the organic composition of carbonaceous meteorites:...

The Organic Composition of CarbonaceousMeteorites: The Evolutionary Story Aheadof Biochemistry

Sandra Pizzarello1 and Everett Shock1,2

1Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-16042School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404

Correspondence: [email protected]

Carbon-containing meteorites provide a natural sample of the extraterrestrial organicchemistry that occurred in the solar system ahead of life’s origin on the Earth. Analyses of40 years have shown the organic content of these meteorites to be materials as diverse askerogen-like macromolecules and simpler soluble compounds such as amino acids andpolyols. Many meteoritic molecules have identical counterpart in the biosphere and, in aprimitive group of meteorites, represent the majority of their carbon. Most of the compoundsin meteorites have isotopic compositions that date their formation to presolar environmentsand reveal a long and active cosmochemical evolution of the biogenic elements. Whetherthis evolution resumed on the Earth to foster biogenesis after exogenous deliveryof meteoriticand cometary materials is not known, yet, the selective abundance of biomolecule precur-sors evident in some cosmic environments and the unique L-asymmetry of some meteoriticamino acids are suggestive of their possible contribution to terrestrial molecular evolution.

INTRODUCTION

Why Meteorites are Part of the Discourseabout the Origin of Life

The studies of meteorites have long been partof investigations and discussions about the

origin of life for the reason that some of theseextraterrestrial bodies have reached the Earthcontaining abundant carbon since its accretion,provide a natural sample of abiotic organicchemistry, and may offer insights on the possibleenvironments and physico-chemical processes

that fostered biogenesis. These conditionsare entirely unknown because geological andbiological processes of over four billion yearshave long eradicated any traces of early Earth’schemistry. On the other hand, we know thatlife has embarked in a long evolutionary pathall through its recorded history and it seems rea-sonable to extend to its unknown beginning thesame evolutionary nature. Albeit a posterioriand without knowledge of the actual chemicalsteps that carried this evolution, therefore, thesingle assessment one can safely make about

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life’s origin on the Earth is that it must have beenan emergent process, through which biogenicatoms and molecules gained the complex asso-ciative and interactive states we observe in eventhe simplest forms of extant life. It is then easy tosee why the discourse about the origins of lifehas been multidisciplinary, broad based, andfostered many theories, all of which, with thenotable exception of the panspermia hypothesis(e.g., Crick and Orgel 1973), accept the funda-mental emergent nature of life from simplemolecules.

In exobiological (as well as astrobiological)terms, it has been proposed that life’s fundamen-tal evolutionary nature might have extendedbeyond its origin and might be rooted in theabioticcosmochemical evolution of the biogenicelements. C, H, N, O, P, and S are known to bepresent as diverse and often complex organicmolecules in a varietyof extraterrestrial environ-ments (Lazcano 2010) and their long cosmichistory has supported the idea of a possible exo-biology. However, its analytical basis comes fromthe study of carbon-containing meteorites thathave provided the only natural sample of chem-ical evolution large enough for direct laboratoryanalyses. Uniquely, therefore, carbon containingmeteorites record for us the abiotic organicchemistry that preceded life’s origin and may asyet reveal whether it is realistic to assume thatthese or similar materials, i.e., either by directdelivery or analogy of formation, might havefostered or even inducted molecular evolutiontoward biogenesis.

The Early Solar System, Meteorites, and thePossible Survival of Cosmochemical Evolution

The meteorites that reach the Earth are for themost part fragments of asteroids, i.e., of thosesmall planetesimals that orbit the Sun in greatnumber between Mars and Jupiter. By theTitius-Bode law of a regular spacing of planetsfrom the Sun, their orbit should be occupiedby a planet; it is believed, however, that the smallchunks of early solar materials reaching this areafell under the strong gravity of the alreadyformed giant planets and were either scatteredthroughout or left unable to coalesce. That is

how we still find them today, joined by icyobjects from more distant locations of the solarsystem that were brought in by further dynam-ical evolution of giant planets’ orbits (Levisonet al. 2009). With their crowding, hazardousorbits, and constant collisions, all of these bodiesput fragments on route to the Earth and havedone so through the ages. The importance ofmeteorites for the study of prebiotic chemistryis a result of this failed planet formation and notjust for their obvious delivery but also becausemany of the asteroid belt objects never had theircomposition drastically transformed by gravita-tional high temperatures and pressures as largerbodies did. Their meteoritic fragments, there-fore, may carry unaltered a pristine record ofearly solar system chemistry as well as allowthe deciphering of its cosmic history.

The meteorites that best fit this descriptionare the carbonaceous chondrites (CCs), a prim-itive subgroup of stony meteorites having anelemental composition that is very similar tothat of the Sun and the universe overall. As theirname indicates, CCs have the distinction of con-taining several percent amounts (�1.5%–4%)of carbon, which is for the most part presentas organic materials. These meteorites are aggre-gate rocks, i.e., consist mainly of a matrix madeof packed together hydrous and anhydrous sili-cates that do not show signs of metamorphismor alteration by high heat. However, as part oftheir small planet parent bodies, CC mineralogyalso shows that these rocks had experienced aliquid water phase as well as the effects of impactshocks. For example, a recent measurement ofthe optical activity of three CC surfaces (Arteagaet al. 2010) showed a circular birefringence biasto negative values that the authors attributeto chiral fractures and distortions in the claysfollowing mechanical forces. The meteorites’aggregation also captured various inclusions;the chondrules, to whose name CCs owe theirclassification, are round beads of glassy appear-ance that have re-crystallized from a melt, i.e.,high heat, and bring witnesses to the variety ofmaterials and processes that must have contrib-uted to CC parent bodies’ formation (Fig. 1).

Overall, these meteorites do not seem todiffer much from terrestrial rocks, a similarity

S. Pizzarello and E. Shock

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that has not helped their collection or preserva-tion because, if not seen to fall and promptlycollected, they easily disappear in the environ-ment. The Murchison meteorite was excep-tional in this respect because it fell at the veryeve of lunar samples’ return in 1969 and wasanalyzed directly by NASA laboratories as a pos-sible analog of those samples. One hundredkilograms of this meteorite were recovered andhave been used in 40 years of analyses for prob-ably the most comprehensive study of any extra-terrestrial organic material to date. As a result ofthis focus, the Murchison meteorite composi-tion has long been considered representativenot only of meteorites of the same type (Pizza-rello et al. 2006) but, often (e.g., Luisi 2007),also of the capabilities of abiotic organic synthe-ses in general. Given our yet tentative knowledgeof cosmochemical environments, it is not sur-prising that the latter assumption turned out tobe premature and a new group of pristine mete-orites found in the ice fields of Antarctica, theRenazzo-family of chondrites or CR, have beenoffering a novel view of the possible syntheticoutcomes of abiotic chemical evolution as wellas of its prebiotic relevance.

