9.2billionyearsa.b.(afterthebang)tothepresentday(13.7billionyearsa.b.)

chapter5theemergenceoflife

stromatolites

thenearestthingonearthtolivingfossils,stromatolites

arerocksmadefrommillionsofmicroscopiclayers

comprisedoftheremainsofbacteria.theycanstill

beseeninsharkbay,australia.

theMoon

thefamiliarnearsideoftheMoon.thedarkareas

arethemare,orseas–ancientlavabasinsfilledwith

darkmaterial–andthewholesurfaceispeppered

withcraters.

formationoftheMoon

twoplanetscollidedandtheircorescoalescedinto

asinglelargerplanet,earth.themantledebris

becametheMoon.

ataround4.6billionyearsago,theearthhadfinallyformedbutitwascompletelymolten,andbeforethesurfacecouldcooladramaticeventtookplacethatresultedintheformationoftheMoon.thefavouredtheoryatthepresenttimeis

thatthiswasduetoagiantimpact,whentheearthcollidedwithabody,perhapsaslargeasMars.thetwomerged,anddebriswasscatteredtoformtheMoon.thefactthattheMoonislessdensethantheearthshowsthattheactualcoresofthetwobodieswerenotinvolvedintheformationoftheMoonbutmergedtoformtheearth’spresentcore.

theroleoftheMoonourMoonappearstobespecial,andhasanessentialroleintheevolutionoflifeonourplanet.theMoonstabilisesthetiltoftheearth’saxis,whichiscurrently23degreesanddoesnotvarybymorethanadegree.iftheMoonhadnotbeenpresent,thistiltwouldhavevariedmarkedly,andtheclimaticsituationwouldbeverydifferent.comparethiswithMars,whichhasnocomparablesatellite;itstwoattendants,phobosanddeimos,aresosmallthattheirinfluenceisnegligible.thereisthusnostabilisingforce,andthetiltoftheMartianaxisvariesfrom11to35degreesoveracycleofroughly100,000years.theevolutionoflifedependsuponthelong-termstabilityofclimate.ifearth’saxisofrotationvariedwildlyoverashortperiod,thisstabilitywouldbelacking,andlifeasweknowitwouldnothavedeveloped.itseemsweoweourgratefulthankstotheMoonformakingourexistencepossible.

themostobviouseffectoftheMoonontheearthisinthetidesthatitraises.thefrictionthiscausesslowstheearth’srotation,andthisprocessiscontinuingtoday.anequallyimportanteffectistoincreasetheseparationbetweentheearthanditssatellite;thedistancebetweenthemisincreasingatarateof4centimetres(11/2inches)peryear.

asonemightexpect,theearthhasasimilareffectontheMoon,andtheearth’smassis80timesthatoftheMoon,soitsinfluencehasbeenevenmoreextensive.longagotheMoon’srotationwastidallysloweduntilitbecame‘captured’,orsynchronous,meaningthatitsperiodofspinbecameexactlyequaltoitsorbitalperiod.theresultisthattheMoonalwayspresentsthesamefacetotheearth.itisimportanttorememberthatalthoughtheMoonkeepsthesamefaceturnedtowardus,itdoesnotkeepthesamefaceturnedtowardthesun,andtheideathatthereisa‘darkside’totheMooniscompletelywrong.dayandnightconditionsontheMoonarethesameinbothhemispheres,apartfromthefactthattheearthwillneverbeseenfromtheavertedhemisphere.

thespeedoftheMoon’srotationquicklybecameaconstant,butthespeedwithwhichittravelsalongitsellipticalpathroundtheearthneverhas.followingtheusualtrafficlawsofthesolarsystem,theMoonmovesfastestwhennear‘perigee’,itsclosestpointtotheearth,andslowerelsewhere.thereforethepositioninorbitandtheamountofrotationbecomeoutofstep.theresultisthat,seenfromtheearth,theMoonseemstorocktoandfro.sometimesweseealittlearoundthemeanWestern‘limb’,andsometimesalittlearoundtheeasternedge.altogether,duetothisandothersmaller‘librations’,asthesewobblesarecalled,wecanexaminefromtheearthatotalof59percentoftheMoon’ssurface,althoughnevermorethan50percentatatime.itisonly41percentofthesurfacethatwecannotsee.

ourplanet–cradleforlifeatfirst,theearthwasmolten,andfartoohotforlifetoappear.itcooleddowngradually

