KEYNOTE: ANIMALS, ACRITARCHS AND MOLECULAR CLOCKS: WHAT MAKES THE PROTEROZOIC DIFFERENT?

The Proterozoic can be broadly delineated by the onset of continental tectonics at or around 2500 Ma, and the (apparent) onset of animal-based ecosystems at or around 540 Ma. The nature of the upper boundary is the subject of heated debate – is this a real evolutionary breakthrough or mere taphonomic appearance? – and has profound implications for understanding the Proterozoic Earth system.

Molecular clocks offer an independent approach to resolving this issue, but differing algorithms and calibration points yield conspicuously discordant divergence times for early metazoan evolution. Using a set of seven nuclear-encoded protein sequences – calibrated with respect to echinoderms, insects, bivalves and gastropods – we have calculated sponge/eumetazoan and cnidarian/bilaterian divergence times using both distance (ME) and likelihood (ML) molecular clocks: Minimum Evolution brackets the appearance of eumetazoans (non-sponge animals) between 634 Ma and 604 Ma, whereas Maximum Likelihood puts it between 867 Ma and 748 Ma.

By virtue of their motility and differentiated gut eumetazoans represent a singularly powerful ecological and evolutionary challenge, capable of driving both extinction and radiation. Thus, we expect the evolution of eumetazoans to be accompanied by a major shift in evolutionary tempo and mode, whether or not they are recognised directly in the fossil record. Significantly, the Proterozoic record of acritarchs (essentially the whole of the eukaryotic record prior to 580 Ma) documents a major regime change in the early Ediacaran (ca. 635 Ma) including: 1) disappearance of low-diversity, evolutionarily static, pre-Ediacaran acanthomorphs; 2) radiation of a high-diversity, short-lived Doushantuo-Pertatataka microbiota; and 3) an order-of-magnitude increase in evolutionary turnover rate. Combined with our ME molecular clock estimate, this pronounced turnover – the first measurable extinction and radiation in the whole of the fossil record – presents positive, and compelling, evidence for the appearance of eumetazoans at or around 635 Ma. By extension, much of the character of the Proterozoic can be ascribed to the bona fide absence of eumetazoans – even small ones.