PRECAMBRIAN-CAMBRIAN EVOLUTIONARY HISTORY OF PROTISTS

Protists, single-celled eukaryotes, have a very early (Archean or Paleoproterozoic) origin as inferred from molecular sequence data, yet their fossils are not known with certainty until much later. Molecular data for many protistan groups, i.e. foraminifera, ciliates, and dinoflagellates, indicate a long unfossilized Precambrian presence. This disparity may result from a search failure but more likely from taphonomic effects resulting from a lack of fossilizable parts. Organic walled cysts appear at 1.8 ga and other complex protists occur later in the Proterozoic. Even groups preserved well later, like foraminifera, were extant at least by the early Neoproterozoic. Thus protists, likely abundant and diverse in shallow seas, were important components of Proterozoic biotas, especially as primary producers. Skeletonized protists appear with metazoans in the Cambrian radiation. The protist record mirrors that of the metazoan radiation (an “explosion”?), which has been attributed to developmental processes, environmental changes or ecological interactions. All of these models are metazoan bound and do not consider the evolutionary events in protists as part of the evidence. Because the protists that responded to the Cambrian events are both benthic and pelagic, and onshore and offshore, the causes likely transgressed all marine ecosystems. A reasonable hypothesis is that a change in the energy flow (nutrient supply and trophic resources) linking these ecosystems took place. This would have increased primary production which may have caused the secondary evolutionary responses of increased complexity of trophic strategies, increasing morphologic and/or developmental complexity, and skeletonization in all eukaryotes. This evidence, derived from all eukaryotes rather than just metazoans, indicates that the oceans changed pervasively. The Proterozoic differed because its homogeneous oceanic structure prevented nutrient \recycling through upwelling processes, as they likely did in the heterogeneous oceans of much of the Phanerozoic. This trophic model is likely a general one, as similar evolutionary patterns took place at later times too. It differs from other plankton hypotheses in that energy availability through nutrient supply is at its root, not grazing or predation on protistan plankton.