human evolution

Barrowclough,D.2015.AShortGuidetotheEvolutionofHumanIntelligence:ATimelinefortheEvolutionofHomoSapiensOvertheLastSixtyMillionYears.Cambridge:RedDaggerPress

AShortGuidetotheEvolutionofHumanIntelligence:ATimelinefortheEvolutionofHomoSapiensOvertheLastSixtyMillionYears

DrDavidBarrowcloughFellow,TutorandDirectorofStudiesWolfsonCollege,UniversityofCambridge.Email:dab32@cantab.netABSTRACTThisteachingaidreviewsthemajorevolutionarylandmarksthatoverthelastsixtymillionyearsmarkedthedevelopment of intelligence in Homo sapiens. The discussion begins with social primates and charts theadaptations that ultimately led to the emergence of modern humans including the growth of the brain,increasedmanual dexterity, and evolution of language.Discussion includes consideration of the followinggene mutations: RNF213 GENE, HAR1 GENE, ARHGAP11B GENE, SLC2A1 GENE, SLC2A4 GENE, HACNS1GENE,MYH16GENE,SRGAP2GENE,FOXP2GENE,AMY1GENEKEYWORDSARCHAEOLOGY, BIOLOGICAL ANTHROPOLOGY, HUMAN EVOLUTION, EVOLUTIONARY BIOLOGY, LANGUAGE, GENETICS,HOMOSAPIEN,HOMOHABILIS,AUSTRALOPITHECINES,PRIMATES,CHIMPANZEE,GORILLA,GELADA,MOYAMOYADISEASE,CAROTID ARTERY, HOMININ, WONDERWERK CAVE, SOUTH AFRICA, STARCH DIET, MEAT DIET, RNF213 GENEMUTATIONS, HAR1 GENE, ARHGAP11B GENE, SLC2A1 GENE, SLC2A4 GENE, HACNS1 GENE, MYH16 GENE,SRGAP2GENE,FOXP2GENE,AMY1GENEINTRODUCTIONThe success of thehuman species,markednot least by its development and applicationof technology,resultsfrommillionsofyearsofevolution.Homosapiensarenotablefortheirlargebrains,inrelationtooverallbodysize,whichfacilitatethemanipulationofabstractconceptsandcommunicationofcomplexideas. These evolutionary advantages did not come without cost, and on the ‘down‐side’ large brainsmakepregnancyadifficultandsometimesdangerousprocess.Otherlessobviousbeneficialadaptationsarethelossofmostbodyhairandjawmuscle.Thispaperpresentsanoverviewofthecurrentstateofknowledge,andisprimarilyforstudentstakingintroductory papers. It is meant to supplement text‐books, teaching and reading lists, and focuses onnamed genetic mutations that can be shown to have had an adaptive effect on human intelligence. A‘FurtherReading’sectionisincludedattheendofthereview.SOCIALPRIMATESThe name given to the group that includesmonkeys and humans is the Primates. They first emergedfollowingtheextinctionofthedinosaurs.Afeatureoftheprimates,thatdatestobetweensixtyandthirtymillion years ago, is that many of their members live in highly social groups (FIGURE 1). Group livingrequiresthatindividualsdevelopanawarenessofthosewithwhomtheylive.Itmeansbeingsensitivetofriendships and rivalries, not justbetweenoneself and theother groupmembers, but also interactionsbetweenthirdparties.Processingthiswebofinformationandrememberingwhereeachgroupmemberstandswithinthegrouphierarchymaywellhavedriventhesteadyevolutionofincreasedbrainpower.

FIGURE1.AsocialgroupofGeladaPrimates.THEBRAINOFACOMMONANCESTORThecommonancestorofHumans,chimpanzeesandgorillasisanunknownextinctape.Inthisancestorbetweenfifteenandtenmillionyearsago,agenecalledRNF213beganevolvingrapidly.Thismayhaveboosted the flow of blood to the brain bywidening the carotid artery (FIGURE 2). In humans, RNF213mutationscauseMoyamoyadisease,inwhichthearteryistoonarrow.

FIGURE2.Anatomyofthecarotidartery.

THESPLITFROMAPESOurancestorssplitfromtheirchimp‐likerelativesbetweenthirteenandsevenmillionyearsago(FIGURE3).At first,externallytheywouldhavelookedsimilar,butwithintheircells,changesweretakingplace.Followingthesplit,boththeASPMandARHGAP11Bgenes,andaregioncalledHAR1begantochange.Itisnotcompletelyclearwhatimpactthechangeshad,buttheHAR1regionandARHGAP11Bareinvolvedinthegrowthofthecerebralcortex.Thissuggeststhatthegenemutationswerelinkedtocognitivechanges.

