Download Your Inner Fish - 09_Chapter Nine

CHAPTERNINE
VISION
OnlyonceinmyentirecareerhaveIfoundtheeyeofa
fossilcreature.Iwasn’tinthefieldonanexpedition,Iwas
inthebackroomofamineralshopinasmalltownin
northeastChina.MycolleagueGaoKeqinandIwere
studyingtheearliestknownsalamanders,beautifulfossils
collectedfromChineserocksabout160millionyearsold.
WehadjustfinishedacollectingtriptosomesitesGao
knewabout.Thelocationsweresecret,becausethese
salamanderfossilshaveseriousmonetaryvalueforthe
farmerswhotypicallyfindthem.Whatmakesthemspecial
isthatimpressionsofthesofttissue,suchasgills,guts,and
thenotochord,areoftenpreserved.Privatecollectorslove
thembecausefossilsofthisqualityareexceedinglyrare.By
thetimeweendedupatthemineralshop,GaoandIhad
alreadycollectedanumberofreallybeautifulancient
salamandersofourownfromhissites.
Thisparticularmineraldealerhadgottenhishandson
oneofthebestsalamanderfossilsofalltime.Gaowantedus
toseeitandspentthebetterpartofadaytryingtowork
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thedeal.Thewholevisithadaterrificallyillicitfeel.Gao
spentseveralhourssmokingcigaretteswiththegentleman,
speakingandgesturinginChinese.Clearlytherewassome
barteringgoingon,butnotknowingChineseIhadnoidea
whatofferswerebeingputonthetable.Afterlotsof
headshakingandultimatelyabighandshake,Iwas
permittedtogotothebackroomandlookatafossilonthe
dealer’sdesk.Itwasastunningsight:thebodyofalarval
salamander,nomorethanthreeincheslong.Init,Icould
seeimpressionsofthewholeanimal,allthewaydownto
thelittleshellsitateasitslastmeal.And,forthefirstand
onlytimeinmycareer,Iwasstaringattheeyeofanancient
fossilanimal.
Eyesrarelymakeitintothefossilrecord.Aswe’veseen,
thebestcandidatesforpreservationasfossilsarethehard
partsoftheanimals—bones,teeth,andscales.Ifwewantto
understandthehistoryofeyes,thenwecanusean
importantfacttoouradvantage.Thereisaremarkable
diversityoforgansandtissuesthatanimalsusetocapture
light,fromsimplephotoreceptororgansininvertebrate
animalstothecompoundeyesofvariousinsectsandour
owncamera-typeeye.Howdoweputthisvariationtouse
inunderstandinghowourabilitytoseedevelopedover
time?
Thehistoryofoureyesisalotlikethatofacar.Takea
ChevyCorvette,forexample.Wecantracethehistoryofthe
modelasawhole—theCorvette—andthehistoryofeachof
itsparts.The’Vettehasahistory,beginningwithitsorigins
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in1953andcontinuingthroughthedifferentmodeldesigns
eachyear.Thetiresusedonthe’Vettealsohaveahistory,
asdoestherubberusedinmakingthem.Thissuppliesa
greatanalogyforbodiesandorgans.Oureyeshavea
historyasorgans,butsodoeyes’constituentparts,thecells
andtissues,andsodothegenesthatmakethoseparts.
Onceweidentifythesemultiplelayersofhistoryinour
organs,weunderstandthatwearesimplyamosaicofbits
andpiecesfoundinvirtuallyeverythingelseontheplanet.
Muchoftheprocessingoftheimagesweseeactually
happensinsideourbrains:theroleoftheeyeistocapture
lightinawaythatitcanbecarriedtothebrainfor
processingasanimage.Oureyes,likethoseofevery
creaturewithaskullandbackbone,arelikelittlecameras.
