The Cambrian Explosion
Explosion of specified complexity
Cambrian explosion, the unparalleled emergence of organisms between 541 million and approximately 530 million years ago at the beginning of the Cambrian Period. The event was characterized by the appearance of many of the major phyla (between 20 and 35) that make up modern animal life. ... By the Early Cambrian the bulk of the biosphere was confined to the margins of the world’s oceans; no life was found on land.*

According to mainstream science the Cambrian explosion was the relatively rapid appearance (over a period of several millions of years) of most major animal phyla, as demonstrated in the fossil record.
At the same time the Cambrian stratum also shows that there was a sudden and massive destruction of life on earth because of the preservation of large quantities of fossils which requires sudden burial under pressure.
Darwin's Dilemma
Charles Darwin:
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 through 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 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.*

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 slow and gradual evolution by natural selection.
With the Cambrian explosion for example brains appear which are considered the most complex of organs. The Cambrain explosion was an explosion of specified complexity. In Darwin's time there was little knowledge from the period before the Cambrian explosion. Darwin expected older strata to show fossils that would explain the sudden appearance of life in the Cambrian. But this is not the case...
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.

The Precambrian strata have been thoroughly examined by scientists since Darwin, but they contain very few fossils with explanatory power. The sudden appearance of phyla in the Cambrian is a serious blow to Darwin's theory of slow and gradual evolution. Evolutionists Niles Eldredge noticed that: 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.
It is easy to explain why the Cambrian explosion was unprecedented in producing a great array of novel body forms: it was close to the origin of multicellular organisms, and so there could have been little previous opportunity. (The rate at which it happened is, however, impressive.) But why has this burst of evolutionary invention never again been equaled? 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?

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.

After the Cambrian explosion no new phyla emerged. There are no intermediates and no macroevolution because from those phyla which appeared in the Cambrian period all subspecies and variety came by means of microevolution. Therefore evolutionists Niles Eldredge and Stephen Jay Gould invented punctuated equilibrium, which contradicts Darwin's gradual evolution, in an attempt to safe there faith. See Fossil Record.

All phyla appeared in a relatively short period of time and after that many phyla died out. That sounds more like degeneration than macroevolution. There are many more burning scientific questions for evolution than there are satisfying scientific answers from it.
Talk Origins Non Sense
According to Talk.Origins, a fanatic evolution club sponsored by mainstream science...
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...
Talk.Origins tries to explain away the Cambrian explosion. However...
1. There is nothing "seemingly" about this sudden appearance. It was definitely sudden in the supposed evolution of billions of years. Nobody says it was the origin of complex life. But it was an explosion of complex life. This is certainly not in accordance with slow and gradual evolution as proposed by Darwin or neo-Darwinism.
2. Transitional fossils are based on metaphysics, not on science.
3. Plants, like Algea, existed before the Cambrian explosion. But all known complex animal phyla appeared with the Cambrian explosion.
4. It is not. Scientists agree upon the timespan depending on what they want to include.
5. Anybody can make up just-so stories based on wishful thinking.
6. Of course, extinction events caused certain types to disappear. Living fossils prove that there was no evolution. Lazarus taxa show that presumed extinct life forms are still alive today. And of course microevolution caused variety.
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.*

TalkOrigins doesn't explain, it tries to explain away. This merely understates the problem for evolution theory.