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TheCambrian explosion was an apparent remarkable increase ofanimal diversity in thefossil record fromca. 540 million toca. 510 million years ago, resulting in theevolution of many phyla of animals that have served as the basic "templates" on which animals today are based.

Creationist claims[edit]

Creationists often identify the Cambrian explosion as giant holes in thetheory of evolution.Jehovah's Witnesses' publicationLife--How did it get here? By evolution or by creation? refers to the Cambrian explosion in order to support its claim that thefossil record does not provide evidence of evolution:^[1]

Thisclaim fails to recognise several aspects of the Cambrian explosion. Despite the vernacular name applied to it, the Cambrian explosion wasnot 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, butyoung-earth creationists delight inconfusing the two. The Cambrian itself is defined as beginning with the first appearance of the traces of a possible worm,Treptichnus;^[2] the first trilobites only appeared roughly 20 million years after the Cambrian period actually started.

Pre-Cambrian fossils[edit]

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,^[3] and stromatoliths (formed from blue-green bacteria and sediment) date back 3500 million years.^[4] Nucleate cells (domainEukaryota) date back at least 2700 million years ago^[5], and many-celled organisms appear by 2100 million years ago.^[6]Sexual reproduction first appeared in red algae around 1200 million years ago.^[7] Testate (shelled) amoebae evolved by 750 million years ago.^[8]

TheEdiacaran 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 andsponges, but others (e.g.,Dickinsonia) do not have known relationships. Although some have proposed affiliations between some members of the Ediacaran biota to modern phyla (e.g.,Spriggina andParvancorina with the Arthropoda;Kimberella with theMollusca), 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,^[9] which may have killed off the Ediacarans, opening the way for the modern animals to "conquer" theEarth. Furthermore, in the uppermost Ediacaran period, simple hard-shelled creatures likeCloudina have been found, being preyed upon by arrow worms likeProtohertzina. Ocean chemistry was also changing around the Pre-Cambrian/Cambrian boundary; increasing levels oflime andoxygen 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[edit]

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,taxonomists classify the animal kingdom into about 35 phyla.^[10] Of these:^[11]

• 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,^[12] Gastrotricha, Hemichordata, Kinorhyncha, Loricifera,Mollusca, 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 athird of all the phyla alive today are known to have appeared in the Cambrian. Among the Cambrian animals themselves, there are several transitional forms. Thelobopods, includingHallucigenia andAnomalocaris, are best described as worms with legs; indeed, their descendants include thevelvet worms.Halkeriids could possibly be close relatives of the ancestor of molluscs, annelids, and brachiopods. It is also important to notice that, even among the phyla thatwere 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, andall trueplants post-date the Cambrian. The onlyfishes in the Cambrian period (so-called myllokunmingids likeHaikouichthys) were nothing like any fishes today. In a creationist model, one would expect everything appearing fully formed all at once in thefossil record.

See also[edit]

• Fossil sorting by the global flood

External links[edit]

• TalkOrigins on the Cambrian explosion
• A guide to the pre-Cambrian eukaryote evolution

References[edit]

• Evolution
• Creationism
• Paleontology

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