EXPLAINING THE CAMBRIAN EXPLOSION: CAUSE, CONSEQUENCE OR COINCIDENCE?

The Proterozoic-Phanerozoic transition (1000-500 Ma) was a time of marked geological and biological perturbation. In addition to the Cambrian explosion of large animals and acritarchs, it witnessed the assembly and breakup of two supercontinents, at least two major glaciation events, a brief but widespread return of banded iron formation, various indications of rising atmospheric oxygen, a long term decline in stromatolite "diversity," the appearance of complex developmental programmes, Ediacaran-type faunas, major accumulations of phosphorites and black shale, and marked shifts in d¹³C and d³⁴S signatures. Some of these phenomena are assuredly causally correlated, but most can be ruled out as causes of the Cambrian explosion.

Current contenders for the cause of the Cambrian explosion include: 1) the Snowball Earth, specifically the genetic isolation associated with runaway icehouse conditions; 2) Oxygen Limitation, constraining animals to small size and/or limited exertion; 3) Nutrient Stimulus, inducing or accelerating animal evolution through an influx of nutrients; 4) developmental innovations allowing the construction of complex organization; and 5) ecological innovation, particularly that induced by complex multicellular organisms.

The Snowball Earth hypothesis offers a potential source of increased genetic variation, but fails to provide any mechanism for the selection of the features that need to be explained, namely large size, skeletalization, and morphological and behavioural complexity. Oxygen limitation may have influenced the timing of the Cambrian explosion, but cannot be considered a cause, per se. Nutrient Stimulus is not supported by ecological theory or secular trends in TOC. Developmental innovations were certainly necessary for the evolution of complex organisms, but do not themselves provide the selection necessary for Darwinian evolution. I will argue here that it was ecological innovation and an escalating biological environment, defined largely by complex multicellular organisms, that gave rise the Cambrian explosion. Some of the accompanying phenomena are likely to be consequences of the radiation, others merely coincidental.