Geological Periods

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Hadean Era (4,600 to 3,800 million years ago)
The Earth formed as a molten mass about 4,600 million years ago. It took about 800 million years for the molten material on the surface of the Earth to cool sufficiently to form rocks.

Archaean Era (3,800 to 2,600 million years ago)
When the Earth first cooled, it had an atmosphere of ammonia, methane and other gases that would be toxic to life today. Nevertheless, it was in this period that life first appeared on Earth. For the fist billion years, until about 2,500 million years ago, the only form of life on Earth was bacteria. The oldest known fossils are 3,500 million year old microfossils of cyanobacteria from Western Australia.

Stromatalites

Colonies of photosynthesising bacteria formed large mounds known as stromatolites, which have been found as fossils in Western Australia and South Africa. The process still occurs today in Shark Bay, Western Australia. 

  

Proterozoic Era (2,600 to 590 million years ago)
During the Protoozoic period, stable continents began to develop. The most common life form was still bacteria but two two new forms emerged. Achaea are microbes, which can inhabit hostile locations, such as extremely acid, alkaline or saline water. Some live near rift vents on the bottom of the oceans at temperatures well over 100 degrees Centigrade; other are quite abundant among the plankton of the open sea.   

Eucaryote CellAbout 1,800 million years ago, eukaryotic cells first appeared. These are the types of cells which form animals, plants and fungi.

 

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Cambrian Period (590 to 505 million years ago)
Trilobite In the Cambrian period, the first multi-celled animals appeared. This led to an "explosion" of forms. Almost all animal phyla originated within a few million years during the Cambrian period. Marine animals, especially brachiopods and trilobites were the most common.

  

Ordovician Period (505 to 434 million years ago)
During the Ordovician Period, most of the land mass of the Earth was in the super-continent of Gondwana while the northern Hemisphere was almost entirely ocean. The dominant species were marine invertebrates including graptolites, trilobites and brachiopods.

Brachiopod

At the end of the Ordovician Period, Gondwana drifted to the north pole causing enormous glaciers to form and sea levels to drop. The resulted in the first of the "great extinctions" which wiped out 60% of all marine invertebrate genera.
 

CrinoidSilurian Period (434 to 408 million years ago)
During the Silurian Period, the Earth's temperature stabilized and the seas rose again. The first coral reef appeared and jawless fish and crinoids ("sea lilies") proliferated. The earliest fossils of land animals, relatives of centipedes, date from the Silurian Period.
 
Devonian Period (408 to 354 million years ago)
Small plants, such as mosses, had existed since the Ordovician Period, but during the Devonian Period, ferns and seed plants appeared, producing the first forests.

During this Period, the first tetrapods (four-footed animals) emerged onto land. These earliest tetrapods were similar to lungfish.

The first arthropods, in the form of wingless insects and the first ancestors of the spiders also appeared.

At the end of the Devonian Period, the second "great extinction" wiped out 70% of all marine species, including many corals and brachiopods; trilobites became less common. There is some evidence that this was caused by an asteroid hitting the Earth.
 

Carboniferous Period (354 to 286 million years ago)
Scale treeEarly in the Carboniferous Period, crinoids ("sea lilies") and blastoids ("sea buds") became very common. Periods of glaciation during the Carboniferous Period, caused vast amounts of plant material from the forests to be preserved at great pressures under ice and water. This eventually formed the world's great coal deposits. The rising and falling seas also caused the deposit of limestone, composed mostly of the remains of crinoids and lime-encrusted algae, to be deposited. Late in Carboniferous Period, amphibians and reptiles began to become common.
 
Permian Period (286 to 248 million years ago)
In the Permian Period, amphibians and reptiles became larger and ammonites (a type of marine invertebrate) became common.

The Permian Period, and the Paleozoic Era, ended with the third and largest of the "great extinctions". Sea levels declined by as much as 150 m, eliminating vast expanses of shallow marine environments. Huge volcanic eruptions (a million times more powerful then Mt Saint Helens) darkened the Sun and produced greenhouse gasses. Up to 97% of marine species, including all trilobites and brachiopods, most crinoids, bryozoans and ancient corals, 75% of land vertebrates (amphibians and reptiles), up to 97% of leafy land plants and 27 orders of insects.
 

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Triassic Period (248 to 213 million years ago)
Eoraptor While some species, such as ammonites,  which had survived the great Permian extinction, continued through the Triassic Period, new species came to dominate the Earth. These included the conifers, cycads (palm-like plants) and small, saurischian ("lizard-hipped") dinosaurs.

The fourth "great extinction" came at the end of the Triassic Period.
 

Jurassic Period (213 to 144 million years ago)
During the Jurrasic Period, an enormous variety of dinosaurs developed - from huge herbivores feeding on ferns and cycads to small carnivores hunting the herbivores. Ichthyosaurs, as well as fish and squid, swam in the oceans and pterosaurs took to the air. The remains of these dinosaurs form most of the world's oil deposits.

Cretaceous Period (144 to 66.4 million years ago)
Ceratopsian (or "bird-hipped") dinosaurs, such as the huge triceratops and huge carnivorous dinosaurs, like the tyrannosaurus rex, roamed the Earth during the Cretaceous Period. Ammonites, belemnoids (a type of shellfish) and the anccestors of oysters were common. At the same time, the first modern mammals, the first flowering plants, and many types of insects, first appeared.

At the end of the Cretaceous Period, came the fifth and most recent of the "great extinctions", probably caused by an asteroid about 6 miles (10 km) in diameter striking the Earth on the Yucatan coast of Mexico. It resulted in the extinction of the dinosaurs, ichthyosaurs, pterosaurs  and ammonoids.
 

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Paleocene Period (66.4 to 58.0 million years ago)
Following the "great extinction" at the end of the Mesozoic Era, mammals, flowering plants, insects, fish and birds dominated the Earth.

In the Paleocene Period, the first primates evolved.
 

Eocene Period (58.0 to 36.7 million years ago)
Herbivores, including the earliest horses, and primitive whales evolved in the Eocene Period.

Basilosaurus (prinitive whale)

Oligocene Period (36.7 to 23.5 million years ago)
Heptodon In the Oligocene Period, vast grasslands developed and primates evolved in isolation in the "new world"
MastadonMiocene Period (23.5 to 5.3 million years ago)
The first hominids split from primates.

 

Pliocene Period (5.2 to 1.8 million years ago)
In the Pliocene Period, homo habilis began to make use of tools.

Glyptodon

NeanderthalPleistocene Period (1.8 million to 10,000 years ago)
The Pleistocene Period was the "Great Ice Age" when glaciers spread over much of Europe, Asia and North America. Mammals grew larger. Homo sapiens appeared in the Pleistocene Period.
 




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