Cambrian_explosion

One possible timescale for events
around the Cambrian/Precambrian boundary.

Axis scale: millions of years ago.

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.Darwin, C (1859). On the Origin of Species by Natural Selection. Murray, London, United Kingdom, 315–316. 

American palæontologist Charles Walcott, who extensively studied North American fossil animals, 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.Walcott, C.D. (1914). "Cambrian Geology and Paleontology". Smithsonian Miscellaneous Collections 57: 14. 

The intense modern interest in the subject 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 very different from any living animals.Whittington, H.B.; Geological Survey of Canada (1985). The Burgess Shale. Yale University Press.  Stephen Jay Gould’s popular 1989 account of this work, Wonderful Life,Gould, S.J. (1989). Wonderful Life: The Burgess Shale and the Nature of History. W.W. Norton & Company.  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. 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.

Difficulty of dating the Cambrian

It has been difficult to work out the chronology of the early Cambrian. 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. 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. 

Relative dating (A was before B) is often good enough for studying processes of evolution, but this has also been difficult, because of the problems involved in matching up rocks of the same age across different continents, particularly around the internationally-defined Precambrian/Cambrian boundary section.e.g. Gehling, James (March 2001). "Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland". Geological Magazine 138 (2): 213–218. doi:10.1017/S001675680100509X.  (the most common technique uses widespread but short-lived fossil species to identify rocks of similar ages)

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

Types of evidence

Body fossils

Body fossils preserve significant parts of organisms and are therefore the most informative type of evidence. Unfortunately they are increasingly rare as one looks further back in time, among other reasons because the rocks in which they are buried are usually covered by more recent rocks and because they may have been eroded before being covered by later rocks. One recent study concluded that “parts of the fossil record are clearly incomplete, but they can be regarded as adequate to illustrate the broad patterns of the history of life.”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 But there is evidence that some types of animals or parts of animals are relatively likely to be preserved as fossils in some environments and times, and extremely unlikely to be preserved in other environments and times. Part of this is due to changes in the chemistry of the oceans, which were partly caused by the on-going evolution of life, and these changes were most significant before the start of the Cambrian – for example any increase in the marine biomass would reduce the concentration of carbon, and the appearance of sponges reduced the concentration of silicon.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. 

Another limitation in the discovery and use of body fossils is the lack of preservation of large portions of the body. In most cases the sole anatomical features that are fossilized are the highly mineralised body parts containing high proportions of silica (sponges\' skeletons), calcium carbonate (the shells of bivalves, gastropods and ammonites and exoskeletons of most trilobites and some crustaceans) or calcium phosphate (the bones of vertebrates). The majority of animal species living now are unlikely ever to leave fossils, since they are soft-bodied invertebrates such as worms and slugs. Of the more than 30 phyla of living animals, two-thirds of these have never been found as fossils.Cowen, R.. History of Life. Blackwell Science. 

The Cambrian fossil record includes an unusually high number of lagerstätten which preserved the fossils\' soft tissues in extremely fine detail, allowing a very informative study of animals that normally would not have left fossils. The fine detail of the deposits has allowed paleontologists to examine the internal workings of animals which in other sediments are only represented by shells, spines, claws, etc. The most significant Cambrian lagerstätten are: the early Cambrian Maotianshan shale beds of Chengjiang (Yunnan, China) and Sirius Passet (Greenland)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. ; the middle Cambrian Burgess Shale (British Columbia, Canada)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. Retrieved on 2007-04-24. ; and the Upper Cambrian Orsten (Sweden) fossil beds.

While lagerstätten are superior to most fossil beds in preserving fine anatomical detail, they are far from perfect. The majority of then-living animals are probably not represented because lagerstätten are restricted to a narrow range of environments (e.g. where soft-bodied organisms can be preserved very quickly such as by mudslides), and the exceptional events that cause quick burial make it difficult to study the normal environments of the animals.Butterfield, N.J. (2001). "Ecology and evolution of Cambrian plankton". The Ecology of the Cambrian Radiation. Columbia University Press, New York: 200–216. Retrieved on 2007-08-19.  In addition, the known lagerstätten cover only a very limited period of time within the Cambrian, and none covers the crucial period just before the start of the Cambrian. Because normal fossil beds are very rare and lagerstätten even rarer, both are very unlikely to show the first occurrence of any type of organism.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(A 84-25651 10-42). Boulder, CO, Geological Society of America, 1982,: 291-296. Retrieved on 2008-01-07. 

