ChemicalEvolution
TheSovietbiochemistAlexanderIvanovichOparin(1894～1980)publishedthebook"TheOriginofLifeonEarth"in1936,andproposedThechemicalevolutionarytheoryoftheoriginoflife.Theso-calledchemicalevolutionreferstotheprocessofgraduallyevolvingfrominorganicmatterandsimpleorganicmattertoprimitivelifeformsundertheconditionsoftheprimitiveearth.Accordingtothepossibledevelopmentmodelofmatterfromsimpletocomplex,thischemicalevolutionprocesscanberoughlydividedintothefollowingfourstages.
Evolutionaryprocess
(1)Smallmoleculelifebuildingblocks
Frominorganicsmallmoleculesubstances(suchashydrogen,ammonia,etc.)toorganicsmallmoleculesubstances(suchasaminoacids),Nitrogenousbases,riboseordeoxyribose,etc.).Thisaspecthasbeenprovedbymoreandmoreexperimentssimulatingprimitiveearthconditions.
In1952,AmericanchemistStanleyLloydMiller(1930-2007)putthechemicalelementsthatmayhavebeenintheearlyearth’satmosphereinalargeflask:water,ammonia,methaneandhydrogenAfterafewdays,anorangeviscousmixturegatheredatthebottomoftheflask.10-15%ofthecarboninthemethanewenttoorganiccompounds,includingthe9aminoacidsthatmakeupproteins.Thepre-lifesoupofinorganicmixturesdischargesandcreatessomelifesubstances.Althoughtheyarenotlifeitself,theyarecompoundsthatmakelifepossible.
(2)Biologicalmacromolecules
Fromorganicsmallmolecularsubstances,biologicalmacromolecularsubstancesareformed.Thesmallbiologicalmolecules(suchasaminoacids,etc.)generatedintheoriginalreducingatmospherearewashedbyrainanddissolvedintheoriginalocean.Ifthesesmallbiologicalmoleculesarefurthertransformedintobiologicalmacromolecules(suchasaminoacidsintoproteins),theymustbedehydratedandcondensed.;Anddehydrationandcondensationintheprimitiveoceanisasdifficultasdryinggrapessoakedinwater.Scientistshaveputforwardvarioushypothesestotrytosolvethisproblem.Therearemainlytwokindsofmorecredibleandexperimentallyproved:
①IsraeliscientistA.KatchalskybelievesthattheaminoacidsintheprimitiveoceanItwascondensedintopeptidesonsomespecialclays(theoriginalearthandthepresentearthhavesuchclays).Theyfirstreactedaminoacidswithadenylateinthelaboratorytoproduce"activated"aminoacids,namely"aminoacyladenylicacid",whichcanbefoundonsomelamellarclayssuchasmontmorillonite.Condensedintopolypeptidechainsofvaryinglengths.
②JapanesescientistAkaboriShiroandothersproposeda"polyglycinetheory"thatcanbypassthedifficultyof"dehydrationcondensation"toillustratetheformationofpolypeptidechains.Theybelievethattheformaldehydeproducedintheoriginalatmospherereactswithammoniaandcyanidetoformanorganicsubstancecalled"aminoacetylcyanide".Thissubstancecanpolymerizeandthenhydrolyzetoformpolyglycine(thatis,multipleglycinesarepolymerizedinThepolypeptidechainformedtogether),andfinallyaproteincomposedofvariousaminoacidresiduesisobtainedthroughthechangeofthesidegroup(R).
(3)Multi-molecularsystem
Themulti-molecularsystemiscomposedoforganicpolymermaterials.Itisconceivablethatorganicpolymersubstancessuchasproteinsandnucleicacidsaccumulatemoreandmoreintheprimitiveocean.Undercertainconditions(suchashightemperatureandappropriatepH,etc.),theyinteracttoformamulti-molecularsystem,boundedmembranesandthesurroundingenvironment.Spacedapart,theyarebigandsmallballs,floatinginthepristineocean.Thisideahasalsobeenprovedbypreliminaryexperiments.
(4)Primitivelife
Evolvedfromamulti-molecularsystemtoprimitivelife.Thisisthemostcriticalstepintheoriginoflife,andthisprocesshasnotbeenverifiedinthelaboratory.Theoreticallyspeaking,theessenceofthisstepisthequestionofhowto"turnfromdeadtolife"inamulti-molecularsystemwithproteinandnucleicacidasthemaincomponents,thatis,howdoesmetabolismandself-proliferationoccur?Fromabiologicalpointofview,therearetwoimportantissuestoberesolved:oneistheproductionofbiofilms,andtheotheristheoriginofgeneticinstitutions.
