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1、实用文档L4Unit2-1Listening物种、环境Theearthwasbornabout4.6millionyearsago.Thefirstlifeappearedaboutabillionyearslater.Thefirstplantdidn’tappearuntilaround2billionyearsago.Untilaround580millionyearsago,lifeformedweresmallandmicroscopic(微笑的).Sincethattime,liveshasevolvedtomanydifferenttypesandspec
2、ies.However,scientistsestimatethatmorethan99%ofthemarenowextinct.Theyhavebeenseveralextinctioneventsinthehistoryoftheearth.Intheextinctionevent,manylifeformsorspeciesoflifedieout.Thecausesofthemassiveextinctioneventsareunsoughtmystery.Theseeventsarenotpredictableandcanhappenatanytime.Sci
3、entiststhinkthatsomeofthatstartedbyasteroidpassorvolcanocorruption.Eventslikethis,canchangetheglobalconditionsthatlifedependson.Onemajorextinctionevent,happenedabout66millionyearsago.Thateventmayhavebeenstartedbyalargeasteroidthathittheearth.Asaresult,thedinosaursandabouthalfoftheallspec
4、iesontheplanet,diedout.Whentheasteroidhittheearth,itstartedachainofevents.Achainofeventisaseriesofeventslinkedtogether,likeachain.Theseparticularchainofeventshappenedoveraperiodofmonthsandyears.Whentheasteroidhittheearth,itcausedhugecloudofdusttoentertheatmosphere.Thiscloudofdustblockedt
5、hesunanddarkentheearthforaperiodofmonths.Withoutenoughsunlight,plantsandplant-eatinganimals,quicklydied.Withthedeathsofsomanyplantsandanimals,thefoodchaincollapsed.Thisledtothedeathsofmanymorespecies.Massextinctioneventshadplayedimportantpartintheevolutionoflife.Infact,someextinctioneven
6、tshadhelpedtoaccelerateevolution.Forexample,somemammalslivedtotheageofdinosaurs,buttheycouldn’tcompete.Whenthedinosaurswereremoved,themammalswereabletocompetemoresuccessfully.Oncetheycouldcompete,mammalevolvedtoahigherstate.Withouttheextinctionevent,perhapswewouldn’tbeheretoday.Scientist
7、estimatethattherehavebeenatleast5massiveextinctionsinthepass540millionyears.Theyalsobelievedthatwehadenteredanewextinctionevent.Speciesarebecomingextinctatamuchfasterratethanbefore.Insteadoftheasteroidandvolcanocorruptions,thecurrenteventistheresultofhumanactivities.Human