FIGURE3.HumanandGorilla.BRAINPOWERAfter the human lineage split from that of chimpanzees, which occurredwithin the last sevenmillionyears,twogenesmutated:SLC2A1andSLC2A4.Bothofthesegenesbuildproteinsthattransportglucoseinandoutofcells.Thetweaksmadebythegeneticchangesmayhavehadtheeffectofdivertingglucoseaway frommusclesand into theearlyhominins'brains. In thebrain theglucosewouldprovidedmoreenergyboostingthebrains,andallowingthemtoincreaseinsize(FIGURE4).

FIGURE4.Anatomyofbrain.

MANUALDEXTERITYThehandsofHomosapiensareunusuallydextrouscomparedtootherprimates(FIGURE5).Thisdexterityallowedthemtodevelopearlystonetooltechnologiesandlaterwriting.ItappearsfromgeneticstudiesthatthisadaptationisatleastinpartduetomutationsinanareaofDNAcalledHACNS1.Duringthelastsevenmillionyears,sincehumansdivergedfromchimpanzees,thisareaofDNAhasevolvedrapidly.Ourcurrentstateofknowledgeleavesitunclearastotheprecisefunction(s)ofHACNS1,butitisactiveinthearmsandhandsastheydevelop,whichsuggeststhatitislinkedtomanualdexterityinHomosapiens.

FIGURE5.Anatomyofthehumanhandrevealingitsdexterity. SLACKJAWED

Compared toother apeshumans can'tbite veryhard, because theyhaveweak jawmuscles (FIGURE6).ThisisdowntoamutationintheMYH16gene,whichcontrolstheproductionofmuscle,whichhappenedbetween5.3and2.4millionyearsago.Thesmallerjawscouldhavefreedupspaceforthebraintogrow.

FIGURE6.Musclesofthehumanjaw.

MEATFEASTHomo sapiens’ oldest ape‐like ancestors mostly ate fruit, but the later species of Australopithecusdiversifiedtheirdiet,aprocessthatlastedfrom3.5to1.8millionyearsago(FIGURE7)Aswellaseatingawiderrangeofplants,suchasgrasses,theyalsoatealotmoremeat.Archaeologicalevidenceshowsthatthey evenused stone tools to butchermeat.Meat ismore calorific than fruit for the sameweight, andrequires less chewing to digest than vegetables and grasses. The additional calories provided bymeatallowedhumanancestorstoincreasethesizeoftheirbrain,whichconsumeslargeamountsofenergy,aswellasreducingthesizeoftheirjawmuscles.

FIGURE7.Thedevelopmentofmeateating.

THEHAIRLESSAPE

Betweenthreeandfourmillionyearsagohumanancestorslostalmostalltheirbodyhair(FIGURE8).Itisbynomeansclearwhythisoccurred,butthedevelopmentcanbedatedbytheappearanceofpubiclice.Theliceevolvedthreetofourmillionyearsago,andsincetheycouldonlyinfestourancestorsoncetheirotherbodyhairwasgonewecanbesurethatalmostallbodyhairhadbeenshedbythistime.Exposedtothe Sun, the skin darkened, and thus all our ancestors were black, until comparatively recently whensome modern humans left the tropics, and adapted to more northerly latitudes by losing skinpigmentation.

FIGURE8.Humans:thehairlessape.

GENETICINTELLIGENCE

Ofallthegeneticchangesthatmadehumanswhattheyare,itisthosethatarelinkedtointelligencethatare ofmost significance.Research shows that a gene called SRGAP2wasduplicatedno less than threetimes.Asaconsequence,theancestorsofHomosapienshadseveralcopies,someofwhichappeartohaveevolvedfreely.Oneofthemutatedcopiesturnedouttobebetterthantheoriginalasitcausedbraincellsto extrudemore spines, allowing them to formmore synaptic connections, and thus increase cognitiveability(FIGURE9).

FIGURE9.Brainsynapsesfiring.TECHNOLOGICALTHINKING

Modernhumansbelongtothe‘genus’(group)ofanimalsknownasHomo.TheoldestknownHomofossil,found in Ethiopia, is 2.8 million years old (FIGURE 10). The first species was probably Homo habilis,although this has been disputed, and new fossil discoveries are continually made. Compared to theirancestors,thesenewhomininshadmuchlargerbrains.