Trace fossils

Trace fossil of the type called Cruziana, possibly made by a trilobite.

Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily-fossilized hard parts. 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. Seilacher, A. (1994). "How valid is Cruziana Stratigraphy?". International Journal of Earth Sciences 83 (4): 752–758. Retrieved on 2007-09-09.  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

The ratios of three major isotopes, ⁸⁷Sr / ⁸⁶Sr, ³⁴S / ³²S and ¹³C / ¹²C, undergo dramatic fluctuations around the beginning of the Cambrian.Magaritz, M.; Holser, W.T., Kirschvink, J.L. (1986). "Carbon-isotope events across the Precambrian/Cambrian boundary on the Siberian Platform". Nature 320 (6059): 258–259. doi:10.1038/320258a0. Retrieved on 2007-04-24. 

Further documentation on these variations is available at the following URLs: [1][2][3][4][5][6] (All listed at this Scholar results page

This chemical signature in the rocks of the Precambrian/Cambrian boundary is difficult to interpret, and may be related to continental break-up, the end of a “global glaciation”, or a catastrophic drop in productivity caused by a mass extinction just before the beginning of the Cambrian.

Carbon has 2 stable isotopes, carbon-12 (¹²C) and carbon-13 (¹³C). Causes often suggested for changes in the ratio of ¹³C to ¹²C found in rocks include:

• A mass extinction. Chemistry is largely driven by electro-magnetic forces, and lighter isotopes such as ¹²C respond to these more quickly than heavier ones such as ¹³C. So living organisms generally contain a disproportionate amount of ¹²C. A mass extinction would increase the amount of ¹²C available to be included in rocks and therefore reduce the ratio of ¹³C to ¹²C.
• A methane “burp”. In permafrosts and continental shelves methane produced by bacteria gets trapped in “cages” of water molecules, forming a mixture called a clathrate. This methane is very rich in ¹²C because it has been produced by organisms. Clathrates may dissociate (break up) suddenly if the temperature rises or the pressure on them drops. Such dissociations release the ¹²C-rich methane and thus reduce the ratio of ¹³C to ¹²C as this carbon is gradually incorporated into rocks (methane in the atmosphere breaks down into carbon dioxide and water; carbon dioxide reacts with minerals to form carbonate rocks).

Comparative anatomy

Cladistics is a technique for working out the “family tree” of a set of organisms, and has most often applied to evidence from comparative anatomy (features of the bodies of organisms). In this kind of analysis it is possible to include both living and fossilized organisms and work out their evolutionary relationships. Sometimes one can conclude that group A must have evolved before groups B and C, because B and C have more similarities to each other than either has to A. On its own this method can say nothing about when A evolved, but if there are fossils of B or C dating from X million years ago, then A must have evolved more than X million years ago.

Molecular phylogenetics

Molecular phylogenetics attempts to reconstruct the relationships between organisms by comparing details of their biochemistry, such as their DNA. In other words, it applies the analysis techniques of cladistics to biochemical rather than anatomical features. It provides an alternative line of evidence about evolution in the Cambrian and Precambrian, although the need for calibration against the fossil record means it is not entirely independent. Further, since the “clocks” measure molecular evolution, a period of rapid evolution is indistinguishable from a longer period of slow change, so it is unwise to rely on molecular phylogeny for estimates of datesL.A. Hug and A.J.Roger, The Impact of Fossils and Taxon Sampling on Ancient Molecular Dating Analyses. Molecular Biology and Evolution 2007 24(8):1889-1897, 2007.

