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CHAPTERTHREE HANDYGENES WhilemycolleaguesandIwerediggingupthefirst TiktaalikintheArcticinJuly2004,RandyDahn,a researcherinmylaboratory,wassweatingitoutonthe SouthSideofChicagodoinggeneticexperimentsonthe embryosofsharksandskates,cousinsofstingrays.You’ve probablyseensmallblackeggcases,knownasmermaid’s purses,onthebeach.Insidethepurseoncelayaneggwith yolk,whichdevelopedintoanembryonicskateorray.Over theyears,Randyhasspenthundredsofhours experimentingwiththeembryosinsidetheseeggcases, oftenworkingwellpastmidnight.Duringthefateful summerof2004,Randywastakingthesecasesand injectingamolecularversionofvitaminAintotheeggs. Afterthathewouldlettheeggsdevelopforseveralmonths untiltheyhatched. Hisexperimentsmayseemtobeabizarrewaytospend thebetterpartofayear,letaloneforayoungscientistto launchapromisingscientificcareer.Whysharks?Whya formofvitaminA? 61 Tomakesenseoftheseexperiments,weneedtostep backandlookatwhatwehopetheymightexplain.Whatwe arereallygettingatinthischapteristherecipe,writtenin ourDNA,thatbuildsourbodiesfromasingleegg.When spermfertilizesanegg,thatfertilizedeggdoesnotcontain atinyhand,forinstance.Thehandisbuiltfromthe informationcontainedinthatsinglecell.Thistakesustoa veryprofoundproblem.Itisonethingtocomparethe bonesofourhandswiththebonesinfishfins.What happensifyoucomparethegeneticrecipethatbuildsour handswiththerecipethatbuildsafish’sfin?Tofind answerstothisquestion,justlikeRandy,wewillfollowa trailofdiscoverythattakesusfromourhandstothefinsof sharksandeventothewingsofflies. Aswe’veseen,whenwediscovercreaturesthatreveal differentandoftensimplerversionsofourbodiesinside theirown,awonderfullydirectwindowopensintothe distantpast.Butthereisabiglimitationtoworkingwith fossils.Wecannotdoexperimentsonlong-deadanimals. Experimentsaregreatbecausewecanactuallymanipulate somethingtoseetheresults.Forthisreason,mylaboratory issplitdirectlyintwo:halfisdevotedtofossils,theother halftoembryosandDNA.Lifeinmylabcanbe schizophrenic.ThelockedcabinetthatholdsTiktaalik specimensisadjacenttothefreezercontainingour preciousDNAsamples. ExperimentswithDNAhaveenormouspotentialto revealinnerfish.Whatifyoucoulddoanexperimentin 62 whichyoutreatedtheembryoofafishwithvarious chemicalsandactuallychangeditsbody,makingpartofits finlooklikeahand?Whatifyoucouldshowthatthegenes thatbuildafish’sfinarevirtuallythesameasthosethat buildourhands? Webeginwithanapparentpuzzle.Ourbodyismadeup ofhundredsofdifferentkindsofcells.Thiscellulardiversity givesourtissuesandorganstheirdistinctshapesand functions.Thecellsthatmakeourbones,nerves,guts,and soonlookandbehaveentirelydifferently.Despitethese differences,thereisadeepsimilarityamongeverycell insideourbodies:allofthemcontainexactlythesameDNA. IfDNAcontainstheinformationtobuildourbodies,tissues, andorgans,howisitthatcellsasdifferentasthosefoundin muscle,nerve,andbonecontainthesameDNA? TheanswerliesinunderstandingwhatpiecesofDNA (thegenes)areactuallyturnedonineverycell.Askincellis differentfromaneuronbecausedifferentgenesareactive ineachcell.Whenageneisturnedon,itmakesaprotein