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1、Page8ManofWisdomStartingaboutonemillionyearsago,thefossilrecordshowsanacceleratinggrowthofthehumanbrain.Itexpandedatfirstattherateofonecubicinchofadditionalgraymattereveryhundredthousandyears;thenthegrowthratedoubled;itdoubledagain;andfinallyitdoubledoncemore.F
2、ivehundredthousandyearsagotherateofgrowthhititspeak.Atthattimethebrainwasexpandingataphenomenalrateoftencubicincheseveryhundredthousandyears.Nootherorganinthehistoryoflifeisknowntohavegrownasfastasthis.Whatpressuresgeneratedtheexplosivegrowthofthehumanbrain?Ach
3、angeofclimatethatsetinabouttwomillionyearsagomaysupplypartoftheanswer.AtthattimetheworldbeganitsdescentintoagreatIceAge,thefirsttoafflicttheplanetinhundredsofmillionsofyears.Thetrendtowardcolderweathersetinslowlyatfirst,butafteramillionyearspatchesoficebegantof
4、orminthenorth.Theicepatchesthickenedintoglaciersasmoresnowfell,andthentheglaciersmergedintogreatsheetsofice,asmuchastwomilesthick.Whentheicesheetsreachedtheirmaximumextent,theycoveredtwo-thirdsoftheNorthAmericancontinent,allofBritainandalargepartofEurope.Manymo
5、untainrangeswereburiedentirely.Somuchwaterwaslockeduponthelandintheformoficethattheleveloftheearth’soceansdroppedbythreehundredfeet.Thoseeventscoincidedpreciselywiththeperiodofmostrapidexpansionofthehumanbrain.Isthecoincidencesignificant,orisithappenstance?Thes
6、toryofhumanmigrationsinthelastmillionyearsprovidesacluetotheanswer.AtthebeginningoftheIceAgeHomolivedneartheequator,wheretheclimatewasmildandpleasant.Laterhemovednorthward.FromhisbirthplaceinAfricahemigratedupacrosstheArabianPeninsulaandthenturnedtothenorthandw
7、estintoEurope,aswellaseastwardintoAsia.Page8Whentheseearlymigrationstookplace,theicewasstillconfinedtothelandsinthefarnorth;buteighthundredthousandyearsago,whenmanwasalreadyestablishedinthetemperatelatitudes,theicemovedsouthwarduntilitcoveredlargepartsofEuropea
8、ndAsia.Now,forthefirsttime,menencounteredthebone-chillingblastsoffreezingwindsthatblewoffthecakesoficetothenorth.TheclimateinsouthernEuropehadaSiberianharshnessthen,andsumme
