High School Earth Science: Earth’s History - History of Earth’s Complex Life Forms — Flashcards

The Paleozoic Era follows the Precambrian and is the first era of the Phanerozoic Eon. The Paleozoic lasted from about 540 to 240 million years ago. It is divided into several periods, beginning with the Cambrian and ending with the Permian. Large extinction events separate the periods. After each extinction event, many new life forms evolved.    
During the Cambrian Period, there was a tremendous diversification of marine life forms, commonly called the   
“Cambrian Explosion.” At the start of the period, shallow seas covered most of the land, and every major type of marine organism evolved during this time. Animals also evolved shells and other hard parts in the Cambrian, so from the Cambrian forward, fossils are much more abundant and better preserved. For example, the Burgess Shale formation in the Rocky Mountains of British Columbia, Canada, contains an amazing diversity of Cambrian life forms.    
During the remainder of the Paleozoic Era, several major evolutionary events occurred, beginning with the first fish about 500 million years ago. By about 425 million years ago, the first land plants and fungi had evolved. This was followed by the first insects (400 million years ago), first amphibians (360 million years ago), and first reptiles (300 million years ago).    
As the Paleozoic was reaching its end about 250 million years ago, the largest mass extinction in Earth’s history occurred. It is called the Permian mass extinction. More than 95 percent of marine species and 70 percent of land species went extinct. This was also the only known mass extinction of insects species. The Permian mass extinction appears to have taken place in three pulses with separate causes. Gradual environmental change, an asteroid impact, intense volcanism, and changes in the composition of the atmosphere may each have played a role.