When the earth was born about 4,600 million years ago, the atmosphere was mostly made up of hydrogen, ammonia, methane and water vapour. There was no oxygen. It took 600 million years for the first form of life to appear - simple, single-celled micro organisms. This was the only form of life for the next 2,000 million years. During this time, some of these early cells developed the ability to photosynthesize. This process, as you know, gives out oxygen as a waste product. In course of millions of years an oxygen-rich atmosphere was created which became the source of life for millions of animal species on earth.
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