the body forms that the "explosion" produced. Alternatively a more subtle aspect, such as the evolution of a new "style" of predation, may have played a role.

Increase in size and diversity of planktonic animals

Geochemical evidence strongly indicates that the total mass of plankton has been similar to modern levels since early in the Proterozoic. Before the start of the Cambrian, their corpses and droppings were too small to fall quickly towards the seabed, since their drag was about the same as their weight. This meant they were destroyed by scavengers or by chemical processes before they reached the sea floor.cite journal
author = Butterfield, N.J.
year = 2001
title = Ecology and evolution of Cambrian plankton
journal = The Ecology of the Cambrian Radiation. Columbia University Press, New York
pages = 200–216
url = http://66.102.1.104/scholar?hl=en&lr=&ie=UTF-8&q=cache:9xeRu1SdF0QJ:www.earthscape.org/r3/ES14785/ch09.pdf+
accessdate = 2007-08-19 ]

Mesozooplankton are plankton of a larger size, and early Cambrian specimens filtered microscopic plankton from the seawater. These larger organisms would have produced droppings and corpses that were large enough to fall fairly quickly. This provided a new supply of energy and nutrients to the mid-levels and bottoms of the seas, which opened up a huge range of new possible ways of life. If any of these remains sunk uneaten to the sea floor they could be buried; this would have taken some carbon out of circulation, resulting in an increase in the concentration of breathable oxygen in the seas. (carbon readily combines with oxygen)

The initial herbivorous mesozooplankton were probably larvae of benthic (seafloor) animals. A larval stage was probably an evolutionary innovation driven by the increasing level of predation at the seafloor during the Ediacaran period. cite journal
title=Tempo and mode of early animal evolution: inferences from rocks, Hox, and molecular clocks
journal=Paleobiology
month=June | volume=31
issue=2 (Supplement)
pages=36–55
doi=10.1666/0094-8373(2005)031 [0036:TAMOEA] 2.0.CO;2
publisher=Paleontological Society
url=http://paleobiol.geoscienceworld.org/cgi/content/abstract/31/2_Suppl/36
author=Peterson, K.J., McPeek, M.A., and Evans, D.A.D.
year=2005
format=abstract ]

Metazoans have an amazing ability to increase diversity through co-evolution. This means that a trait of one organism can cause another to evolve in response; a number of responses are possible, and a different species can potentially emerge for each. As a simple example, the evolution of predation may have caused one organism to develop defence while another developed motion to flee. This would cause the predator lineage to split into two species: one that was good at chasing prey, and another that was good at breaking through defences. Actual co-evolution is somewhat more subtle, but in this fashion, great diversity can arise: three quarters of living species are animals, and most of the rest have formed by co-evolution with animals.

Discredited hypotheses

As our understanding of the events of the Cambrian becomes clearer, data has accumulated to make some hypotheses look improbable. Causes that have been proposed but are now discounted include the evolution of herbivory, vast changes in the speed of tectonic plate movement or of the cyclic changes in the Earth's orbital motion, or the operation of different evolutionary mechanisms from those that are seen in the rest of the Phanerozoic eon.Fact|date=August 2008

No explanation required

The explosion may not have been a significant evolutionary event. It may represent a threshold being crossed; for example a threshold in genetic complexity that allowed a vast range of morphological forms to be employed.Verify source|date=June 2008 cite journal
title=Adaptive walks in a gene network model of morphogenesis: insights into the Cambrian explosion
year=2003
journal=Int. J. Dev. Biol.
volume=47
issue=7
pages=685–693
pmid=14756344
author=Solé, R.V., Fernández, P., and Kauffman, S.A.]

Uniqueness of the explosion

The "Cambrian explosion" can be viewed as two waves of metazoan expansion into empty niches: first, a co-evolutionary rise in diversity as animals explored niches on the Ediacaran sea floor, followed by a second expansion in the early Cambrian as they became established in the water column.cite journal
doi = 10.1111/j.1475-4983.2006.00613.x
author = Butterfield, N.J.
journal = Palaeontology
volume = 50
issue = 1
pages = 41–55
year = 2007
title = Macroevolution And Macroecology Through Deep Time] The rate of diversification seen in the Cambrian phase of the explosion is unparalleled among marine animals: it affected all metazoan clades of which Cambrian fossils have been found. Later radiations, such as those of fish in the Silurian and Devonian periods, involved fewer taxa, mainly with very similar body plans.