thatcanaffectwhatthecelllookslikeandhowitbehaves. Therefore,tounderstandwhatmakesacellintheeye differentfromacellinthebonesofthehand,weneedto knowaboutthegeneticswitchesthatcontroltheactivityof genesineachcellandtissue. Here’stheimportantfact:thesegeneticswitcheshelpto assembleus.Atconception,westartasasinglecellthat containsalltheDNAneededtobuildourbody.Theplanfor thatentirebodyunfoldsviatheinstructionscontainedin 63 thissinglemicroscopiccell.Togofromthisgeneralizedegg celltoacompletehuman,withtrillionsofspecializedcells organizedinjusttherightway,wholebatteriesofgenes needtobeturnedonandoffatjusttherightstagesof development.Likeaconcertocomposedofindividualnotes playedbymanyinstruments,ourbodiesareacomposition ofindividualgenesturningonandoffinsideeachcell duringourdevelopment. GenesarestretchesofDNAcontainedineverycellof ourbodies. Thisinformationisaboontothosewhoworkto understandbodies,becausewecannowcomparethe activityofdifferentgenestoassesswhatkindsofchanges areinvolvedintheoriginofneworgans.Takelimbs,for example.Whenwecomparetheensembleofgenesactivein thedevelopmentofafishfintothoseactiveinthe developmentofahumanhand,wecancataloguethegenetic differencesbetweenfinsandlimbs.Thiskindof comparisongivesussomelikelyculprits—thegenetic 64 switchesthatmayhavechangedduringtheoriginoflimbs. Wecanthenstudywhatthesegenesaredoinginthe embryoandhowtheymighthavechanged.Wecanevendo experimentsinwhichwemanipulatethegenestoseehow bodiesactuallychangeinresponsetodifferentconditions orstimuli. Toseethegenesthatbuildourhandsandfeet,weneed totakeapagefromascriptfortheTVshowCSI:CrimeScene Investigation—startatthebodyandworkourwayin.We willbeginbylookingatthestructureofourlimbs,andzoom allthewaydowntothetissues,cells,andgenesthatmake it. MAKINGHANDS Ourlimbsexistinthreedimensions:theyhaveatopanda bottom,apinkysideandathumbside,abaseandatip.The bonesatthetips,inourfingers,aredifferentfromthe bonesattheshoulder.Likewise,ourhandsaredifferent fromonesidetotheother.Ourpinkiesareshaped differentlyfromourthumbs.TheHolyGrailofour developmentalresearchistounderstandwhatgenes differentiatethevariousbonesofourlimb,andwhat controlsdevelopmentinthesethreedimensions.WhatDNA actuallymakesapinkydifferentfromathumb?Whatmakes ourfingersdistinctfromourarmbones?Ifwecan understandthegenesthatcontrolsuchpatterns,wewillbe 65 privytotherecipethatbuildsus. Allthegeneticswitchesthatmakefingers,armbones, andtoesdotheirthingduringthethirdtoeighthweekafter conception.Limbsbegintheirdevelopmentastinybuds thatextendfromourembryonicbodies.Thebudsgrow overtwoweeks,untilthetipformsalittlepaddle.Inside thispaddlearemillionsofcellswhichwillultimatelygive risetotheskeleton,nerves,andmusclesthatwe’llhavefor therestofourlives. Thedevelopmentofalimb,inthiscaseachickenwing. Allofthekeystagesinthedevelopmentofawing skeletonhappeninsidetheegg. Tostudyhowthispatternemerges,weneedtolookat embryosandsometimesinterferewiththeirdevelopment toassesswhathappenswhenthingsgowrong.Moreover, weneedtolookatmutantsandattheirinternalstructures andgenes,oftenbymakingwholemutantpopulations 66 throughcarefulbreeding.Obviously,wecannotstudy humansintheseways.Thechallengeforthepioneersin