Introduction

Possible Causes of the Cambrian Explosion
"Despite the evidence that moderately complex animals existed before and possibly long before the start of the Cambrian, it seems that the pace of evolution was exceptionally fast in the early Cambrian. Possible explanations for this fall into three broad categories: environmental, developmental, and ecological changes. Any explanation must explain the timing and magnitude of the explosion. It is also possible that the "explosion" requires no special explanation."

from Wikipedia

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Stephen J. Gould:
"I therefore feel that Conway Morris has misrepresented my views by vague allusion (for he can cite no true source for arguments I never made) when he states that I hint "at a special mechanism at work, some unusual genetic happenstance gone wild" or when he floats an even vaguer charge about unorthodox mechanisms that he 'believes' I have 'implied.'"

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CAMBRIAN STRATIGRAPHY:
The Cambrian is a short period in earth history, starting approximately 545 m.y. ago and ending about 490 m.y. ago. Nevertheless, it was without any doubt one of the most important and dramatic periods. The lower boundary of the Cambrian is not only the beginning of a new system but also the start of the Paleozoic and the Phanerozoic. And the Early Cambrian saw the extremely rapid diversification of multicellular animals, the Cambrian Explosion, that determined the animal evolution and is indirectly responsible for the present-day wildlife.

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The oldest fossils of metazoan aspect are remains that belong to the so-called Ediacara fauna. The peculiar character of this fauna was first recognized in the Pound Quartzite, Ediacaran Hills, South Australia, although the same type of fauna was earlier discovered in the present-day southern Namibia. Ediacaran-type fossils are now known from numerous sites worldwide (such as Namibia, Ireland, England, northwestern Russia, South Australia, Newfoundland and the Canadian Northwest Territories) in rocks dated between 610 and 510 m.y., thus ranging from the Late Proterozoic (Vendian or late Neoproterozoic) to the Middle Cambrian.

Coexisting with latest Neoproterozoic Ediacaran faunas are simple trace fossils ("worm burrows") that were created by multicellular animals, the first evident Metazoa. Indeed, the stratigraphical occurrence of trace-fossils depicts an evolution to more complicated traces, which, in turn, proves the progressive evolution to more anatomically complicated animals that were able to perform a progressively complex behaviour. The first trace with a somewhat complicated pattern is Trichophycus pedum (formerly known as "Phycodes pedum"). It occurs nearly worldwide, and its first occurrence is with late fossils of typical Ediacaran aspect or, usually, in strata above them, whereas the first shelly fossils appeared clearly later. Hence, the ichnofossil assemblage with Trichophycus pedum marks the first occurrence of well-developed, fairly complex metazoan animals, and this is today regarded as the most useful landmark to characterize the boundary between the Precambrian and the Phanerozoic and, synchronously, the Proterozoic and the Cambrian.

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No animals are known from the very base of Cambrian that had hard parts, either as an external skeleton or simply spicules.

Another type of fossil archive is found in the Upper Cambrian of Sweden. Most of these fossils represent early and often previously unknown arthropod groups,...

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The most prolific example for such a fossil archive is the Middle Cambrian Burgess Shale from the Burgess Pass, Yoho National Park, British Columbia. However, recent studies have shown that many of the organisms had strange and previously unknown morphologies and have no modern analogues.

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The latest Neoproterozoic to Early Cambrian fossil record indicates that multicellular life evolved into a large number of possible bauplans as soon as it got a foothold. These bauplans (body designs), or types of organization, characterize high-ranked taxa such as phyla. Although life developed to a huge diversity as seen today, probably no new phyla developed in post-Cambrian times and the number of phyla has actually decreased since. The Middle Cambrian may thus represent the time with the organizational diversity at a maximum. What are the reasons of the Cambrian Explosion? This is a question that nobody can answer with enough certainty at the moment.

Physical examination of latest Proterozoic and Cambrian rocks indicate that there was:

(1) a distinct fluctuation of carbon isotopes around the Proterozoic-Cambrian,
(2) a dramatic increase of the d34S curve,
(3) an increase of the global sea-level,
(4) a distinct rise of the phosphorite production, and
(5) a slow increase of oxygen in the atmosphere from late Proterozoic to early Phanerozoic times.

These facts form the frame of a probably complex scenario, which ecologically equals the filling of an ecological barrel. However, we only hypothesize factors that may be responsible for a dramatic increase of phylogenetic development, such as possibly simpler Cambrian genomes or a more direct translation of gene to product, which may have enabled early diversification. Other hypotheses are needed to explain the rapid evolution and diversification of hard parts. Most of those hypothesis focus on changes in the physico-chemical environment and ecological stimuli (such as the evolution of the first predators). Regardless of the reasons, the novelty of hard parts led to more efficiency and improvements in the performance of animals and so is directly related to "advanced" animal groups such as arthropods and the group to which we belong, the chordates.*

*from the International Subcommission on Cambrian Stratigraphy

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Glacial deposits that formed on tropical land areas during snowball Earth episodes around 600 million years ago lead to questions about how the glaciers that left the deposits were created.*

*from the Daily University Science News

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December 17, 2003 -- Scientists at the University of California, Riverside and Columbia University have found evidence of the release of an enormous quantity of methane gas as ice sheets melted at the end of a global ice age about 600 million years ago, possibly altering the ocean's chemistry, influencing oxygen levels in the ocean and atmosphere, and enhancing climate warming because methane is a powerful greenhouse gas. The study was published in today's issue of the journal Nature.

The global ice age is of particular interest to paleobiologists because it took place shortly before the first appearance of animals in the fossil record, and may have supplied an environmental drive to evolution. The Earth's most severe climate is thought to have occurred about 600 million years ago with ice sheets stretching to the tropics. Some scientists have referred to times of such extreme cold as a "snowball Earth" condition, assuming that the ocean would have been totally ice covered.

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"The Burgess Shale animals have now been supplemented by a range of new Cambrian discoveries -- even earlier -- from China and Greenland. It is clear that the Cambrian evolutionary 'explosion' went down to the very base of that time period. This suggests that there must have been a much extended period of 'pre-explosive' evolution and, as I write, this hunch has been confrimed by the discovery of tiny, PreCambrian animal embryos in China."

Richard Fortey, paleontologist at the Natural History Museum, Oxford

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Eldredge, N., 1989 Macro-Evolutionary Dynamics: Species, Niches, and Adaptive Peaks. McGraw-Hill Publishing Company, New York, p. 22

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"My domain is the Proterozoic Era, pre-Cambrian. My understanding of life's development and activity, from its earliest beginnings up to the Cambrian Explosion, in no way allows for such a sudden, radical and permanent change to the direction of life. And, as far as the meaning and significance of the recent discovery in Eastern Siberia is concerned, with respect and relation to the Cambrian Explosion, if it had any influence on it, that is, I have no idea, but it's certainly a curious coincidence."

Professor S. Samuelson, Ph.D,
Chairman of Paleontology Department, Berkeley;
Curator of Berkeley Museum of Natural History

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