1 The Nature and Origin of Life Instructor: Carl B. Pilcher OLLI at SSU/Oakmont Life in the Universe: The Science of Astrobiology Week 2—January 21, 2016
1
The Nature and Origin of Life
Instructor: Carl B. Pilcher
OLLI at SSU/Oakmont
Life in the Universe: The Science of Astrobiology
Week 2—January 21, 2016
Outline• DiversityofLife• An2quityofLife• EarlyLifeandtheLastUniversalCommonAncestor(LUCA)
• Originoflife• Canwedefinelife?• Doesithavetobecarbonandwater?• Detec2ngalienlife
Temperature (°C)
47 27 37
Normal
Medical Emergency
Death
Human Physiological Limits
Normal 7.4
7.3
7.5
Blood pH
Acidosis
Alkalosis
SlidecourtesyofT.Hoehler
0
2
4
6
8
10
12
14
-30 10 50 90 130
1
1
1
Temperature (°C)
pH
You Are Here
(salinity, pressure, radiation, dessication…)
Range of Documented
Biological Activity
SlidecourtesyofT.Hoehler
“LostCity”pH10-11T~50-90CCarbonatechimneys
BlackSmokerspH3-5T~350CMetalsulfide
chimneys
SubmarineHydrothermalVents
Life on Earth as we thought about it 30 years ago
Five Kingdoms Fungi Animals Plants
Protists
Monera (Bacteria)
Cow Mushroom Tomato
Amoeba, Ciliates, Giardia
E. coli, Bacillus, bacteria in general
Euka
ryot
es
Prokaryotes
The Universal"Tree of Life"(adapted from Pace;"Science 276, 734, 1997)"
Bacteria
Archaea
Eucarya
Youarehere
Most Energy
(chocolate!)
Least Energy
(celery!)
Worst Oxidant
Best Oxidant
MetabolicDiversity
Reductants
Narrow, 2-5µm Cylindrical Cells APEX CHERT Microfossils (~3,465 Ma)
Medium, 3-6µm Discoidal Cells Broad, 6-9µm Cylindrical Cells
SlidecourtesyJ.WilliamSchopf
Clement Conditions in the Hadean (>3.9 BY)? Mark Harrison (UCLA), Steve Mojzsis (U. Colorado) and coworkers John Valley (U. Wisc.), Aaron Cavosie (U. Puerto Rico) and coworkers
Twopossibleearlymetabolisms
4H2+CO2CH4+2H2Omethanogenesis
H2+S0 H2Ssulfurrespira2on
Thesetwometabolismsarerelatedbiochemically:bothinvolvebreakingbondsbetweensulfuratoms
Wastheancestorofalllifeacommunityofmethanogensandsulfur-u6lizingmicrobesthatlivedathigh
temperature?
AllLifeonEarthSharesManyCommonCharacteris2cs
• Gene2cinforma2onencodedinDNAmadeupofapar2cular5-carbonsugar(ribose),phosphategroups,andfourpar2cularnucleo2des(the“lehers”inourgene2calphabet)
• Biochemicalreac2onscatalyzedbyproteinsmadeupoftwentyspecificlej-handedaminoacids
• Cellmembranesmadeofphospholipidbilayers• EnergycarryingmoleculeisATP(adenosine
triphosphate)producedviatheproton-mo2veforce
• Gene2cinforma2onistranscribed(copied)intoRNAandtranslatedintoproteinsbytheribosome
• Energyisstoredincarbohydratesmadeupofasmallnumberof6-carbonright-handedsugars
These must all have been characteristics of LUCA
• HorizontalgenetransfermayhavebeenmuchmoreprevalentearlyinthehistoryoflifeonEarthwhencellsweremuchlessdeveloped(e.g.,“leaky”membranes)thantheyarenow.
• Thismeansthat“ver2cal”inheritance/evolu2onimpliedbythetreestructuremaybeanoversimplifica2on.
TheLastUniversalCommonAncestor(LUCA)
• LUCAwasnotadiscreteorganism.
• Itwasacommunityofprimi2vecellsrapidlyexchanginggene2cinforma2on.
• ItdidnotexhibitDarwinianevolu2on(dominanceofver2calgenetransfer)
• Over2me,LUCArefinedintoincreasinglycomplexcelltypesthatledtotheancestorsofthreedomainsoflife.
LUCAisveryfarremovedfromtheoriginoflife!