BACKGROUND

The Abiotic Organic Composition ofMeteorites: Prebiotic Traits and BiochemicalCounterparts

In spite of exhaustive chemical analyses, we stillhave a very vague idea of where Murchisonorganic materials are actually located vis-a-vis the inorganic components in the mete-orite. The only successful description so far was

obtained by X-ray microscopy of the meteoritesurfaces after their exposure to selective stainingwith OsO4 vapors (Pearson et al. 2007). Fromthese analyses, they appear to be broadlydistributed within the matrix, intermixed withhydrous silicate components. As in other CCs,Murchison organic materials can be broadlydescribed in terms of their solubility in aqueousand organic solvent systems, a practical charac-terization that nevertheless leaves room formissed analytical targets and the possibility ofunknowns (e.g., Deamer 1985). Insoluble andsoluble components represent respectively 70%and 30% of total carbon and, within their mole-cular range, are both very complex and funda-mentally heterogeneous.

Murchison Insoluble Organic Material (IOM)

The larger portion of Murchison organic carbonis often referred to as kerogen-like because, liketerrestrial kerogens, it is an insoluble macromo-lecular material of complex composition that isnot known in much molecular detail; its averageelemental abundances are C100H46N10O15S4.5.The bulk of the IOM can be inferred only indi-rectly from spectroscopy (e.g., nuclear magneticresonance and infrared) and by decompositionstudies, where it is pyrolyzed by heat or oxidizedinto its fragments. These analyses suggest a gen-eral structure composed of aromatic ring clus-ters, bridged by aliphatic chains containing S,N, and O, with peripheral branching and func-tional groups. By transmission electron micro-scopy, most of the IOM appears dispersed andamorphous but �10% of it is found as self-contained nanostructures (Fig. 2), spheres aswell as tubes, of diverse elemental compositionthat varies from close to pure graphitic C(. 99%) to containing several percent amountsof O, N, and S. The IOM also contain minuteamounts of “exotic” carbon, so called becauseit was likely formed in the envelopes of starsprior to the formation of the solar system.

On the whole, the large compositional het-erogeneity of the IOM as well as the diversity ofits phases strongly suggest that this material isthe complex end product of cosmochemicalregimes and environments that varied greatly.

Figure 1. A CR2 meteorite stone found in theAntarctica Graves mountains (GRA 95229). The openfaces show the large chondrules that characterize thisfamily of meteorites. Chondrule and other inclusionabundance reduce the amount of matrix whereorganics are found to about 30% of the geology.

The Organic Composition of Carbonaceous Meteorites

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On the other hand, in spite of being insolublein acids and solvents, the IOM can free severalindividual compounds under conditions of hightemperature and pressure similar to those ofterrestrial hydrothermal vents (3008C, 100 MPa)(Yabuta et al. 2007). These are mainly a varietyof aromatic and heteroaromatic hydrocarbonsbut also smaller noncondensed molecular spe-cies and a suite of alkyl dicarboxylic acids upto C18 chain length. In addition, the hydrother-molysis changed the IOM’s chiral responseto the Soai autocatalytic reaction in that itdisplayed a statistical R-chiral bias prior to thetreatment but not afterwards (Kawasaki et al.2006), suggesting that some chiral species arepresent in Murchison IOM but cannot bedetected at the molecular level. These experi-ments show that portions of the IOM macro-molecular structure can be modified at themolecular level, exchange species with the solu-ble organic pool, and possibly represent materi-als caught in flux between aggregation states. Wemay also assume that, were meteoritic materialsexposed to hydrothermal conditions or pro-longed exposure to water upon their fall, IOMrelease might have made an important contri-bution to the organic pool of the early Earthwhen CCs delivered an estimated 1%–3% oftheir weight in carbon during the early impactperiod (Mautner et al. 1995).

Murchison Soluble Organic Compounds

The soluble organic compounds of the Murch-ison meteorite make up an abundant anddiverse group of well over a thousand molecularspecies that vary from smaller water-solublecompounds such as amino acids and polyolsup to 30-carbon-long nonpolar hydrocarbonsextracted only with solvents (Table 1). As theirlarge number indicates, they are also presentin multiple isomeric forms up to the limit oftheir solubility. This diversity is observed thro-ughout most of the various compound typesand is often a sign of their indigeneity becauseit contrasts starkly with the structural and func-tional selectivity displayed in biochemistry. Ithas been analyzed in particular detail forMurchison amino acids. For example, all thepossible a-amino alkylamino acids up to seven-carbon were identified in Murchison extractsbased on the reference of synthesized standardsand several eight- and nine-carbon homolo-gous species could also be easily recognizedby chromatography-mass spectroscopy on thebasis of their spectra even if their standardswere not available. Similar large abundancesof N-substituted, cyclic, b-, g-, d-, and 1-aminoacids were also found and the total number ofmeteoritic amino acids can be placed at overone hundred. In contrast, the whole of terres-trial protein is made up of just 20 amino acids.

Within this overall diversity, several compo-nents of Murchison’s organic suite have identi-cal counterparts in the biosphere. Eight of themeteorite amino acids are also found in pro-teins (glycine, alanine, proline, valine, leucine,isoleucine, aspartic acid, and glutamic acid)and numerous other compounds are encoun-tered in terrestrial metabolisms, as shown inTable 1. A very interesting similarity with bio-chemical traits was found in a group of chiralamino acids not present in terrestrial proteins,the 2-methyl 2-amino acids, which displayin Murchison L-enantiomeric excesses (ee)that, if not as large, have the same configuration(L-) of terrestrial protein. The ee were firstdiscovered in the diastereomers of the seven-carbon 2-amino 2,3-dimethylbutaoic acid(Fig. 3) (Cronin and Pizzarello 1997) and were

1000 nm

Figure 2. Scanning electron microscope image of theGRA 95229 acid residue showing an abundance ofsubmicron-sized spherical carbonaceous particles.The particles are solid, single, and agglomeratedwith the largest close to 500 nm in diameter. Theresidue is deposited onto a carbon planchette andimaged with 5 kV electrons and current of 98 pA(reprinted with express permission from LaurenceGarvie).





Table 1. Classes of organic compounds in the Murchison meteorite.