Afterlightfromtheoutsideenterstheeye,itisfocusedona
screenatthebackoftheeyeball.Lighttravelsthrough
severallayersasittraversesthispath.Firstitpasses
throughthecornea,athinlayerofcleartissuethatcovers
thelens.Theamountoflightthatenterstheeyeis
controlledbyadiaphragm,calledtheiris,whichdilatesand
contractsbytheactionofinvoluntarymuscles.Thelight
thenpassesthroughthelens,which,asacameradoes,
focusestheimage.Tinymusclessurroundthelens;asthese
musclescontract,theychangethelens’sshape,thus
focusingimagesfromnearandfar.Ahealthylensisclear
andmadeupofspecialproteinsthatgiveititsdistinctive
shapeandlight-gatheringproperties.Theseproteins,
knownaslenscrystallins,areexceptionallylong-lived,
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allowingthelenstocontinuefunctioningasweage.The
screenonwhichallofthelightisprojected,theretina,is
loadedwithbloodvesselsandlightreceptors.Theselight
receptorssendsignalstoourbrainthatwetheninterpret
asimages.Theretinaabsorbsthelightviasensitivelightgatheringcells.Therearetwotypesofsuchcells:oneis
verysensitivetolight,theotherlessso.Themoresensitive
cellsrecordonlyinblackandwhite;thelesssensitivecells
recordincolor.Ifwelookaroundtheanimalworld,wecan
assesswhetheranimalsarespecializedfordaylightornight
bylookingatthepercentagesofeachtypeoflight-sensing
cellintheireyes.Inhumansthesecellsmakeupabout70
percentofallthesensorycellsinourbody.Thatisaclear
statementabouthowimportantvisionistous.
Ourcamera-likeeyeiscommontoeverycreaturewitha
skull,fromfishtomammals.Inothergroupsofanimalswe
finddifferenteyes,rangingfromsimplepatchesofcells
specializedtodetectlight,toeyeswithcompoundlenses
suchasthosefoundinflies,toprimordialversionsofour
owneye.Thekeytounderstandingthehistoryofoureyes
istounderstandtherelationshipbetweenthestructures
thatmakeourcamera-eyeandthosethatmakealltheother
kindsofeyes.Todothis,wewillstudythemoleculesthat
gatherlight,thetissuesweusetosee,andthegenesthat
makethewholething.
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Eyescomeintofocus:fromprimitivelight-capturing
devicesininvertebratestoourcamera-typeeyewitha
lens.Aseyesevolve,visualacuityincreases.
LIGHT-GATHERINGMOLECULES
Thereallyimportantworkinthelight-gatheringcells
happensinsidethemoleculethatactuallycollectslight.
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Whenthismoleculeabsorbslight,itchangesshapeand
breaksupintotwoparts.Onepartisderivedfromvitamin
A,theotherfromaproteinknownasanopsin.Whenthe
opsinbreaksoff,itinitiatesachainreactionthatleadstoa
neuronsendinganimpulsetoourbrain.Weusedifferent
opsinstoseeinblackandwhiteandincolor.Justasan
inkjetprinterneedsthreeorfourinkstoprintincolor,we
needthreelight-gatheringmoleculestoseeincolor.For
black-and-whitevision,weuseonlyone.
Theselight-gatheringmoleculeschangeshapeinthe
light,thenrechargeinthedarkandgobacktotheirnormal
state.Theprocesstakesafewminutes.Weallknowthis
frompersonalexperience:gofromabrightplaceintoadark
roomanditisvirtuallyimpossibletoseefaintobjects.The
reasonisthatthelight-gatheringmoleculesneedtimeto
recharge.Afterafewminutes,visioninthedarkreturns.
Despitethestunningvarietyofphotoreceptororgans,
everyanimalusesthesamekindoflight-capturing
moleculetodothisjob.Insects,humans,clams,andscallops
alluseopsins.Notonlycanwetracethehistoryofeyes
throughdifferencesinthestructureoftheiropsins,butwe
havegoodevidencethatwecanthankbacteriaforthese
moleculesinthefirstplace.
Essentially,anopsinisakindofmoleculethatconveys
informationfromtheoutsideofacelltotheinside.Topull
offthisfeat,itneedstocarryachemicalacrossthe
membranethatencirclesacell.Opsinsuseaspecialized
kindofconductorthattakesaseriesofbendsandloopsas
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ittravelsfromtheoutsidetotheinsideofthecell.Butthis
twistedpaththereceptortakesthroughthecellmembrane
isnotrandom—ithasacharacteristicsignature.Where
elseisthistwistedpathseen?Itisidenticaltopartsof
certainmoleculesinbacteria.Theveryprecisemolecular
similaritiesinthismoleculesuggestaveryancient
propertyofallanimalsextendingallthewaytoourshared
historywithbacteria.Inasense,modifiedbitsofancient
bacterialieinsideourretinas,helpingustosee.
