EVOLUTIONARY HISTORY OF PROTISTS

Protists, the vast majority of eukaryotes, do not fossilize well. Millions of extant species would not fossilize. Yet the protist fossil record, containing hundreds of thousands of species, mirrors that of other groups, starting in the Archean with an uncertain presence, showing abundance in the Mesoproterozoic and radiations in the Neoproterozoic and into the Phanerozoic when some developed skeletons. Their early origin is inferred from molecular sequences, but fossils appear much later. For many groups (foraminifera, ciliates, dinoflagellates) molecular data indicate a long unfossilized Precambrian presence, although biomarkers date at 2.7 Ga. Acritarchs appear at 1.8 Ga and other complex protists occur later in the Proterozoic. Thus protists, likely abundant and diverse in shallow seas, were important components of Precambrian biotas, especially as primary producers.

Skeletonized protists (foraminifera, radiolaria, tintinnids?) appear in the Cambrian radiation, as acritarchs also diversify, in some kind of trophic cascade. From then on, the record shows radiations and extinctions, with protists diversifying and becoming extinct at all the major biological events recorded by metazoans in the fossil record. They occupied and even dominated major environments (carbonate shelves and reefs) and contributed to the growing diversity of animals as trophic resources. In the Mesozoic, dinoflagellates, diatoms, coccolithophorids, silicoflagellates, and planktic foraminifera appeared and radiated in pelagic environments, and foraminifera were abundant and diverse in benthic habitats. The modern protist biota reflects an important role at all trophic levels in marine environments. A huge number of largely unknown terrestrial, parasitic and symbiotic protists must have existed for much of geologic time as well. These were surely important, as indicated by the impact of disease-causing forms in humans and other organisms.