Abstract
The major features of protist evolution are fraught with controversies, problems and few answers, especially in early Earth history. In general they are based on molecular data and fossil evidence that respectively provide a scaffold and details of eukaryotic phylogenetic and ecologic histories. 1. Their origin, inferred from molecular sequences, occurred very early (>;3Ga). They are a chimera of different symbiont-derived organelles, including possibly the nucleus. 2. The initial diversification of eukaryotes may have occurred early in geologic time. Six supergroups exist today, each with fossils known from the Proterozoic and Phanerozoic. 3. Sex, considered an important development, may have been inherited from bacteria. 4. Precambrian protists were largely pelagic cyst-bearing taxa, but benthic forms were probably quite diverse and abundant. 5. Protists gave rise to animals long before 600 Ma through the choanoflagellates, for which no fossil record exists. 6. Acritarchs and skeletonized protists radiated in the Cambrian (544-530 my). From then on, they radiated and became extinct at all the major events recorded in the metazoan fossil record. 7. Protists dominated major environments (shelves and reefs) starting with a significant radiation in the Ordovician, followed by extinctions and other radiations until most died out at the end of the Permian. 8. In the Mesozoic, new planktic protozoa and algae appeared and radiated in pelagic environments. 9. Modern protists are important at all trophic levels in the oceans and a huge number terrestrial, parasitic and symbiotic protists must have existed for much of geologic time as well. 10. The future of protists is likely in jeopardy, just like most reefal, benthic, and planktic metazoans. An urgent need to understand the role of protists in modern threatened oceans should be addressed soon.
Highlights
Protists are single-celled eukaryotes of wide genetic, phylogenetic, morphologic, ecologic, and functional variety (Cavalier-Smith, 2004, 2005; Patterson, 1999)
While their evolutionary rules may vary somewhat, they show remarkable historical patterns preserved in the fossil record that commonly mirror those in metazoans
A good part of the evidence for all eukaryotes comes from the history that molecular phylogenetics reveals and, together with the fossil record, provides a framework for their evolution
Summary
Protists are single-celled eukaryotes of wide genetic, phylogenetic, morphologic, ecologic, and functional variety (Cavalier-Smith, 2004, 2005; Patterson, 1999). Fossil protists offer substantial tests of evolutionary hypotheses and environmental conditions through time due to their abundance and continuity through the geologic record, especially in the late Neoproterozoic and Phanerozoic Their record is superlative compared with the prokaryotic or multicellular organisms. No fossil record exists at all, yet like metazoans and higher plants, a sufficient number of them left fossils so that we may broadly know their history (Figure 2) They are mostly microscopic, many manufacture characteristic biomolecules, shells or other cell coverings (Table 2) that preserve well and abundantly in the geologic record. A good part of the evidence for all eukaryotes comes from the history that molecular phylogenetics reveals and, together with the fossil record, provides a framework for their evolution Much of this remains controversial and problematic, so it is in need of enhancement, especially for the early parts of their history. It is an overview because so much of it is controversial and problematic for a variety of molecular and taphonomic reasons, and most answers remain tentative
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