懂你英语L4-Unit21ExtinctionEventsLevel4Unit21Listening?ExtinctionEventsTheEarthwasformedabout4.6billionyearsago.Thefirstlifeappearedaboutabillionyearslater.Thefirstplantsdidntappearuntilaround2billionyearsago.Untilabout580millionyearsago,lifeformsweresmallandmicroscopic.Sincethattime,lifehasevolvedintomanydifferenttypes,orspecies.However,scientistsestimatethatmorethan99%ofthemarenowextinct.TherehavebeenseveralextinctioneventsinthehistoryoftheEarth.Inanextinctioneventmanylifeforms,orspeciesoflife,dieout.Thecausesofmassextinctioneventsareunsolvedmysteries.Theseeventsarenotpredictableandcanhappenatanytime.Scientiststhinkthatsomeofthemwerestartedbyasteroidimpactsorvolcaniceruptions.Eventslikethesecanchangetheglobalconditionsthatlifedependson.Onemajorextinctioneventhappenedabout66millionyearsago.ThateventmayhavebeenstartedbyalargeasteroidthathittheEarth.Asaresult,thedinosaursandabouthalfofallspeciesontheplanetdiedout.Whentheasteroidhittheearth,itstartedachainofevents.Achainofeventsisaseriesofeventslinkedtogether,likeachain.Thisparticularchainofeventshappenedoveraperiodofmonthsandyears.WhentheasteroidhittheEarth,itcausedahugecloudofdusttoentertheatmosphere.ThiscloudofdustblockedthesunanddarkenedtheEarthformanymonths.Withoutenoughsunlight,plantsandplant-eatinganimalsquicklydied.Withthedeathsofsomanyplantsandanimals,thefoodchaincollapsed.Thisledtothedeathsofmanymorespecies.Massextinctioneventshaveplayedanimportantpartintheevolutionoflife.Infact,someextinctioneventshavehelpedtoaccelerateevolution.Forexample,somemammalslivedthroughtheageofthedinosaurs,buttheycouldntcompete.Whenthedinosaurswereremoved,themammalswereabletocompetemoresuccessfully.Oncetheycouldcompete,mammalsevolvedtoahigherstate.Withoutextinctionevents,perhapswewouldntbeheretoday.Scientistsestimatethattherehavebeenatleast5massextinctionsinthelast540millionyears.Theyalsobelievethatwehaveenteredanewextinctionevent.Speciesarebecomingextinctatamuchfasterratethanbefore.Insteadofasteroidsorvolcaniceruptions,thecurrenteventistheresultofhumanactivities.Humanactivities,forexample,haveledtoglobalwarming.Globalwarmingistheresultofanincreaseincarbongasesinouratmosphere.Thesegasestrapheatfromthesun,sotheEarthisgettingwarmer.Asaresult,sealevelsarerisingandweatherpatternsarechanging.Inaddition,wehumansaredestroyingthehabitatsofmanyplantsandanimals.Cuttingdownforestsandpollutingriversdestroysthehabitatsofmanyformsoflife.Thereisnolongeranydoubtthatthisishappening.Speciesoflifearedisappearingatanincreasingrate.Globaltemperaturesarerisingandweatherpatternsarechanging.Scientistsarewarningusthatfastactionisnecessarytosaveourfuture.Ifwedontact,wemaygothewayofthedinosaurs.(Thisnewextinctioneventiscausedbyhumans.)BodySystemsLevel4 Unit2 1 VocabularyThebodyscirculatorysystemincludestheheartandbloodvessels.Theheartpumpsbloodtoallpartsofthebodythroughanetworkofarteriesandveins.(Thebloodvesselsthatcarryoxygen-richbloodfromtheheartarecalledarteries.)Thedigestivesystembreaksdownandabsorbsfoodintothebody.Itssuppliestheenergythatthebodyneedsforgrowthandrepair.Afterdigestion,excessfoodisturnedintowasteandeliminatedfromthebody.(Thedigestivesystemiswherefoodisbrokendownandabsorbedintothebody.)Thenervoussystemincludesthebrain,spinalcordandnerves.Thisisyourbodyscontrolsystem.Itcontrolsyourmusclesandorgans,includingyourcirculatoryanddigestivesystem.(Thenervoussystemregulatesyourheartbeatanddigestion.)Therespiratorysystemincludesthenoseandlungs.Itbringsairandoxygenintothebodyandremovescarbondioxide.Thelungsarewhereoxygenentersthebloodstream,andcarbondioxideisremoved.Theimmunesystemprotectsthebodyagainstinfectionsanddiseases.Sneezingandfeverareexamplesofhowtheimmunesystemworks.Ifaninfectioninvadesthebody,theimmunesystemattacksit.GreatDiscoveriesThecontrolleduseoffireallowedpeopletocookfoodandsurviveinthecolderclimates.Fireprovidedlightsothatpeoplecouldworkandsocializeforlongerhours.Withtheinventionofagriculture,peoplecouldgrowfoodandsettleintocommunities.Bygrowingfood,peoplecouldspendlesstimehunting,andmoretimeforotherthingssuchasart.(Withthedevelopmentofagriculture,peoplehadmoretimeforotheractivities.)Thedomesticationofanimalsallowedpeopletoraiseanduseanimalsforfoodandwork.Byraisingherdsofanimalssuchassheep,humanshadaregularsupplyofmeatandclothing.Thestrengthofanimalssuchashorsesandoxenwasusedforworkandtransportation.Thedevelopmentoftoolsandmachineshasresultedinmoreefficient?manufacturingandconstruction.Withbettertoolsandmachines,workcanbedonewithfewerpeopleandatmuchlowercosts.(Theuseofmachineshasreducedthenumberofpeopleneededtomakethings.)Thedevelopmentofscienceandengineeringhasimprovedmanythings,includingcommunications.Withanunderstandingofelectricity,telephonesandradiosallowpeopletocommunicateoverlongdistances.(Scienceandengineeringhavechangedthewaywecommunicatewitheachother.)Level4Unit2Part1DialogueSmartphoneAddictionItookavacationlastnight.Whatareyoutalkingabout?Wheredidyoutakeyourvacation?Ididntgoanywhere.Ijustspendane