Cambrian explosion

{{evolution}}
{{cquote|I cannot doubt that all the Silurian [the Cambrian was not yet recognised] trilobites have descended from some one crustacean, which must have lived long before the Silurian age… Consequently, if my theory be true, it is indisputable that before the lowest Silurian strata was deposited, long periods elapsed, as long as, or probably longer than, the whole interval from the Silurian to the present day… The case must at present remain inexplicable; and may be truely urged as a valid argument against the views here entertained…|||[[Charles Darwin]], ''[[On the Origin of Species|The Origin of Species]]'', 1859, pp. 313-314}}
The '''Cambrian explosion''' was an apparent remarkable increase of [[animal]] diversity in the [[fossil record]] from ''ca.'' 540 million to ''ca.'' 510 million years ago, resulting in the [[evolution]] of many phyla of animals that have served as the basic "templates" on which animals today are based.

==Creationist claims==
[[Creationist]]s often identify the Cambrian explosion as giant holes in the [[theory of evolution]]. [[Jehovah's Witnesses]]' publication ''[[Life--How did it get here? By evolution or by creation?]]'' refers to the Cambrian explosion in order to support its claim that the [[fossil record]] does not provide evidence of evolution:<ref>''Life--How did it get here? By evolution or by creation?'', Watchtower Bible and Tract Society of New York, Inc., Brooklyn, NY, 1985, p. 60</ref>

{{cquote|''Thus, at the start of what is called the [[Cambrian]] period, the fossil record takes an unexplained dramatic turn. A great variety of fully developed, complex sea creatures, many with hard outer shells, appear so suddenly that this time is often called an "explosion" of living things.''}}

This [[creationist claims|claim]] fails to recognise several aspects of the Cambrian explosion. Despite the vernacular name applied to it, the Cambrian explosion was ''not'' an event that occurred overnight. The very shortest timeframe given for the Cambrian explosion is five million years, and the current evidence is indicative that the "explosion" occurred over anywhere from 20 million to 40 million years. This was sudden only on geological timescales, not human ones, but [[Young Earth creationism|young-earth creationists]] delight in [[equivocation|confusing the two]]. The Cambrian itself is defined as beginning with the first appearance of the traces of a possible worm, ''Treptichnus'';<ref>http://www.trilobites.info/biostratigraphy.htm</ref> the first trilobites only appeared roughly 20 million years after the Cambrian period actually started.

=== Pre-Cambrian fossils ===
[[File:DickinsoniaCostata.jpg|thumb|left|''Dickinsonia'' was one of the members of the mysterious Ediacaran biota that died and gave way to the life of the Cambrian period.]]
Pre-Cambrian fossils actually show quite a bit of evolution that occurred before the Cambrian period. Organic signatures in rocks indicate that life was abundant by 3800 million years ago,<ref> Bjornerud, Marcia, (2005), "Reading the Rocks: the autobiography of the Earth".</ref> and stromatoliths (formed from blue-green bacteria and sediment) date back 3500 million years.<ref>Allwood, A. C. et al. 2006. Stromatolite reef from the Early Archaean era of Australia. Nature 441: 714-718. See also Awramik, Stanley M. 2006. Respect for stromatolites. Nature 441: 700-701.</ref> Nucleate cells (domain [[Eukaryote|Eukaryota]]) date back at least 2700 million years ago<ref>''The Geology of Australia'', David Johnson [http://books.google.co.uk/books?id=RSIbLENyT2wC&pg=PA81&lpg=PA81&dq=australia+eukaryote+pilbara+2700+ma#v=onepage&q=australia%20eukaryote%20pilbara%202700%20ma]</ref>, and many-celled organisms appear by 2100 million years ago.<ref>Han, Tsu-Ming, and Bruce Runnegar. "[http://europepmc.org/abstract/MED/1631544 Megascopic eukaryotic algae from the 2.1-billion-year-old Negaunee Iron-Formation, Michigan.]" Science (New York, NY) 257.5067 (1992): 232.</ref> [[Sexual reproduction]] first appeared in red algae around 1200 million years ago.<ref>Butterfield, Nicholas J. "[http://paleobiol.geoscienceworld.org/content/26/3/386.short Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes.]" Paleobiology 26.3 (2000): 386-404.</ref> Testate (shelled) amoebae evolved by 750 million years ago.<ref>Porter, Susannah M. and Andrew H. Knoll, 2000. Testate amoebae in the Neoproterozoic Era: evidence from vase-shaped microfossils in the Chuar Group, Grand Canyon. Paleobiology 26(3): 360-385.</ref>