110

overroughly500millionyears,andformedasolidcrust.theoriginalatmospherewasmadeuplargelyofhydrogen,butthiscouldnotlast.theenergeticatomssoonleakedawayintospace,astheearth’sgravitationalpullwas,andis,tooweaktoretainthem.theremayevenhavebeenaperiodwhentheearthhadnoatmosphereatall,butthischanged.Volcanicactivitywouldhavebeenmuchmorecommon–andmuchmoreviolent–anderuptionsfrominsidetheglobesoonsentoutsufficientquantitiesofgasestoproduceanewatmosphere.ofcourse,thisatmospherewasverydifferentfromthatoftoday,notablyinitslackofoxygen.however,astheatmospherecooledwaterbegantocondenseandtherefollowedtheperiodwecouldcallthegreatrains,whichlastedforlongenoughtofillthelowerlyingregionswithwaterandproducethefirstoceans.

therewasalsoaperiodofbombardmentfrommaterialleftover,sotospeak,whentheplanetswereformed.thisisveryevidentwhenwelookatthelunarsurface,wherethecraterswereproducedbybombardmentduringthisperiod.ofcourse,theearthwasequallybombardedbuterosionhasremovedmostofitsscars.itisworthnotingthat,haditnotbeenforthecontinuingtectonicactivity,thecrashingtogetherofplatesandthesqueezingupofmountains,waterwouldhavecoveredthewholeofasmoothglobetothisday.thetectonicforcesaredrivenbyheatfromthedecayofuraniumandotherunstableheavyelementsdeepwithintheearth.thesematerialsmust,aswehaveseen,havecomefrompreviouscataclysmicstardeaths.somanydistanteventshavebeenresponsibleformakingitpossibleforthestagetobesetfortheemergenceoflife.

lifeemergedmuchearlierthanhasoftenbeenbelieved.thefirstorganismabletoreproduceitselfprobablyappearedabout4.3billionyearsago.theearliestevidenceoflife,attributedtothefirstorganismswhichwereclearlyprimitive,isamarkedriseinlevelsofoxygenintheatmosphere.thefactthatthepresenceofsignificantquantitiesofoxygenisanunmistakeablesignoflifegiveshopetothosescientistsdevelopingmissionstosearchforearth-sizedplanetsaroundotherstars;interstellartravelmaybesomewayoff,butwemaybeabletoseethesignatureoflifefromagreatdistance.theoldestevidenceoflifeyetdiscovereddatesfrom3.8billionyearsagoandisfoundinancientrocksfromtheislandofakiliainWesterngreenland.

theexactprocessbywhichlifearoseisstillunclear;contrarytopopularmyth,noonehasyetcomeclosetorepeatingthisfeatinthelaboratory.thetheory(unproven)goesthatchemicalreactionsweredrivenbyenergyfromsourcessuchaslightningstrikesandshort-waveradiationfromthesun.astimewentby,moreandmorecomplexmoleculeswereproduced,untileventuallyamoleculeappearedwhichcouldreplicateitself.theabilitytoreplicate,orreproduce,isfundamentaltoanythingthatwethinkofaslife.thereplicationwasnotperfect;eachgenerationbroughtwithitachanceofrandomvariations–errorsinthecopyingprocess.someoftheserandommutations,astheyarecalled,were

panspermia

thelatefredhoyle,andhiscolleaguechandra

Wickramasinghe,buildingonconjecturebythe

swedishscientistarrenhuis,maintainedthatcomets

coulddumpvirusesintotheupperatmosphere,

causingwidespreadepidemics.(Virusesarestrands

ofdnaorrnathatusetheapparatusoflivingcells

toreplicate.somebiologistsdisputethattheyare

actuallyaliveintheclassicsense.)againtherehas

beenlittlesupportforthisidea,andtheideahas

neverbeentakenseriouslybymedicalexperts.

probablythemostunusualserioustheoryin

connectionwithlifefromspacewasproposedbyno

lessaneminencethanfranciscrick,co-discoverer

ofthedouble-helixstructureofdna.Withthe

9.2billionyearsa.b.–present

chemistleslieorgel,heputforwardthetheory

of‘directedpanspermia’,accordingtowhichlife

wasdeliberatelysenttoearthbybeingsfroman

advancedtechnologicalcivilizationfromfaracross

thegalaxy.itwaspointedoutthatthechancesof

micro-organismsbeingpassivelytransportedfrom

worldtoworldacrossinterstellarspacewereslight,

butthingswouldbedifferentifpreparationswere

made.differenttypesofmicro-organismcould

becarriedinaspaceshipanddepositedhereto

flourishanddevelop.Whenthetheoryappeared,it

isperhapsfairtosaythatscientistsingeneralwere

stunnedratherthanenthusiastic,butideasofthis

kindareextremelydifficulttodisprove.