Whatever triggered the early Cambrian diversification opened up an exceptionally wide range of previously-unavailable ecological niches. When these were all occupied, there was little room for such wide-ranging diversifications to occur again, because there was strong competition in all niches and wict|incumbents usually had the advantage. If there had continued to be a wide range of empty niches, clades would be able to continue diversifying and become disparate enough for us to recognise them as different phyla; when niches are filled, lineages will continue to resemble one another long after they diverge, as there is limited opportunity for them to change their life-styles and forms. cite journal | title=Why No New Phyla after the Cambrian? Genome and Ecospace Hypotheses Revisited | author=Valentine, J.W. | journal=Palaios | voluime=10 | issue=2 | month=April | pages=190–194 | doi=10.2307/3515182 | url=http://links.jstor.org/sici?sici=0883–1351(199504)10%3A2%3C190%3AWNNPAT%3E2.0.CO%3B2-H#abstract | year=1995 | volume=10 | format=abstract]

There is a similar one-time explosion in the evolution of land plants: after a cryptic history beginning about Ma|450, land plants underwent a uniquely rapid adaptive radiation during the Devonian period, about Ma|400.

Further reading

*
* Collins, Allen G. [http://www.ucmp.berkeley.edu/phyla/metazoafr.html “Metazoa: Fossil record”] . Retrieved Dec. 14, 2005.
*
* cite journal
author = Conway Morris, S.
year = 2006
title = Darwin’s dilemma: the realities of the Cambrian ‘explosion’
journal = Philosophical Transactions of the Royal Society B: Biological Sciences
volume = 361
issue = 1470
pages = 1069–1083
doi =10.1098/rstb.2006.1846
url = http://www.journals.royalsoc.ac.uk/index/QU151T4722902768.pdf An enjoyable account.
*
*
*
* [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B83WC-4N0HJMK-2&_user=1300184&_coverDate=12%2F31%2F2007&_rdoc=6&_fmt=summary&_orig=browse&_srch=doc-info(%23toc%2333783%232007%23999839995%23671853%23FLA%23display%23Volume)&_cdi=33783&_sort=d&_docanchor=&_ct=9&_acct=C000052237&_version=1&_urlVersion=0&_userid=1300184&md5=d9c2663e7fbd6a77385d61334953d75d Alexander V. Markov, and Andrey V. Korotayev (2007) “Phanerozoic marine biodiversity follows a hyperbolic trend” "Palaeoworld" 16(4): pp. 311–318] .
*
*Timeline References:
*

External links

* [http://www.ijdb.ehu.es/web/contents.php?vol=47&issue=7–8&doi=14756326 The Cambrian “explosion” of metazoans and molecular biology: would Darwin be satisfied?]
* [http://genome6.cu-genome.org/andrey/GouldComment.pdf On embryos and ancestors] by Stephen Jay Gould
* &ndash; ^{[http://scholar.google.co.uk/scholar?hl=en&lr=&q=intitle%3AThe+Cambrian+%E2%80%9Cexplosion%E2%80%9D%3A+Slow-fuse+or+megatonnage%3F&as_publication=&as_ylo=&as_yhi=&btnG=Search Scholar search]}
* [http://www.bbc.co.uk/radio4/history/inourtime/ram/inourtime_20050217.ram The Cambrian Explosion] – "In Our Time", BBC Radio 4 broadcast, 17 February 2005
* [http://www.kumip.ku.edu/cambrianlife/ Utah's Cambrian life] – new (2008) website with good images of a range of Burgess-shale-type and other Cambrian fossils.

Notes

Older marks found in billion-year old rockscite journal
title=Animals More Than 1 Billion Years Ago: Trace Fossil Evidence from India
journal=Science
volume=282
number=5386
pages=80–83
year=1998
url=http://www.sciencemag.org/cgi/content/abstract/282/5386/80
doi = 10.1126/science.282.5386.80
accessdate=2007-08-20
author=Seilacher, A., Bose, P.K. and Pflüger, F.
format=abstract
pmid=9756480] have since been recognised as non-biogenic.cite journal
author = Budd, G.E.
coauthors = Jensen, S.
year = 2000
title = A critical reappraisal of the fossil record of the bilaterian phyla
journal = Biological Reviews
volume = 75
issue = 02
pages = 253–295
doi = 10.1017/S000632310000548X
url = http://www.journals.cambridge.org/abstract_S000632310000548X
accessdate = 2007-06-27
format = abstract ] cite journal
author =Jensen, S.
title =The Proterozoic and Earliest Cambrian Trace Fossil Record; Patterns, Problems and Perspectives
journal =Integrative and Comparative Biology
volume =43
issue =1
pages =219–228
url=http://icb.oxfordjournals.org/cgi/content/abstract/43/1/219
doi=10.1093/icb/43.1.219
year =2003
format=abstract ]

The analysis considered the bioprovinciality of trilobite lineages, as well as their evolutionary rate.cite journal
author = Lieberman, B.
year = 2003
title = Taking the Pulse of the Cambrian Radiation
journal = Integrative and Comparative Biology
volume = 43
issue = 1
pages = 229–237
doi = 10.1093/icb/43.1.229
url = http://icb.oxfordjournals.org/cgi/content/abstract/43/1/229]

References