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The Earth’s atmosphere today is made up of 78% nitrogen (N2), 21% oxygen (O2) and 1% other, including argon, water vapour and carbon dioxide (CO2). This is much more O2 and much less CO2 than in the early atmosphere.
O2 is required for humans and animals to respire, and CO2 is required for photosynthesis. CO2 is also the byproduct of respiration.