moresuccessful,survivinglongerorreproducingmoreeasilythanothers,andsoweremorelikelytoformthenextgeneration.thiscompetitionbetweenslightlydifferentformsisattheheartofwhathasbecomeknownasevolution.thelongstatelyprocesswhichmusthaveledfromthesesimplereplicators,nomorethancomplexmolecules,tothevastvarietyoflifethatweseearoundustoday,hadbegun.

theearliestknownfossilsareofbacteria.theseorganismsprobablylivedinthehotoceansfoundonearthatthistime.Wecanbefairlyconfidentabouttheirage,becausegeologicalmethodscantellustheageoftherocksinwhichtheremainsoftheseprimitiveorganismsarefound.inrocksofthistimewealsofindwhatarecalledstromatolites,

Volcano

earth'searlyoxygen-freeatmosphereisbelievedtohave

formedasaresultofaperiodofprolificvolcanicactivity.

blacksmoker

Moreproperlyknownashydrothermalvents,black

smokersaresitesoveramilebelowthesurfaceof

theoceanwheresuperheatedgasesareescaping

fromfissuresintheearth’scrust.theyarefoundin

mid-oceanridges,andsurprisinglyahugevariety

oforganismssurroundthem,thrivingdespitehigh

temperatures,acompleteabsenceofsunlight,andhigh

acidity.thesecreaturesformafoodchaincompletely

independentofenergyfromthesun,butthatisinstead

basedonchemicalsemergingfromtheventitself.

9.2billionyearsa.b.–present

theemergenceoflife 113

rock-likestructuresbuiltbyblue-greenalgae,alsoknownascyanobacteria;stromatolitestoodatebacktoaround3.5billionyearsago,and,remarkably,sometypessurvivetoday,notablyinpartsofthenorthernterritoryofaustralia.duringtheearlystagesofearth’shistory,cyanobacteriaplayedamajorroleinproducingfreeoxygen,andthisbegantheprocessofproducingabreathableatmosphere.

Wehavefoundthatlifeisextremelyversatile;someformsareamazinglytolerant,andcanthriveinthemostunlikelyplaces.forexample,oneofthefirstplaceslifemayhaveappearedisaroundwhataretermedhydrothermalvents,oftenknownasblacksmokers.thesearefissuresinthefloorsoftheoceansthatleakouthot,acid-richwaterfrombelow;theeffluviaareoftenblack–hencethenickname.thetemperatureofthewateremergingfromthesefissures,atleastamileunderthesea’ssurface,maybeashighas400°c.Waterisabletoreachthistemperature,higherthanitsnormalboilingpoint,duetothepressure,whichis25timesthatduetotheatmosphereatgroundlevel.remarkably,thefissuresareteemingwithspecializedlifeformssuchastubeworms,shrimpsandevenclams,whichsurviveinanenvironmentasacidicasvinegarthat

colonyoftubeworms

relativesofthesecreaturesliveneartheblacksmokers

inextremelyhostileconditions.theyhavenomouth

orstomachandsurvivebyabsorbingchemicalsinthe

waterthroughtheirskin.

inspirationalhistory

itwasabookbypatrickMoore,theearth,intheschool

library,thatintroducedbriantotheamazingstoryof

thetrilobites,andinspiredhimtoalife-longpassionfor

astronomy.therewereonce15,000speciesoftrilobite,

andbythetimetheywerewipedoutbythepermian

extinction250millionyearsago,theyhadroamedthe

earthfor300millionyears.bycomparison,wehumans

havesofarbeenaroundforlessthan200,000years.

thetrilobites’nearestlivingrelativeisthehorse-shoe

crab.thisparticulartrilobiterecentlyfounditswayto

anatureshopinnewyork,forsalealongwithafine

selectionofmeteorites,dinosaurbones,andotherclues

toourdistantpast.

theemergenceoflife 115

wouldbeinstantlyfataltomostotherformsofsealife,andwithoutreceivinganyenergyfromthesun.

