TheRiseofThecityofTeotihuacán,whichlayabout50kilometersnortheastofmodern-dayMexicoCity,beganitsgrowthby200-100B.C.Atitsheight,betweenaboutA.D.150and700,itprobablyhadapopulationofmorethan125,000peopleandcoveredatleast20squarekilometers.Ithadover2,000apartmentcomplexes,agreatmarket,alargenumberofindustrialworkshops,anadministrativecenter,anumberofmassivereligiousedifices,andaregulargridpatternofstreetsandbuildings.Clearly,muchnningandcentralcontrolwereinvolvedintheexpansionandorderingofthisgreatmetropolis. Moreover,thecityhadeconomicandperhapsreligiouscontactswithmostpartsofMesoamerica(modernCentralAmericaandMexico).Howdidthistremendousdevelopmenttakece,andwhydidithappenintheTeotihuacánValley?AmongthemainfactorsareTeotihuacán’sgeographiclocationonanaturaltraderoutetothesouthandeastoftheValleyofMexico,theobsidianresourcesintheTeotihuacánValleyitself,andthevalley’spotentialforextensiveirrigation.Theexactroleofotherfactorsismuchmoredifficulttopinpoint―forinstance,Teotihuacán’sreligioussignificanceasashrine,thehistoricalsituationandaroundtheValleyofMexicotowardtheendofthefirstmillenniumB.C.,theingenuityandforesightednessofTeotihuacán’selite,and,finally,theimpactofnaturaldisasters,suchasthevolcaniceruptionsofthelatefirstmillenniumB.C.ThislastfactorisatleastcircumstantiallyimplicatedinTeotihuacán’srise.Priorto200B.C.,anumberofrelativelysmallcenterscoexistedinandneartheValleyofMexico.Aroundthistime,thelargestofthesecenters,Cuicuilco,wasseriouslyaffectedbyavolcaniceruption,withmuchofitsagriculturallandcoveredbylava.WithCuicuilcoeliminatedasapotentialrival,anyoneofanumberofrelativelymodesttownsmighthaveemergedasaleadingeconomicandpoliticalpowerinCentralMexico.Thearchaeologicalevidenceclearlyindicates,though,thatTeotihuacánwasthecenterthatdidariseasthepredominantintheareabythefirstcenturyA.D.ItseemslikelythatTeotihuacán’snaturalresources,alongwiththecityelite’sabilitytorecognizetheirpotential,gavethecityacompetitiveedgeoveritsneighbors.Thevalley,likemanyothercesinMexicanandGuatemalanhighlands,wasrichinobsidian.Thehardvolcanicstonewasaresourcethathadbeeningreatdemandformanyyears,atleastsincetheriseoftheOlmecs(apeoplewhoflourishedbetween1200and400B.C.),anditapparentlyhadasecuremarket.Moreover,recentresearchonobsidiantoolsfoundatOlmecsiteshasshownthatsomeoftheobsidianobtainedbytheOlmecsoriginatednearTeotihuacán.Teotihuacánobsidianmusthavebeenrecognizedasavaluablecommodityformanycenturiesbeforethegreatcityarose.Long-distancetradeinobsidianprobablygavetheeliteresidentsofTeotihuacánaccesstoawidevarietyofexoticgood,aswellasarelativelyprosperouslife.SuchsuccessmayhaveattractedimmigrantstoTeotihuacán.Inaddition,Teotihuacán’selitemayhaveconsciouslyattemptedtoattractnewinhabitants.Itisalsoprobablethatasearlyas200B.C.Teotihuacánmayhaveachievedsomereligioussignificanceanditsshrine(orshrines)mayhaveservedasanadditionalpopulationmagnet.Finally,thegrowingpopulationwasprobablyfedbyincreasingthenumberandsizeofirrigatedfields.ThepictureofTeotihuacánthatemergesisaclassicpictureofpositivefeedbackamongobsidianminingandworking,trade,populationgrowth,irrigation,andreligioustourism.Thethrivingobsidianoperation,forexample,wouldnecessitatemoreminers,additionalmanufacturersofobsidiantools,andadditionaltraderstocarrythegoodstonewmarkets.Allthisledtoincreasedwealth,whichinturnwouldattractmoreimmigrantstoTeotihuacán.Thegrowingpoweroftheelite,whocontrolledtheeconomy,wouldgivethemthemeanstophysicallycoercepeopletomovetoTeotihuacánandserveasadditionstothelabor.Moreirrigationworkswouldhavetobebuilttofeedthegrowingpopulation,andthisresultedinmorepowerandwealthfortheelite.特坎的崛于公元前200到100年前的 公元150到700，它可能有超過12.5的人口至少覆蓋圓20平方公里。它擁有超過2,000座、一座大型市場(chǎng)、大至特坎城與洲的大部分都保持著經(jīng)濟(jì)和的聯(lián)系。 坎地處墨西哥峽谷南部和東部的自然形成的通 心，Cuicuilco遭到火山爆發(fā)的嚴(yán)重影響，其大部分農(nóng)田被巖漿覆蓋了。隨著人(一個(gè)在公元前1200到400繁榮過的名族)的崛起之后就是這樣了，顯然崛起之前，特坎的黑曜巖工具一定已經(jīng)作為極有價(jià)值的商品聞名數(shù)世紀(jì)長(zhǎng)距離的黑曜巖可能就使得特坎的精英們有機(jī)會(huì)得到外來的商品和繁榮的生活，這種成功可能會(huì)吸引到特坎。