Geological Era Timeline
= Geological Eras and Life on Earth: the major stages in the history of life on Earth.
URL = http://palaeos.com/timescale/index.html
"The four and a half billion year history of planet Earth is divided into six segments."
Timeline
Chaotian Era / Eon
This era begins with the formation of the Solar System and the Earth from planetesimals in the solar nebula. Mineral evolution begins at this time (although presumably it has already occurred many times in other, early, solar systems.
Hadean Era / Eon
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. This era begins when a Mars-sized body struck the original Earth, pulverizing both, and reforming into the current Earth-moon binary. Gradually the molten Earth cools, outgassing of first atmosphere and oceans, bombardment by left-over planetesimal and debris. The Hadean eon was characterized by extensive volcanism and formation of the first crust. Following a second period of cosmic bombardment, by the end of the Hadean, the Earth had an atmosphere (unbreathable to most organisms today), and oceans filled with water.
Archean Era / Eon
Lasting more than twice as long as the Phanerozoic eon, the Archean was a time when diverse microbial life flourished in the primordial oceans, and the continental shields developed from volcanic activity. The reducing (anaerobic) atmosphere enabled Archaea (anaerobic microbes) to develop, and plate tectonics followed a regime of continental drift different to that of the Proterozoic and later. 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.
Proterozoic Era / Eon
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 Ediacarian biota and finally the appearance of first metazoa.
Paleozoic Era
Early in the 300 million year history of the Paleozoic, atmospheric oxygen reached its present levels, generating the ozone shield that screens out ultraviolet radiation and allows complex life to live in the shallows and finally on land. 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. 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. The Paleozoic was brought to an end by the end Permian mass-extinction, perhaps the most severe extinction the planet has seen.
Mesozoic Era
Lasting little more than half the duration of the Paleozoic, this was a spectacular time. The generalized archosaurian reptiles of the Triassic gave way to the dinosaurs, a terrestrial megafauna the like of which the Earth has not seen before or since. While dinosaurs dominated the land, diverse sea-reptiles ruled the oceans, and invertebrates, especially ammonites, were extremely diverse. Pterosaurs and later birds took to the sky. Mammals however remained small and insignificant. Climatic conditions remained warm and tropical worldwide. The supercontinent of Pangea broke up into Laurasia and Gondwana, with different dinosaurian faunas evolving on each. During this era modern forms of corals, insects, new fishes and finally flowering plants evolved. At the end of the Cretaceous period the dinosaurs and many other animals abruptly died out, quite likely the result of an asteroid impact and associated extensive volcanism (acid rain)
Cenozoic Era
With the extinction of the dinosaurs and the end of the Mesozoic, the mammals swiftly inherit the Earth. Archaic mammals co-existed with birds and modern reptiles and invertebrates. The current continents emerged, and the initial tropical conditions were replaced by a colder drier climate, possibly caused by the Himalayan uplift. The appearance of grass meant the rise of grazing mammals, and the cooler drier world allowed modern mammalian groups to evolve, along with other lineages now extinct and a few archaic hold-overs. Among the newcomers were the anthropoid apes that culminated in the australopithecine hominids of Africa. Decreasing temperatures and a polar landmass of Antarctica resulted in a new Ice Age. Most recently, in the blink of an eye geologically speaking, this era saw the rise of Man (Homo erectus, Neanderthal and Cro-Magnon) and use of stone tools and fire, the extinction of Megafauna, and civilization and human activities that have transformed the globe, but at a cost of great environmental destruction."