Worldwide,thefossilrecordallowsustotracetheevolutionoflivingcreatures.generallyspeaking,lifeevolvedratherslowly;foralongtimeitwasconfinedtothesea,andonlyduringwhatistermedthedevonianperiod,around400millionyearsago,didlifespreadtotheland–plantsfirst,thenarthropods(suchasinsects,spidersandcrustaceans)andvertebrates.theplantgrowthonlandcontinuedtoproducechangestothecompositionoftheatmosphere.plantssurviveusingphotosynthesis,whichremovescarbondioxidefromtheairandusesittobuildfoodintheformofsugarmolecules.awasteproductofthisprocessisoxygen,whichplantsreleaseintotheair.

graveyardofthedinosaursthegreatestcatastropheinlife’shistoryoccurredattheendoftheeraknowntogeologistsasthepermian,250millionyearsago.thepermianlastedforaroundsixtymillionyears,andseemstohavebeenatimeofwidespreaddeserts.Mostoftheworld’slandmasseswerejoinedtogetherinavastcontinent,whichhasbeencalledpangaea.itseemsthatthispermianextinction,oftenknownmorepoeticallyasthe‘greatdying’,wasthegreatestinhistoryandwipedoutmostoflifeonearth.this,ofcourse,canbeestablishedbythefossilrecord,butthereisnocraterlefttoguideusastothecauseofthedisaster.instead,wemustdependoncertaincarbonmoleculesknownasfullerenes.thesemoleculesformacage-likestructure,mostoftenintheshapeofaball,andinsidethiscagesingleunreactiveatomsaretrappedatthetimeofformation.theheliumandargonfoundinfullerenesattheendofthepermianseemtohavecomefromspace,producedintheatmosphereofastarthatexplodedasasupernovabeforethesunwasformed.thesechemicalsmaybetheremainsofameteoritethatcarriedmaterialleftoverfromthebeginningofthesolarsystem.itissuggestedthat,asaconsequenceoftheimpact,theremayalsohavebeenhugeamountsofvolcanicactivity,coveringtheentirelandsurfaceinlavatoadepthofthreemetres(ninefeet).itisthereforefarfromsurprisingthat90percentofallmarinespeciesand70percentoflandvertebratesfailedtosurvive.

reptilesbegantoappearthroughoutthepermian,andwecometotheageofthedinosaurs,someofwhichwerehugeandferocioushunterswhileothersweresmallandherbivorous(planteaters).onesmallharmlessdinosaur,nolargerthanacanary,hasbeennicknamedthetweetieosaurus.

dinosaursruledtheworldforalmost200millionyears(incomparison,humanbeingshavebeenonearthforlessthan200,000years),butthen,attheendofthegeologicalcretaceousperiod,65millionyearsago,thegreatdinosaurssuddenlyvanished.yettheextinctionmaynothavebeentotal;itnowseemscertainthatsomeofthesmallerspeciessurvivetothepresentdayintheformoftheirfeatherydescendants,thebirds.thedepartureofthedinosaursmayhavebeenagoodthingfromourpointofview,becauseitmeantthatmammalscoulddiversifyfromsmallshrew-likeanimalsintothewidevarietyofspeciesweseetoday.theapesthatevolvedintheMioceneperiod(25to5millionyearsago)areourdirectancestors.

investigationofthecauseofextinctionsisapopularpursuit,andopinionsvary.sofarasthedemiseofthedinosaursisconcerned,thefavouredtheoryoftodayisthatalargemeteoritestrucktheearth,throwingupacolossalamountofdust,andcausingglobaldevastation–ithasevenbeenclaimedthatthesiteoftheimpacthasbeenidentified;

chicxulubbasin

thechicxulubcraterinMexicomaybethesiteofthe

impactthatendedthereignofthedinosaurs.

lightcurveartwork

Marsexpress

formanyyears,peoplehavespeculatedaboutthe

possibilityoflifeonMars,whichistheonlyworld

inthesolarsystemthatisnottoounliketheearth.

itismuchsmaller,sothatithaslostmuchofthe

atmosphereitmayoncehavehad,anditiscolder,

becauseitisaround44millionmilesfurtheraway

fromthesun,buttherearenotoxiccloudsorlethal

radiationzones.resultsobtainedin2004bythe

Marsexpressorbiter,andthetwoUsroversspirit

andopportunity,provethatsaltyseasoncecovered

partsofthesurface,andpresumablyconditions

thenweresuitableforlife.theMartianengineers

invokedbypercivallowellamerehundredyears

agotoexplainthecanal-likefeatureshethought

hesawonthesurfacehavebeenbanishedtothe

realmofsciencefiction,althoughmanypeople,

exploringthesurfaceofMars

inthisreconstruction,nasa’srovervehicle,spirit,

standsproudlyonthesurfaceofMars,halfwayup

husbandhill.thisrobotgeologist,alongwithitstwin,

opportunity,hasprovidedthebestevidenceyetthat

therewasonceliquidwaterontheplanet’ssurface.