SomeChallengesfortheOriginofLife• Manybiochemicalmolecules(polymers)areformedbyremovingawatermoleculefromapairofsmallermolecules(monomers)
• Butitisverydifficulttodothisabio2callyinthepresenceoflotsandlotsofwater,e.g.,anearlyocean.
• Ifyoutakeamixtureoforganicmolecules(the“rawingredientsoflife”)andaddenergy(e.g.,heat),youdon’tgetlife,yougettar!
• Akeypropertyoflifeisthatthemoleculesinourcellsarecapableofreproducingthemselves,aprocesscalled“autocatalysis.”Itisverychallengingtoproduceautocataly2csystems(wecan’treallydoitinthelaboratory).
LifeasanEmergentPhenomenoninaComplexSystem
1. Acomplexsystemhasmanyinterac2ngpar2clesor“agents.”
2. Whenenergyflowsthroughthesystemofpar2cles,newpahernsorbehaviorsemergethatarenotmanifestbytheindividualagents.
3. Emergentphenomena,bydefini2on,cannotbepredictedevenwithperfectknowledgeabouttheindividualpar2clesinacomplexsystem.
4.Forexample,perfectknowledgeofanindividualsandgrainwouldnotallowyoutopredictthepahernsofsanddunesinthedesert(createdbywindenergy).
Severalwaystothinkaboutandinves2gatetheoriginoflife
• Lifeisanemergentpropertyofacomplexprebio2cchemicalsystemindisequilibrium.1. Onecanstudytheprebio2ccomponentsofthesystemtotryto
understandhowcomplexityleadstotheproper2esoflivingsystems,e.g.,autocatalysis(self-reproduc2on),encapsula2on.{BoCom-up}
2. Onecanstudylivingsystemsandahempttoinferwhatweretheproper2esoftheveryfirstlivingsystems.{Top-down}
• Onecanconductbohom-uportop-downinves2ga2onsfromthreedifferentperspec2ves:1. Lifebeganwithmetabolism;gene2cmoleculeswereincorporated
later.2. Lifebeganwithself-replica2nggene2c(informa2onal)molecules;
metabolismwasincorporatedlater.3. Lifebeganasacoopera2vechemicalphenomenonarisingbetween
metabolismandgene2cs.
Miller-UreySynthesis• Originalexperimentsusedsimulatedatmosphereof
hydrogen(H2),methane(CH4),ammonia(NH3),andH2O• Thiscomposi2onisnowthoughttobemuchmore
reducedthanEarth’searlyatmosphere,whichwaslikelycomposedofcarbondioxide(CO2),nitrogen(N2),andtracesofreducedgases.
• Similarexperimentswithmorerealis2cmodelatmospheresalsoproduceaminoacidsandotherbiologicallyrelevantmolecules,butinsmalleramounts.
• H2OandCO2canreacttoformformaldehyde(CH2O),whichcaninturnpolymerizetoformcarbohydrates
• Conclusion:itisnotdifficulttoformprebio6cmoleculesunderchemicalcondi6onsthoughtrealis6cforearlyEarth
Jason P. Dworkin et al. PNAS 2001;98:815-819
SpontaneousVesicleForma2on
Laboratorysimula2onsofinterstellaricesJ.P.Dworkinetal.,
2001
Muchisonmeteorite
organicextractD.W.Deamer,1999
Phase Fluorescence
Iron-Sulfur(FeS)Catalysts• SomeoftheearliestproteinsthatenablelifetousesimplemoleculeslikeH2andN2andtotransferelectrons(e.g.,inmetabolism)containFe-Sclustersattheircataly2ccenters(e.g.,Fe2S2)
• ThestructureoftheseclustersisverysimilartothestructureofFe-Smineralssuchasgreigite(Fe3S4)
• Fe-Sminerals,includingpyrite(FeS2),areknowntobeexcellentcatalystsandwerepresentontheearlyEarth
• Wereprebiologicalchemicalnetworks,catalyzedbyFe-Sminerals,theprecursorsofbiologybasedonthesamechemistry?
ThePossibleRoleofClayMinerals
• Claysarecommonmineralsformedbytheac2on(weathering)ofwateronigneousandmetamorphicrocks.
• Theyarehighlylayeredwithastrongsurfaceelectrosta2ccharge.
• Duetoitslayeredstructure,agramofclaycanhaveasurfaceareaofseveralsquarefeet!
