![]() What few clues exist indicate that life first appeared on the planet some 3.9 billion years ago in the form of single-celled organisms. Little is known about this time period since very few fossils or unaltered rocks have survived. The first of these, the Precambrian, extends from about 4.6 billion years ago to 541 million years ago. In contrast, the Carboniferous and Pennsylvanian & amp Mississippian periods all enjoy official status, with the latter pair being more commonly used in the US. The Tertiary period, though it was officially phased out in 2008 by the ICS, remains on the scale in the Guides, since “Tertiary” is present extensively in past literature. The time scale in The TeacherFriendly Guides™ follows that of the International Commission on Stratigraphy (ICS). It spans Earth’s entire history and is separated into four principle divisions. The geologic time scale is an important tool used to portray the history of the Earth-a standard timeline used to describe the age of rocks and fossils, and the events that formed them. Time periods were named after dominant rock types, geography, mountain ranges, and even ancient tribes like the Silurese of England and Wales, from which the “Silurian period” was derived. The names you see for the different periods on the geologic time scale have diverse origins. Rock units were then correlated with similarly aged rock units from around the world. Gradually, geologists discovered evolutionary successions of fossils that helped them determine the relative ages of groups of rocks. Geology as a science grew as geologists studied individual sections of rock. No rock record in any one place contains the complete sequence of rocks from Precambrian to present. How did geologists come up with the timeline for the history of the Earth? Over the course of many years and through the combined work of geologists around the world, the geologic time scale was developed (Figure 1.1).
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