Geologic Time

• 4.6 to 3.8 billion years ago: The Azoic period (also known as the Hadian) of the Precambrian era includes the first 800 million years of Earth's history when the planet was formed from a molten  mass. The oldest rocks found on earth date back to about 3.96 billion years (although older meteorites and moon rocks have been found). Rocks older than 4 billion years are not found since they are either too deep or have been melted and recycled. Recent evidence of life as old as 3.85 billion years will probably push back the end of the azoic even further.
• 2.5 to 3.8 billion years ago: The Archeozoic period of the Precambrian era is when we see the first stirrings of life. The earliest fossils (stromatolytes and spherical bacteria) are from 3.0 to 3.5 billion years old. Chemical fossils (organic compounds that are known to be produced by life processes and are unlikely to be produced through geological processes have been found in rocks as old as 3.85 billion years.
• 570 to 2500 million years ago: During the Proterozoic period of the Precambrian Earth's atmosphere began to accumulate oxygen (essentially photosynthetic waste). We also see our first eucaryotes and multicellular organisms. The first animals show up (the Ediacarans ). A global glaciation event may have wiped out the edicarans and made way for the Cambrian explosion. The supercontinent, Rhodinia began breaking up about 650 MYBP.
• 500 to 570 million years ago. During the Cambrian, the supercontenent, Gondwana, was located near the south pole.. The Cambrian period of the Paleozoic era is a time of great expansion in the numbers of species (the Cambrian explosion). Fossils are much more common than those from the Precambrian and all the major phyla are represented. Many hard-bodied organisms are seen including trilobites, mollusks, corals, and brachiopods (Pictures are here). A mass extinction at the end of the Cambrian wiped out many of the species.
• 425 to 500 million years ago: The Ordivucian Period is the second of the Paleozoic era. Marine invertebrates  were the dominant life form, although a few plants managed to gain a foothold on land (and perhaps a few arthropods too). The first primitive fish called ostrcoderms. with bony armored plates developed in the oceans. North America was warm and occupied a moist tropical environment. A mass extinction occurred during the Ordovician (more on that here).
• 410 to 425 million years ago: During the Silurian period the southern continents were grouped together into a supercontinent (Gonwannaland). Climate varied from ice-covered areas to hot and dry. Terrestrial plants had gained a good foothold and included leafless vascular plants. Animal life was mainly marine (jawed fish and invertebrates) and terrestrial arthropods were well-established.
• 360 to 410 million years ago: North America, Eurasia, and Gondwanaland collided during the Devonian period. Land plants, including the first seed plants) are well-developed by this time (although there are as yet, no flowering plants). Some of the most extraordinary marine animals include ammonites and the fish were well-developed, with scales, modern fins, and jaws. The first air-breathing fish (relatives of the lobe-finned coelocanth) evolved into the first tetrapods   (amphibians). Yet another mass extinction occurred near the end of the Devonian when perhaps 70% of the species were lost.
• 315 to 360 million years ago: The Carboniferous period was originally named for the coal-bearing strata found in England. Some texts include the carboniferous period as part of the Mississippian period.  The dominant plants of the Carboniferous swamps were mainly flowerless, spore-forming plants such as club-mosses, horsetails, and ferns (some as huge as trees). Many insects, and amphibians inhabited the Carboniferous swamps and the first sharks show up in the oceans. Early and later Carboniferous land forms are shown.
• 315 to 285 million years ago: During the Mississippian movement of the earth's crust resulted in the collision of continents to produce the supercontinent, Pangia. The resulting interruption in air and ocean currents resulted in a global ice ace.
• 250 to 285 million years ago: During the early Permian, glaciers spread over much of Pangia and Gondwanaland, and then retreated. Reptiles, with their more advanced jaws, teeth, scales, and reproduction started to take over the vertebrate niche for land animals and we see our first dinosaur-like reptiles. The great forests of spore-producing plants and seed ferns shifted to gymnosperms. At the same time, small mammal-like reptiles called therapsids began their evolution. Yet another mass extinction took place during the Permian, wiping out 90-95% of marine species. More on this extinction event can be found here.
• 200 to 250 million years ago: The Triassic period began with the impact of the largest object ever to hit the earth, along with several other major impacts. The Triassic is the first period of the Mesozoic era. During this time the supercontinent Pangea began to tear apart. The climate was relatively warm and the terrain was dominated by conifers. The first true dinosaurs are seen in this period. The first birds may have evolved near the end of the Triassic. As many as one fourth of all families may have gone extinct during a Triassice mass extinction.
• 135 to 200 million years ago: During the Jurassic, palm-like plants called cycads dominated the forests. The Jurassic Period is when dinosaurs flourished including those in the oceans and air. The first birds definitely were found in the Jurassic. A Jurrasic map is here.
• 65 to 135 million years ago: The Cretaceous is the latest period of the Mesozoic Era. During the early Cretaceous, there were two supercontinents (Laurasia and Gondwanaland) while by the end of the Cretaceous, these large land masses began to drift apart. Many of the genera of trees we have today had developed in the mild climate of the time. Several mass extinctions eventually wiped out the dinosaurs along with 85% of the other terrestrial and marine species.
• 1.6 to 65 million years ago: The Tertiary is the first period of the Cenozoic Era. During this time most of the world's modern plant and animal species, including those leading to Homo sapiens, evolved.
• Present to 1.6 million years ago. We live in the Quaternary period.

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