• Asaresult,clayreadilyadsorbsorganicmolecules.• Whenmoleculesadsorbinproximityonaclaysurface,they
readilyreact,forexample,bypolymerizing.• Claysmaythushaveplayedanimportantcataly2crolein
crea2ngproto-biopolymers,byconcentra2ngreactantsonclaysurfacesandpromo2ngchemicalreac2ons.
TheRNAWorld• Inmodernlife,DNAstoresgene2cinforma2onandproteinscatalyze
biochemicalreac2ons.• Butproteinsynthesisrequiresthegene2cinforma2onofDNA,andDNA
synthesisrequiresproteinsascatalysts.• Thisleadstoa“chickenandegg”problem:Whichcamefirst?• RNAcanbothstoregene2cinforma2onandcatalyzebiochemical
reac2ons.It’saverysimilarmoleculetoDNA,butmuchmorechemicallyreac2ve.– Thecataly2cac2vityoftheribosome,theproteinfactoryinalllivingcells,is
duetoanRNAcatalyst(a“ribozyme”)• Aleading“informa2onfirst”theoryoftheoriginoflifepositsthatthe
DNA-proteinworldwasprecededbyanRNAworldinwhichRNAperformedthefunc2onsofbothoftheotherbiopolymers.
• TheproblemwiththistheoryisthatthesynthesisofRNAfromsimplemoleculesinaprebio2cworldisverydifficult.AnRNAworldwasprobablyprecededbyasimplerlivingworldthat“invented”RNA,leavinguss2llwithalargegaptoexplainbetweentheabio2candlivingworlds.
CampbellEssen6alBiologywithPhysiology
EricJ.Simon,JeanL.Dickey,JaneBReece
Manydefini2onsoflifearelistsofproper2es.Here’sonesuchlistfrommydaughter’sbiologytextbook:
• Proper2esofLife– Order– Regula2on(internalenvironment)– Growthanddevelopment– Energyprocessing– Responsetotheenvironment– Reproduc2on– Evolu2on
• Bythisdefini2on,isonerabbitalive?Isamule?– Onerabbitcan’treproduce,norcananynumberofmules.
• Whataboutaforestfire?– Itregulatesitsinternalenvironment,growsanddevelops,processesenergy,respondstotheenvironment,andreproduces.
ANASAcommiheecameupwithadefini2onthatahemptedtoavoidthesedifficul2es:
“Lifeisaself-sustainingchemicalsystemcapableofDarwinianevolu6on.”
NotethatDarwinianevolu2oninvolves
– Amoleculargene2csystem– Imperfectreplica2on– Thereplica2onof“mistakes”– Differen2alfitnessbetweenreplicates
G.F.Joyce,1994
Wouldpre-LUCAlifesa2sfythisdefini2on?
• ClelandandChybaarguethatwecannotdefinelifebecausewedon’thaveanunderlyingtheoryofbiology.
• Theylikenourignorancetothatofsomeonetryingtodefine“water”priortothedevelopmentofmodernchemistryinthe19thCentury.
• Onecoulddescribewater’sproper2es(liquid,quenchesthirst,putsoutfires,freezes,boils,etc.)
• Butnotun2lthedevelopmentofatomicandmoleculartheorycouldwesaythatwaterisH2Oandexplainthedetailsofthestructurethataccountsforitsproper2es.
• Soasa2sfactorydefini2onoflifemayhavetoawaitanunderlyingtheoryofbiology.
C.E.ClelandandC.F.Chyba,OriginsofLifeandEvolu6onoftheBiosphere32:387-393,2002
WaterandCarbon• Waterismadefromthefirst(hydrogen)andthird(oxygen)mostabundantelementsintheuniverse.
• Wateriseverywhere!– Andweknowit’sanexcellentsolventforlife.
• Carbonistheonlyelementthatformscomplex,long-chainmoleculesinwater.
• Earth’scarbon-basedlifeisextremelyspecific!• Onecaneasilyimaginecarbon-basedlifeinwaterthatisverydifferentfromlifeonEarth.
• Socarbonandwaterareagoodstar2ngpoint,butweshouldnotexcludeothercombina2onsfromourconsidera2on.
Abiotic distributions are smooth Biotic distributions are spiked
McKay 2004 PLoS Biol 2(9)1260-12623
McKay 2004 PLoS Biol 2(9)1260-12623
alien!
Abiotic distributions are smooth Biotic distributions are spiked