Our present measure of around 4.5 billion years for the Earth's age is only some 50 years old, astonishingly recent considering that scientists have been trying to work it out for well over 200 years. Everyone now knows that dinosaurs lived in Jurassic times and some enthusiasts may know that this period of Earth Time persisted for around 54 million years from 199.6 to 145.5 million years ago, but what of the rest of Earth Time? How is the history of the Earth divided up and dated? Surprisingly few people seem to have much knowledge about the life and times of our blue planet - Earth.

Maybe it is not entirely comfortable to be reminded just how old the planet is and how recent we are by comparison? Perhaps as egocentric humans we do not like to feel diminished in any way? After all, the Inquisition had the Italian philosopher Vanini burned at the stake in 1616 for suggesting that humans might originate from apes, and in 1632 it had Galileo recant his support for Copernicus's claim that the Sun and not the Earth is at the centre of the Solar System. More recently there was a popular belief that the Earth is only some 6000 years old, a belief that persists today in the minds of some fundamentalists.

We are all familiar with subdivisions of historic time based on a variety of events and people such as pharoaic dynasties, political administrations such as the Kennedy years, epochs of colonisation such as Roman Britain and periods of war such as the 1914-18 First World War. By comparison, we have very little knowledge or understanding of the subdivisions of the history of the Earth, or Earth Time as I am calling it.

As we shall see, there is some overlap with history, especially through phases of technological change such as the stone age, iron age and so on. Recent discoveries have pushed the beginning of the stone age back as far as 2.5 million years ago in Africa. In geological terms this is late Pliocene in age, but who, outside the professional world of geology and archaeology, has heard of the Pliocene? Not nearly as many people as are familiar with the Jurassic period of Earth Time, I'll warrant.

So how has the depth of Earth Time been 'plumbed', carved up and named? Who, what or where were the Jurassic, Cambrian, Pliocene and so on? When were these divisions created and how is it that they can be recognised around the world? Nowadays geologists from the USA, Britain and China all use the same major divisions of Earth Time such as Jurassic. However, there has not always been the same degree of unanimity and even today there are still ongoing boundary disputes about many of the finer subdivisions of Earth Time. How is it that a division of Earth Time, based on rock strata forming a series of hills on the western flank of the European Alps, is taken to represent a period of time dominated by extinct dinosaurs?

As we shall see, the basis for the division of Earth Time is not hugely different from historic time. Major eras, for example the Mesozoic, are based on major developments in the history of life and their boundaries are often linked to catastrophic events or relatively sudden changes in the environment, both of which have had a significant impact on life and the raw data of Earth Time - the rocks of the Earth.

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