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1、L4Unit2-1Listening物种、环境Theearthwasbornabout4.6millionyearsago.Thefirstlifeappearedaboutabillionyearslater.Thefirstplantdidn’tappearuntilaround2billionyearsago.Untilaround580millionyearsago,lifeformedweresmallandmicroscopic(微笑的).Sincethattime,liveshasevolvedtomanydifferentty
2、pesandspecies.However,scientistsestimatethatmorethan99%ofthemarenowextinct.Theyhavebeenseveralextinctioneventsinthehistoryoftheearth.Intheextinctionevent,manylifeformsorspeciesoflifedieout.Thecausesofthemassiveextinctioneventsareunsoughtmystery.Theseeventsarenotpredictablea
3、ndcanhappenatanytime.Scientiststhinkthatsomeofthatstartedbyasteroidpassorvolcanocorruption.Eventslikethis,canchangetheglobalconditionsthatlifedependson.Onemajorextinctionevent,happenedabout66millionyearsago.Thateventmayhavebeenstartedbyalargeasteroidthathittheearth.Asaresul
4、t,thedinosaursandabouthalfoftheallspeciesontheplanet,diedout.Whentheasteroidhittheearth,itstartedachainofevents.Achainofeventisaseriesofeventslinkedtogether,likeachain.Theseparticularchainofeventshappenedoveraperiodofmonthsandyears.Whentheasteroidhittheearth,itcausedhugeclo
5、udofdusttoentertheatmosphere.Thiscloudofdustblockedthesunanddarkentheearthforaperiodofmonths.Withoutenoughsunlight,plantsandplant-eatinganimals,quicklydied.Withthedeathsofsomanyplantsandanimals,thefoodchaincollapsed.Thisledtothedeathsofmanymorespecies.Massextinctioneventsha
6、dplayedimportantpartintheevolutionoflife.Infact,someextinctioneventshadhelpedtoaccelerateevolution.Forexample,somemammalslivedtotheageofdinosaurs,buttheycouldn’tcompete.Whenthedinosaurswereremoved,themammalswereabletocompetemoresuccessfully.Oncetheycouldcompete,mammalevolve
7、dtoahigherstate.Withouttheextinctionevent,perhapswewouldn’tbeheretoday.Scientistestimatethattherehavebeenatleast5massiveextinctionsinthepass540millionyears.Theyalsobelievedthatwehadenteredanewextinctionevent.Speciesarebecomingextinctatamuchfasterratethanbefore.Insteadofthea
8、steroidandvolcanocorruptions,thecurrenteventistheresultofhumanactivities.Humanacti
