Nitrogen, Oxygen, Carbon dioxide and Argon are the primary elements that make up the Earth's atmosphere.
With argon, carbon dioxide, helium, neon, and other gases making up just trace amounts, nitrogen and oxygen make up 99 per cent of the gases in dry air. The atmosphere of Earth also contains dust and water vapor. Many of the gases in our atmosphere, according to scientists, were expelled into the atmosphere by early volcanoes. There would have been little to no free oxygen encircling Earth at that period.
The term "free oxygen" refers to oxygen atoms that are not joined to another element, such as carbon (to create carbon dioxide) or hydrogen (to create water). Primitive creatures, most likely bacteria, may have contributed free oxygen to the atmosphere during photosynthesis. A plant or other autotroph uses photosynthesis to convert carbon dioxide and water into food and oxygen.
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