thisfieldwastofindtheanimalsthatcouldbeuseful windowsintoourowndevelopment.Thefirstexperimental embryologistsinterestedinlimbsinthe1930sand1940s facedseveralproblems.Theyneededanorganisminwhich thelimbswereaccessibleforobservationandexperiment. Theembryohadtoberelativelylarge,sothattheycould performsurgicalproceduresonit.Importantly,theembryo hadtogrowinaprotectedplace,inacontainerthat sheltereditfromjostlingandotherenvironmental disturbances.Also,andcritically,theembryoshadtobe abundantandavailableyear-round.Theobvioussolutionto thisscientificneedisatyourlocalgrocerystore:chicken eggs. Inthe1950sand1960sanumberofbiologists,including EdgarZwillingandJohnSaunders,didextraordinarily creativeexperimentsonchickeneggstounderstandhow thepatternoftheskeletonforms.Thiswasaneraofslice anddice.Embryoswerecutupandvarioustissuesmoved abouttoseewhateffectthishadondevelopment.The approachinvolvedverycarefulmicrosurgery,manipulating patchesoftissuenomorethanamillimeterthick.Inthat way,bymovingtissuesaboutinthedevelopinglimb, SaundersandZwillinguncoveredsomeofthekey mechanismsthatbuildlimbsasdifferentasbirdwings, whaleflippers,andhumanhands. Theydiscoveredthattwolittlepatchesoftissue 67 essentiallycontrolthedevelopmentofthepatternofbones insidelimbs.Astripoftissueattheextremeendofthelimb budisessentialforalllimbdevelopment.Removeit,and developmentstops.Removeitearly,andweareleftwith onlyanupperarm,orapieceofanarm.Removeitslightly later,andweendupwithanupperarmandaforearm. Removeitevenlater,andthearmisalmostcomplete, exceptthatthedigitsareshortanddeformed. Anotherexperiment,initiallydonebyMaryGasselingin JohnSaunders’slaboratory,ledtoapowerfulnewlineof research.Takealittlepatchoftissuefromwhatwillbecome thepinkysideofalimbbud,earlyindevelopment,and transplantitontheoppositeside,justunderwherethefirst fingerwillform.Letthechickdevelopandformawing.The resultsurprisednearlyeverybody.Thewingdeveloped normallyexceptthatitalsohadafullduplicatesetofdigits. Evenmoreremarkablewasthepatternofthedigits:the newfingersweremirrorimagesofthenormalset. Obviously,somethinginsidethatpatchoftissue,some moleculeorgene,wasabletodirectthedevelopmentofthe patternofthefingers.Thisresultspawnedablizzardofnew experiments,andwelearnedthatthiseffectcanbe mimickedbyavarietyofothermeans.Forexample,takea chickenembryoanddabalittlevitaminAonitslimbbud, orsimplyinjectvitaminAintotheegg,andlettheembryo develop.IfyousupplythevitaminAattheright concentrationandattherightstage,you’llgetthesame mirror-imageduplicationthatGasseling,Saunders,and 68 Zwillinggotfromthegraftingexperiments.Thispatchof tissuewasnamedthezoneofpolarizingactivity(ZPA). Essentially,theZPAisapatchoftissuethatcausesthe pinkysidetobedifferentfromthethumbside.Obviously chicksdonothaveapinkyandathumb.Theterminology weuseistonumberthedigits,withourpinky correspondingtodigitfiveofotheranimalsandourthumb correspondingtodigitone. MovingalittlepatchoftissuecalledtheZPAcausesthe fingerstobeduplicated. TheZPAdrewinterestbecauseitappeared,insomeway, tocontroltheformationoffingersandtoes.Buthow?Some peoplebelievedthatthecellsintheZPAmadeamolecule thatthenspreadacrossthelimbtoinstructcellstomake