Abstract James Hutton’s proclamation that he saw “no vestige of a beginning” was an irresistible challenge. Geologists soon began searching downward through die “interminable greywackes” of Murchison for just such vestiges, especially of life. After Logan’s demonstration in the mid-1800s of thick sedimentary successions beneath fossiliferous Cambrian, Dawson described the generally discredited but perhaps in part biosedimentary Eozoon from rocks then thought to be among Canada’s most ancient. Stromatolites eventually became widely accepted as evidence of pre-Phanerozoic life. They are now recorded in carbonate rocks from perhaps 3.5 billion years (Gyr) ago onward. Many early reports also claimed the occurrence of Metazoa in rocks below the Cambrian. All reports turned out to be mistaken or dubious, however, except for those that now define the Ediacarian System which is basal to the Paleozoic Era and, in my view, a part of that era. We see at last why searches for older Metazoa failed. The records of pre-Ediacarian life are paleomicrobiological, sedimentological, and biogeochemical. Microbial evidence is demonstrably present back to ~2 Gyr ago and still convincing back to ~ 2.8 Gyr ago. Stromatolites are presumptive evidence (not proof) for still older life. Geochemical interpretation is consistent with the presence of life back to the oldest known sediments. Metazoa began with the Ediacarian. They were the last main event of biological evolution. Thereafter, it involved primarily elaboration on the multicellular theme. The significant older records are cratonal, edited by the caprices of preservation, erosion, discovery, and subduction.

The sialic plutonism that transformed prevailing petrographic, sedimentary, and paleogeographic styles from Archean to Proterozoic aspects lasted for 600 million years. It created the first extensive cratonal surfaces, with all their potential for interaction among a diversity of microbiotas and their physical surroundings. Earth, in fact, was essentially completed during the Proterozoic. Continental and oceanic crusts and waters achieved their approximate present dimensions and chemistries. O 2 -releasing photosynthesis became established. Life at the cellular level differentiated, culminating in eucaryotic heredity and leading to Metazoa at the Proterozoic-Phanerozoic transition. A kinetic lag between sources and sinks of O 2 eventually created an oxygenous atmosphere, its development being reflected by such sedimentary peculiarities as detrital uraninite, banded-iron-formation, distinctive soil profiles, red beds, and perhaps sedimentary phosphorites and metal sulfides. Combined with biological evidence, the temporal distribution of such materials suggests an O 2 growth curve connecting these provisional points: (1) near zero O 2 until ~2.3 × 10 9 years ago, (2) 1 percent present atmospheric level (PAL) O 2 2-2.3 × 10 9 years ago, (3) ~7 percent PAL 670 × 10 6 years ago, (4) ~ 10 percent PAL 550 × 10 6 years ago, and (5) present atmospheric levels beginning ~ 400 × 10 6 years ago.