chicxuluboffthecoastofMexico,wherewecandetecttheerodedtractsofavastcrater.theevidenceisbasedmainlyonthefactthatrockslaiddownatthisperiodoveralargeareacontainmorethantheexpectedamountofiridium–anelementcomparativelyrareonearthbutcharacteristicofmeteorites.Wecannotbecertaintheimpactwipedoutthedinosaurs,butthetheoryiswidelysupported.

ithasbeenusefultospendsometimeindiscussionoflifeonearth,becausethequestionmustnowbeaskedwhetherthissequenceofeventshasbeenduplicatedelsewhere.if,elsewhere,thereisanearth-likeplanetorbitingasun-likestar,therehavebeensuggestionsthatweshouldexpecttofindsomekindoflife,eventhoughwehavenoideahowlifestarted.butwewillneverbesureuntil–if–wedetectsignalsfromanother

civilization.thesearchison;however,alldirectsearchesforsignalsfromotherintelligentbeings(seti–thesearchforextraterrestrialintelligence)havesofardrawnablank.

istherelifeonMars?Whatfactorsdoweneedtoconsiderwhencalculatingtheoddsofsucceedinginoursearch?onepointmustbeclearedupatonce:wearediscussinglifeasweknowit.alllifeofthekindwecanunderstandisbasedupononetypeofatom,theatomofcarbon;onlyitcanlinkupwithenoughotheratomstoformthecomplexatomgroupsormoleculesthatareneeded.itfollowsthatlife,whereveritexists–here,onMars,oronaplanetinadistantgalaxy–mustbecarbon-based.WorldssuchastheairlessMoonmustberejected

Marsexpressrover

theeuropeanspaceagency'sMarsexpress

probecarriesastereocamerathatproducesthree-

dimensionalimages.

includingscientists,weresorrytoseethemleave

thestage,butlowlyorganismscouldexistthere

evennow.itisnotunreasonabletosupposethatif

lifeappears,itwillevolveasfaraslocalconditions

permit.itmaywellbethatlifeappearedonMars

andbegantoevolvebuthadlittleopportunityto

diversifybeforeconditionsdeteriorated.

itwasrecentlyclaimedthatevidenceofmicro-

organismshadbeenfoundinrocksthoughtto

havebeenthrownfromMarstoearthbythe

impactofahugemeteoriteonthesurfaceofMars.

however,thereisdoubtastotheaccuracyofthe

conclusions,anditwouldtakeasampleofthe

Martiansurfacebroughtbackbyaprobe,showing

clearsignsoflife,toallowustoconcludethatlife

willappearwhereverconditionsarefavourable.

118

outofhand.inoursolarsystem,perhapsonlytheearthissuitedtocomplex,intelligentlifeforms.thecounter-argument,ofcourse,isthatwemaybecompletelywrong,andthattherecanbeintelligentbeingswhosebodiesarebaseduponatomsofgold,andwhobreatheinatmospheresofsulphuricacid.beingsofthiskind(beMsorbug-eyedMonsters)aremuchlovedbyscience-fictionwriters,fromhgWellsonward,butiftheydoexistthenthewholeofourmodernscienceiswrong,andthisseemswildlyimprobable.

Wehaveatleastestablishedthatmanystarsdoindeedhaveplanetarysystems,butforaplanettobearlifethereareseveralconditionsthatmustbefulfilled.(again,letusstressthatweareconsideringonlylifeofthekindwecanunderstand.onceweentertherealmofcompletelyalienlifeforms,speculationbecomesendlessand,fornow,weintendtorestrictthediscussiontocarbon-basedlife.)theplanetmusthaveanatmospherethatcontainssufficientfreeoxygen;itmusthaveasolid(orpossiblyliquid)surface;theremustbeanadequatesupplyofwater;afairlyequabletemperature;andalongperiodoverwhichconditionsdonotchangedramatically.alltheseconditionsarefulfilledbytheearth,butbynootherbodyinthesolarsystem.

however,theremaybeotherlessobviousrequirements.forexample,afairlyregularalternationofdayandnightseemsadvantageous.ifonehemisphereoftheplanetisinpermanentdarkness,andtheotherpermanentlyilluminatedbythesun,violentwindswouldensue,rainwouldbeimpossibleandtemperatureswouldhardlybeconduciveto

coprateschasma

thestereocameraontheeuropeanspaceagency’s

Marsexpressspacecraftallowsscientiststoreconstruct

theviewofMarsfromanyperspective,likethis.

coprateschasmaislocatedinthemiddleoftheValles

Marineris,aseriesofcanyonsthatdwarfearth’sgrand

canyon.