Compound Class Structure & Example Molecule

Carboxylic acids H3C—COOH Acetic acid

Amino acidsH3C–C–COOH

NH2–

H

–

Alanine

Hydroxy acidsH3C–C–COOH

OH

–

H

–

Lactic acid

KetoacidsH3C–C–H

O

– –

Pyruvic acid

Dicarboxylic acids

HOOC–C–COOH

H2–

Succinic acid

Sugar alcohols & acids

– –H2C–C–CHO

OHOH

H

Glyceric acid

Aldehydes & Ketones O

– –

H3C–C–HAcetaldehyde

Amines & Amides H3C.CH2NH2Ethyl amine

Pyridine carb. acids

N

COOH Nicotinic acid

Purines & Pyrimidines NH2

NH

NN

N

Adenine

Hydrocarbons:Alyphatic H3C–CH2–CH3 Propane

NaphthaleneAromatic

Polar

N

Isoquinoline

N

N

O

SO

S

OO

O

COOH InsolubleMaterial(estimated)





later established for the whole homologousseries of these chiral compounds up to eight-carbon long; their magnitude varies within themeteorite and is largest, up to 18%, for isovaline(2-methyl-2-aminobutyric acid). As the bio-chemical structures and functions of all lifetoday are dependent upon the exclusive chiralhomogeneity of their polymers, it appearsreasonable to assume that a homochirality,albeit of unknown origin, was also essential tothe origin and/or evolution of life. The ee ofmeteorites represent the only case so far ofmolecular asymmetry ever measured outsidethe biosphere and their indigeneity is supportedby compound specific carbon-, and hydrogenisotopic data obtained for D-, and L-isovalineenantiomers (Pizzarello et al. 2003; Pizzarelloand Huang 2005).

Overall, the study of Murchison has dis-closed detailed insights on the capabilities andpossible range of abiotic syntheses in cosmo-chemical environments. We have learned thatthis abiotic chemistry can form organic materi-als of considerable complexity and includecompounds similar or identical to biomole-cules. Particularly captivating is the findingof chiral asymmetry in abiotic amino acidsand, although less defined at the molecularlevel, the fact that some macromolecular andinorganic phases of the meteorite show signsof optical activity is intriguing as well. Con-sidered as a whole, these data support the

conclusion that molecular chiral asymmetrypreceded biochemistry.

Nevertheless, these studies also leave manyquestions unanswered as to the prebiotic poten-tial of an organic suite of Murchison-like com-position. In fact, the large heterogeneity ofMurchison organic inventories and the appar-ent randomness involved in their formationled to question the means and opportunitiesby which such a diverse mixture of molecules,a majority of which are thermodynamicallystable end products (e.g., carboxylic acidsand hydrocarbons), could find an evolutio-nary path toward the selectivity and functio-nal specificity displayed by even the simplestbiochemistry.

RECENT RESULTS

The CR Antarctica Finds

Recorded falls of carbonaceous chondrites havebeen few (37 to date, since the first registered in1806) and this record is needed, because thesemeteorites resemble terrestrial rocks, are porousin nature, and quickly disappear into the envi-ronments if not spotted soon. For the samereason, their organic analyses have also becomeincreasingly limited in scope with their years ofterrestrial residence, due to the ease with whichCCs acquire biochemical contaminants. Fortu-nately, several of the meteorites recovered in

COOH

CH3

2R3S[D]

H3C

H3C

NH2∗

∗

COOH

CH3

23.0 23.50

0.20.40.6A

bund

ance

0.81.01.21.41.6

(D)2R, 3R

(L)2S, 3S

(D)2R, 3S

(L)2S, 3R

24.0Time (min)

24.5 25.0

2R3R[D]

H3C NH2

CH3

∗

∗

COOH

CH3

2S3R[L]

H2N

CH3

NH2∗

∗

COOH

CH3

2S3S[L]

H2N CH3

H3C

∗∗

m/Z 246

Figure 3. Chemical structure and chromatographic elution of the Murchison 2-amino 2,3-dimethylbutyric aciddiastereomers.





Antarctica are found unspoiled because of theunique shelter of the glaciers, where fallingmeteorites are quickly covered by snow, remainburied within the ice, and resurface only whenthe ice sheets, flowing toward the sea, encounterthe obstacle of a mountain.

Renazzo family of meteorites make up arecent classification (CR) of several Antarctic“finds” that have petrology closely similar tothat of the Renazzo meteorite, a CC that fell in1864 and long remained unclassified. Two CR21

meteorites (the GRA95229 and LAP023422)were analyzed recently for the major groupsof organic compounds known to be presentin the Murchison meteorite (Pizzarello et al.2008; Pizzarello and Holmes 2009) and haveshown an organic composition that differs dra-matically from that of Murchison and, in fact,from any seen before in carbonaceous meteor-ites (Fig. 4). Their organic suite is composedmainly of water-soluble compounds, betweenwhich N-containing amino acids and aminesare predominant. Ammonia is the single largestcomponent of the suite, whereas hydrocarbonsand carboxylic acids are only minor compo-nents. Novel were also the abundant distribu-tions found within CR2 amino acids, wherethe shorter chain-length molecules of a homol-ogous series, e.g., glycine, alanine, and a-aminoisobutyric acid, are overabundant comparedto longer chain species and, in effect, accountfor most of these compounds’ abundance.Several reactive compounds are found inthese meteorites as well, such as aldehydes,tertiary amines, and the hydroxy amino acidsserine, threonine, allothreonine, and tyrosine(Pizzarello and Holmes 2009); the latter twogroups of compounds were never detected inMurchison.

Another difference between CR2 and CMmeteorites is found in their respective contentof enantiomerically enriched chiral molecules.In CR2s, the same amino acid species having

ee in CMs display this trait to less extent(GRA95229) or not at all (LAP02342), whereasthe abundance distribution of some of themeteorites’ diastereomer amino acids allowedthe inference of an original asymmetry of theirprecursor aldehydes (Pizzarello et al. 2008).This somewhat indirect reasoning concernsthe diastereomers of the amino acid isoleucineand can be explained as follows. The molecule[2-amino3-methylpentanoic acid, CH3CH2-C�H(CH3)-C�H(NH2)-COOH] contains twochiral centers (C�) and can be present as two dif-ferent compounds (depending on the possibledistribution of the methyl branching alongthe alkyl chain), each with two enantiomers,i.e., the pairs of D-, L-isoleucine (ile) andD-, L-alloisoleucine (allo) (called diastereomersand shown schematically in Fig. 5B). OnlyL-ile is present in terrestrial proteins, whereasall four diastereomers are found in meteorites.

A possible reaction for the formationof amino acids in meteorites is the addition ofHCN to ketones and aldehydes in the presenceof water and ammonia (Fig. 5A) (e.g., Peltzerand Bada 1978). Although producing an asym-metric carbon in most cases (and thereforea chiral molecule), this type of synthesis is

200

Amine

s

Amm

onia

Amino

acids

Aldehy

des

& Ket

ones

Hydro

carb

ons

Carbo

xylic

acids

0

600

1000

Abu

ndan

ces

in p

pm

1000

1,800

2,200

GRA CR2 LAP CR2 Murchison

Figure 4. Comparative plot of major soluble organiccompound abundances in the Murchison and CR2meteorites (GRA 95229 and LAP 02342 shown).

