ion ▁years , ▁m amm al ian ▁life ▁divers ified . ▁ ▁Several ▁million ▁years ▁ago , ▁a ▁species ▁of ▁small ▁African ▁a pe ▁gained ▁the ▁ability ▁to ▁stand ▁u pr ight . ▁The ▁subsequent ▁advent ▁of ▁human ▁life , ▁and ▁the ▁development ▁of ▁agricult ure ▁and ▁further ▁civil ization ▁allowed ▁humans ▁to ▁affect ▁the ▁Earth ▁more ▁rapidly ▁than ▁any ▁previous ▁life ▁form , ▁affect ing ▁both ▁the ▁nature ▁and ▁quantity ▁of ▁other ▁organ isms ▁as ▁well ▁as ▁global ▁climate . ▁By ▁comparison , ▁the ▁Great ▁O xygen ation ▁Event , ▁produced ▁by ▁the ▁pro l ifer ation ▁of ▁alg ae ▁during ▁the ▁S ider ian ▁period , ▁required ▁about ▁ 3 0 0   mill ion ▁years ▁to ▁cul min ate . ▁ ▁The ▁present ▁era ▁is ▁class ified ▁as ▁part ▁of ▁a ▁mass ▁ext inction ▁event , ▁the ▁Hol oc ene ▁ext inction ▁event , ▁the ▁fast est ▁ever ▁to ▁have ▁occurred . ▁Some , ▁such ▁as ▁E . ▁O . ▁Wilson ▁of ▁Harvard ▁University , ▁predict ▁that ▁human ▁destruction ▁of ▁the ▁b ios phere ▁could ▁cause ▁the ▁ext inction ▁of ▁one - half ▁of ▁all ▁species ▁in ▁the ▁next ▁ 1 0 0   year s . ▁The ▁extent ▁of ▁the ▁current ▁ext inction ▁event ▁is ▁still ▁being ▁research ed , ▁deb ated ▁and ▁calculated ▁by ▁bi olog ists . ▁ ▁At mos phere , ▁climate , ▁and ▁weather ▁ ▁The ▁Earth ' s ▁atmosphere ▁is ▁a ▁key ▁factor ▁in ▁sust aining ▁the ▁e cos ystem . ▁The ▁thin ▁layer