Wecaneventracesomemajoreventsinthehistoryof
oureyesbyexaminingopsinsindifferentanimals.Takeone
ofthemajoreventsinourprimatepast,thedevelopmentof
richcolorvision.Recallthathumansandourclosestape
relatives,theOldWorldmonkeys,haveaverydetailedkind
ofcolorvisionthatreliesonthreedifferentkindsoflight
receptors.Eachofthesereceptorsistunedtoadifferent
kindoflight.Mostothermammalshaveonlytwokindsof
receptorsandsocannotdiscriminateasmanycolorsaswe
can.Itturnsoutthatwecantracetheoriginofourcolor
visionbylookingatthegenesthatmakethereceptors.The
twokindsofreceptorsmostmammalshavearemadeby
twokindsofgenes.Ofourthreereceptor-makinggenes,
twoareremarkablylikeoneofthoseinothermammals.
Thisseemstoimplythatourcolorvisionbeganwhenone
ofthegenesinothermammalsduplicatedandthecopies
specializedovertimefordifferentlightsources.Asyou’ll
remember,asimilarthinghappenedwithodorreceptor
genes.
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Thisshiftmayberelatedtochangesinthefloraofthe
earthmillionsofyearsago.Ithelpstothinkwhatcolor
visionwaslikelygoodforwhenitfirstappeared.Monkeys
thatliveintreeswouldbenefitbecausecolorvisionenabled
themtodiscriminatebetteramongmanykindsoffruitsand
leavesandselectthemostnutritiousamongthem.From
studyingtheotherprimatesthathavecolorvision,wecan
estimatethatourkindofcolorvisionaroseabout55
millionyearsago.Atthistimewefindfossilevidenceof
changesinthecompositionofancientforests.Beforethis
time,theforestswererichinfigsandpalms,whicharetasty
butallofthesamegeneralcolor.Laterforestshadmoreofa
diversityofplants,likelywithdifferentcolors.Itseemsa
goodbetthattheswitchtocolorvisioncorrelateswitha
switchfromamonochromaticforesttoonewitharicher
paletteofcolorsinfood.
TISSUES
Animaleyescomeintwoflavors;oneisseenin
invertebrates,theotherinvertebrates,suchasfishand
humans.Thecentralideaisthattherearetwodifferent
waysofincreasingthelight-gatheringsurfaceareaineye
tissue.Invertebrates,suchasfliesandworms,accomplish
thisbyhavingnumerousfoldsinthetissue,whileour
lineageexpandsthesurfaceareabyhavinglotsoflittle
projectionsextendingfromthetissueliketinybristles.A
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hostofotherdifferencesalsorelatetothesedifferentkinds
ofdesigns.Lackingfossilsattherelevantphaseofhistory,it
wouldseemthatwewouldneverbeabletobridgethe
differencesbetweenoureyesandthoseofinvertebrates.
Thatis,until2001,whenDetlevArendtthoughttostudythe
eyesofaveryprimitivelittleworm.
Polychaetesareamongthemostprimitivelivingworms
known.Theyhaveaverysimplesegmentedbodyplan,and
theyalsohavetwokindsoflight-sensingorgans:aneye
and,buriedundertheirskin,apartoftheirnervoussystem
thatisspecializedtopickuplight.Arendttooktheseworms
apartbothphysicallyandgenetically.Knowledgeofthe
genesequenceofouropsingenesandthestructureofour
light-gatheringneuronsgaveArendtthetoolstostudyhow
polychaetesaremade.Hefoundthattheyhadelementsof
bothkindsofanimalphotoreceptors.Thenormal“eye”was
madeupofneuronsandopsinsliketheeyeofany
invertebrate.Thetinyphotoreceptorsundertheskinwere
anothermatteraltogether.Theyhad“vertebrate”opsins
andcellularstructureevenwiththelittlebristle-like
projections,butinprimitiveform.Arendthadfoundaliving
bridge,ananimalwithbothkindsofeyes,oneofwhich—
ourkind—existedinaveryprimitiveform.Whenwelook
toprimitiveinvertebrates,wefindthatthedifferentkinds
ofanimaleyessharecommonparts.
