Scientists believe that the Earth is almost five billion years old, which means that the third rock from the Sun has seen a lot of history. Human history has taken up only a small fraction of the Earth's total existence, so scientists use the geologic time scale to divide Earth's overall history into a number of major segments. Much like humans think of periods like the Middle Ages and the Renaissance, scientists break the Earth's geological history into supereons, eons, eras, and periods such as the Precambrian supereon, Mesozoic era, and the Paleogene period.
Geological history has been established with a number of techniques, many of which revolve around stratigraphy, the study of rock strata. In stratigraphy, geologists examine the layers of geologic material which have been deposited over the eons, dating these layers with scientific techniques and using the dates to establish various critical points in time. Rock strata can also be used to track the movement of the Earth's tectonic plates, the age of geological features like mountains, and the overall age of landscapes.
The largest unit of time in geological history is the supereon. Each supereon is broken into a series of smaller eons, which are divided into eras, periods, epochs, and then ages. Dating can get a bit fuzzy with geological history; unlike in human history, where being off by 100 years makes a big difference, geological history deals with such vast spans of time that 100 years is a trivial error. The goal is to provide a rough timeline which geologists can use to establish a framework of events.
A number of useful things can be accomplished with the study of geological history. Being able to date various rock strata, for example, has allowed geologists to date the appearance of historic organisms. Students of paleobiology use this information to determine when organisms first appeared, and to look for information on major evolutionary shifts, such as the first appearance of mammals. Paleobotany and paleoclimatology also take advantage of information on the geologic time scale to determine what the Earth's climate has been like at various points in history, and how long shifts in the climate have taken to occur.
Geologists are also interested in how the Earth formed and changed over the course of geological history. Their studies have explored everything from the periodic reversal of the magnetic poles to the reasons why some elements are rare and others are abundant.
There are occasional disputes about geological history. Imprecise dating has led to challenges about various finds and beliefs, and sometimes not enough information is available to draw clear, incontrovertible conclusions. As with other sciences, the goal of geology is to gather information dispassionately, contributing to overall knowledge of the world, and geological history can sometimes include contradictions or evidence which defies some conclusions and beliefs.