(more on these subdivisions) We know these intervals lasted for millions of years, or even tens or hundreds of millions, because they can be dated them with a fair degree of accuracy according to the amount of radioactivity that occurs in the rocks.We even have a rough idea of geological timescales on the moon and Mars.The Paleozoic was brought to an end by the end Permian mass-extinction, perhaps the most severe extinction the planet has seen.Lasting little more than half the duration of the Paleozoic, this was a spectacular time.During this era, one type of organism, the Cyanobacteria (blue-green algae) produced oxygen as a metabolic by-product; the eventual build-up of this highly reactive gas was to eventually prove fatal to many life-forms, and converted the atmosphere from.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.
Here the four and a half billion year history of planet Earth is divided into six segments, although this is semi-informal classification, mixing eons and eras. The name says it all; a hellish period lasting some 760 million years, when the Earth was subject to frequent bombardment by comets, asteroids, and other planetary debris.
The generally mild to tropical conditions with their warm shallow seas were interspersed with Ordovician and Permo-Carboniferous ice ages.
Towards the end of the Paleozoic the continents clustered into the supercontinent of Pangea, and increasingly aridity meant the end of the great Carboniferous swamps and their unique flora and fauna.
This era witnessed the age of invertebrates, of fish, of tetrapods, and (during the Permian) reptiles.
From the Silurian on, life emerged from the sea to colonize the land, and in the later Paleozoic pteridophyte and later gymnospermous plants flourished.