另外，特坎的們也可能會(huì)有意的吸引新的。也有可能是早在公元前200年前，特坎的就達(dá)到了一定的高度，所以其神殿就是另一種對(duì)的。最后，不 展將需要的礦工，的黑曜巖工具的制造商和的商人將工具運(yùn)往新的市場(chǎng)。所有的這一切導(dǎo)致了的增加，而的增加反過來又會(huì)吸引的人到特坎。而那些掌控者經(jīng)濟(jì)命脈的社會(huì)精英們的力量的增長(zhǎng)就會(huì)為他ExtinctionofthePaleozoicEra334to248millionyearsagoMesozoicEra245to65millionyearsago—JurassicCenozoicEra65millionyearsagotothePaleontologistshavearguedforalongtimethatthedemiseofthedinosaurswascausedbyclimatical tionsassociatedwithslowchangesinthepositionsofcontinentsandseasresultingfromtetectonics.OffandonthroughouttheCretaceous(thelastperiodoftheMesozoicera,duringwhichdinosaursflourished),largeshallowseascoveredextensiveareasofthecontinents.Datafromdiversesources,includinggeochemicalevidencepinseafloorsediments,indicatethattheLateCretaceousclimatewasmilderthantoday’s.Thedayswerenottoohot,northenightstoocold.Thesummerswerenottoowarm,northewinterstoofrigid.Theshallowseasonthecontinentsprobablybufferedthetemperatureofthenearbyair,keeitrelativelyconstant.AttheendoftheCretaceous,thegeologicalrecordshowsthattheseseawaysretreatedfromthecontinentsbackintothemajoroceanbasins.Nooneknowswhy.Overaperiodofabout100,000years,whiletheseaspulledback,climatesaroundtheworldbecamedramaticallymoreextreme:warmerdays,coolernights;hottersummers,colderwinters.Perhapsdinosaurscouldnottoleratetheseextremetemperaturechangesandbecameextinct.Iftrue,though,whydidcold-bloodedanimalssuchassnakes,lizards,turtles,andcrocodilessurvivethezingwintersandtorridsummers?Theseanimalsareatthemercyoftheclimatetomaintainalivablebodytemperature.It’shardtounderstandwhytheywouldnotbeaffected,whereasdinosaurswerelefttoocrippledtocope,especiallyif,assomescientistsbelieve,dinosaurswerewarm-blooded.CriticsalsopointoutthattheshallowseawayshadretreatedfromandadvancedonthecontinentsnumeroustimesduringtheMesozoic,sowhydidthedinosaurssurvivetheclimaticchangesassociatedwiththeearlierfluctuationsbutnotwiththisone?Althoughinitiallyappealing,thehypothesisofasimpleclimaticchangerelatedtosealevelsisinsufficienttoexinallthedata.Dissatisfactionwithconventionalexnationsfordinosaurextinctionsledtoasurprisingobservationthat,inturn,hassuggestedanewhypothesis.ManyntsandanimalsdisappearabruptlyfromthefossilrecordasonemovesfromlayersofrockingtheendoftheCretaceousupintorocksrepresentingthebeginningoftheCenozoic(theeraaftertheMesozoic).BetweenthelastlayerofCretaceousrockandthefirstlayerofCenozoicrock,thereisoftenathinlayerofclay.Scientistsfeltthattheycouldgetanideaofhowlongtheextinctionstookbydetermininghowlongittooktodepositthisonecentimeterofclayandtheythoughttheycoulddeterminethetimeittooktodeposittheclaybydeterminingtheamountoftheelementiridium(Ir)itcontained.IrhasnotbeencommonatEarth’ssincetheverybeginningofthenet’sBecauseitusuallyexistsinametallicstate,itwaspreferentiallyincorporatedEarth’scoreasthenetcooledandconsolidated.Irisfoundinhighinsomemeteorites,inwhichthesolarsystem’soriginalchemicalcompositionp.Eventoday,microscopicmeteoritescontinuallybombardEarth,fallingonbothlandandsea.BymeasuringhowmanyofthesemeteoritesfalltoEarthoveragivenperiodoftime,scientistscanestimatehowlongitmighthavetakentodeposittheobservedamountofIrintheboundaryclay.Thesecalculationssuggestthataperiodofaboutonemillionyearswouldhavebeenrequired.However,otherreliableevidencesuggeststhatthedepositionoftheboundaryclaycouldnothavetakenonemillionyears.SotheunusuallyhighconcentrationofIrseemstorequireaspecialexInviewofthesefacts,scientistshypothesizedthatasinglelargeasteroid,about10to15kilometersacross,collidedwithEarth,andtheresultingfalloutcreatedtheboundaryclay.Theircalculationsshowthattheimpactkickedupadustcloudthatcutoffsunlightforseveralmonths,inhibitingphotosynthesisinnts;decreasedsurfacetemperaturesoncontinentstobelowzing;causedextremeepisodesofacidrain;andsignificantlyraisedlong-termglobaltemperaturesthroughthegreenhouseeffect.Thisdisruptionoffoodchainandclimatewouldhaveeradicatedthedinosaursandotherorganismsinlessthanfiftyyears.

