Figure 1 Comparison of the Phanerozoic and Precambrian histories of life.

of having specialized organ systems for exploitation of specific ecologic niches, members of the most successful group of these early-evolving microorganisms—photoautotrophic cyanobacteria—were ecologic generalists, able to withstand the rigors of a wide range of environments. In contrast with normal evolution, the "primitive rules" of prokaryotic evolution appear to have been speciation, generalization, and exceptionally long-term survival.

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