Although this rapidly developing science must be treated with a degree of caution,Ayala, F.J. (1999). "Molecular clock mirages". BioEssays 21 (1): 71–75. doi:10.1002/(SICI)1521-1878(199901)21:1%3C71::AID-BIES9%3E3.3.CO;2-2.  it has yielded some useful results. For example, it provides evidence that the three major animal groups diverged some time before the Cambrian, then independently underwent a rapid Cambrian diversificationDe Rosa, R.; Grenier, J.K.; Andreeva, T.; Cook, C.E.; Adoutte, A.; Akam, M.; Carroll, S.B.; Balavoine, G. (1999). "Hox genes in brachiopods and priapulids and protostome evolution". Nature 399 (6738): 772–776. doi:10.1038/21631.  – although the reliability and implications of this apparent finding are still being debated.Adoutte, A.; Balavoine, G.; Lartillot, N.; Lespinet, O.; Prud’homme, B.; De Rosa, R. (2000). "The new animal phylogeny: Reliability and implications". PNAS 97 (9): 4453–4456. Retrieved on 2007-09-09.  The current state of molecular phylogenetics seems not to support the Cambrian Explosion theory, but rather a considerably earlier evolutionary radiation.

Evidence in rocks

This lists the main items in order of the time when the relevant rocks were formed, because timing is the central issue in the Cambrian explosion – but remember that dating rocks from the Cambrian and earlier rocks is very difficult. The survey also starts well before the start of the Cambrian and finishes in the early Ordovician, because some scientists think that the diversification of animal life started before and finished after the Cambrian.Odontogriphus omalus

It covers body fossils, trace fossils and geochemical evidence, because these are all found in rocks which can be dated at least approximately. Arguments based on molecular phylogenetics will appear in a separate section, because this type of evidence is much harder to date with confidence.

Explanation of a few scientific terms

To avoid becoming even longer this article uses some scientific terms, and this is a good place for some simple explanations.Marshall, C.R. (2006). "Explaining the Cambrian “Explosion” of Animals". Annu. Rev. Earth Planet. Sci. 34: 355–384. doi:10.1146/annurev.earth.33.031504.103001. 

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.Valentine, James W. (2004). On the Origin of Phyla. Chicago: University Of Chicago Press, 7. 0226845486. "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 organizations.Parker, Andrew (2003). In the blink of an eye: How vision kick-started the big bang of evolution. Sydney: Free Press, 1–4. 0743257332. "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 crabs both belong to the phylum Arthropoda; but earthworms and tapeworms, although similar in shape, are members of the Annelida and Platyhelminthes respectively.

But the word "phylum" does not describe a fundamental division of nature (not like the difference between electrons and protons). It simply refers to a very high level in the classification system created by Linnaeus in the 18th century to describe all the animals which are alive to-day. This system is not perfect 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. Classification systems based on living organisms, including Linneus\', do not accommodate extinct organisms well, or even at all.Jefferies, R.P.S. (1979), House, M.R.,, ed., The origin of chordates — a methodological essay, London: Academic Press, pp. 443–477 summarised in Budd, G.E. (2003). "The Cambrian Fossil Record and the Origin of the Phyla". Integrative and Comparative Biology 43 (1): 157-165. doi:10.1093/icb/43.1.157. 

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.).

Bilaterian means having 2 sides; this implies that they also have top and bottom surfaces and, perhaps more importantly, distinct front and back ends. All known bilaterian animals are triploblastic, and all known triploblastic animals are bilaterian except for echinoderms (but sea cucumbers do have distinct front and back ends; and echinoderm larvae have 2 sides). Porifera (sponges) and Cnidaria (jellyfish, sea anemones, etc.) are radially symmetrical (like wheels).

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 (which includes us vertebrates) - the non-coelomate priapulids are an important exception. All coelomate animals are triploblastic, but some triploblastic animals do not have a coelom (e.g. flatworms; their organs are surrounded by unspecialized tissues). Some bilaterian animals are not coelomates (e.g. flatworms). Echinoderms are coelomates; living species do not look bilaterian (they are radially symmetrical, although sea cucumbers) have distinct front and rear ends), but the earliest echinoderms are still poorly understood and some may have been bilaterally symmetrical. (June 2002) "Paired gill slits in a fossil with a calcite skeleton". Nature (417): 841-844. doi:10.1038/nature00805. 