1The number represents a classification of petrographic typeand estimates asteroidal secondary processes (were 2,1).2The acronyms stand for the names of the Antarctica loca-tions where the meteorites were found: Graves mountainand LaPaz ice fields, respectively.





R

A

B

C

RC C

H

NH2

COOH

H

O + NH3 + HCNH2O

∗

(S) (R)

HCN

H2ONH3

D-allo(RS)

D-ile(RR)

L-ile(SS)

∗

∗

∗

∗

∗

∗

∗

∗

∗

∗

Mirror images

Mirror images

L-allo(SR)

1000

Time

D-ile

L-ile

22.5 23.0 23.5

3000

5000

7000

Ion 182.00Ion 153.00Ion 171.00

D-allo

L-allo

Abundance

Figure 5. Possible formative pathway of the isoleucine (ile)-alloisoleucine (allo) diastereomers in meteorites. (A)The cyanohydrine reaction. (B) Schematic of the distribution of ile and allo following the same reaction with a2-methylbutyraldehyde precursor. (C) A chromatogram of the ile-allo diatereomers in the GRA 95229 meteorite.





nonstereospecific because the HCN additionwould be random and give equal amounts ofD- and L-enantiomers. However, the reactionresults become more complex for longeraldehydes that already contain an asymmetriccarbon, for example, in the synthesis of thefour ile and allo diastereomers from DL 2-methylbutanal. In this case, were an ee present inthe aldehyde, e.g., of the (S) configuration,those amino acids that carried the S-portionof the molecule through their synthesis (shownbetween dotted squares in Fig. 2B) will be moreabundant than their respective enantiomers. Inthe above example, this would be the (RS) alloand (SS) ile compounds or, in the formalismused for amino acids, D-allo and L-ile. Suchwas the distribution of isoleucine diastereomersfound in the CR2 extracts (Fig. 5C) that, on thebasis of the above formative premise, was inter-preted to signify that their precursor aldehydeenantiomers carried an original S-bias to themeteorite’s parent body.

Overall, the compositional differences bet-ween CR2- and Murchison-type meteoritesmake stark contrasts. Where the heterogeneityof Murchison compounds easily points tothe difficulties that a primordial “soup” wouldencounter in molecular evolution, CR2 organicdistributions and abundances have the unques-tionable prebiotic appeal of being over abun-dantly water-soluble, N-containing, and oflow molecular weight. Regardless of how CR2organic material came to be, it is also clear thatan unknown combination of elemental compo-sition, energetic availabilities, and cosmic con-tingencies made CR2 precursor environmentscapable of a de facto selectivity of such “prebioti-cally desirable” molecular species.

Whether CR2 meteorite parent bodies wouldfit the new category (Levison et al. 2009)of “trans-Neptunian” objects or not, certainlythe formative environments and histories oftheir organic materials must have differed fromthose of CMs. That the known ee-carryingamino acids as well as their ee are in lower abun-dance in CR2 than CM meteorites, whereas eeappear larger in a precursor aldehydes, wouldseem to further allow the general inferencethat abiotic organic pools in chemical evolution

were diverse and differed in both their compo-sition and exposure to asymmetric effects.

The Long Cosmic History of Meteorites’Organic Materials

The formation of meteoritic organic com-pounds was actively debated after Murchison’sfall and the revelation that a large variety ofextraterrestrial organic molecules with counter-parts in the biosphere could be made abioti-cally. Clearly, to know the syntheses and localsresponsible for their formation may have pro-found significance for the origin of extant lifeand even a broaderexobiology. The earlier hypo-theses all focused on solar system processes and,of these, the more influential were the sugges-tion of possible Miller-Urey type (Miller et al.1976) syntheses in small planets, following pro-duction and recombination of radicals, and ofcatalytic, FisherTropsch-type, processes in theearly stages of the solar nebula, where carbonmonoxide could have undergone hydrogena-tion to hydrocarbons and other compounds(Lancet and Anders 1970). Eventually, the his-tory of the organic compounds in carbonaceousmeteorites was elucidated, at least in generalterms, by the stable isotope analyses of severalcompounds and compound classes in theMurchison meteorite.

The isotopic composition is a good indica-tor of any molecule’s synthetic history becausethe mass differences between isotopomers resultin energy differences in their bond formationand may lead to a mass dependent fractiona-tion, which becomes diagnostic of the physico-chemical conditions affecting those reactions.Ultimately, isotopic fractionation is a functionof zero point energy difference between iso-topomers (DE) and local temperature, in theform: exp(-DE/T). In other words, the largerthe energy difference between isotope bondsand the lower the temperature, the greater thepotential for heavy isotope enrichment. Betweenthe biogenic elements, therefore, hydrogen hasthe potential for the most enrichment in 2H(deuterium, D) at low T, because of the high rel-ative mass difference of D/H isotope pair (2/1 u,e.g., compared to 13C/12C ¼ 1.084). The most





dramatic demonstration of these capabilities isgiven by the spectroscopic observations of theD/H ratios of molecules formed in the denseclouds of the interstellar medium where tem-peratures are in the 10–30K range. Over a hun-dred such molecules have been described (e.g.,Roueff and Gerin 2002), many of which showextremely high D/H ratios. For example, theaverage D/H ratio in terrestrial organic com-pounds is approximately 1.5 � 1024, whereasa D/H ratio as high as 0.33 has been observedin the interstellar molecule D2CO/DHCO(Loinard et al. 2000).

Most Murchison compounds were foundenriched in D and 13C to varying degreesand these data, which alone suggest a relationbetween such molecules or their direct precur-sors and cold synthetic environments, led to ageneral theory of formation of meteorite organ-ics that involved interstellar as well as parentbody processes. By this hypothesis, icy aster-oidal bodies accreted with abundant volatiles,including water and deuterium-rich inter-stellar organics that, upon warming and a sub-sequent period of aqueous phase chemistry,yielded the various soluble organic compoundsof meteorites.

The possibility of parent body aqueous syn-theses seems confirmed by the likelihood thatat least some of Murchison amino acids wereformed via a Strecker-like reaction of precursoraldehydes and ketones, ammonia and HCN(Fig. 5A). The evidence supporting this hypoth-esis is the finding in the Murchison meteorite ofcomparable suites of a-amino and a-hydroxylinear acids (although this correspondenceis not valid for the a-methyl compounds) andof imino acids (e.g., Pizzarello and Cooper2001). These are compounds in which twocarboxyl-containing alkyl chains are bonded atthe same amino group and would likely resultfrom a Strecker synthesis, e.g., when an aminoacid product becomes the reactant in place ofammonia (Fig. 5).

