Cambrian explosion

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… 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
Creationists 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:

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 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 creationists delight in confusing the two. The Cambrian itself is defined as beginning with the first appearance of the traces of a possible worm, Treptichnus; the first trilobites only appeared roughly 20 million years after the Cambrian period actually started.

Pre-Cambrian fossils
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, and stromatoliths (formed from blue-green bacteria and sediment) date back 3500 million years. Nucleate cells (domain Eukaryota) date back at least 2700 million years ago, and many-celled organisms appear by 2100 million years ago. Sexual reproduction first appeared in red algae around 1200 million years ago. Testate (shelled) amoebae evolved by 750 million years ago.

The, 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 sponges, 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., ' and ' with the Arthropoda;  with the Mollusca), 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, 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  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 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, taxonomists classify the animal kingdom into about 35 phyla. Of these:


 * 3 phyla appear in pre-Cambrian time (Porifera, Cnidaria, Ctenophora).
 * 12 phyla appear in the Cambrian (Annelida, Arthropoda, Brachiopoda, Chaetognatha, Chordata, Echinodermata, Entoprocta, Hemichordata, Mollusca, Nematoda, Onchyophora, Priapulida).


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


 * 18 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 including Hallucigenia and Anomalocaris, are best described as worms with legs; indeed, their descendants include the  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 fishes 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.