Decline of stromatolites over 1 billion years ago

Modern stromatolites in Shark Bay, Western Australia.

Stromatolites are not organisms, they are stubby pillars of sediment built by photosynthesizing microorganisms, especially cyanobacteria. They are now restricted to hostile environments such as extremely salty lagoons, because in less hostile environments they are eliminated by grazing and burrowing invertebrates.

Stromatolites are an important part of the fossil record for about the first 3 billion years of life on earth, peaking about 1250 million years ago, but after then they declined in abundance and diversity, and by the start of the Cambrian had fallen to 20% of their peak. The most widely-supported explanation is that stromatolite-building organisms were the victims of grazing animals, which would imply that sufficiently complex animals were common over 1 billion years ago.McNamara, K.J. (20 December 1996). "Dating the Origin of Animals". Science 274: 1993–1997. doi:10.1126/science.274.5295.1993f.  Awramik, S.M. (19 November 1971). "Precambrian columnar stromatolite diversity: Reflection of metazoan appearance". Science 174: 825–827. doi:10.1126/science.174.4011.825. Retrieved on 1 Dec 2007.  This connection is supported by the facts that: stromatolites declined again when the abundance and diversity of marine animals increased in the Ordovician evolutionary radiation; and stromatolite abundance increased after the end-Ordovician and end-Permian extinctions decimated marine animals, but fell back to earlier levels as marine animals recovered. (2004) "Microbialite resurgence after the Late Ordovician extinction". Nature 430: 75–78. doi:10.1038/nature02654. Retrieved on 1 Dec 2007. 

Increase in abundance and spininess of acritarchs

Acritarchs include the remains of a wide range of quite different kinds of organisms - ranging from the egg cases of small metazoans to resting cysts of many different kinds of chlorophyta (green algae). They first appear in rocks about 2 billion years old, but about 1 billion years they started to increase in abundance, diversity, size, complexity of shape and especially size and number of spines. Their populations crashed during the Snowball Earth episodes, but they reached their highest diversity in the Paleozoic era. Their increasingly spiny forms in the last 1 billion years probably result from the need for defense against predators, especially predators large enough to swallow them or tear them apart. Other groups of small organisms from the Neoproterozoic era also show signs of anti-predator defenses.Bengtson, S. (2002), "Origins and early evolution of predation", in Kowalewski, M., and Kelley, P.H., The fossil record of predation. The Paleontological Society Papers 8, The Paleontological Society, pp. 289– 317, <http://www.nrm.se/forskningochsamlingar/fossil/paleozoologi/personal/stefanbengtson/stefanbengtsonpublikationer.4.4e32c81078a8d924980007638.html>

Trace fossils 1 billion years ago?

Trace fossils found in rocks about 1 billion years old in India may represent marks of creatures moving across and below soft surfaces. The organisms making the traces were clearly not exploiting deep sediments, but only the layers immediately below the mat of cyanobacteria that covered the seabed. The researchers concluded that the burrows were produced by the peristaltic action of triploblastic metazoans up to 5 mm wide—in other words by animals about the diameter of earthworms, about as complex and possibly coelomates. (1998) "Animals More Than 1 Billion Years Ago: Trace Fossil Evidence from India". Science 282: 80–83. Retrieved on 2007-08-20.  But other researchers have dismissed this and other purported finds of trace fossils older than about 600 million years ago, usually on the grounds that they were produced by physical processes rather than by organisms.Jensen, S. (2003). "The Proterozoic and Earliest Cambrian Trace Fossil Record; Patterns, Problems and Perspectives". Integrative and Comparative Biology 43 (1): 219–228. doi:10.1093/icb/43.1.219. 

Cryogenian glaciations

The Cryogenian Period between 750 and 600 million years ago was cold, with a few major glaciations:New evidence supports three major glaciation events in the distant past (21 April 2004).

