Based on examinations of rocks worldwide, geologists have constructed a timescale that sets in sequence the main events in the history of the Earth. The timescale suggests that Earth was formed about 4.6 billion years ago.
To understand how the geologists constructed this timescale imagine that newspapers were printed and discarded every day since the planet was formed. These newspapers were designed as we know them today in all but one way - the issue date was never printed.
As the Earth aged, the newspapers accumulated in an ever-deepening pile. The oldest newspapers lie at the bottom, the youngest at the top and the relative age of any newspaper in between is roughly proportional to its depth in the pile.
Now imagine that you made a detailed study of the newspapers in the pile, noting the weather reports, types of goods advertised, pictures of flora and fauna etc, characteristically found at different depths in the pile. If someone pulled a newspaper at random from the pile, you could accurately tell from what depth the paper came by comparing its news and other content with your observations which correlate newspaper content with depth in pile.
You now know the sequence in which events unfolded on Earth, but you cannot tell how many years ago any particular event happened - the newspapers have no dates. To do this, you must be able to match certain events reported in the newspapers with absolute time counted backwards from the present.
For example, the newspaper's Science Today pages will have faithfully recorded the occurrence of astronomical events such as cometary appearances, solar eclipses, asteroid visits etc, the dates of whose occurrences you can accurately determine from independent calculations. Once you can do this for several deep, intermediate and shallow depths in the newspaper pile, you can estimate the ages of intervening depths by extrapolation.
The key for moving from the newspaper analogy to the geological timescale is that the layers of newspapers stand for layers of rock in the Earth's crust. The details printed in the newspapers stand for the detailed characteristics, principally the fossil record, present in the rocks. Sedimentary rocks such as limestone form when mineral and organic fragments are deposited and compacted by water or wind.
Early geologists recognised that these rocks are deposited in layers, called strata, and the deeper the rock lies underground the older it is. Igneous rocks such as granite form when magma (molten rock) rises from below the crust to the surface and solidifies. It can cool as strata, but is often arranged less regularly. By examining the strata, geologists have established the stratigraphic ages of Earth, the order of events which have occurred since the beginning. This is analogous to ordering the sequence of human historical eras: the Georgian era preceded the Victorian era, which preceded the Edwardian era.
This does not tell us how long ago these eras occurred, but geologists have also worked this out and established the chronological ages. They have relied principally on radiometric methods to measure the absolute ages of rocks, which measure the residual radioactivity of certain elements in the rock.
Radioactive elements are unstable and break down to form different elements or daughter products in a predictable way. The rate of breakdown is measured in terms of half life, i.e. the time required for half of any given number of radioactive atoms to break down. Uranium in igneous rocks has a half life of 4.5 billion years and eventually breaks down to lead. By measuring the ratio of uranium to lead present in the rocks today, it is possible to calculate its age.
Geologists have divided the ages of Earth into broad aeons, subdivided into eras, subdivided into periods. Formed about 4.6 billion years ago, its oldest rocks are about 4 billion years old and it is estimated that life began about 3.8 billion years ago.
Early geologists could see no fossil evidence of life before 540 million years ago and chose this time to mark the start of the Cambrian period. The entirety of earlier time was called Precambrian. Geologists later found earlier signs of life in Precambrian rocks. The Cambrian period, lasting only 35 mil lion years, produced an amazing burst of evolution during which creatures emerged showing all the basic invertebrate body plans of present life on Earth.
The rock strata do not lie in conveniently identical layers everywhere around the globe. How do geologists match layers of equivalent age found in different parts of the world? The most convenient way is to examine the fossils preserved in the rocks. Fossils are the remains of biological organisms that were living on Earth when the rocks formed. The vast majority of biological species which arose on Earth became extinct after a time.
For example, trilobites, small sea creatures which look like woodlice, thrived from about 540 to 505 million years ago. Therefore any rocks which contain trilobite fossils must have been formed during this period. The trilobites are widely used for identifying the Cambrian period.
Unravelling the details of the physical history of Earth is a milestone in the history of science. The story is still far from complete. For example, it was only in the 1970s that a global iridium-rich layer of dust was discovered in the rock strata. This is widely interpreted now as the telltale signal of a massive asteroid impact with Earth 65 million years ago, resulting in the extinction of the dinosaurs.
William Reville is a senior lecturer in biochemistry and director of microscopy at University College Cork.