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Actually, oxygen was in the atmosphere long before plants evolved. Anaerobic prokaryotes produce oxygen as a waste product. Since oxygen is a deadly poison, especially to anaerobes, the atmosphere they created killed them of in all but a few remote locations.
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But the atmosphere only gained the oxygen for land life forms to appear after the first plants began colonising it and oxygenating the air. I'm not saying oxygen wasn't present, just not nearly as much until this point.
http://www.tenchford.com/_images/Hobbitvisit.jpg