▁in ert ▁g ases , ▁and ▁was ▁lack ing ▁in ▁free ▁o xygen . ▁There ▁is , ▁however , ▁evidence ▁that ▁an ▁o xygen - rich ▁atmosphere ▁existed ▁since ▁the ▁early ▁Ar che an . ▁ ▁At ▁present , ▁it ▁is ▁still ▁believed ▁that ▁mole cular ▁o xygen ▁was ▁not ▁a ▁significant ▁fraction ▁of ▁Earth ' s ▁atmosphere ▁until ▁after ▁photos yn th etic ▁life ▁forms ▁evol ved ▁and ▁began ▁to ▁produce ▁it ▁in ▁large ▁quantities ▁as ▁a ▁by product ▁of ▁their ▁met abol ism . ▁This ▁radical ▁shift ▁from ▁a ▁chem ically ▁in ert ▁to ▁an ▁ox id izing ▁atmosphere ▁caused ▁an ▁e colog ical ▁crisis , ▁sometimes ▁called ▁the ▁o xygen ▁cat ast rop he . ▁At ▁first , ▁o xygen ▁would ▁have ▁quickly ▁combined ▁with ▁other ▁elements ▁in ▁Earth ' s ▁cr ust , ▁primarily ▁iron , ▁removing ▁it ▁from ▁the ▁atmosphere . ▁After ▁the ▁supply ▁of ▁ox id izable ▁surfaces ▁ran ▁out , ▁o xygen ▁would ▁have ▁begun ▁to ▁accum ulate ▁in ▁the ▁atmosphere , ▁and ▁the ▁modern ▁high - ox y gen ▁atmosphere ▁would ▁have ▁developed . ▁Ev idence ▁for ▁this ▁lies ▁in ▁older ▁rocks ▁that ▁contain ▁massive ▁band ed ▁iron ▁form ations ▁that ▁were ▁laid ▁down ▁as ▁iron ▁ox ides . ▁ ▁Sub div isions ▁ ▁A ▁termin ology ▁has ▁evol ved ▁covering ▁the ▁early ▁years ▁of ▁the ▁Earth ' s ▁existence , ▁as ▁radi ometric ▁d ating ▁has ▁allowed ▁real ▁dates ▁to ▁be ▁assigned ▁to ▁specific ▁form ations ▁and ▁features . ▁The ▁P rec amb rian