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Sorry, this answer is incorrect, nitrogen is the major component in the atmospheric gas mixture. The Earth's atmosphere includes several layers: the troposphere, the stratosphere with the ozone layer, the mesosphere and outermost ionosphere where most atoms are ionized and where the aurora takes place. The atmosphere only contains 20% dioxygen, 79% dinitrogen and 1% other gases including ozone. Oxygen is an essential component of the Earth's atmosphere and is a prerequisite for life as we know it. Almost all of the oxygen in the atmosphere was and is produced by cyanobacteria (blue-green algae). These architects of today's atmosphere are also the oldest known fossils (more than 3.5 billion years old). Cyanobacteria are an essential part of the Earth's ecosystem and one of the largest and most important groups of bacteria.
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