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.
Figures & Tables
Geologists and Ideas
An unusually coherent, well-written volume. Prepared for DNAG by the History of Geology Division of GSA. Spotlights events, ideas, and people, and sheds light on the history of North American geology as a whole. With its many intellectual jewels on the evolution of scientific concepts, this book will provide many happy hours of entertainment and instruction for anyone interested in the history of science, especially that of the earth sciences. Thirty-four papers are organized into four categories: (1) The Evolution of Significant Ideas; (2) Contributions of Individuals; (3) Contributions of Organized Groups; and (4) Application of Significant Ideas. Excellent as a course-book or for additional reading for classes related to the history of geology or general science.