Life on Earth can be traced back over three thousand million years into the past. Many examples of the Earth’s past inhabitants are to be found in rocks, preserved as beautiful and fascinating fossils. The earliest life forms were bacteria and algae; these produced the oxygen that enabled more complex life forms to develop. About 600 million years ago multi-cellular organisms appeared on Earth, some of which could protect themselves with hard parts such as shells. Many of these life forms were .
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