懂你英语Level 4 unit 2Word格式文档下载.docx

懂你英语Level 4 unit 2Word格式文档下载.docx

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Listening

Theearthwasformedbyabout4.6billionyearsago.Thefirstlifeappearedaboutabillionyearslater.Thefirstplaintsdidn’tappearuntilaround2billionyearsago.Untilabout580millionyearsago,lifeformsweresmallandmicroscopic.Sincethattime,lifehasevolvedintomanydifferenttypesorspecies.However,scientistsestimatethatmorethan99%ofthemarenowextinct.-Whendidthefirstplantappearonearth?

–about2billionyearsago.

TherehavebeenseveralextinctioneventsinthehistoryoftheEarth.Inanextinctionevent,manylifeforms,orspeciesoflife,dieout.Thecausesofmassextinctioneventsareunsolvedmysteries.Theseeventsarenotpredictable,andcanhappenatanytime.Scientiststhinkthatsomeofthemwerestartedbyasteroidimpactsorvolcaniceruptions.

Eventslikethiscanchangetheglobalconditionsthatlifedependson.-Whathappensinanextinctionevent?

–Manyspeciesoflifebecomeextinct.

Onemajorextinctioneventhappenedabout66millionyearsago.ThateventmayhavebeenstartedbyalargeasteroidthathittheEarth.Asaresult,thedinosaursandabouthalfofallspeciesontheplanet,diedout.WhentheasteroidhittheEarth,itstartedachainofevents.Achainofeventsisaseriesofeventslinkedtogether,likeachain.-Whatmayhavecausedtheextinctionofthedinosaurs?

–Anasteroidhittheearthandstartedachainofevents.

Thisparticularchainofeventshappenedoveraperiodofmonthsandyears.WhentheasteroidhittheEarth,itcausedahugecloudofdusttoentertheatmosphere.ThiscloudofdustblockedthesunanddarkenedtheEarthformanymonths.Withoutenoughsunlight,plantsandplant-eatinganimalsquicklydied.Withthedeathsofsomanyplantsandanimals,thefoodchaincollapsedThisledtothedeathsofmanymorespecies.-Whatcausedthefoodchaintocollapse?

–Thedeathsofmanyplantsandplant-eatinganimals.

Massextinctioneventshaveplayedanimportantpartintheevolutionoflife.Infact,someextinctioneventshavehelpstoaccelerateevolution.Forexample,somemammalslivedthroughtheageofthedinosaurs,buttheycouldn’tcompete.Whenthedinosaurswereremoved,themammalswereabletocompetemoresuccessfully.Oncetheycouldcompete,mammalsevolvedtohigherstate.Withoutextinctionevents,perhapswewouldn’tbeheretoday.-Whatmayacceleratetheevolutionofmammals?

–theextinctionofdinosaurs.

Mammalscouldn’tcompetesuccessfullywiththedinosaurs,sotheirevolutionwasblocked.Scientistsestimatedthattherehadbeenatleast5massextinctionsinthelast540millionyears.Theyalsobelievedthatwehaveenteredanewextinctionevent.Speciesarebecomingextinctatamuchfasterratethanbefore.Insteadoftheasteroidorvolcaniceruptions,thecurrenteventisaresultofhumanactivities.

Humanactivities,forexample,haveledtoglobalwarming.-Whatiscausingthecurrentmassextinctionevent?

–humanactivities-Whatdoscientistsbelievehasalreadystarted?

–anewextinctionevent.

Globalwarmingisaresultofanincreasingincarbongasesinoutatmosphere.Thesegasestrapheatfromthesun,sotheEarthisgettingwarmer.Asaresult,sealevelsarerising,andweatherpatternsarechanging.Inaddition,wehumansaredestroyingthehabitatsofmanyplantsandanimals.Cuttingdownforests,andpollutingriversdestroysthehabitatsofmanyformsoflife.-Whyaresealevelsrising?

–Withtheriseintemperature,thepolaricecapsaremelting.

Thereisnolongeranydoubtthatthisishappening.Speciesoflifearedisappearingatanincreasingrate.Globaltemperaturesarerisingandweatherpatternsarechanging.Scientistsarewarningusthatfastactionisnecessarytosaveourfuture.

Ifwedon’tact,wemaygothewayofthedinosaurs.Thisnewextinctioneventiscausedbyhumans.

Vocabulary

Thebody’scirculatorysystemincludestheheartandbloodvessels.Theheartpumpsbloodtoallpartsofthebodythroughanetworkofarteriesandveins.Thedigestivesystembreaksdownandabsorbsfoodintothebody.Itsuppliestheenergythatthebodyneedsforgrowthandrepair.Afterdigestion,excessfoodisturnedintowasteandeliminatedfromthebody.Thenervoussystemincludesthebrain,spinalcord,andnerves.Thisisyourbody’scontrolsystem.Itcontrolsyourmusclesandorgans,includingyourcirculatoryanddigestivesystems.Therespiratorysystemincludesthenoseandlungs.Itbringsairandoxygenintothebodyandremovescarbondioxide.Thelungsarewhereoxygenentersthebloodstreamandcarbondioxideisremoved.Theimmunesystemprotectsthebodyagainstinfectionsanddiseases.Sneezingandfeverareexamplesofhowtheimmunesystemworks.Ifaninfectioninvadesthebody,theimmunesystemattacksit.Thenervoussystemregulatesyourheartbeatanddigestion.Forsomedisease