9.2billionyearsa.b.–present

theemergenceoflife 119

life;freezingononesideandboilingontheother.ofcoursetheremightbeafavourablezoneontheterminator(theboundarybetweenthedarkandlightsides).

letusfocusontemperature.aroundastarthereisaregionknownasthehabitable,or‘goldilocks’zoneorecosphere,whereaplanetwillbeneithertoohotnortoocoldforlifetoexist.neitheroftheorbitsofVenusorMarsliewithintheecosphere;Venusistooclosetothesun,andtoohot,andMarsistoofaraway,andtoocold.onlyourplanetmovescomfortablyinthemiddleofthezone;theearth,likebabybear'sporridge,isjustright.astarlessluminousthanthesunwillhaveitsecospherecloser-in;withamorepowerfulstar,theecospherewillbefurtherout.Manyrequirementsareselfevident,andruleoutmanystarsascandidatesforplanetarysystems;astronglyvariablestar,forexample,wouldgiveanyplanetamostchangeableclimate.

Wehavealreadyseenthatourgalaxyhasaroundahundredbillionstars,anaveragenumberforalargegalaxy.itseemslikelyfrompresentdayobservationsthatmostsinglestarshaveplanets,leavinguswitharoundfortybillionsolarsystems.howmanyoftheseplanetsliewithintheirstar’secosphere?ontheevidenceofourownsolarsystem,theonlyonethatwehavesufficientknowledgeof,wemightsurmisethateachsolarsystempossessesoneplanetwithinitsecosphere.Weshouldruleoutplanetsaroundstronglyvariablestars,however,sowemightbeleftwithtwentybillionsuitablyplacedplanets.howmanyofthesewillberocky?thisisanewproblem–aswehaveseen,othersolar

christmasonanalienworld

oneoftheplayfulvisionsofwhatlifemightbelikeon

anotherplanetbygertrudeMoore,patrick'smother.

factandfiction

renownedforhisscientificwriting,patrickhasalso

writtenmanyscience-fictionnovels.

120

systemsseemtohavegasgiantsinthegoldilocksregion.itisdifficulttodeterminetheprobabilityoffindingarockyplanetinasuitableplace,butoutofthe120orsosystemsknown,30havenogiantplanetintheway,sousingthatfractionasouroptimisticestimatewehavefivebillionplanetsonwhichwethinkconditionscouldallowlifetoform.onhowmanyofthesediditform?toanswerthisquestion–perhapsthemostdifficultofall–weneedtoknowandunderstandtheexactmechanismbywhichlifestarted.itisfairtosaythatbiologistshavenoexperimentallytesteddetailedtheories,andthismakesreducingtheprobabilitytoasinglenumberverydifficult.iftheprobabilityisevenoneinathousandbillion,findingjustourcivililizationwithinourgalaxyseemsanamazingfluke.if,assomebelieve,itisclosertooneinahundred,thenwewillhavemanymillionsofpromisingplanetstosearch.thisproblemisoneofthemainreasonsthediscoveryoflifeonMarswouldbesoimportant–iflifebeganindependentlytwiceinthesamesolarsystem,theoddsofitbeginningthroughoutthegalaxymustbegreatlyimproved.eventhen,though,ourproblemswouldnotbeover!

