The Proterozoic Eon of Precambrian Time:
2500 to 542 million years ago
|Up: Geological Timescale|
|Previous: Archean | Next: Phanerozoic|
The Proterozoic, which lasted even longer than the Archean Era, saw the atmosphere changes from reducing to oxygenated, driving the original anaerobic inhabitants of the Earth into a few restricted anoxic refuges and enabling the rise of aerobic life (both prokaryote and the more complex eukaryotic cell, which requires the high octane boost that oxygen enables.) Stromatolites (colonial cyanobacteria), which had appeared during the Archean, were common. The modern regime of continental drift began, and saw the formation of supercontinent of Rodinia, and several extensive ice ages. Late in the Proterozoic a runaway icehouse effect meant that the preceding warm conditions were replaced by a "Snowball Earth" with ice several kilometers deep covering the globe. Warming conditions saw the short-lived Edicarian biota and finally the appearance of first metazoa.
The Proterozoic or "First Life" was originally the third and youngest era of the Precambrian Eon, but has more recently been reclassified as an eon. It includes the period from about 2500 to a little over 540 million years ago. During this very long span of Earth's history, the atmosphere switched from reducing to oxygenated, and new types of microorganisms, and the first multicelluar life, evolved. The Proterozoic also includes several periods of severe glaciation, including one period - Snowball Earth - when it is thought that the Earth was covered in a layer of ice several kilometers thick. It was in or following these harsh conditions that the first animal life appeared.
| Proterozoic eon|
| Paleoproterozoic era|
| Mesoproterozoic era|
| Neoproterozoic era|
The Proterozoic is, roughly speaking, the time when plate tectonics began to govern over other processes in determining the form of the Earth's crust. Although continents were small, they consisted of stable cratons. Mid-ocean spreading ridges did a good deal of the moving, just as they do today. However, everything happened a good deal faster. The magma on which the continents floated was hotter, less viscous, and closer to the surface. Hot spots were probably hotter. The continents moved more swiftly, collided more often and tended to fracture or suture with greater frequency.
Life developed from the infant stage of single celled organisms to an adolescence of Eukarya and early plants, fungi, animals. Perhaps other forms developed as well which we know less about because they failed to explode in the Cambrian Explosion. Like all other adolescents, Life grew much larger, discovered sex, and changed its mind frequently about what it was going to be when it grew up. Undoubtedly it tried out many forms and lifestyles which, had we learned of them at the time, we would have sternly disapproved. Life engaged in risky behaviors, such as carelessly spewing so much oxygen into the atmosphere that it nearly poisoned itself until it learned to adapt. It moved out from the warm geothermal vents where, perhaps, it was raised, and nearly froze to death once or twice by wandering into very serious Ice Ages without its mittens. Somehow, in spite of a number of these very close calls, it grew up into the sort of grown-up Life we know today.
 Veevers, J.J.(Ed.), Billion-year Earth History of Australia and neighbours in Gondwanaland. GEMOC Press, 2000.
Credits: ATW040930 public domain Palaeos com, Transferred with minor modifications MM060920 Palaeos org, Intro MAK060920