懂你英语Level-4-unit-2

Level4unit2part1ListeningTheearthwasformedbyabout4.6billionyearsago.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.VocabularyThebody’scirculatorysystemincludestheheartandbloodvessels.Theheartpumpsbloodtoallpartsofthebodythroughanetworkofarteriesandveins.Thedigestivesystembreaksdownandabsorbsfoodintothebody.Itsuppliestheenergythatthebodyneedsforgrowthandrepair.Afterdigestion,excessfoodisturnedintowasteandeliminatedfromthebody.Thenervoussystemincludesthebrain,spinalcord,andnerves.Thisisyourbody’scontrolsystem.Itcontrolsyourmusclesandorgans,includingyourcirculatoryanddigestivesystems.Therespiratorysystemincludesthenoseandlungs.Itbringsairandoxygenintothebodyandremovescarbondioxide.Thelungsarewhereoxygenentersthebloodstreamandcarbondioxideisremoved.Theimmunesystemprotectsthebodyagainstinfectionsanddiseases.Sneezingandfeverareexamplesofhowtheimmunesystemworks.Ifaninfectioninvadesthebody,theimmunesystemattacksit.Thenervoussystemregulatesyourheartbeatanddigestion.Forsomediseases,theimmunesystemuseswhitebloodcellsandantibodiestoattackit.Thedigestivesystemiswherefoodisbrokendownandabsorbedintothebody.Thecontrolleduseoffireallowedpeopletocookfoodandsurviveincolderclimates.Fireprovidedlightsothatpeoplecouldworkandsocializeforlongerhours.Withtheinventionofagriculturepeoplecouldgrowfoodandsettleintocommunities.Bygrowingfood,peoplecouldspendlesstimehuntingandmoretimeforotherthingssuchasart.Thedomesticationofanimalsallowedpeopletoraiseanduseanimalsforfoodandwork.Byraisingherdsofanimalsuchassheep,humanshadaregularsupplyofmeatandclothing.Thestrengthofanimalssuchashorseandoxwasusedforworkingtransportation.Thedevelopmentoftoolsandmachineshasresultedintomoreefficientmanufactureandconstruction.Withbettertoolsandmachinesworkcanbedonewithfewerpeopleatmuchlowercost.Thedevelopmentofscienceandengineeringhasimprovedmanythi