differentfingers.Thekeyproposalwasthatitwasthe concentrationofthisunnamedmoleculethatwasthe 69 importantfactor.InareasclosetotheZPA,wherethereisa highconcentrationofthismolecule,cellswouldrespondby makingapinky.Intheoppositesideofthedevelopinghand, fartherfromtheZPAsothatthemoleculewasmore diffused,thecellswouldrespondbymakingathumb.Cells inthemiddlewouldeachrespondaccordingtothe concentrationofthismoleculetomakethesecond,third, andfourthfingers. Thisconcentration-dependentideacouldbetested.In 1979,DenisSummerbellplacedanextremelysmallpieceof foilbetweentheZPApatchandtherestofthelimb.Theidea wastousethisbarriertopreventanykindofmoleculefrom diffusingfromtheZPAtotheotherside.Summerbell studiedwhathappenedtothecellsoneachsideofthe barrier.CellsontheZPAsideformeddigits.Cellsonthe oppositesideoftendidnotformdigits;iftheydid,the digitswerebadlymalformed.Theconclusionwasobvious. SomethingwasemanatingfromtheZPAthatcontrolled howthedigitsformedandwhattheylookedlike.To identifythatsomething,researchersneededtolookatDNA. THEDNARECIPE Thatprojectwaslefttoanewgenerationofscientists.Not untilthe1990s,whennewmoleculartechniquesbecame available,wasthegeneticcontrolfortheZPA’soperation unraveled. 70 Amajorbreakthroughhappenedin1993,whenCliff Tabin’slaboratoryatHarvardstartedhuntingforthegenes thatcontroltheZPA.Theirpreywasthemolecular mechanismsthatgavetheZPAitsabilitytomakeourpinky differentfromourthumb.Bythetimehisgroupstartedto workintheearly1990s,anumberofexperimentslikethe onesI’vedescribedhadledustobelievethatsomesortof moleculecausedthewholething.Thiswasagrandtheory, butnobodyknewwhatthismoleculewas.Peoplewould proposeonemoleculeafteranother,onlytofindthatnone wasuptothejob.Finally,theTabinlabcameupwitha novelnotion,andoneveryrelevanttothethemeofthis book.Looktofliesfortheanswer. Geneticexperimentsinthe1980shadrevealedthe wonderfulpatternofgeneactivitythatsculptsthebodyofa flyfromasingle-celledegg.Thebodyofafruitflyis organizedfromfronttoback,withtheheadatthefrontand thewingsattheback.Wholebatteriesofgenesareturned onandoffduringflydevelopment,andthispatternofgene activityservestodemarcatethedifferentregionsofthefly. Tabindidn’tknowitatthetime,buttwoother laboratories—thoseofAndyMacMahonandPhilIngham— hadalreadycomeupwiththesamegeneralidea independently.Whatemergedwasaremarkablysuccessful collaborationamongthreedifferentlabgroups.Oneofthe flygenescaughttheattentionofTabin,McMahon,and Ingham.Theynotedthatthisgenemadeoneendofabody segmentlookdifferentfromtheother.Flygeneticists 71 namedithedgehog.Doesn’tthefunctionofhedgehoginthe flybody—tomakeoneregiondifferentfromanother— soundlikewhattheZPAdoesinmakingthepinkydifferent fromthethumb?Thatparallelwasnotlostonthethree labs.Soofftheywent,lookingforahedgehoggenein creatureslikechickens,mice,andfish. Becausethelabgroupsknewthestructureofthefly’s hedgehoggene,theyhadasearchimagetohelpthemsingle outthegeneinchickens.Eachgenehasadistinctive sequence;usinganumberofmoleculartools,the researcherscouldscanthechicken’sDNAforthehedgehog sequence.Afteralotoftrialanderror,theyfoundachicken hedgehoggene. Justaspaleontologistsgettonamenewspecies, geneticistsgettonamenewgenes.Theflygeneticistswho