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Geologists are scientists who study the structure of rocks and the history of the earth. By looking at and examining layers of rocks and the fossils they contain they are able to tell us what the earth looked like at a certain time in history and what kind of plants and animals lived at that time. Scientists think that the earth probably was formed at the same time as the rest of our solar system, about 4.6 billion years ago. The solar system may have begun as a cloud of dust, from which the sun and the planets evolved. Small particles crashed into each other to create bigger objects, which then turned into smaller or larger planets. Our earth is made up of three basic layers. The centre has a core made of iron and nickel. Around it is a thick layer of rock called the mantle and around that is a thin layer of rock called the crust. When the earth formed over 4 billion years ago it was totally different from the planet we live on today. There were no plants or animals, only rock, desert, water and ice. The atmosphere probably consisted of carbon dioxide and steam with almost no oxygen to breathe. The oldest period of the earth's history lasted from the beginnings four and a half billion years ago to about 600 million years ago. At first simple forms of one-celled life developed in the oceans. Later on bacteria and algae evolved. Towards the middle of the Precambrian, about 2 billion years ago, more complex organisms, sponge-like creatures and soft-bodied animals lived in the seas. During this time there was no life on land because there was not enough oxygen to breathe. As the Precambrian came to an end the oceans were full of life. Plants started absorbing the carbon dioxide from the atmosphere and turned it into oxygen. Early continents formed, but they looked

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