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1、Level4unit2part1ListeningTheearthwasformedbyabout4.6billionyearsago.Thefirstlifeappearedaboutabillionyearslater.Thefirstplaintsdidn’tappearuntilaround2billionyearsago.Untilabout580millionyearsago,lifeformsweresmallandmicroscopic.Sincethattime,lifehasevolvedintomanydifferenttype
2、sorspecies.However,scientistsestimatethatmorethan99%ofthemarenowextinct.-Whendidthefirstplantappearonearth?–about2billionyearsago.TherehavebeenseveralextinctioneventsinthehistoryoftheEarth.Inanextinctionevent,manylifeforms,orspeciesoflife,dieout.Thecausesofmassextinctioneventsa
3、reunsolvedmysteries.Theseeventsarenotpredictable,andcanhappenatanytime.Scientiststhinkthatsomeofthemwerestartedbyasteroidimpactsorvolcaniceruptions.Eventslikethiscanchangetheglobalconditionsthatlifedependson.-Whathappensinanextinctionevent?–Manyspeciesoflifebecomeextinct.Onemaj
4、orextinctioneventhappenedabout66millionyearsago.ThateventmayhavebeenstartedbyalargeasteroidthathittheEarth.Asaresult,thedinosaursandabouthalfofallspeciesontheplanet,diedout.WhentheasteroidhittheEarth,itstartedachainofevents.Achainofeventsisaseriesofeventslinkedtogether,likeacha
5、in.-Whatmayhavecausedtheextinctionofthedinosaurs?–Anasteroidhittheearthandstartedachainofevents.Thisparticularchainofeventshappenedoveraperiodofmonthsandyears.WhentheasteroidhittheEarth,itcausedahugecloudofdusttoentertheatmosphere.ThiscloudofdustblockedthesunanddarkenedtheEarth
6、formanymonths.Withoutenoughsunlight,plantsandplant-eatinganimalsquicklydied.Withthedeathsofsomanyplantsandanimals,thefoodchaincollapsedThisledtothedeathsofmanymorespecies.-Whatcausedthefoodchaintocollapse?–Thedeathsofmanyplantsandplant-eatinganimals.Massextinctioneventshaveplay
7、edanimportantpartintheevolutionoflife.Infact,someextinctioneventshavehelpstoaccelerateevolution.Forexample,somemammalslivedthroughtheageofthedinosaurs,buttheycouldn’tcompete.Whenthedinosaurswereremoved,themammalswereabletocompetemoresuccessfully.Oncetheycouldcompete,mammalsevol
8、vedtohigherstate.Withoutextinctionevents,perhapswewoul