懂你英语Level4unit2Word文件下载.docx

懂你英语Level4unit2Word文件下载.docx

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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,thefoodchaincollapsed

Thisledtothedeathsofmanymorespecies.-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