Geologic Time Geologists have divided Earth's history into a series of time intervals. These time intervals are not equal in length like the hours in a day. Instead the time intervals are variable in length. This is because geologic time is divided using significant events in the history of the Earth.
The History of Life ~ The Fossil Record Fossil record, history of life as documented by fossils, the remains or imprints of the organisms from earlier geological periods preserved in sedimentary rock.
Evolution of Animals Many questions regarding the origins and evolutionary history of the animal kingdom continue to be researched and debated, as new fossil and molecular evidence change prevailing theories. Some of these questions include the following: How long have animals existed on Earth? What were the earliest members of the animal kingdom, and what organism was their common ancestor? Animal diversity increased during the Cambrian period of the Paleozoic era, 542 million years ago.
Amphibians to Mammals Amphibians: The first major groups of amphibians developed in the Devonian period, around 370 million years ago, from lobe-finned fish which were similar to the modern coelacanth and lungfish.
Reptiles: Reptiles were the first true terrestrial vertebrates. Reptiles were the first animals to lay shelled eggs. These new animals - the reptiles - had evolved from amphibians who lay jelly like eggs. Although the first reptiles laid their shelled eggs on the land, they were still mostly aquatic creatures.
Mammals: The earliest known mammals were the morganucodontids, tiny shrew-size creatures that lived in the shadows of the dinosaurs 210 million years ago. They were one of several different mammal lineages that emerged around that time. All living mammals today, including us, descend from the one line that survived.
Mass Extinctions - The Big Five The big five mass extinctions. The big five mass extinctions. Late Devonian, 375 million years ago, 75% of species lost. End Permian, 251 million years ago, 96% of species lost. End Triassic, 200 million years ago, 80% of species lost. End Cretaceous, 66 million years ago, 76% of all species lost.