We know little about the origin of our atmosphere, just as we know little about the origin of our planet. In the absence of any reliable evidence, one can only speculate. According to cosmologists, our planet most likely originated from the sun about 4.6 billions of years ago in the wake of the latter's encounter with a passing star, following a cosmic event, popularly known as the Big Bang. After its separation from the sun, it started revolving around the sun under the effect of the sun's gravitational pull and rotating about its axis like a fiery ball, surrounded by an extremely hot gaseous envelope which may be called the primordial atmosphere.
However, it may be imagined that the primordial atmosphere at that stage must have been at a great upheaval as it hurled through space and its hot gaseous envelope rapidly cooled, condensed and solidified forming a solid crust in the surface layers after giving out considerable amount of volatile hot gases and vapours from the molten material at the surface. Part of the hot gases and vapours which cooled and condensed into water formed the world's oceans. The remainder formed a gaseous envelope around the planet or was stored in rocks. The atmosphere thus formed had a preponderance of hydrogen and little or no oxygen, so could not support life of the kind that we know on the earth to-day. However, as the earth cooled down further, complex chemical actions and reactions in the crust and the interactions between the crust and the atmosphere gradually led to the formation of an atmosphere which could support an early form of life such as single-celled microbes which required little oxygen for their survival. Such microbial forms of life, also known as blue-green algae, perhaps, first appeared in the oceans where they absorbed carbon dioxide and in the presence of water and sunlight released oxygen by a bio-chemical process known as the green-plant photosynthesis. The accumulation of oxygen in the atmosphere facilitated the evolution of more complex and multi-celled forms of life which we observe on our planet to-day. It is believed that all these developments occurred within the first one billion years of the earth's history and that, since then, our atmosphere has gradually stabilized to its present state.
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