Scientists believe the earth was formed around 4,600 million years ago. By about 4,000 million years ago, the earth had cooled sufficiently for liquid water to appear, and the first life appeared soon after.
For nearly 3,500 million years, all life was single-celled, but eventually multi-celled life evolved.
During the Paleozoic Era (542 to 248 million years ago), which is itself divided into various periods (Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian), the first fish, and later amphibians, reptiles and mammal-like reptiles, all appeared. There were some disasters during the Paleozoic Era, such as the Ordovician-Silurian mass extinction, and the late Devonian mass extinction, but life continued to evolve and diversify.
At the very end of the Paleozoic Era, there was some terrible disaster (known as the "Permian mass extinction", or the "Permian-Triassic mass extinction") when many creatures became extinct. Some entire groups of animals such as Trilobites and Sea Scorpions were completely wiped out, whereas others, for example, the mammal-like reptiles lost many species.
Next came the Mesozoic Era (248 to 65 million years ago), which is itself divided into three periods:
Additionally, during the Mesozoic, many other types of reptile (which are not generally believed to be closely related to dinosaurs or other archosaurs) returned to the seas. These include Nothosaurs, Placodonts, Thalattosaurs, Ichthyosaurs, Plesiosaurs and Pliosaurs, as well as Mosasaurs.
During the early part of Mesozoic Era, those mammal-like reptiles who had survived from Paleozoic Era dramatically declined, apparently out-competed by the dinosaurs, however they did give rise to the first mammals, which were mostly tiny mouse-sized creatures.
Birds (such as Archaeopteryx) first appear in the fossil-record during the Jurassic period. It is believed that they evolved from dinosaurs (see family tree).
During the Mesozoic Era, there were various extinctions, the largest of which occurred between the Triassic period, and the Jurassic period (known as the "Triassic-Jurassic mass extinction"). At each of these extinctions various animals (including some types of dinossaurs) became extinct, but others then evolved to take the place, and as a whole, the dinosaurs and other large reptiles continued to thrive.
At the very end of the Mesozoic Era, another great disaster occurred (the Cretaceous-Tertiary mass extinction), where the dinosaurs, Pterosaurs, Mosasaurs, Plesiosaurs, Pliosaurs, and many other creatures (such as Ammonites, and Belemnites) were wiped out.
The period since the dinosaurs' extinction is known as the Cenozoic and is divided into 2 periods: the Paleogene period (which is itself divided into the Paleocene, Eocene and Oligocene epochs), and the Neogene period (which is itself divided into the Miocene, Pliocene, Pleistocene and Holocene epochs).
Note: some scientists prefer a classification system where the Neogene period consists of just the Miocene and Pliocene epochs, and the more recent past is term the Quaternary period. Additionally in older classification systems (and therefore in older books), the Paleocene, Eocene, Oligocene. Miocene and Pliocene epochs are placed together as a single geological period known as the Tertiary period.
Here is a chart that shows when some of the main groups of dinosaurs and other prehistoric animals lived:
Note: This diagram is greatly simplified, based in part on speculation, and the depicted dates of branches/evolutionary-splits and extinctions may not always be exactly correct.
During the Mesozoic Era, when dinosaurs ruled the earth, not all dinosaurs lived at the same time. The different times that various types of dinosaurs lived at, is shown in the following chart:
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