懂你英语Level4unit2.docx
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懂你英语Level4unit2
Level4unit2part1
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,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