Wemustthenaskhowlikelyitisthatlife,oncebegun,willformintelligentbeingsthatwecouldcommunicatewith.somebiologistsbelievethatoncelifehasstarted,intelligenceisinevitable,whileothersargueequallyconvincinglythatintelligencelikeoursisatrueone-off.howmanyoftheseintelligencescouldwedetect?theymusthavereachedorsurpassedaleveloftechnologyachievedbythehumanraceonlyinthelasthundredyears.andthenwemustwonderhowlongacivilizationcapableofcommunicationwilllastbeforebeingdestroyed,eitherbynaturaldisasterorbyitsownfolly.inourowncaseatpresent,thelatterseemsmorelikely.Wehavereachedthepointwheretheuncertaintiesarebiologicalratherthanastronomical,andweawaitfurtherdevelopments.now,rememberwehavesofarconsideredonlyourowngalaxy–oneofbillions.thethoughtthatinallofthisvastnesswecouldbealoneisliterallyawe-inspiring.

ifintelligenceexistselsewhere,arethereanywaysofhavingameaningfulinteraction?Wecandismissmodern-typespacecraftwithoutfurtherado.evenifwecouldtravelatthespeedoflight,reachingeventheneareststarthatmightbeattendedbyplanetswouldtakeyears,andaccordingtoeinstein'stheoryofrelativity,travelatthefullspeed

projectozma

the84-foot(25.9metre)radiotelescopeatgreen

bank,WestVirginia,wasusedbyfrankdrakeand

histeamin1960tocarryoutthefirstsearchfor

extraterrestrialintelligence.

Jodrellbank

the250-foot(76m)radiotelescopewasbuiltat

Jodrellbankbybernardlovellin1957,andhas

subsequentlybeeninvolvedinsetiprojects.

cetianderidani

twonearbystarsseemedparticularlygood

candidatesascentresofplanetarysystems:tauceti

andepsiloneridani,bothofwhichareeasilyvisible

withthenakedeye.bothareofsolartype,though

lessluminousthanthesun.tauceti,atarangeof

11.9light-yearshas40percentoftheluminosity

ofthesun,whileepsiloneridanilies10.7light-

yearsawayandhas30percentofsolarluminosity.

thesewerethetargetstarsforthefirstexperiment

forseti,carriedoutbyfrankdrakeandhis

teamin1960.theinvestigators‘listenedout’at

awavelengthof21cm(1420megahertz),the

frequencyofemissionssentoutfromthecloudsof

coldhydrogenspreadthroughoutthegalaxy,but

theresultswerenegative.drakeformulated

thefamous‘drakeequation’tocalculatethe

probabilityoflifeexistingelsewhereintheUniverse.

Unfortunately,asdrakehimselfwasthefirstto

pointout,theformulacontainstoomanyunknowns

forfirmconclusionstobedrawn.theexperiment

wasknownofficiallyasprojectozma,afterthefake

wizardinfrankbaum’sclassicchildren’sbook,the

Wizardofoz.

taucetihasturnedouttobesomethingofa

disappointment.likethesun,itisayellowdwarf

star,itsmassis80percentofthesun’s,and

itssurfacetemperatureismuchthesame.its

ecosphereshouldextendbetween0.6and0.9

9.2billionyearsa.b.–present

theemergenceoflife 121

astronomicalunits(1a.U.istheaveragedistance

oftheearthfromthesun),sothatifitreplaced

thesuninoursolarsystem,Venuswouldbe

comfortablyplaced.butwhentaucetiwasstudied

withtheJamesclerkMaxwelltelescopeinhawaii,

themostpowerfultelescopeofitstypeintheworld,

thingsbegantolooklesspromising.thestaris

associatedwithadiskofdebris,andsoanyplanets

orbitingtauceti,wouldbesubjecttoconstant

bombardmentfromasteroidsofthekindwebelieve

wipedoutthedinosaurs.Withsomanylarge

impacts,itseemsunlikelylifecouldhavesurvived.

epsiloneridani,ontheotherhand,isvery

differentinasmuchasitreallyisthecentreofa

planetarysystem.in1998adustdiskaroundit

wasdiscovered,ataboutthesamedistanceas

thatbetweenoursunandtheKuiperbelt;there

areclumpsinit,whichsuggestthepresenceofa

planet.in2000alargeplanetwastrackeddown

bythewobbletechnique;itsmassisrathergreater

thanthatofJupiter,andithasaveryeccentricorbit.

athirdplanethasbeensuspected,furtheroutand

muchmoremassive.alltheseareunsuitedtolife,

butthereseemstobenodustclosetothestar,

andithasbeensuggestedthatanymaterialthere

hasbeensweptupbyearth-sizedplanets.the

ecosphereoferidaniliesatarangeofjustover

halfthedistancebetweentheearthandthesun.

