INTHEBEGINNING... 　　TheEarthbeganasatwinkleintheSolarNebula'seyesome4.5billionyearsagoandit-alongwiththerestoftheplanets,asteroids,meteors,comets-formed,itisthought,throughthetendencyofmattertoclumptogether,evermoreuntilfinallythereweresubstantialbodies,theplanetsandtheirmoons,sweepingupallleft-oversintheirorbits.Duringthisera,approximatelyonebillionyearslong,thenewly-borningEarthwaspummeledmercilesslybytheseleft-overs.Thiswastheso-called"HadeanPeriod"(andwellnamedatthat!),a"hell-ish"timeindeedwhentheEarth'ssurfacewasperiodicallybroiled,flash-friedsotospeak.Incomingasteroidsofsufficientsizewouldactuallyvaporize,themselvesandthepartofthesurfacetheyimpactedandthiswouldturnintoaseeringplasmathatwouldtsusamiaroundtheglobe-notaprettypicture.Nottomentionvolcaniceruptions. 　　Thismadeitalittlehard,onewouldthink,forlifetogetatoeholdontheyoungplanet'ssurface. 　　AbioticProductionofOrganicMolecules 　　Theclassicexperimentdemonstratingthemechanismsbywhichinorganicelementscouldcombinetoformtheprecursorsoforganicchemicalswasthe1950experimentbyStanleyMiller.Heundertookexperimentsdesignedtofindouthowlightning--reproducedbyrepeatedelectricdischarges--mighthaveaffectedtheprimitiveearthatmosphere.Hedischargedanelectricsparkintoamixturethoughttoresembletheprimordialcompositionoftheatmosphere.Inawaterreceptacle,designedtomodelanancientocean,aminoacidsappeared.Aminoacidsarewidelyregardedasthebuildingblocksoflife. 　　Althoughtheprimitiveatmosphereisnolongerbelievedtobeasrichinhydrogenasoncethought,thediscoverythattheMurchisonmeteoritecontainsthesameaminoacidsobtainedbyMiller,andeveninthesamerelativeproportions,suggestsstronglythathisresultsarerelevant.TheBeginningsofLifeonEarth 　　Othershavemadesimilarexperiments.AgroupattheDepartmentofChemistryandBiochemistryattheUniversityofCalifornia,SanDiego,exposedsulfur-bearingmoleculeslikethosethoughttohavebeenpresentbeforetheEarthformedtolowlevelsoflight.Thepresenceofthelightwasenoughtogenerateorganiccompounds-moleculescontainingcarbon,whichformthechemicalbasisoflifeasweknowit.MeteoriteRevealsLifeNotDifficulttoMake 　　Thenewcompoundshadadistinctisotopic(atomicmakeup)signature,notnormallyfoundonEarth.Infact,thepeculiarpartisthattheseisotopeshaveonlybeenfoundoneothertime,incompoundsremovedfromtheMurchisonmeteorite. 　　HypothesesonOriginofMacromolecules 　　1.RNA-FirstHypothesis 　　RNAcouldcarryoutprocessesassociatedwithLife 　　NobelPrize1989(Cech,UnofColorado&Altman,Yale) 　　RNAcanactasasubstrateand/oranenzyme 　　2.Protein-FirstHypothesisSidneyFox(above) 　　Proteinoidsformfromaminoacidsat180o 　　ProteinoidscanformMicrospheres 　　3.ClaycatalyzedRNA&Proteinsynthesis(BothFirst) 　　GrahamCairns-Smith(UniversityofGlasgow) 　　ClayishelpfulinpolymerizingProteins&NucleicAcids 　　Attractssmallorganicmolecules 　　Containszinc&iron(metalcatalysts) 　　CollectsenergyfromradioactivedecayandreleasesitwhenTemperatur