• The Sturtian, for which evidence was found in South Australian deposits, occurred about 720 million years old.
• The Changan (glacial deposits found in China)
• The Tiesiao (glacial deposits found in China) ended before 633 million years ago.
• The Nantuo (glacial deposits found in China) began later than 633 million years ago and is probably equivalent to the Marinoan glaciation in South Australia, which is dated at 630 million years ago.

Doushantuo Formation

The Doushantuo Formation in China contains one of the oldest known lagerstätten. These rocks range from about 635 million to about 551 million years ago, but their animal fossils are mostly less than 580 million years old, predating by perhaps 5 million years the earliest of the \'classical\' Ediacaran faunas (see below) from Mistaken Point, Newfoundland. (1 April 2005) "U-Pb Ages from the Neoproterozoic Doushantuo Formation, China". Science (5718): 95–98. doi:10.1126/science.1107765.  Doushantuo fossils are all marine, microscopic and highly preserved. They include algae, giant acritarchs and what may be phosphatised embryos of bilaterian animals; but some scientists think the “embryos” are fossils of giant sulfur-metabolising bacteria like Thiomargarita, which is so large that it is visible to the naked eye.:Xiao, S., Zhang, Y. & Knoll, A. H. “Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite”. Nature 391 553–558 (1998).

Hagadorn, J. W. et al. “Cellular and Subcellular Structure of Neoproterozoic Animal Embryos”. Science. 314: 291–294 (2006).

Bailey, J. V., et al. “Evidence of giant sulphur bacteria in Neoproterozoic phosphorites”. Nature 445: 198–201 (2007).

One Doushantuo fossil from about 580M years ago, Vernanimalcula (0.1 to 0.2 mm in diameter), has been described as a possible adult triploblastic coelomate bilaterian, in other words about as complex as an earthworm or a mollusc;Chen, J.Y.; Bottjer, D.J.; Oliveri, P.; Dornbos, S.Q.; Gao, F.; Ruffins, S.; Chi, H.; Li, C.W.; Davidson, E.H. (2004-07-09). "Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian". Science 305 (5681): 218–222. doi:10.1126/science.1099213.  others think it was more probably created by non-biological rock-forming processes;Bengtson, S.; Budd, G. (2004). "Comment on ‘‘Small bilaterian fossils from 40 to 55 million years before the Cambrian’’". Science 306: 1291a. doi:10.1126/science.1101338.  but the team that discovered Vernanimalcula have defended their conclusion that it was an animal, pointing out that they found 10 specimens of the same size and configuration, and stating that non-biological processes would be very unlikely to produce so many specimens that were so alike. (2004) "Response to Comment on “Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian”". Retrieved on []. 

The Gaskiers glaciation, known from glacial deposits in Newfoundland and Massachusetts, is later than the earliest Doushantuo fossils although it is regarded as the last of the Cryogenian series of glaciations.

The most recent Doushantuo rocks show a sharp decrease in the ¹³C/¹²C carbon istope ratio. Since this change appears to be worldwide but its timing does not match that of any other known major event such as a mass extinction, it may represent “possible feedback relationships between evolutionary innovation and seawater chemistry” in which metazoans (multi-celled organisms) removed carbon from the water, this increased the concentration of oxygen, and the increased oxygen level made possible the evolution of new metazoans such as Namapoikia (see below).

Ediacaran organisms

Dickinsonia costata, an Ediacaran organism of unknown affinity, with a quilted appearance.

Main article: Ediacaran biota

Strange-looking fossils were found first in the Ediacara Hills in Australia and then in marine sediments from many parts of the world including Charnwood Forest (England) and the Avalon Peninsula (Canada), with dates between 610 million and 543 million years ago (right up to the start of the Cambrian). Most of the Ediacaran biota were at least a few centimeters long, significantly larger than previous finds. The Mackenzie Mountains of northwestern Canada contain 3 distinct assemblages (sets) of Ediacaran fossils: (1) the oldest, dating between 610M and 600M years ago, before the last of the Cryogenian