The Cambrian explosion was the relatively rapid appearance (over a period of many millions of years), around 530 million years ago, of most major animal phyla, as demonstrated in the fossil record...

The fossils from the Cambrian period can cause a real headache for evolutionary biologists. Instinct tells us to expect simple organisms evolving over time to become increasingly more complex. However during the Cambrian period there was an apparent explosion of different major groups of animals, all appearing simultaneously in the fossil record.

Equally challenging is answering why the Cambrian period provided such a rich interval for the redeployment of genes that led to more complex bodyplans.

The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the 1840s, and in 1859 Charles Darwin discussed it as one of the main objections that could be made against his theory of evolution by natural selection.

The abrupt manner in which whole groups of species suddenly appear in certain formations, has been urged by several paleontologists, for instance, by Agassiz, Pictet, and by none more forcibly than by Professor Sedgwick, as a fatal objection to the belief in the transmutation of species. If numerous species, belonging to the same genera or families, have really started into life all at once, the fact would be fatal to the theory of descent with slow modification though natural selection. For the development of a group of forms, all of which have descended from some one progenitor, must have been an extremely slow process; and the progenitors must have lived long ages before their modified descendants.

The fossil record contains no trace of these preliminary stages in the development of many-celled organisms. The first clues to the existence of relatively advanced forms of life consist of a few barely discernible tracks, presumably made in the primeval slime by soft, wriggling wormlike animals. These are found in rocks about one billion years old. These meager remains are the earliest traces of many-celled animal life on the planet.

Biological structures were first recognized in the Longmyndian Supergroup more than 150 years ago where they were interpreted as body and trace fossils. Despite a variety of interpretations through the years, many of these bedding plane markings are now recognized as biological and include the sedimentary record of ancient microbial mats at the sediment-water interface. John Salter had therefore made the key discovery of life prior to the Cambrian radiation.

One outstanding fact of the fossil record that many of you may not be aware of; that since the so called Cambrian explosion... during which essentially all the anatomical designs of modern multicellular life made their first appearance in the fossil record, no new Phyla of animals have entered the fossil record.

And it has been the paleontologist—my own breed—who have been most responsible for letting ideas dominate reality: ... We paleontologist have said that the history of life supports that interpretation [gradual adaptive change], all the while knowing that it does not.

Why, in subsequent periods of great evolutionary activity when countless species, genera, and families arose, have there been no new animal body plans produced, no new phyla? (Lewin, 1988)

1. The Cambrian explosion was the seemingly sudden appearance of a variety of complex animals about 540 million years ago, but it was not the origin of complex life...

2. There are transitional fossils within the Cambrian explosion fossils...

3. Only some phyla appear in the Cambrian explosion. In particular, all plants postdate the Cambrian...

4. The length of the Cambrian explosion is ambiguous and uncertain...

5. There are some plausible explanations for why diversification may have been relatively sudden...

6. Cambrian life was still unlike almost everything alive today...

7. Major radiations of life forms have occurred at other times, too...

None of the organisms mentioned by TalkOrigins represent transitional precursors to the forms that appear so abruptly during the Cambrian explosion. Five to ten million years (which is the most likely time span) is extremely abrupt relative to the time that would be needed to accomplish those radical innovations in animal form resulting in the origin of many different animal body types. ... In summary, TalkOrigins has failed to provide us with a feasible naturalistic explanation for the Cambrian explosion. The facts that the article draws upon are either irrelevant to the subject at hand or accentuate the problem. The Cambrian explosion continues to represent a formidable challenge to the neo-Darwinian theory of evolutionary gradualism.

The difficulty with “explaining the Cambrian explosion” efforts is that the explainers are not trying to explain it. They are trying to explain it away. That is a very different procedure. ... what we really hear from explainers away is a non-credible account that satisfies the emotional needs of the people with a vested interest in avoiding the difficulty that the attested event presents to Darwinism.

This rather sudden appearance, relatively speaking, of essentially all animal phyla during the Cambrian period is referred to as the 'Cambrian Explosion'. After this period essentially no new phyla are thought to have evolved over the course of hundreds of millions of years. Compared with the 35 or so animal phyla that still exist today, some people estimate that the Cambrian explosion may have generated as many as 100 different phyla.