discoveredhedgehoghadnameditthatbecausetheflies withamutationinthegenehadbristlesthatreminded themofalittlehedgehog.Tabin,McMahon,andIngham namedthechickenversionofthegeneSonichedgehog,after theSegaGenesisvideogame. Nowcamethefunquestion:WhatdoesSonichedgehog actuallydointhelimb?TheTabingroupattachedadyetoa moleculethatwouldsticktothegene,enablingthemto visualizewherethegeneisactiveinthelimb.Totheirgreat surprise,theyfoundthatonlycellsinatinypatchofthe limbhadgeneactivity:theZPA. Sothenextstepsbecameobvious.Thepatternsof activityintheSonichedgehoggeneshouldmimicthoseof 72 theZPAtissueitself.Recallthatwhenyoutreatthelimb withretinoicacid,aformofvitaminA,yougetaZPAactive ontheoppositeside.Guesswhathappenswhenyoutreata limbwithretinoicacid,thenmapwhereSonichedgehogis active?Sonichedgehogbecomesactiveonbothsides— pinkyandthumb—justastheZPAdoeswhenitistreated withretinoicacid. KnowingthestructureofthechickenSonichedgehog gaveotherresearchersthetoolstolookforitineverything elsethathasfingers,fromfrogstohumans.Everylimbed animalhastheSonichedgehoggene.Andineverysingle animalthatwehavestudied,Sonichedgehogisactiveinthe ZPAtissue.IfSonichedgehoghadn’tturnedonproperly duringtheeighthweekofyourowndevelopment,thenyou eitherwouldhaveextrafingersoryourpinkyandthumb wouldlookalike.Occasionally,whenthingsgowrongwith Sonichedgehog,thehandendsuplookinglikeabroad paddlewithasmanyastwelvefingersthatalllookalike. WenowknowthatSonichedgehogisoneofdozensof genesthatacttosculptourlimbsfromshouldertofingertip byturningonandoffattherighttime.Remarkably,workin chickens,frogs,andmicewastellingusthesamething.The DNArecipetobuildupperarms,forearms,wrists,anddigits isvirtuallyidenticalineverycreaturethathaslimbs. HowfarbackcanwetraceSonichedgehogandtheother bitsofDNAthatbuildlimbs?Isthisstuffactiveinbuilding theskeletonoffishfins?Orarehandsgenetically completelydifferentfromfishfins?Wesawaninnerfishin 73 theanatomyofourarmsandhands.WhatabouttheDNA thatbuildsit? EnterRandyDahnwithhismermaid’spurses. GIVINGSHARKSAHAND RandyDahnenteredmylaboratorywithasimplebutvery elegantidea:treatskateembryosjustthewayCliffTabin treatedchickeneggs.Randy’sgoalwastoperformallthe experimentsonskatesthatchickenbiologistshad performedonchickeneggs,fromSaundersandZwilling’s tissuesurgeriesallthewaytoCliffTabin’sgene experiments.Skatesdevelopinaneggwithakindofshell andayolk.Skatesevenhavebigembryos,justaschickens do.Becauseoftheseconvenientfacts,wecouldapplyto skatesmanyofthegeneticandexperimentaltoolspeople haddevelopedtounderstandchickens. Whatcouldwelearnbycomparingthedevelopmentofa sharkfintothatofachickenleg?Evenmorerelevant,what couldwelearnaboutourselvesfromallthis? Chickens,asSaunders,Zwilling,andTabinshowed,area surprisinglygoodproxyforourownlimbs.Everythingthat wasdiscoveredbySaundersandZwilling’scuttingand graftingexperimentsandbyTabin’sDNAworkappliesto ourownlimbsaswell:wehaveaZPA,wehaveSonic hedgehog,andbothhaveagreatbearingonourwell-being. Aswesaw,amalfunctioningZPAoramutationinSonic 74 hedgehogcancausemajormalformationsinhumanhands. Randywantedtodeterminehowdifferenttheapparatus