The {{wpl|Ediacaran biota}}, which first appeared around 580 million years ago and faded away at the beginning of the Cambrian, were a group of organisms, currently believed to be animals, but with relationships to modern phyla unknown. Some of them may have been relatives of modern jellyfish and [[sponge]]s, but others (e.g., <i>Dickinsonia</i>) do not have known relationships. Although some have proposed affiliations between some members of the Ediacaran biota to modern phyla (e.g., ''{{wpl|Spriggina}}'' and ''{{wpl|Parvancorina}}'' with the Arthropoda; ''{{wpl|Kimberella}}'' with the [[Mollusc]]a), but these attempts to link the Ediacarans to modern groups have failed. At the end of the Ediacaran period, there was an event known as the Baykonur glaciation,<ref>Chumakov, Nickolay M. "[http://mem.lyellcollection.org/content/36/1/303.short Glacial deposits of the Baykonur Formation, Kazakhstan and Kyrgyzstan.]" Geological Society, London, Memoirs 36.1 (2011): 303-307.</ref> which may have killed off the Ediacarans, opening the way for the modern animals to "conquer" the [[Earth]]. Furthermore, in the uppermost Ediacaran period, simple hard-shelled creatures like ''{{wpl|Cloudina}}'' have been found, being preyed upon by arrow worms like ''Protohertzina''. Ocean chemistry was also changing around the Pre-Cambrian/Cambrian boundary; increasing levels of {{wpl|calcite|lime}} and [[oxygen]] were to be found. All these factors combined would have allowed the primitive bilaterians to rapidly diversify and fill the seas in only 20 million years (which is still a long time, if short geologically).

===Appearance of phyla within the Cambrian===
Although multiple phyla of animals do appear in the Cambrian, claims that all phyla appeared during an interval of 20 million years is misleading. Depending on who one asks, [[taxonomy|taxonomists]] classify the animal kingdom into about 35 phyla.<ref>See {{wpl|Phylum#Animal_phyla|Wikipedia}}.</ref> Of these:<ref>Modified from Collins, 1994.</ref>

* 3 phyla appear in pre-Cambrian time (Porifera, Cnidaria, Ctenophora (possibly Chaetognatha and Echinodermata as well, being inferred from earliest Cambrian fossils)).
* 14 phyla appear in the Cambrian (Annelida, Arthropoda, Brachiopoda, Chaetognatha, [[Chordata]], Echinodermata, Entoprocta,<ref>Zhang, Zhifei, et al. "[http://www.nature.com/srep/2013/130117/srep01066/full/srep01066.html#/f1 A sclerite-bearing stem group entoproct from the early Cambrian and its implications.]" Scientific Reports 3 (2013).</ref> Gastrotricha, Hemichordata, Kinorhyncha, Loricifera, [[Mollusc]]a, Nematoda, Onchyophora, Priapulida)

* 1 phylum appears after the Cambrian but has an extensive fossil record (Bryozoa).

* 8 phyla (Placozoa, Rotifera, Nemertea, Tardigrada, Platyhelminthes, Phoronida, Acanthocephala, Nematomorpha) appear after the Cambrian with only minimal fossils.

* 9 phyla have no significant fossil record, being soft-bodied.

This means only about a '''third''' of all the phyla alive today are known to have appeared in the Cambrian. Among the Cambrian animals themselves, there are several transitional forms. The {{wpl|Lobopodia|lobopods|,}} including ''Hallucigenia'' and ''Anomalocaris'', are best described as worms with legs; indeed, their descendants include the {{wpl|Onchyophora|velvet worms|.}} {{wpl|Halkeriid}}s could possibly be close relatives of the ancestor of molluscs, annelids, and brachiopods. It is also important to notice that, even among the phyla that ''were'' present in the Cambrian, they are not at all like their descendants in the present half a billion years later; [[mammals]], [[birds]], reptiles, amphibians, insects, arachnids, cephalopods, and ''all'' true [[plants]] post-date the Cambrian. The only [[fish]]es in the Cambrian period (so-called myllokunmingids like ''[[Haikouichthys]]'') were nothing like any fishes today. In a creationist model, one would expect everything appearing fully formed all at once in the [[fossil record]].

==See also==
* [[Fossil sorting by the global flood]]

==External links==
* [http://www.talkorigins.org/indexcc/CC/CC300.html TalkOrigins on the Cambrian explosion]
* [http://www.geol.ucsb.edu/faculty/porter/Papers/Porter_2006_heterotrophs.pdf A guide to the pre-Cambrian eukaryote evolution]

==References==
{{reflist|2|90%}}

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[[Category:Evolution]]
[[Category:Paleontology]]

Categories: Creationism, Evolution, Paleontology