GENES
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Arendt’sdiscoveryleadstoyetanotherquestion.Itisone
thingforeyestosharecommonparts,buthowcaneyes
thatlooksodifferent—suchasthoseofworms,flies,and
mice—becloselyrelated?Fortheanswer,letusconsider
thegeneticrecipethatbuildseyes.
Attheturnofthetwentiethcentury,MildredHogewas
recordingmutationsinfruitflieswhenshefoundaflythat
hadnoeyeswhatsoever.Thismutantwasnotanisolated
case,andHogediscoveredthatshecouldbreedawholeline
ofsuchflies,whichshenamedeyeless.Later,asimilar
mutationwasdiscoveredinmice.Someindividualshad
smalleyes;otherslackedwholeportionsoftheheadand
face,includingtheireyes.Asimilarconditioninhumansis
knownasaniridia;affectedindividualsaremissinglarge
piecesoftheireyes.Intheseverydifferentcreatures—flies,
mice,andhumans—geneticistswerefindingsimilarkinds
ofmutants.
Abreakthroughcameintheearly1990s,when
laboratoriesappliednewmoleculartechniquesto
understandhoweyelessmutantsaffectedeyedevelopment.
Mappingthegenes,theywereabletolocalizethebitsof
DNAresponsibleforthemutations.WhentheDNAwas
sequenced,itturnedoutthatthefly,mouse,andhuman
genesresponsibleforeyelessnesshadsimilarDNA
structuresandsequences.Inaveryrealsense,theyarethe
samegene.
Whatdidwelearnfromthis?Scientistshadidentifieda
singlegenethat,whenmutated,producedcreatureswith
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smalleyesornoeyesatall.Thismeantthatthenormal
versionofthegenewasamajortriggerfortheformationof
eyes.Nowcamethechancetodoexperimentstoaska
wholeotherkindofquestion.Whathappenswhenwemess
withthegene,turningitonandoffinthewrongplaces?
Flieswereanidealsubjectforthiswork.Duringthe
1980s,anumberofverypowerfulgenetictoolswere
developedthroughworkonflies.Ifyouknewagene,ora
DNAsequence,youcouldmakeaflylackingthegeneor,the
reverse,aflywiththegeneactiveinthewrongplaces.
Usingthesetools,WalterGehringstartedplayingaround
withtheeyelessgene.Gehring’steamwasabletomakethe
eyelessDNAactiveprettymuchanywheretheywanted:in
theantenna,onthelegs,onthewings.Whenhisteamdid
this,theyfoundsomethingstunning.Iftheyturnedonthe
eyelessgeneintheantenna,aneyegrewthere.Ifthey
turnedontheeyelessgeneonabodysegment,aneye
developedthere.Everywheretheyturnedonthegene,they
wouldgetaneweye.Totopitalloff,someofthemisplaced
eyesshowedanascentabilitytorespondtolight.Gehring
haduncoveredamajortriggerintheformationofeyes.
Gehringdidn’tstopthere;hebeganswappinggenes
betweenspecies.Theytookthemouseequivalentofeyeless,
Pax6,andturneditoninafly.Themousegeneproduceda
neweye.Andnotjustanyeye—aflyeye.Gehring’slabfound
theycouldusethemousegenetotriggertheformationofan
extraflyeyeanywhere:ontheback,onawing,nearthe
mouth.WhatGehringhadfoundwasamasterswitchfor
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eyedevelopmentthatwasvirtuallythesameinamouse
andafly.Thisgene,Pax6,initiatedacomplexchain
reactionofgeneactivitythatultimatelyledtoanewflyeye.
Wenowknowthateyeless,orPax6,controls
developmentineverythingthathaseyes.Theeyesmay
lookdifferent—somewithalens,somewithout;some
compound,somesimple—butthegeneticswitchesthat
makethemarethesame.
Whenyoulookintoeyes,forgetaboutromance,creation,
andthewindowsintothesoul.Withtheirmolecules,genes,
andtissuesderivedfrommicrobes,jellyfish,worms,and
flies,youseeanentiremenagerie.
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