That there is a distinction in evolutionary tempo and mode between the Phanerozoic and Precambrian histories of life is not a new idea (Schopf, 1978), but it is one that has recently received additional impetus (Schopf, 1992a) and therefore deserves careful scrutiny. However, evaluation of this generalization hinges critically on the quality and quantity of the fossil evidence available, and because active studies of the Precambrian fossil record have been carried out for little more than a quarter century (Schopf, 1992b) a thorough comparison of the early history of life with that of later geologic time is not yet possible. Therefore, as a first approximation, the approach used here is to analyze the known fossil record of Precambrian cyanobacteria: well-studied, widespread, abundant, commonly distinctive, and evidently dominant members of the early prokaryotic biota. Microscopic fossils regarded as members of other prokaryotic groups are also known from the Precambrian (Schopf, 1992c, 1992d), but their documented record is minuscule. Hence, conclusions drawn here about the early fossil record apply strictly to free-living cyanobacteria, the evolutionary history of which may or may not be representative of prokaryotes in general. To evaluate the generalization, two central questions must be addressed. First, was the tempo of Precambrian cyanobacterial evolution markedly slower than that typical of Phanerozoic eukaryotes? Second, if so, how can this difference be explained?

Tempos of Evolution

In Tempo and Mode, Simpson coined terms for three decidedly different rate distributions in evolution, inferred from morphological comparisons of Phanerozoic and living taxa: tachytelic, for "fast"-evolving lineages; horotelic, the standard rate distribution, typical of most Phanerozoic animals; and bradytelic, for "slow" morphological evolution (Simpson, 1944). Included among the bradytelic lineages are so-called living fossils (such as linguloid brachiopods, horseshoe crabs, coelacanth fish, crocodilians, opossums), "groups that survive today and show relatively little change since the very remote time when they first appeared in the fossil record" (Simpson, 1944, p. 125). Simpson's bradytely closely approximates Ruedemann's earlier developed concept of "arrested evolution'' (Ruedemann, 1918, 1922a, 1922b), both based on comparison of modern taxa with fossil forms that are virtually indistinguishable in morphology but are 100 Ma or more older.

Hypobradytely. Recently, a fourth term—hypobradytely—has been added to this list of rate distributions (Schopf, 1987) "to refer to the exceptionally low rate of evolutionary change exhibited by cyanobacte-

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