isthatbuildsourhands.Howdeepisourconnectiontothe restoflife?Istherecipethatbuildsourhandsnew,ordoes it,too,havedeeprootsinothercreatures?Ifso,howdeep? Sharksandtheirrelativesaretheearliestcreaturesthat havefinswithaskeletoninside.Ideally,toanswerRandy’s question,youwouldwanttobringa400-million-year-old sharkfossilintothelaboratory,grinditup,andlookatits geneticstructure.Thenyou’dtrytomanipulateitsfossil embryostolearnwhetherSonichedgehogisactiveinthe samegeneralplaceasinourlimbstoday.Thiswouldbea wonderfulexperiment,butitisimpossible.Wecannot extractDNAfromfossilssoold,and,evenifwecould,we couldneverfindembryosofthosefossilanimalsonwhich todoexperiments. Livingsharksandtheirrelativesarethenextbestthing. Nobodywouldeverconfuseasharkfinwithahumanhand: youcouldn’taskfortwomoredifferentkindsof appendages.Notonlyaresharksandhumansverydistantly related,butalsotheskeletalstructuresoftheirappendages looknothingalike.NothingevenremotelysimilartoOwen’s onebone–twobones–lotsablobs–digitspatternisinsidea shark’sfin.Instead,thebonesinsideareshapedlikerods, longandshort,thinandwide.Wecallthemboneseven thoughtheyaremadeofcartilage(sharksandskatesare knownascartilaginousfish,becausetheirskeletonsnever turnintohardbone).IfyouwanttoassesswhetherSonic 75 hedgehog’sroleinlimbsisuniquetolimbedanimals,why notchooseaspeciesutterlydifferentinalmosteveryway? Inaddition,whynotchoosethespeciesthatisthemost primitivelivingfishwithanykindofpairedappendage, whetherfinorlimb?Sharksfitbothbillsperfectly. Ourfirstproblemwasasimpleone.Weneededareliable sourcefortheembryosofsharksandskates.Sharksproved difficulttoobtainwithanydegreeofregularity,butskates, theircloserelatives,wereanothermatter.Sowestarted withsharksandusedskatesasoursupplyofsharks dwindled.Wefoundasupplierwhowouldshipusevery monthortwoabatchoftwentyorthirtyeggcaseswith embryosinside.Webecameavirtualcargocultaswe waitedeachmonthforourshipmentofpreciouseggcases. WorkbyTabin’sgroupandothersgaveRandyimportant cluestobeginhissearch.SinceTabin’sworkin1993, peoplehadfoundSonichedgehoginanumberofdifferent species,everythingfromfishtohumans.Withthe knowledgeofthestructureofthegene,Randywasableto searchalltheDNAoftheskateandsharkforSonic hedgehog.Inaveryshorttimehefoundit:asharkSonic hedgehoggene. ThekeyquestionstoanswerwereWhereisSonic hedgehogactive?,and,evenmoreimportant,Whatisit doing? TheeggcaseswereputtouseasRandyvisualizedwhere andwhenSonichedgehogisactiveinthedevelopmentof skates.HefirststudiedwhetherSonichedgehogturnsonat 76 thesametimeinskatefindevelopmentasitdoesin chickenlimbs.Yes,itdoes.Thenhestudiedwhetheritis turnedoninthepatchoftissueatthebackendofthefin, theequivalentofourpinky.Yesagain.Nowhedidhis vitaminAexperiment.Thiswasthemillion-dollarmoment. Ifyoutreatthelimbofachickenormammalwiththis compound,yougetapatchoftissuethathasSonichedgehog activityontheoppositeside,andthisresultiscoupledwith aduplicationofthebones.Randyinjectedtheegg,waiteda dayorso,andthencheckedwhether,asinchickens,the vitaminAcausedSonichedgehogtoturnonintheopposite sideofthelimb.Itdid.Nowcamethelongwait.Weknew thatSonichedgehogwasbehavingthesamewayinour