Scientists divide the Earth's history into five major time spans called eras: Archeozoic, Proterozoic, Paleozoic (the longest one), Mesozoic, and Cenozoic. The first two ones are the earliest, which cover the events from the beginning of the Earth to about 600 million years ago. Until recently it was believed that the Proterozoic was the era before life began, but it was discovered that certain forms of life, such as bacteria and algae existed even then.The Paleozoic ("ancient life") era covers the time span from approximately 600 million to 230 million years ago. This era is subdivided into seven periods which correspond to important geological events during this era; they are Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian. The first fish appeared during the Ordovician period and reptiles during the Mississipian.The Mesozoic ("middle life") era began some 230 million years ago, extending to 75 million years ago. It is divided into the Triassic, Jurassic, and Cretaceous periods. The Triassic and Jurassic were the periods in which the dinosours throve.Finally, the last era is the present one, the Cenozoic ("recent life"), which is divided into the Tertiary and the Quaternary periods, which are in turn subdivided into epochs: Paleocene, Eocene, Oligocene, Miocene, and Pliocene in the Tertiary; and Pleistocene and Holocene, which are the epochs of the Quaternary period of the Cenozoic era. The Andes, the Rocky, the Alps, and the Himalaya mountain ranges arose during the Tertiary orogeny; while the first human beings appeared during the Pleistocene epoch of the Quaternary period of the Cenozoic era.
read more: Time Spans of Earth's History