Question

Which highly reactive gas was probably absent from the Earth's primitive atmosphere? O2 (oxygen gas) water vapor methane carbon dioxide

Answer 1

Oxygen gas was most likely absent from Earth's primitive atmosphere.  The current theory is that the Earth's early atmosphere was composed of mainly carbon dioxide and methane due to the high volcanic activity.  Cyanobacteria and their use of photosynthesis was what caused earth's atmosphere to become oxygen enriched.
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