handsandinskates’andsharks’fins.Butwhatwouldthe effectofallthisbeontheskeleton?Wewouldhavetowait twomonthsfortheanswer. Theembryosweredevelopinginsideanopaqueeggcase. Allwecouldtellwaswhetherthecreaturewasalive;the insideofthefinwasinvisibletous. Theendresultwasastunningexampleofsimilarity amongus,sharks,andskates:amirror-imagefin.The dorsalfinsduplicatedtheirstructuresinawonderfulfrontto-backpattern,thesamekindwesawwithexperimentsin limbs.Limbsduplicatealimbstructure.Sharkfinsduplicate asharkfinstructureasdoskates.Sonichedgehoghasa similareffectineventhemostdifferentkindsofappendage skeletonsfoundonearthtoday. OneeffectofSonichedgehog,youmayrecall,istomake 77 thefingersdistinctfromoneanother.Aswesawwith respecttotheZPA,whatkindofdigitdevelopsdependson howclosethedigitistothesourceofSonichedgehog.A normaladultskatefincontainsmanyskeletalrods,which alllookalike.Couldwemaketheserodsdifferentfromone another,likeourdigits?Randytookasmallbead impregnatedwiththeproteinmadebySonichedgehogand putitinbetweentheseidenticalskeletalrods.Thekeyto hisexperimentisthatheusedmouseSonichedgehog.So nowwehavearealcontraption:askateembryowithabead insidethatisgraduallyleakingmouseSonichedgehog protein.Wouldthatmouseproteinhaveanyeffectona sharkoraskate? Therearetwoextremeoutcomestoanexperimentlike this.Oneisthatnothinghappens.Thiswouldmeanthat skatesaresodifferentfrommicethatSonichedgehog proteinhasnoeffect.Theotherextremeoutcomewould presentastunningexampleofourinnerfish.Thisoutcome wouldbethattherodsdevelopdifferentlyfromone another,demonstratingthatSonichedgehogdoes somethingsimilarinskatesandinus.Andlet’snotforget thatsinceRandyisusingtheproteinfromamammal,it meansthatthegeneticrecipewouldbereally,reallysimilar. Notonlydidtherodsenduplookingdifferentfromone another,theyrespondedtoSonichedgehog,muchasfingers do,onthebasisofhowclosetheyweretotheSonic hedgehogbead:thecloserrodsdevelopedadifferentshape fromtheonesfartheraway.Totopmattersoff,itwasthe 78 mouseproteinthatdidthejobsoeffectivelyintheskates. Normalfins(left)andRandy’streatedfins.Thetreated finsshowedamirror-imageduplicationjustaschicken wingsdid.PhotographscourtesyofRandallDahn, UniversityofChicago. The“innerfish”thatRandyfoundwasnotasinglebone, orevenasectionoftheskeleton.Randy’sinnerfishlayin thebiologicaltoolsthatactuallybuildfins.Experimentafter experimentoncreaturesasdifferentasmice,sharks,and fliesshowsusthatthelessonsofSonichedgehogarevery general.Allappendages,whethertheyarefinsorlimbs,are builtbysimilarkindsofgenes.Whatdoesthismeanforthe problemwelookedatinthefirsttwochapters—the 79 transitionoffishfinsintolimbs?Itmeansthatthisgreat evolutionarytransformationdidnotinvolvetheoriginof newDNA:muchoftheshiftlikelyinvolvedusingancient genes,suchasthoseinvolvedinsharkfindevelopment,in newwaystomakelimbswithfingersandtoes. Butthereisadeeperbeautytotheseexperimentson limbsandfins.Tabin’slabusedworkinfliestofindagenein chickensthattellsusabouthumanbirthdefects.Randy usedtheTabinlabdiscoverytotellussomethingaboutour connectionstoskates.An“innerfly”helpedfindan“inner chicken,”whichultimatelyhelpedRandyfindan“inner skate.”Theconnectionsamonglivingcreaturesrundeep. 80