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How scientists are able to calculate the Earth's age and key moments in its history. Earth was first formed 4,500 million years ago, life began 550 million years ago, 420 million years ago plants grew, and dinosaurs became extinct 65 million years ago. It has only been since the 1940s that scientists were able to date the Earth. This is a programme from the ÃÛÑ¿´«Ã½ World Service Archive and was originally broadcast in 1998.