恐龍的有人明白為什么。大100000年內(nèi)，海洋收縮了，世界的氣候也隨之變得更這種嚴(yán)峻的氣溫變化因而。難理解它們?yōu)槭裁春敛皇苡绊懀欢铸垍s如此的以至于無法適應(yīng)，尤其是有些科學(xué)家認(rèn)為恐龍是熱血?jiǎng)游铩Ｅu(píng)者們也淺海在中生代曾有過無數(shù)次的 米厚的粘土層中元素銥的含量來推測(cè)其的沉積時(shí)間，進(jìn)而推測(cè)大所用的時(shí)表明撞擊揚(yáng)起的灰塵遮擋了陽(yáng)光達(dá)幾個(gè)月之久，了植物的光合作用，將陸地50年的時(shí)RunningWateronPhotographicevidencesuggeststhatliquidwateronceexistedingreattyonthesurfaceofMars.Twotypesofflowfeaturesareseen:runoffchannelsandoutflowchannels.Runoffchannelsarefoundinthesouthernhighlands.Theseflowfeaturesareextensivesystems—sometimeshundredsofkilometersintotallength—ofinterconnecting,twistingchannelsthatseemtomergeintolarger,widerchannels.TheybearastrongresemblancetoriversystemsonEarth,andgeologiststhinkthattheyaredried-upbedsoflong-goneriversthatoncecarriedrainfallonMarsfromthemountainsdownintothevalleys.RunoffchannelsonMarsspeakofatime4billionyearsago(theageoftheMartianhighlands),whentheatmospherewasthicker,thesurfacewarmer,andliquidwaterwidespread.OutflowchannelsareprobablyrelicsofcatastrophicfloodingonMarslongago.Theyappearonlyinequatorialregionsandgenerallydonotformextensiveinterconnectednetworks.Instead,theyareprobablythepathstakenbyhugevolumesofwaterdrainingfromthesouthernhighlandsintothenorthernins.Theonrushingwaterarisingfromtheseflashfloodslikelyalsoformedtheoddteardrop-shaped“islands”(resemblingtheminiatureversionsseeninthewetsandofourbeachesatlowtide)thathavebeenfoundontheinsclosett dsoftheoutflowcannels.Judgingfromthewidthanddepthofthechannels,theflowratesmusthavebeentrulyenormous—perhapsasmuchasahundredtimesgreaterthanthe105tonspersecondcarriedbythegreatAmazonriver.Floodingshapedtheoutflowchannelsapproximay3billionyearsago,aboutthesametimesasthenorthernvolcanicinsformed.SomescientistsspeculatethatMarsmayhaveenjoyedanextendedearlyPeriodduringwhichrivers,lakes,andperhapsevenoceansadorneditssurface.A2003MarsGlobalSurveyorimageshowswhatmissionspecialiststhinkmaybeadelta—afan-shapednetworkofchannelsandsedimentswhereariveronceflowedintoalargerbodyofwater,inthiscasealakefillingacraterinthesouthernhighlands.Otherresearchersgoevenfurther,suggestingthatthedataprovideevidenceforlargeopenexpensesofwaterontheearlyMartiansurface.Acomputer-generatedviewoftheMartiannorthpolarregionshowstheextentofwhatmayhavebeenanancientoceancoveringmuchofthenorthernlowlands.TheasBasin,whichmeasuressome3,000kilometersacrossandhasafloorthatliesnearly9kilometersbelowthebasin’srim,isanothercandidateforanancientMartiansea.Theseideasremaincontroversial.Proponentspointtofeaturessuchastheterraced“beaches”showninoneimage,whichcouldconceivablyhavebeenleftbehindasalakeoroceanevaporatedandtheshorelinereceded.Butdetractorsmaintainthattheterracescouldalsohavebeencreatedbygeologicalactivity,perhapsrelatedtothegeologic sthatdepressedtheNorthernHeispherefarbelowthelevelofthesouth,inwhichcasetheyhavenothing