Arth’s surface became fully solidified within the first billion years of its history. By about 2.5 billion years ago, much of the framework for Earth as we know it today—a planet with distinct internal zonation, an outer skin fractured into a mosaic of mobile plates, well-defined continental and oceanic crustal areas, and mostly covered by water—had emerged from the chaotic events of the Archean. This turning point in Earth’s history marks the beginning of a new eon, the Proterozoic.
Encompassing the time interval from 2.5 billion years ago to 542 million years ago, the Proterozoic Eon represents approximately 43% of geologic time. In contrast to the Archean, whose story written in the rocks is split up, jumbled, and in many places overwritten by later events, the Proterozoic shows a much more orderly pattern of stratigraphic “pages.” Sprawling cratonic areas covered by shallow seas became sites of sedimentary deposition, and mountain-building events were much like those of the younger Phanerozoic. The Proterozoic witnessed Earth’s first glacial episodes and the transition to an oxygen-rich atmosphere-ocean system.
Microbes played a central role in oxygenating the atmosphere, a critical prerequisite for the evolution of animals.
The appearance of complex biologic organisms, such as the Dickinsonia fossils shown here, led to a fascinating new subplot in Earth’s story, one that would ultimately dominate the story. In less than 2 billion years, life on Earth made the transition from minute cells—cryptic except for the stromatolitic structures some cells constructed—to some of the most obvious features of sedimentary strata.
World History
