The Cambrian Explosion is the seemingly sudden appearance of a number of new complex organisms between 543 and 530 million years ago (MYA). However, newer research shows that this radiation of animal phyla started much earlier, from about 570 MYA, 30 million years before the beginning of the Cambrian geologic period. In 1994, triploblastic animals (organisms with more than two layers, and who therefore rely on internal organs and systems for their cells' supplies of food and waste disposal), were discovered preserved as phosphatized embryos in rocks from southern China [Xiao et al. 1998]. These fossils were estimated to be 570 million years of age and thus were even older than the Ediacaran fauna found in strata about 10 million years younger.

The extinctions connected with the Varangian glaciation that preceded this radiation of newer phyla (popularized as 'Snowball Earth'), along with subsequent greenhouse warming of the Earth, is theorized to have provided the evolutionary impetus. It is thought that the development of sexual reproduction increased the rate of evolutionary change.

Evidence for earlier multicellular animal forms, which may have been the precursors of this radiation, date from 600 million years ago. Notable among these are trace fossils in the form of imprints of animals and their activities, such as burrows in mud, produced by animals that paleontologists call the Ediacaran fauna. These organisms were soft-bodied and are found with various strange body forms. The so-called Small shelly fauna of the ensuing Tommotian period included Cloudinia and its kin.

The original and most widely-publicized source of fossils from the actual radiation period is the Burgess Shale in British Columbia. Some Burgess Shale organisms display strikingly unusual body plans that are not easily connected with any phyla known since.

A popular account of the paleontological analysis of the Burgess Shale is given in Wonderful Life by Stephen Jay Gould.

The Cambrian Explosion has recently been a controversial topic regarding the history and evolution of life, with the idea posited that the Burgess Shale preserved such a wide variety of life and that the "Cambrian Explosion" was actually a slower radiation of animal forms than previously thought. The idea of an "explosion" of life in the Cambrian period is still being debated.

The debate centers in part around an earlier notion that all phyla in existence today (and all others now extinct) except one were first found in this period. This would be as if one were pacing off the length of a football field (starting 4 bya), when between paces 78 and 79 all the different phyla suddenly sprang into existence.

According to more recent research, only some phyla appear in the Cambrian explosion. On talkorigins.org, in response to the Creationist claim of sudden appearance, Mark Isaak [Isaak 2004] gives the following summary:

Only some phyla appear in the Cambrian explosion. In particular, all plants post-date the Cambrian, and flowering plants, by far the dominant form of land life today, only appeared about 140 Mya [Brown 1999].

Even among animals, not all types appear in the Cambrian. Cnidarians, sponges, and probably other phyla appeared before the Cambrian. Molecular evidence shows that at least six animal phyla are Precambrian [Wang et al. 1999]. Bryozoans appear first in the Ordovician. Many other soft-bodied phyla don't appear in the fossil record until much later. Although many new animal forms appeared during the Cambrian, not all did. According to one reference [Collins 1994], 11 of 32 metazoan phyla appear during the Cambrian, one appears Precambrian, 8 after the Cambrian, and 12 have no fossil record.

And that just considers phyla. Almost none of the animal groups that people think of as groups, such as mammals, reptiles, birds, insects, and spiders, appeared in the Cambrian. The fish that appeared in the Cambrian was unlike any fish alive today.

References

Brown, Kathryn S., 1999. Deep Green rewrites evolutionary history of plants. Science 285: 990-991.

Collins, Allen G., 1994. Metazoa: Fossil record. http://www.ucmp.berkeley.edu/phyla/metazoafr.html .

Isaak, Mark, 2004. "CC300: Cambrian Explosion." http://www.talkorigins.org/indexcc/CC/CC300.html. Copyright © 2004.

Wang, D. Y.-C., S. Kumar and S. B. Hedges, 1999. Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi. Proceedings of the Royal Society of London, Series B, Biological Sciences 266: 163-71.

Xiao, Zhang, and Knoll, "Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite," 1998, Nature, vol. 391, pp. 553-58.