已领WhyAreMostofUsRightHanded？

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　　Wehumansdon’ttypicallyagreeonallthatmuch，butthereisatleastonethingthatanimpressiveamountofusaccept：whichhandiseasiesttocontrol。Ifyouuseonehandforwriting，youprobablyusethesameoneforeatingaswell，andmostofusaround85ofourspeciespreferourrighthands。Infact，，accordingtoarchaeologistNatalieUominiattheUniversityofLiverpoolintheUK。
　　Lateralisationoflimbusethatis，abiastowardsonesideortheotherusuallybeginsinthebrain。Weknowthatsometasksarelargelycontrolledbybrainactivityinthelefthemisphere，whiletherighthemispheregovernsothertasks。Confusingly，thereissomecrossingofnervesbetweenthebodyandthebrain，whichmeansit’sactuallytheleftsideofthebrainthathasmorecontrolovertherightsideofthebodyandviceversa。Inotherwords，thebrain’slefthemispherehelpscontroltheoperationoftherighthand，eye，legandsoon。
　　Somearguethatthisdivisionofneurologicallabourhasbeenafeatureofanimalsforhalfabillionyears。Perhapsitevolvedbecauseitismoreefficienttoallowthetwohemispherestocarryoutdifferentcomputationsatthesametime。Theleftsideofthebrain，forinstance，mighthaveevolvedtocarryoutroutineoperationsthingslikeforagingforfoodwhiletherightsidewaskeptfreetodetectandreactrapidlytounexpectedchallengesintheenvironmentanapproachingpredator，forinstance。Thiscanbeseeninvariousfish，toadsandbirds，whichareallmorelikelytoattackpreyseenintherighteye。
　　Soitispossible（thoughhardtoprove）thatasourhomininancestorsbeganwalkingontwolegsratherthanfour，freeinguptheirhandstoperformnewtaskslikemakingtools，theywerepredisposedtobeginusingthosehandsdifferently。Or，ascognitivescientistStephanieBracciniandcolleaguesputitinaJournalofHumanEvolutionstudy，astrengtheningofindividualasymmetry〔mayhave〕。
　　Insupportoftheidea，Bracciniandhercolleagues’lookedathandednessinchimpanzees，andfoundthatwhentheapesstandonallfours，theydisplayednorealhandpreferences。Itwasonlywhenforcedtoassumeanuprightstancethatalateralpreferenceemergedalthoughindividualchimpsinthestudywereequallylikelytobelefthandedasrighthanded。
　　Evidently，then，somethingelsewasneededtopushearlyhumansfromalateralpreferenceingeneraltotheextremelyhighlevelsofrighthandednessweseetoday。
　　Weknowroughlywhenthatchangeoccurredfromexperimentsinwhichresearchersmadetheirownversionsofancientstonetoolsusingeithertheirleftorrighthandstochiporknapthetoolintoshape，beforecomparingthemwiththetoolsmadebyearlyhominins。Doingsosuggeststhereisonlylimitedevidencethathominintoolmakersworkingmorethan2millionyearsagowereprimarilyrighthanded。
　　However，stonetoolsthatweremadesome1。5millionyearsagoinKoobiFora，Kenya，bytwoancienthumanspeciesHomohabilisandHomoerectusdoshowsomeevidenceofspecieswiderighthandedness。AndbythetimeaspeciescalledHomoheidelbergensishadappeared，perhapsaround600，000yearsago，therewasaclearrighthandedpreferenceinprehistoricsocieties。WearonthepreservedteethofHomoheidelbergensis，forinstance，suggestthatfoodwasusuallybroughttothemouthwiththerighthand。
　　Thistellsuswhenthatshiftoccurred，butnotwhy。Somehavearguedthatitallcomesdowntolanguage。Justasmostpeoplearerighthandedatrait，remember，controlledbytheleftsideofthebrainsodomostpeopledothebulkoftheirlinguisticprocessingintheirbrain’slefthemisphere。Indeed，thisleftbrainedspecialisationforlanguageisevenmorecommonthanrighthandednesswhichmightsuggestthatasthelefthemisphereevolvedforlanguage，thepreferencefortherighthandmayhaveintensifiedsimplyasasideeffect。ThisiscalledtheHomoloquenshypothesis：lateralisationingeneralwasdrivenbytheevolutionofanupright，bipedalstance，whiletherightwardpreferencewasdriven，sometimelater，bytheevolutionoflanguage。
　　Righthandedness，then，maysimplybeanaccidentalbyproductofthewaymostofourbrainsarewiredup。Butprovingthehypothesisisdifficult，orevenimpossible，sinceitwouldideallyinvolverunningneurologicaltestsonourlongdeadancestors。Tllprobablyneverquiteknowwhatthesequenceofeventswasthatledourspeciestoleansooverwhelminglyontherightsidesofourbodiesandtheleftsidesofourbrains。
　　Asforthelefthandersoutthere？Takeheart！Accordingtoa1977paperinthejournalPsychologicalBulletin，thereisremarkablylittleevidenceforanyassociationoflefthandednesswithdeficit，。Infact，someresearchshowsthatlefthandedfolksmightevenhaveaneasiertimerecoveringfrombraindamage。Andtheirlefthandseemstohavetheadvantageofsurpriseinafight，whichmeanstheycanbebetteratcombatsports。Allofwhichsuggeststhereareadvantagestobreakingfromthenorm。