122

Voyager2

launchedin1977,Voyager2flewpastUranus(1986)

andneptune(1989),returningthefirsteverclose-up

imagesoftheseouterplanets.itisthefirstman-made

objecttoleavethesolarsystemaltogetherandhead

forthestars.itcarriesgold-platedgramophonediscs

containingimagesandsoundsfromearth,incaseit

encountersalienlife.therecordsincludeaschematic

diagramshowingearth'spositioninthegalaxy,sothat

anyrecipientsoftheseinterstellargiftswillbeableto

thankusinperson.

theanthropicprinciple

inmoderncosmologytherehasbeenanattemptto

examinewhatareknownas‘anthropicarguments’.

thesearebasedontheso-calledanthropic

principle,whichstatesthattheUniversemustbe

thewayitis,becauseifitweretobeanydifferent,

wewouldnotbeheretoobserveit!togiveatrivial

example,iftheUniversewerethesizeofanatom,

creaturescomplexenoughtobeconsciouscould

notexist.sophisticatedversionsofthisargument

havebeendeveloped,andusedtosupportthe

hypothesisthatwereallyareunique.however,itis

difficulttoseehowsuchtheoriesmaybetested.

oflightwoulduseupaninfiniteamountofenergy–whichisanotherwayofsayingthatitsimplycannotbedone.obviously,usingourrocketswouldinvolveajourneylastingforcenturies,anddevicessuchasspace-arks,wheretheoriginaltravellersdieearlyinthevoyageandonlytheirdescendentssurvivetomake‘planetfall’,arefornowlikelytostayassciencefiction.interstellartravelwillrequireatechnologicalbreakthrough,whichmaycometomorrow,withinayear,withinacentury,withinamillionyears–ornever.Untilitdoes,fromamaterialpointofviewweareconfinedtothesolarsystem.

sofarascommunicationisconcerned,wehaveasyettriedonlyonemethod:radio.radiowavestravelatthesamespeedaslight,andthereforethetimeoftravelbetweenourselvesandthenearestpromisingstarsamountstoonlyafewyears.Moreover,radiocommunicationoveragoodmanylight-yearswouldbepossiblewithourpresentequipment.iftherewereastronomerslivingon,say,aplanetorbitingthestartauceti,11light-yearsaway,theycouldpickupsignalsofthestrengthwearecapableofsending.similarlywecouldpickupsignalsfromthem.

Wemightexpectanyartificialtransmissiontobebaseduponmathematics;afterallwedidnotinventmathematics–wemerelydiscoveredit.Variousingenioussystemshavebeentargetedandcodedmessageshavebeentransmitted,notonlytoε (epsilon)eridanibuttomanyotherstars.contactmustbeslow;ifwesendamessagetoepsiloneridaniin

9.2billionyearsa.b.–present

theemergenceoflife 123

triton

triton,neptune’slargestsatellite,wasthelastobject

tobephotographedbyVoyager2beforeitleftthe

solarsystem.Voyager2hadtravelled5.6billionmiles

in12yearstoreachit.

2006,itwillarrivetherein2017,sonoreplycanbeexpectedbefore2028,makingquick-firereparteeratherdifficult.butitisameasureofourchangedattitudethatexperimentsofthiskindareconsideredworthattempting.ifnoreplyiseverreceived,itmayindicatethatwearetryingthewrongexperiment,thatthereisnotechnologicalcivilizationwithinreasonablerange,orthatmankindreallyisunique.

Wecannotsendspacecrafttootherstars,butamoreadvancedcivilizationmightwellbecapableofinterstellartravel.Wehaveyettobeconvincedbyanystoriesofflyingsaucers,alienabductionsandinvadersfromalphacentauri,butwemustrememberthatoursisanewandnodoubtprimitivetechnology.ithasbeensuggestedthatweshoulddoourbesttoremainundetected,andevenrecallthefewprobessuchasVoyager2thatarenowleavingthesolarsystempermanently,butthiswouldbeillogicalevenifitwerepracticable(whichitisnot).perhapsweshouldbecomfortedbythewordsofpercivallowell;‘acivilizationcapableofreachingearthwouldhaveleftwarfarbehindandwouldcomeinpeace’.inanycaseitistoolateforusto‘keepsilent’.Wehavebeenbroadcastingsinceabout1920,sotoanyonewithinarangeof80light-yearsweare‘radionoisy’.

Weknowthatlifeonearthhasalimitedfuture,andthateventuallytheincreaseintheluminosityofthesunwillrenderourworlduninhabitable.Wemustlookahead,soletusconsiderthefutureoftheUniverse.