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Evolution of the protists is discussed on the basis of their known fossil record and the morphological, biochemical, metabolic, physiological, and distributional data concerning modern representatives. Periods of explosive evolution result from the chance appearance of a major innovation and are followed by extensive exploitation of the possibilities of each innovation. A billion years or more following the origination of life were characterized by a biochemical and metabolic stage of differentiation, procaryote structure, rapid biotic turnover, and simple carbon cycle. This was succeeded by a period of intercellular and physiologic differentiation, about 1.5 b.y. in duration, with continued rapid carbon cycling, eucaryotic protist asexual clonal reproduction, and evolution based on mutation-induced variations.

The last 0.6 b.y. have been characterized by intercellular differentiation (metaphytes and metazoans), with increased size of the individual, dominance of the diploid phase, and hence increased relative importance of genetic recombination in evolution, with smaller populations, and development of an extremely complex carbon cycle that is more susceptible to periodic disruption.

Evolutionary rates in protists are comparable to those of multicellular organisms, although attainment of variability, isolation mechanisms, and selection processes differ. The apparent exponential increase of phytoplankton taxa at successive periods in their geologic history is related to their mutation-based evolution. A hypothetical phylogenetic tree is plotted against the geologic time scale, showing the known extent of the protist fossil record and that of other major groups of organisms, grades of structural and metabolic differentiation, relative age of important Precambrian fossil occurrences, and probable time of appearance of an aerobic environment.

The Phanerozoic fossil record of phytoplankton shows periods of maximum productivity interspersed with times of greatly reduced microfloras. Modern distributions and productivity, together with experimental culture data, are related to the geologic past to indicate both possible causes and effects of these fluctuations, in elaboration of a previously proposed Phytoplankton Periodicity Model.

The photosynthetic activity of phytoplankton, by which CO2 and H2O are utilized to produce organic compounds and free oxygen, forms the basis of the oceanic carbon and oxygen cycles and thereby influences atmospheric composition. The areal extent of the oceans and the abundance and rapid growth of the phytoplankton confer prime status on oceanic production in the present-day oxygen balance. Quantitatively, oceanic production was even more important in the geologic past. Oxygen-producing algae appeared some 3 b.y. ago, and a highly oxygenic atmosphere probably was attained geologically soon.

Through the interrelated processes of carbon fixation and oxygen production, periods of high oceanic phytoplankton productivity had a somewhat more oxygenic atmosphere, reduced CO2 pressure, and higher oceanic pH. These conditions are recorded by abundant dispersed organic carbon in sediments and petroleum accumulation (both of isotopically light carbon), lime deposition with increasingly heavy carbon isotope ratio, and an abundance of marine filter-feeding, detrital feeding, and carnivorous animal life. Atmospheric conditions were favorable for land animals.

Times of greatly limited phytoplankton production resulted in relative oceanic increase of procaryotes, particularly bacteria, increased CO2 levels, lowered pH, and oxygen depletion, silica deposition, less organic carbon in sediments, limestones of increasingly light 12C/13C ratio and eventual lime solution, and deposition of gypsum, barite, and other sulfates of isotopically light sulfur. The marine food web was drastically tightened, filter feeders and reef-forming invertebrates were largely eliminated, and only the more efficient food gatherers survived. Relative atmospheric depletion of oxygen affected both marine and terrestrial animals, eliminating many, whereas the increased CO2 pressure stimulated terrestrial plants.

The world-wide contemporaneity of certain geochemical, sedimentological, and marine and terrestrial biologic events is suggested by data obtained in numerous unrelated studies. Changes in phytoplankton quantitative distribution, discussed herein, also coincide with these events and are suggested to have formed the connecting link between them.

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