Darwin—thewarmlittlepoolwherelifeoriginated
In1871,theBritishnaturalistDarwindiscussedinalettertotheBritishbotanistJosephDaltonHookerWhenitcomestothequestionoftheoriginoflife,hewrote,“Peopleoftensaythatalltheconditionsforthefirstgenerationoflivingthingsarenowavailable,andtheywillbethesameinthepast.Butif(goodguy!Whatagreatif!)wecancomeupwithacertainAgentlelittlepondwithallthingslikeammonia,phosphate,light,heat,electricity,etc.,andimagineformingaprotein-likecompound,readytoundergomorecomplexchanges;suchsubstanceswillbeswallowedorabsorbedimmediatelyThissituationwillnothappenbeforetheformationoforganisms"(Meyer1990).
Darwin'sviewthatlifemayoriginatefromasmallwarmpoolhasbeensoughtafterbysomescholars.Althoughtheexperimentalscienceofthe20thcenturyhasconfirmedthatorganicmolecules(aminoacids,peptides)canindeedbeformedfrominorganicsubstances(ammonia,etc.)undersuitablelight,heatorelectricityconditions,butthisactuallyrevealsthetrueoriginoflife.It'sstillafarcry.
TheAmericanchemistRobertH.Shapiro(1935-2004)notedDarwin’sseeminglyinadvertentstatementinhis1986writing,"Itisquiteconsistentwithtoday’sknowledge.Ifitisnotbecauseofhisvision,itmeansthattheresearchonthesourceoflifehasnotmadeanyprogressovertheyears"(JanPhilip1999).
Thebirthofthecell
Buthowdidthecelloriginate?Thisisthebiggestproblemfacingtheoriginoflife.Theoriginofthefirstcellalsoproclaimedtheoriginoflifeonearth.Therefore,inthissense,theoriginofthecellisequivalenttotheoriginoflife.
Recently,somescholarshaveproposedthehypothesisoftheoriginofphototrophiccells(XiePing2014).Inacertainsense,theoriginoflifebeginswiththeestablishmentofapersonalizedsystemoforganicmatter.Inotherwords,withoutindividuality,therewillbenobirthofreallife.Justask,iforganicmoleculesjustripplinginthe"soup"erratic,howcanlifeappear?Whatkindoflifedoesnotexistasanindividual?Therefore,theemergenceofsimpleplasmamembranestructuresthatcanleadtotheformationofindependentlivingsystems(regardlessofthemechanismorprocess)mustbethemostcriticalstepincellevolution.Notonlytheoverallindependenceoftheprocellularbodyisrequired,butsomeimportantlifeprocesses(especiallyphotosynthesis)alsorequireintracellularindependence,thatis,theinnermembranesystem.Lookatthespecializedphotosyntheticmachineryofcyanobacteria—thylakoids.
Intheprecellularbodywithasimplemembranestructure,theselectionprocessdirectedbyphotosynthesispromotesthefunctionaldifferentiationofthemainlivingsubstances-lipidsconstructcellmembranes,proteinsmainlyactascatalysts,DNAstoresgeneticinformation,andRNAconstitutesThebridgebetweenDNAandprotein,sugarstemporarilystorethechemicalenergytransformedfromphotosynthesis...Thisisnotjustarandomprocess(althoughitistoalargeextent),itisacleverfusionofchemistryandlifewithapurposefulorientation(themosteffectiveuseofsolarenergy).Itistheorderingandorderingofbiochemicalreactions.Theproceduralprocessoflifelaidthefoundationstone.
Here,Iborrowthedevelopmentmodelofreactioncycle→catalyticcycle→supercycledescribedbyAigen.Lifeisconnectedtoatime-limitedlifebalancesystemthroughatime-limitedso-called"eternal"reactioncyclebasedonaseriesofcomplexanddelicatepositiveandnegativefeedbackrelationships,inordertomosteffectivelyconvertsunlightenergyintochemicalenergy.Takealookattheseveralcyclicreactionsthatoccurinthephotochemicalreactioncenterofcyanobacteria(thesplittingofwater,thecirculationofelectrons,etc.),thefixationofcarbon(theCalvincycle)andrespiration(thetricarboxylicacidcycle)...andtheconnectionsbetweenthesecyclesAndpositiveandnegativefeedbackcontrol,etc.,thisistheorderingprocessofbiochemicalreactions.
Anotheressentialfeatureoflifeistoprogramgeneticmaterialtodirectthelife-buildingprocess,thatis,tostorethelife-buildingprocessintheDNAinaninformationmanner.Thisistheneedforpersonalizedlifetoobtainself-reproductioncharacteristics.Importantstep.Thelastisthegradualformationofthecelldivisionmechanism,whichbenefitsfromthesemi-permeabilityofthecellmembrane(theentryofsmallmoleculesofnutrientsandtheaccumulationoflargemolecularcarbohydrates),thecontradictionbetweenthetwoandtheinevitableresultofthecellruptureprocessCountlessrepetitions.Onlypreciseandcyclicalcelldivisioncanmakestableinheritancepossible.Onlythencancellslikewhatwecanseeinexistingorganismsbesuccessfullyborn,andthecelloperatingmechanismcanbeprogrammed.