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Downloaded by guest on October 2, 2021 www.pnas.org/cgi/doi/10.1073/pnas.1710339114 Pearce in D. K. Ben of ponds fate little The warm world: RNA the of Origin n hmn)hv enfre ntesrae fiyIPana- , IDP icy (, of nucleobases surfaces the five on formed the been of have ) three and IDPs, iden- in been not tified have nucleobases Although (11). respectively bon, origins condi- ago. these years Under billion 4.17 cycles. before wet–dry appeared RNA likely few polymers into a RNA to polymerization tions, one their that just and mean in periods, occurred dry of during synthesis peri- photodissociation wet the UV during to seepage pond and to ods, nucleobases of Moreover, losses rates. rapid bombardment the for global declining “targets” early by of offset the availability was of increasing out this par- rose but dust continents ocean, interplanetary pri- as from were appeared not Ponds nucleobases and ticles. constituent origin Their in years). meteoritic marily few a meteorites than of deposition (less polymers the RNA after quickly that very prebi- find emerged complex have We must environments. with these in early chemistry ’s otic couple We nucleotides first in RNA. the nucleobases of and of the to evolution leading ponds com- the little warm a for build whose model We ponds numerical polymerization. prehensive little rapid char- warm promoted (the to cycles nucleobases wet–dry nucleotides) as of such interplanetary organics acteristic and meteorites delivered environ- time, particles favorable this dust in At Earth. polymerize early and on RNA ments form of blocks and to building Denmark, the had M., life, Odense (nucleotides) cellular Denmark, simple Southern of of origin University the 2017) Evolution, Before 7, Earth June for review Center for Nordic (received and 2017 28, of August Institute approved Canfield, E. Donald by Edited Germany Heidelberg, 69120 Heidelberg, Astronomy for Center Astrophysics, Canada; 4M1, a ils(Ds n eerts uigteeerytms these times, early these During is delivered par- meteorites. solution dust bodies and One interplanetary by (IDPs) (11). delivered organics ticles were producing nucleobases the at are that efficient reactions very Miller–Urey-type atmosphere, not reducing weakly CO a by dominated likely was questions these to answers unknown. the largely 9), are ideal- (8, highly conditions in laboratory Although strands ized RNA conditions? simple such first produced have the under experiments form molecules and envi- RNA polymerize What RNA? functioning to of nucleotides blocks building enabled ocean. the global ronments that of the of source At out dry the rising was (7). and were What gases, today continents volcanic as than scarce by was greater dominated land was magnitude atmosphere of the orders time, 11 meteoritic to would of undergoing ∼8 world rate Earth RNA a early the at thus violent bombardment and a 5), ear- on (P) (4, appeared developed (Ga) phosphorous life have ago first reduced Gy that 3.7 a suggests than and evidence lier The 3). reaction a (2, through with source formed be different a can four which of of life of sequences catalyze first latter of of up the and basis made nucleotides, information are the molecules formed genetic RNA probably store (1). RNA both replication, own to its ability its and O rgn nttt,MMse nvriy aitn NLS41 Canada; 4M1, L8S ON Hamilton, University, McMaster Institute, Origins st h ore fncebss al at’ atmosphere Earth’s early nucleobases, of sources the to As rteegdo at.Gvnisuiut nlvn cells living in ubiquity its Given life Earth. how is on science in emerged questions first fundamental most the of ne | c lntadSa omto eatet a lnkIsiuefrAtooy 91 edleg emn;and Germany; Heidelberg, 69117 Astronomy, for Institute Planck Max Department, Formation Star and Planet ∼6 a,b,1 × ap .Pudritz E. Ralph , | 10 lntr science planetary 7 o1,000 to ∼1 and 2 N , ∼2 2 SO , × 10 2 | × n H and , a,b,c,d 3 10 kg 12 mtyA Semenov A. Dmitry , ·y gy() hc is which (6), kg/y −1 2 1) nsuch In (10). O | fitc car- intact of N world RNA b eateto hsc n srnm,MMse nvriy aitn NL8S ON Hamilton, University, McMaster Astronomy, and Physics of Department 1073/pnas.1710339114/-/DCSupplemental at online information supporting contains article This 1 11264. page on Commentary See Submission. Direct PNAS a is article This interest. of conflict no declare authors The T.K.H. and D.A.S., R.E.P., B.K.D.P., paper. and the data; wrote analyzed R.E.P. and B.K.D.P. research; formed (13, carriers nucleobase ter- be aster- reaching to of meteorites known of distribution are fraction the that the the velocity 21), and using minal 11, (21), estimated (6, masses are mete- oid rates record The carbonaceous emer- delivery cratering conditions. are whose the lunar model Earth IDPs and our early and in them, orites under nucleobases WLPs, to polymers of of delivered sources RNA evolution nucleobases of the of for gence model fates well-posed the a compute WLPs Evolving We in Nucleobases of Fates Model: cycles. vents wet–dry prob- hydrothermal of critical near absence chains A the is RNA (20). long long polymerizing for few lem a pro- chains up in succeeded RNA chains only ducing have simulating of vents experiments linking hydrothermal of Conversely, the conditions in (19). the promote long minerals WLPs nucleotides clay of 55 Furthermore, to bases pos- (8). and chains links walls shown into 300 the been nucleotides than have of greater cycles polymerization sibly dry for the and promote environments wet to their candidate because excellent process are Warm this (WLPs) occurred. nucleotides ponds of polymerization little the how is and world concentrated of (18). lack environments these the in is sources here floors reactive problem ocean Earth’s potential young second A on A cracks (18). spreading (15–17). vents from hydrothermal stars up in well young synthesis that the is all in nucleobases around of observed dust source are possible be and that could gas HCN them of and within disks nitrogen as of (ppb) such billion source and molecules per ultimate parts , The 515 14). to (, (13, 0.25 meteorites of concentrations in with (12). uracil) radiation found UV to are exposure Nucleobases through laboratory the in logues c uhrcnrbtos ...... n ...dsge eerh ....per- B.K.D.P. research; designed T.K.H. and D.A.S., R.E.P., B.K.D.P., contributions: Author owo orsodnesol eadesd mi:[email protected]. Email: addressed. be should correspondence whom To n hmsK Henning K. Thomas and , a odtos h ieclso ecin n h emergence the polymers. and RNA reaction, first of of environmen- timescales the the world biomolecules, conditions, RNA of tal the source of the timeline constraining sites. and by latter story the the a in advances construct RNA model of we polymer- Our origin RNA), promote the into for to nucleotides model of known comprehensive wet case, well lacks this are former (in which ization the cycles, Because dry ponds. and little warm deep the and at in vents ocean site hydrothermal the emerged: world for RNA hypotheses an which competing two currently are There Significance eta usincnenn h mrec fteRNA the of emergence the concerning question central A PNAS | coe 4 2017 24, October c . | o.114 vol. www.pnas.org/lookup/suppl/doi:10. d nttt o Theoretical for Institute | o 43 no. | 11327–11332