However, there are isotopic as well as molec-ular trends within the Murchison organic suitethat reveal significant formative distinctionsbetween individual compounds and cannotbe accounted for by any simplified model. For

example, not all of Murchison amino acids fallin the same range of deuterium enrichment,and asymmetry-carrying 2-methyl aminoacids display far larger dD values than the 2-Hisomers (Fig. 6). Because a similar branchedversus linear difference in D-enrichment wasalso observed between 3- and 4-amino isomers,it seems reasonable to assume that branchedmolecular species were processed in cold envi-ronments to a different degree than the linearones. On the other hand, 2-, 3- and 4-aminoacids also show different trends of 13C abun-dance with increasing chain lengths, whichdecreases in the case of the 2-amino acids whileremains level, or even slightly increases, in thecase of the others. That is, within each level ofD-enrichment, various processes of chain elon-gation seem to have been possible. The obviousconclusion from these Murchison detailedanalyses is that diverse cosmic regimes and syn-thetic processes might have participated inproducing the organic composition of thistype of meteorites.

The isotopic analyses of CR meteorites ad-ded to the above scenario. The dD differencesbetween 2-amino acid types are still presentand further magnified, with the two GRA952292-methyl amino acids analyzed showing thehighest dD values (þ7200‰) ever measuredfor an extraterrestrial molecule by direct analy-ses. However, d15N values determined for CR2amino acids have a distribution between molec-ular subgroups that is opposite to the one oftheir dD values, with 2-H amino acids havinghigher d15N than 2-methyl amino acids (Pizza-rello and Holmes 2008).

Because of the near absence of molecular15N values for cosmic environments3, onlytheoretical considerations can be offered forthe CR2 findings. The ones offered by Charnleyand Rodgers (2002, 2004, 2008) describe amechanism for higher nitrogen fractionationsin regions of the ISM, where the enhanceddensity and pressure that precede star formationwould cause the freeze-out of most carbon- and

3The possibility of different stellar nucleosynthetic pathwaysfor the element of nitrogen (e.g., Wannier et al. 1981) wouldalso further complicate their interpretation.





oxygen containing molecules; with their dis-appearance, the disruption of N2 formationpathways in clouds of lesser density would resultin a prevalence of gas-phase atomic nitrogen.In turn, this would lead to the efficient pro-duction of ammonia and 15NH3/

14NH3 ratioshigher than the cosmic 15N/14N ratios (to asmuch as by 80%).

These predictions are interesting in thatthey appear to match, albeit in broad terms,the findings in meteorites and the currentinterpretation of meteoritic amino acid for-mation. In fact, if the distinctly higher dDvalues of 2-methyl amino acids seem to pointto their syntheses in cold ISM environmentsand the lower values of 2-H amino acids tosuggest that their syntheses took place at a laterstage in the presence of liquid water, their d15Nopposite trends would also fit with earlier(ISM) and later (prestellar cores) cosmochemi-cal processes, albeit removed from a parent bodyenvironment.

Very little is known of the molecular seq-uence of events that would have taken place ina prestellar core; however, we can expect thatseveral stages of temperature, pressure, andensuing chemical regimes followed the initialcollapse of the presolar portion of the ISM(e.g., Ceccarelli et al. 2007). We could hypothe-size, therefore, that some of the warmer stagesof star formation might have allowed selectedenvironments, where the desorption, mixing,and reactions of radical, precursor molecules,water, and ammonia led to the syntheses ofhigher 15N amino acids and favored shortermolecular species formation. It also appearsthat such locals and the kinetic processes theyallow to envision could, rather than parentbody reactions, explain some of the moleculardistributions seen in the CR meteorites, suchas: the far from unity diastereomer ratios seenfor the thermodynamically similar amino acidsalloile and ile (Chaban and Pizzarello 2007),their erratic levels of enantiomeric excesses as

100020003000

D δ

δ13

C

40005000600070008000

A

B

0

60

50

40

30

20

10

02 3 4 5 6

2 3 4

Interstellar molecules

range

CM2 2-H 2-aaCM2 2-me 2-aaCM2 2-H 2-aa CM2 2-me 2-aa

C #

C #

5 6

2-H 2-amino a.2-me 2-amino a.3-amino a.2-amino di-a.

Figure 6. The hydrogen (A) and carbon (B) isotopic distributions of Murchison and CR2 amino acids.





well as the preponderance of lower chain lengthspecies and the abundance of unreacted carbonylcontaining molecules (Pizzarello and Holmes2009).

Abiotic Pathways to Biomolecules

Meteorites probably present just a minusculesample of the prebiotic potential of cosmicsynthetic processes but, through their studies,we may be able to infer how common or wide-spread they may be. Transformations of organiccompounds, or their synthesis from inorganiccompounds, occurs in response to thermody-namic drives, modulated by the kinetic proper-ties of individual reactions. Setting asidethe mechanistic details for a moment, it is usefulto examine how reactions may or may not befavored by the thermodynamic properties ofthe system. Reactions involving organic com-pounds and occurring in aqueous solutionmay have occurred on meteorite parent bodies,smaller icy aggregates on their way to formasteroids or comets, and in selected prestellarenvironments; therefore, investigating relativestabilities of aqueous organic compounds mayyield clues to these processes. This approachcan help to answer specific questions aboutrelative abundances of organic compoundsfound in carbonaceous meteorites. The follow-ing discussion illustrates this approach, withthe specific goal of understanding the relativeabundances of ammonia, amino acids, andaldehydes.

Stabilities of amino acids relative to otherorganic compounds during aqueous alterationcan be assessed by considering a set of hypo-thetical overall reactions involving amino acidsand other aqueous organic compounds. As anexample, the stability of alanine relative to thealdehyde propanal can be assessed by consider-ing a reaction in which carbon is conserved inthe two aqueous organic compounds, given by

CH3CH2CHOðaqÞ þ H2Oþ NH3ðaqÞpropanal

¼ CH3CHNH2COOHðaqÞ þ 2 H2ðaqÞ;alanine (1)

where the (aq) indicates that the compoundsof interest are all dissolved in H2O. This reactionis not meant to depict a specific syntheticprocess, but instead delineates relative stabil-ities. It is evident from reaction (1) that therecould be abundances of NH3(aq) that wouldfavor the stability of alanine relative to propanal.Likewise, at strongly reduced conditions, wherethere may be considerable H2(aq) present,alanine would become unstable relative to pro-panal and NH3(aq). Quantifying the activities(and concentrations) of NH3(aq) and H2(aq),where such transformations become possible,can be accomplished by considering the equil-ibrium constant for reaction (1), and manipu-lating its law of mass action expression. Thatexpression, in its logarithmic form, is given by

log K ¼ log aCH3CHNH2COOHðaqÞþ 2 log aH2ðaqÞ� log aCH3CH2CHOðaqÞ� log aNH3ðaqÞ � log aH2O: (2)

In most dilute aqueous solutions (salinity,

seawater, as a rule of thumb), it can be safelyassumed that the activity of H2O is so close to 1that setting it equal to 1 introduces only trivialuncertainty. With this assumption, Equation (2)can be rearranged to give

log aNH3ðaqÞ ¼ 2 log aH2ðaqÞ � log K

þ logaCH3CH2CHOðaqÞ

aCH3CHNH2COOHðaqÞ

� �;

(3)

which represents the equation of a line on aplot of log aNH3(aq) vs log aH2(aq) with aslope of 2 and an intercept equal to

� log Kþ logaCH3CH2CHOðaqÞ

aCH3CHNH2COOHðaqÞ

� �:

At constant temperature and pressure, log K isa constant, which means that various linescan be determined based on the activity ratioof alanine to propanal.





