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1、PuzzlesofEvolutionASINGLEmutationmayhaveclearedthewayforrapidbrainevolution.Otherprimateshavestrongjawmusclesthatexertaforceacrossthewholeskull,constrainingitsgrowth.Butaround2millionyearsagoamutationweakenedthisgripinthehumanline.Abraingrowthspurtbegansoonafter(Nat
2、ure,vol428,p415).Whatdrovethisspurtisanothermatter.Theenvironmentprobablypresentedmentalchallenges.Socialdevelopmentswouldhaveplayedapart,too.Totesttherelativeimportanceofthesepressures,DavidGearyattheUniversityofMissouriinColumbiacomparedtheskullsizeofvarioushomini
3、nsagainstenvironmentalconditionseachlivedin,suchastheestimatedvariationinannualtemperatures,andagainstproxiesforsocialpressure,suchasgroupsize.Bothwereassociatedwithbiggerbrains,butthedifficultiesofnavigatingalargersocialnetworkhadthegreatestimpact(HumanNature,vol20
4、,p67).Abigbrainisincrediblyhungry,soearlyhumansneededtochangetheirdiettosupportit.Thetransitiontoeatingmeatwouldhavehelped.Sowouldtheadditionofseafoodabout2millionyearsago,providingomega-3fattyacidsforbrainbuilding(ProceedingsoftheNationalAcademyofSciences,vol107,p1
5、0002).Cookingmighthavehelpedtoo,byeasingdigestion.Thiswouldhaveallowedancestralhumanstoevolvesmallergutsanddevotethespareresourcestobrainbuilding.Bigbrainscomeataprice,however,includingthedangersofgivingbirth.Bythetimethebenefitsnolongeroutweighedthecosts,wehada1.3k
6、ilogramlumpofjellysmartenoughtoquestionitsownexistence.OURancestorshaveachievedsomeepicmigrations.HomoerectusmadethefirstgreattrekoutofAfricaandintoeastAsia1.8millionyearsago.Aroundamillionyearslater,thepredecessorsofNeanderthalsturnedupinEurope.And125,000yearsago,H
7、omosapiensmadeanearlyforayintotheMiddleEast.Noneofthesepopulationslasted.Butsome65,000yearsago,onegroupofmodernhumansleftAfricaandconqueredtheworld-anextraordinaryachievementforanyspecies,letaloneapuny,furlessape.Whatpossessedthemtospreadsofarandwide?Itmayhavebegunw
8、ithabigsqueeze.Allhumansbelongtooneoffourmitochondriallineages(L0,L1,L2andL3)correspondingtofourancestralmothers,butonlyL3isfoundoutsideAf
