Explanations for the Cambrian Explosion

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The Cambrian fossil record includes an unusually high number of lagersttten, which preserve soft tissues.

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Stromatolites, stubby pillars built by colonies of microorganisms, are a major constituent of the fossil record from about 2,700 million years ago, but their abundance and diversity declined steeply after about 1,250 million years ago. This decline has been attributed to disruption by grazing and burrowing animals.

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Precambrian marine diversity was dominated by small fossils known as acritarchs. This term describes almost any small organic walled fossil - from the egg cases of small metazoans to resting cysts of many different kinds of green algae. After appearing around 2,000 million years ago, acritarchs underwent a boom around 1,000 million years ago, increasing in abundance, diversity, size, complexity of shape and especially size and number of spines.

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The earliest Cambrian trilobite fossils are about 530 million years old, but the class was already quite diverse and worldwide, suggesting that they had been around for quite some time.

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However, evidence of Precambrian metazoa is gradually accumulating. If the Ediacaran Kimberella was a mollusc-like protostome (one of the two main groups of coelomates),[55][17] the protostome and deuterostome lineages must have split significantly before 550 million years ago (deuterostomes are the other main group of coelomates).[73] Even if it is not a protostome, it is widely accepted as a bilaterian.[73][59] Since fossils of rather modern-looking Cnidarians (jellyfish-like organisms) have been found in the Doushantuo lagersttte, the Cnidarian and bilaterian lineages must have diverged well over 580 million years ago.

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The presence of Precambrian animals somewhat dampens the "bang" of the explosion: not only was the appearance of animals gradual, but their evolutionary radiation ("diversification") may also not have been as rapid as once thought. Indeed, statistical analysis shows that the Cambrian explosion was no faster than any of the other radiations in animals' history.4

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There is little doubt that disparity - that is, the range of different organism "designs" or "ways of life" - rose sharply in the early Cambrian.[5] However recent research has overthrown the once-popular idea that disparity was exceptionally high throughout the Cambrian, before subsequently decreasing.[75] In fact, disparity remains relatively low throughout the Cambrian, with modern levels of disparity only attained after the early Ordovician radiation.[5]

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The diversity of many Cambrian assemblages is similar to today's.

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Despite the evidence that moderately complex animals (triploblastic bilaterians) 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.

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