Plots of this type are shown in Figure 7 for08C and 258C both at 1 bar, with contours ofthe activity ratio ranging from 0.001 to 1000.The bold contour labeled 1 in each plot showsthe position of equal activities of the twoorganic solutes at equilibrium. Ranges of rela-tive predominance of propanal and alanine areindicated, with that of alanine in each plot fall-ing at higher activities of NH3(aq) and loweractivities of H2(aq), consistent with Le Chat-lier’s principle applied to reaction (1). Alsoshown in these diagrams are the values of loga H2(aq), at which hematite (Fe2O3) would bereduced to magnetite (Fe3O4), consistent with

3 Fe2O3 þ H2ðaqÞ ¼ 2 Fe3O4 þ H2O; (4)

and where magnetite would be reduced to theferrous silicate fayalite (Fe2SiO4) in the presenceof quartz (SiO2) according to

Fe3O4 þ H2ðaqÞ þ 3=2 SiO2

¼ 3=2 Fe2SiO4 þ H2O: (5)

Magnetite, which is one of the aqueous altera-tion products identified in CI, CM, CO, CR,CV meteorites and some LL3 chondrites(Zolensky et al. 2008) would be stable betweenthe two vertical dashed lines on each plot.

The presence of magnetite brackets theequilibrium activities of H2(aq) that couldhave attained during at least a portion of theaqueous alteration processes occurring onthe Murchison parent body. If this alterationoccurred at 08C, then the equilibrium activityof H2(aq) fell between about 1025.2 and1029.1. If, on the other hand, temperatureswere warmer, these activities would change. Asan example, at 258C, the equilibrium activitiesof H2(aq) fall between 1024.9 and 1028.5. Indilute solutions, activities of neutral solutes cor-respond closely to concentrations (Amend andShock 2001).

These plots reveal the ranges of H2(aq)concentrations that are consistent with theoccurrence of magnetite, and the NH3(aq) con-centrations that would provide a thermody-namic drive for the formation of an aminoacid rather than an aldehyde, and vice versa.

–10–8

–6

–4

–2

0

2

–9 –8 –7log a H2(aq)

log

a N

H3(

aq)

–5–6 –4

0.001

Alanine

Propanal

FM

Q

HM

1000

1

–10–8

–6

–4

–2

0

2

–9 –8 –7log a H2(aq)

log

a N

H3(

aq)

–5–6 –4

Alanine

Propanal

FM

Q

HM

1

0.001

1000

Figure 7. Equilibrium activity diagrams showing the relative stabilities of aqueous alanine and propanal in termsof the activities of NH3(aq) and H2(aq) at (left) 08C and 1 bar and (right) 258C and 1 bar. Selected contours ofthe equilibrium ratio of activities of alanine to propanal from 1000 to 0.001 are indicated. Equilibrium constantsfor reaction (1) were calculated with the revised Helgeson-Kirkham-Flowers equation of state (Shock et al. 1992)using data and parameters from Shock et al. (1989); Shock and Helgeson (1990) and Shulte and Shock (1993).Also shown are activities of H2(aq) corresponding to equilibrium between hematite and magnetite (HM,reaction 4), as well as magnetite, quartz, and fayalite (FMQ, reaction 5). Thermodynamic data for mineralscome from Helgeson et al. (1978). All calculations were conducted with the software package SUPCRT92(Johnson et al. 1992).





At, for example, log a H2(aq)¼27 (equal toabout 100 nanomolar dissolved H2), conditionsin the middle of the range of magnetite stability,activities of NH3(aq).1022 at 08C, and .1023

at 258C, would favor the formation of alanineat abundances greater than those of propanal.Whether or not equilibrium is actually attainedamong organic compounds during aqueousalteration events on meteorite parent bodies,the persistent thermodynamic drive to formamino acids or aldehydes depends on thechemical composition of the system. The plotsin Figure 7 reveal the quantitative nature ofthose thermodynamic drives. They also makeit possible to begin to understand the amountsof NH3(aq) that would be required if aminoacid concentrations were similar to aldehydeconcentrations or vastly different.

Comparison of the data from the CR2 mete-orites and Murchison shown in Figure 4 indi-cates that the ratio of total amino acids toaldehydesþketones is on the order of 12 forLAP and about two for GRA and Murchison. Itcan also be seen that ammonia abundances aregreatest in GRA, similarly large in LAP, andvery low in Murchison. These data can be com-binedwiththethermodynamicanalysisdepictedin Figure 7 in an attempt to evaluate what con-ditions were like during aqueous alteration, ifthe relative abundances of aldehydes and aminoacids were influenced by that stage of meteoritehistory. It should be kept in mind that the datathat exist are for what is present in the meteoritesand not what may have been present in aqueoussolution during the alteration process. Adsorp-tion equilibria among solutions and variousmineral phases may differ for these two classesof organic compounds, and much could havehappened to alter ratios inherited from suchan early stage in the history of the solar system.Let us assume that the relative abundances oforganic compounds in meteorite extracts reflectsconditions on the parent bodies at the time thecompounds formed and that they were not radi-cally reset by subsequent history.

Starting with LAP, conditions consistentwith the overall amino acid to aldehyde ratiowould fall just above and to the left of the 10contour, which is the first above the equal

activity (¼1) contour in a plot like those shownin Figure 7 for the presently unknown temper-ature of aqueous alteration. If the amino acidto aldehyde ratio is a result of aqueous altera-tion, then it provides us with this locus of pos-sibilities in log a NH3(aq) versus log a H2(aq)space. Likewise, ratios from GRA and Murchi-son indicate that conditions during aqueousalteration may have generated conditions nearor slightly above the equal activity contour.If there were estimates of the activity of eitherH2(aq) or NH3(aq) that prevailed duringaqueous alteration, then the equilibrium valueof the other would be uniquely defined by theratio of organic compounds.

