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1、Level4unit2part1ListeningTheearthwasformedbyabout4.6billionyearsago.Thefirstlifeappearedaboutabillionyearslater.Thefirstplaintsdidn’tappearuntilaround2billionyearsago.Untilabout580millionyearsago,lifeformsweresmallandmicroscopic.Sincethattime,lifehasev
2、olvedintomanydifferenttypesorspecies.However,scientistsestimatethatmorethan99%ofthemarenowextinct.-Whendidthefirstplantappearonearth?–about2billionyearsago.TherehavebeenseveralextinctioneventsinthehistoryoftheEarth.Inanextinctionevent,manylifeforms,ors
3、peciesoflife,dieout.Thecausesofmassextinctioneventsareunsolvedmysteries.Theseeventsarenotpredictable,andcanhappenatanytime.Scientiststhinkthatsomeofthemwerestartedbyasteroidimpactsorvolcaniceruptions.Eventslikethiscanchangetheglobalconditionsthatlifede
4、pendson.-Whathappensinanextinctionevent?–Manyspeciesoflifebecomeextinct.Onemajorextinctioneventhappenedabout66millionyearsago.ThateventmayhavebeenstartedbyalargeasteroidthathittheEarth.Asaresult,thedinosaursandabouthalfofallspeciesontheplanet,diedout.W
5、hentheasteroidhittheEarth,itstartedachainofevents.Achainofeventsisaseriesofeventslinkedtogether,likeachain.-Whatmayhavecausedtheextinctionofthedinosaurs?–Anasteroidhittheearthandstartedachainofevents.Thisparticularchainofeventshappenedoveraperiodofmont
6、hsandyears.WhentheasteroidhittheEarth,itcausedahugecloudofdusttoentertheatmosphere.ThiscloudofdustblockedthesunanddarkenedtheEarthformanymonths.Withoutenoughsunlight,plantsandplant-eatinganimalsquicklydied.Withthedeathsofsomanyplantsandanimals,thefoodc
7、haincollapsedThisledtothedeathsofmanymorespecies.-Whatcausedthefoodchaintocollapse?–Thedeathsofmanyplantsandplant-eatinganimals.Massextinctioneventshaveplayedanimportantpartintheevolutionoflife.Infact,someextinctioneventshavehelpstoaccelerateevolution.
8、Forexample,somemammalslivedthroughtheageofthedinosaurs,buttheycouldn’tcompete.Whenthedinosaurswereremoved,themammalswereabletocompetemoresuccessfully.Oncetheycouldcompete,mammalsevolvedtohigherstate.Withoutextinctionevents,perhapswewoul