FIGURE10.HomoHabilis,reconstructionandfossilskull.

BIGHEADSForHomo sapiens, childbirth is difficult and dangerous. Unlike other primates, humanmothers almostalwaysneedhelpwhengivingbirth.Thisisbecauseadaptationstowalkingontwolegshaveincludedanarrowingof thepelvic canal, leaving less space for thebaby topass throughduringbirth (FIGURE11).Furthermore,theincreaseinbrainsize,discussedabove,hasleadtoanincreaseinthesizeoftheheadinmodernhumans.Inordertopartiallycompensatefortheincreaseddifficultyofbirth,humanbabiesareborn whilst still small and helpless, and so need more and longer nursing care compared to otherprimates.

Figure11.Anatomyofpregnant femaleHomosapien.The largeheadof thebabyhas topass through thenarrowbirthcanal,causingpotentialdangertobothmotherandinfant.FIRESTARTERSomething else that separatesHomo fromother primates and from its closest cousins is the ability tocontrol fire(FIGURE12). It isnotknownexactlywhenhumanancestorsfirst learnedtocontrol fire.TheoldestdirectevidencecomesfromWonderwerkCaveinSouthAfrica,whichcontainsashesandburnedbonesfromonemillionyearsago,tutthereisevidencehomininswereprocessing,andpossiblycooking,foodevenearlier.

FIGURE12.Akeydevelopmentalstep,theabilitytocontrolfire.

TALKTHETALK

Unlikeallothergreatapes,humansdonothaveairsacsontheirvocaltracts.Inthegreatapesthesesacsallow them to make loud bellows. However the air sacs also make it impossible to produce differentvowelsounds. It isonlybecausehumanancestors lost thesesacs thatmodernhumansareable to talk.Analysis suggests that the air sacswere lost before humans diverged from their Neanderthal cousins,suggestingtheNeanderthalscouldalsospeak(FIGURE13).

FIGURE13.ThedatasuggeststhatNeanderthals,likemodernhumans,wereabletospeak.GRAMMARANDPRONUNCIATION

A few people havemutations in a gene called FOXP2. As a result they struggle to grasp grammar andpronouncewords. That suggests FOXP2 is crucial for learning andusing language. ThemodernFOXP2evolved in the common ancestor of humans and Neanderthals, about half a million years ago, asNeanderthalFOXP2looksjustlikeours(FIGURE14).

FIGURE14.BothHumansandNeanderthalscouldtalk.

STARCHYFOODSSalivacontainstheenzymeamylase,whichdigestsstarch(FIGURE15).ThisenzymeismadebytheAMY1gene.Studieshaveshownthatmodernhumans,whoseancestorswerefarmers,havemorecopiesoftheAMY1genethanpeoplewhoseancestorscontinuedwithahunter‐gatherer‐fisherwayoflife.Theabilityto digest starch is essential to an urbanway of life, and the increase in frequency of AMY1may havehelpedstartfarming,settledvillagesandmodernsocieties,allofwhichhaveoccurredsincetheendofthelasticeageonlytenthousandyearsago.

FIGURE15.Oneofthemostplentifulsuppliesofstarchinthemoderndietiswheat.DISCUSSIONTheforegoinghasprovidedabriefreviewofthecurrentstateofknowledgeconcerningtheevolutionofintelligenceinmodernhumans,dealinginparticularwithnamedgeneticmutations.Furtherinformationaboutthesemutationscanbefoundbyfollowing‐upthereferencesinthe‘FurtherReading’sectionbelow.FURTHERREADING:GENEMUTATIONSAMY1:http://www.ncbi.nlm.nih.gov/gene/276ARHGAP11B:http://www.ncbi.nlm.nih.gov/gene/89839FOXP2:http://www.ncbi.nlm.nih.gov/gene/93986HACNS1:http://mbe.oxfordjournals.org/content/28/11/3005.fullHAR1:http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2441984/MYH16:http://www.ncbi.nlm.nih.gov/gene/84176RNF213:http://www.ncbi.nlm.nih.gov/gene/57674SLC2A1:http://www.ncbi.nlm.nih.gov/gene/6513SLC2A4:http://www.ncbi.nlm.nih.gov/gene/6517SRGAP2:http://www.ncbi.nlm.nih.gov/gene/23380

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