Assuming that the relative abundances ofammonia in the extracts are analogous to therelative abundances during aqueous alterationleads to the following assessment of relativeoxidation-reduction (redox) states during aqu-eous alteration events. The amino acid to alde-hyde ratios in GRA and Murchison are aboutequal, but the abundance of ammonia that canbe extracted from GRA is much greater. There-fore, it seems likely that conditions during aque-ous alteration of the Murchison parent bodywould plot at a lower activity of NH3(aq) thanthose that attained during alteration of theGRA parent body. If so, then the fact that bothmeteorites fall on about the same contour meansthat the activity of H2(aq) was much greaterduring alteration of GRA than during alterationof Murchison if alteration processes happenedat similar temperatures on both parent bodies.The abundance of ammonia in the LAP extractsis nearly as great as the GRA extracts, but theamino acid to aldehyde ratio is also greater. Ifthe temperature of alteration of LAP was similarto that of GRA, then conditions during thealteration of LAP would fall somewhat lowerin log a NH3(aq), but also considerably lowerin the activity of H2(aq) to maintain the higheramino acid to aldehyde ratio. All else beingequal, indications are that conditions weremost oxidized during alteration of the Murchi-son parent body, most reduced during altera-tion of GRA, and intermediate during thealteration of LAP. Corroborating evidencemay be found in the relative abundances of





carboxylic acids, which are more oxidized thaneither amino acids or aldehydes. Redox condi-tions during alteration directly affect the poten-tial for abiotic organic synthesis (Shock 1990;1992a; Shock and Schulte 1990; 1998; Amendand Shock 1998; Shock and Canovas 2010). Ifthe analysis outlined above survives deeperscrutiny, the overall potential for abiotic organicsynthesis from inorganic starting compoundsmay have been greatest on the GRA parentbody, despite its lower abundances of aminoacids.

There are several ways that these predictionsof relative redox states can be tested. One wouldbe to examine the mineralogy of the alterationproducts in all three meteorites for evidence ofmineral assemblages that could indicate redoxconditions that prevailed during alteration.Another would be to seek evidence from min-eral assemblages, organic compound associa-tions, and isotopes (oxygen in pairs or suitesof minerals that formed together, for example)that could bracket the temperatures of thealteration events on each parent body so thatquantitatively appropriate versions of the plotsin Figure 7 could be built. Also, experimentalstudies of the adsorption of ammonia, aminoacids, aldehydes, and other organic compoundscommonly extracted from meteorites on miner-als found in meteorite alteration assemblageswould enable estimation of aqueous concentra-tions or activities from the abundances of thesecompounds in the meteorites.

Exogenous Delivery and Molecular Evolution

The Monomers and Their Potential

If we trust the record of impact craters observedin most of solar planets and satellites, meteor-ites have showered the Earth throughout geo-logical ages and certainly did so soon after itsaccretion (e.g., Chyba and Sagan 1992). Abun-dant organic materials were just as certainlydelivered to the early Earth and, it is reasonableto assume, a good portion of them survived theprocess. We have learned from the analyses oftwo largely different types of meteorites thatthis exogenous input delivered both complexmacromolecules of uncertain composition and

free soluble compounds. Various molecularspecies must have interacted in the meteoritesalready prior to their fall, to a certain degree,because some derivative compounds such asthe carboxamides (Cooper and Cronin 1995)are released from their extracts upon hydrolysis;however, peptides have been carefully searchedfor in the Murchison meteorite and not found,with the exception of diglycine (Shimoyamaand Ogasawara 2002). If we are trying to esti-mate the potential of this delivery for prebioticevolution and we believe that such evolutionhad to gain some polymeric complexity forlife to ensue, then, we have to conclude thatthe bulk of meteoritic compounds could haveprovided, at best, monomeric constituents.In general, however, any evolutionary path hasto rely on monomeric material as well and,just comparing with other early planetary proc-esses that could have led to organic compoundssuch as atmosphere-mediated Miller-Urey-typesyntheses or the environment of hydrothermalvents, the molecular species ready-made inmeteorites would not appear as too bad of astart. Of these, meteoritic amino acids appearas likely candidates for further molecular evo-lution, particularly considering their selectiveand abundant suites found in CR2 chondrites.

Amino acids, the components of extantproteins, are able to polymerize under a varietyof laboratory conditions and could have done soin early Earth environments. For example, Oro’and Guidry (1961) first showed that glycinereadily polymerizes in the presence of ammoniaand little water at temperatures of about 1408C.Also Leman, Orgel, and Ghadiri (2004) showedthat the presence of carbonyl sulfide, such as it isfound around volcanoes, could lead to easy for-mation of peptides. When of the type foundnonracemic in Murchison, amino acids readilyform an activated carboxyl, e.g., as an oxazoloneby intramolecular dehydration, and polymerizeconforming into helixes at lengths as short asthree-amino acid units (Crisma et al. 2004).

These findings suggest that it is plausiblethat exogenous amino acids acquired at leastsome polymeric complexity during early ter-restrial evolution; it is as likely that theiroverall molecular properties might have been





evolutionary factors as well. For example, all eefound so far for meteoritic amino acids havejust one configuration, L-, whereas thoseobtained for chiral molecules in natural proc-esses, designed experiments, or via theoreticalschemes are all subjected to chance outcomein the absence of asymmetric influences. Simi-larly, several terrestrial crystals such as quartzare chiral but their world-wide productionis about equal in d- and l-forms. Also, so far,ee have been found in amino acids that donot racemize4, meaning that their ee couldhave been preserved in prebiotic aqueousenvironments.

Most importantly, amino acids as well aspeptides are molecules with diverse catalyticproperties that are readily displayed experimen-tally and in biochemistry. They can also beasymmetric catalysts, a fact suggesting that theunique molecular asymmetry of meteoriticamino acids might have been a particularly use-ful evolutionary tool. A set of experiments havebeen conducted with this theme, to assess thepossibility that the nonracemic amino acids ofmeteorites could have acted as catalysts duringearly Earth molecular evolution and transferredtheir asymmetry to other prebiotic buildingblocks such as sugars. It was found that bothamino acids and dipeptides can catalyze theasymmetric aldol condensation of glycolalde-hyde, or glycolaldehyde and glyceraldehyde, toproduce tetrose (Pizzarello and Weber 2004;Weber and Pizzarello 2006) and pentose (Pizza-rello and Weber 2010) sugars with significantee. It is interesting that these syntheses singledout D-erythrose and D-ribose in forming eewith LL dipeptides catalysts, whereas all othersugars acquired either ee of the same configura-tion as the catalyst or, in some cases, no ee at all.These reactions were conducted in bufferedwater solution, made use of simple reactantrealistically available to the early Earth, andimplied likely catalytic pathways under mild

conditions. They have, therefore, some prebio-tic credibility and support the conclusion that,whereas the extent to which meteoritic catalystsmight have been effective in a mixture is enti-rely unknown, their possible inductive effecttoward chiral asymmetry in the monomericinteractions of molecular evolution cannot bedisregarded.

