The 41 contributors to this monograph are members of the Precambrian Paleobiology Research Group (PPRG), a multidisciplinary, international consortium of physical and biological scientists actively investigating the Precambrian history of the earth, and of life. Initially organized more than a decade ago, the PPRG includes geologists and biologists of diverse backgrounds and expertise: sedimentologists, petrologists, geochemists, and atmospheric scientists, microbiologists, phycologists, molecular phylogenists, and paleontologists. In 1979–80–supported by NASA and by funds provided by the National Science Board's 1977 Alan T. Waterman Award to J. W. Schopf–the group worked together at UCLA for 14 months on problems centered on the paleobiology of the Archean, the earlier half of Precambrian time encompassing the initial 40% of the history of the planet (viz., extending from 4500 to 2500 Ma ago). This first PPRG project culminated with publication in 1983 of Earth's Earliest Biosphere (J. W. Schopf, ed.; Princeton University Press), a monographic work concerned with the origin and evolution of the Archean biota and the development of the Archean environment.

Stemming from what was viewed by the group as the success of this initial venture–a remarkable educational experience that both broadened and deepened the perspectives of the two dozen participants–in the mid-1980s the group elected to undertake a follow-on study, a sequel to be devoted to the paleobiology of the Proterozoic, the latter half of Precambrian time extending from the close of the Archean, 2500 Ma ago, to the first appearance in the geologic record of skeletonized invertebrate metazoans, about 550 Ma ago, that marks the beginning of the richly fossiliferous (and paleobiologically well-documented) Phanerozoic Eon of earth history.