REASSESSING THE RECORD OF PROTEROZOIC EUKARYOTES:IMPLICATIONS FOR THE CAMBRIAN EXPLOSION

The problem of the Cambrian explosion is not so much the speed at which it proceeded, but why it failed to express itself much earlier. The 600+ million-year gap separating the middle Mesoproterozoic appearance of crown-group eukaryotes and the late Neoproterozoic appearance of crown-group metazoans is widely attributed to external constraints such as limiting oxygen or nutrients, but the evidence is circumstantial at best. Re-examination of the Proterozoic record of eukaryotes points to a significant inflation of true paleodiversity and a predominance of exceedingly long lived taxa, to the extent that most contemporaneous diversity can be considered “living fossils”. With Proterozoic rates of evolutionary turnover at least an order of magnitude slower than those typical of the Phanerozoic (e.g., Knoll 1994), the “delay” in the appearance of eumetazoans converts to no more than 60 million years of Phanerozoic evolutionary time. In other words, there is no significant delay, and thus no need to invoke ad hoc, externalist explanations for the Cambrian explosion.

The best explanation for the order-of-magnitude increase of evolutionary rates in the terminal Proterozoic is the appearance of animals and their unique contributions to ecology: predation to be sure, but also a host of associated direct and indirect effects. In the absence of such ecological interactions there was limited opportunity or impetus for evolutionary innovation or speciation, hence the low-diversity, evolutionarily lethargic aspect of Proterozoic eukaryotes. We argue that it is animals, in particular, that have served as the principal motors of eukaryote evolution, and that the Cambrian explosion (of both diversity and evolutionary rate) was triggered simply by the terminal Proterozoic evolution of animals, itself a relatively direct consequence of the Mesoproterozoic evolution of crown-group eukaryotes. The conspicuous stasis of Proterozoic eukaryotes stands as strong evidence against claims for deep metazoan roots.