Energetic Contingencies

Knowing that life ensued rather quickly in earlyEarth history, it seems also realistic to assumethat the planet environments might have beenpart of the unknown contingencies that fosteredthe transition from abiotic chemistry to themolecular evolution that preceded the emer-gence of life. If so, these environments wouldhave combined available organic compoundswith favorable catalysts, which might havebeen organic as well as inorganic. Just as allknown life forms have habitats, the emergenceof life may have had a habitat as well (Shocket al. 1998, 2000).

Because the record of Earth’s first geologicalera was lost to ensuing diagenetic and metamor-phic changes, clays represent the first alterationproducts of basaltic glass under hydrous condi-tions and would be good candidates for aidingsimple abiotic molecules, such as those foundin meteorites, in undertaking evolutionarysteps of prebiotic significance. As detailed inDeamer and Weber (2010), these minerals areknown to adsorb organic molecules and activelyparticipate as catalysts in their syntheses andreactions (Williams et al. 2005). In particular,the smectite group of expandable clays, suchas montmorillonite and saponite, can undergosurface energy changes during diagenesis thatwill affect their surface H-bonding at key sitesand form complex aromatic and polyaromatichydrocarbons of up to C20 from methanol(e.g., Williams et al. 2005).

Also, as mentioned above, conditions ofvery low H2O activities or elevated temperaturesin aqueous solution can drive polymerizationreactions that involve dehydration such as pep-tide formation. The latter possibility has in-spired several experimental investigations of the

4Racemization, the reversal of configurations in water,involves the loss and reacquisition of hydrogen by the car-bon adjacent to the carboxyl group, which is slightly acidic,and is not allowed when the H at C-2 is substituted with amethyl group.





potential for amino acid polymerization underhydrothermal conditions (Shock 1992b, 1993).Starting with amino acids, it has repeatedlybeen shown that dipeptides and cyclic dipepti-des form rapidly in hydrothermal experiments(Imai et al. 1999b; Alargov et al. 2002; Li andBrill 2003; Lemke et al. 2009; Cleaves et al.2009). Occasionally, these experimental studiesalso obtain small concentrations of tripeptidesand longer oligomers (Imai et al. 1999a; Tsuka-hara et al. 2002), but the formation of cyclicdipeptides, which is thermodynamically fav-ored (Shock 1992b), often dominates. It hasalso been shown that if experiments are startedwith somewhat larger oligomers, say three orfour amino acids in length, then the peptidescan be lengthened by hydrothermal reactionsinvolving the monomers (Kawamura et al.2005), and that polymers containing up to 20amino acids can be generated hydrothermallyfrom glutamic acid or aspartic acid, which donot form cyclic dipeptides (Kawamura and Shi-mahashi 2008). In addition, hydrothermaldehydration reactions involving alkanoic acidsand glycerol produce lipid-like molecules capa-ble of self-assembly (Simoneit et al. 2007).Taken together, these recent results show thatcondensation, polymerization, and peptidebond formation may commonly occur inhydrothermal conditions. If so, planetary proc-essing of materials supplied from meteoritesmay have been integral to the emergence ofliving systems.

CHALLENGES AND FUTURE RESEARCHDIRECTIONS

Carbonaceous chondrites are natural samplesof limited and unpredicted availability that,once reaching the Earth, are under the constantthreat of terrestrial contamination. Their studyhas obviously met with challenges of materialpreservation, designing of analytical methodol-ogies, identification of indigenous materials,and more; these will remain, mutatis mutandis,much the same in the future. Nonetheless, thesemeteorites have been analyzed successfully ingreat detail and their studies have been invalu-able in determining the prebiotic possibilities

of cosmochemical environments; however, theyhave not answered the basic exobiological ques-tion of whether extraterrestrial organic com-pounds contributed to molecular evolution onthe early Earth and to the emergence of life.

That answer may never be possible, but, ifwe believe with Eschenmoser (2008) that life’sorigin “ . . . cannot be discovered, as other thingsin science, it can only be re-invented”, meteoriteanalyses will offer realistic molecular tools toattempt just that and much still can be done.After forty years of studying Murchison-typemeteorites, a new group of Antarctic finds hasshown that within the diverse cosmic environ-ments may reside the capabilities of formingorganic suites enriched in biomolecule precur-sors and of high prebiotic appeal. The CR2organic compounds are still poorly character-ized but new studies will define their extentand distribution. Many small molecules thatcould be useful for initiating molecularevolution could have escaped detection in ear-lier studies of these pristine meteorites andhave not yet been targeted for analyses: glycolal-dehyde, glyceraldehydes (detected but notquantified or unpublished), HCN, formamide,urea, and small peptides are all “stuff” requiredfor modeling early evolutionary biology. Hope-fully, we shall know soon their distribution inspace also.

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2010; doi: 10.1101/cshperspect.a002105Cold Spring Harb Perspect Biol Sandra Pizzarello and Everett Shock Evolutionary Story Ahead of BiochemistryThe Organic Composition of Carbonaceous Meteorites: The

Subject Collection The Origins of Life

Constructing Partial Models of Cells

et al.Norikazu Ichihashi, Tomoaki Matsuura, Hiroshi Kita,

The Hadean-Archaean EnvironmentNorman H. Sleep

RibonucleotidesJohn D. Sutherland

An Origin of Life on MarsChristopher P. McKay

Characterize Early Life on EarthHow a Tree Can Help−−Deep Phylogeny

RandallEric A. Gaucher, James T. Kratzer and Ryan N.

Primitive Genetic PolymersAaron E. Engelhart and Nicholas V. Hud

Medium to the Early EarthCosmic Carbon Chemistry: From the Interstellar

Pascale Ehrenfreund and Jan Cami

Membrane Transport in Primitive CellsSheref S. Mansy

Origin and Evolution of the RibosomeGeorge E. Fox

The Origins of Cellular LifeJason P. Schrum, Ting F. Zhu and Jack W. Szostak

BiomoleculesPlanetary Organic Chemistry and the Origins of

Kim, et al.Steven A. Benner, Hyo-Joong Kim, Myung-Jung

From Self-Assembled Vesicles to ProtocellsIrene A. Chen and Peter Walde

the Origins of LifeMineral Surfaces, Geochemical Complexities, and

Robert M. Hazen and Dimitri A. Sverjensky

The Origin of Biological HomochiralityDonna G. Blackmond

Historical Development of Origins ResearchAntonio Lazcano

Earth's Earliest AtmospheresKevin Zahnle, Laura Schaefer and Bruce Fegley

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