EARTH, ATMOSPHERIC, AND SEE COMMENTARY PLANETARY SCIENCES 22). The WLPs are “targets” in which molecular evolution of to not dry up too quickly but small enough that high nucleobase nucleobases into nucleotides and subsequent polymerization into concentrations can be achieved (see SI Text). RNA occur. The evolution of ponds due to precipitation, evap- oration, and seepage constitutes the immediate environments Meteorite Sources and Targets in which the delivered nucleobases must survive and polymer- In Fig. 1A, we show the history of meteoritic mass delivery rates ize. [We don’t emphasize groundwater-fed ponds (hot springs) on early Earth, and of WLP targets. From total meteoritic mass, because their small number on Earth today—approximately we extract just the nucleobase sources, i.e., only carbonaceous thousands—compared with regular lakes/ponds—∼304 million whose fragments slow to terminal velocity in the early (23)—suggests they didn’t contribute greatly to the combined atmosphere. In Fig. 1B, we show the history of depositions of WLP target area for meteorite deposition.] The data for these such nucleobase sources into 1- to 10-m-radius WLP targets from sources and sinks are gathered from historical precipitation 4.5 Ga to 3.7 Ga. records (24), or are measured experimentally (25, 26). Sinks The lunar cratering record either shows a continuously decreas- for these molecules include destruction by unattenuated UV ing rate of impacts from 4.5 Ga forward or shows a brief outburst rays (as early Earth had no ), in the pond beginning at ∼3.9 Ga known as the Late Heavy Bombardment , as well as seepage from the bases of ponds. The data (LHB). Due to the lack of lunar impact melts with ages before for these sinks are measured experimentally (26–28). The nucle- ∼3.92 Ga, the rate of mass delivered to early Earth preceding this obases that survive go on to form nucleotides that ultimately date is uncertain (30). Therefore, we compare three models for polymerize. mass delivery to early Earth based on analytical fits to the lunar To calculate the number of carbonaceous meteorite source cratering record (6, 21): an LHB and a minimum and a maximum depositions in target WLPs on early Earth, we combine a con- bombardment model. The latter two models represent lower and tinental crustal growth model (29), the lake and pond size dis- upper limit fits for a sustained declining bombardment, and are tribution and ponds per unit crustal area estimated for ponds on subject to considerable uncertainty (31). The total target area of Earth today (23), the belt mass distribution (21), and WLPs increases as the rising continents provide greater surface three possible mass delivery models based on the lunar crater- area for pond sites. An ocean world, in this view, is an implausi- ing record (6, 21). This results in a first-order linear differential ble environment for the emergence of an RNA world. Iron and equation (Eq. S18), which we solve analytically. The details are rocky bodies impact the solid surface at speeds high enough to in SI Text, and the main results are discussed in Meteorite Sources form small craters (32), contributing to the WLP population. and Targets. Our calculations show that 10 to 3,840 terminal velocity Nucleobase abundances in WLPs from IDPs and meteorites carbon-rich meteoroids would have deposited their fragments are described in our model by first-order linear differential equa- into WLPs on Earth during the Hadean Eon. Given the large tions (Eqs. S34 and S38, respectively). The equations are solved uncertainty of the ponds per unit area and the growth rate of numerically (see SI Text for details). continental crust, we vary the WLP growth function by ±1 order Our fiducial model WLPs are cylindrical (because sedimenta- of magnitude. From this, we get a minimum of 1 to 384 depo- tion flattens their initial bases) and have a 1-m radius and depth. sitions and a maximum of 100 to 38,400 WLP depositions from We find these ponds to be optimal because they are large enough 4.5 Ga to 3.7 Ga. The optimal model for WLP depositions—the

AB

Fig. 1. History of carbonaceous meteorite deposition in WLPs. (A) History of mass delivery rate to early Earth and of effective WLP surface area. Three models for mass delivered to early Earth are compared: an LHB model and a minimum and a maximum bombardment model. All mass delivery models are based on analyses of the lunar cratering record (6, 21). The effective WLP surface area during the Hadean Eon is based on a continental crust growth model (29), the number of lakes and ponds per unit crustal area (23), and the lake and pond size distribution (23). (B) Cumulative WLP depositions for the small fragments of carbonaceous meteoroids of diameter 40 m to 80 m landing in any 1- to 10-m-radius WLP on early Earth. A deposition is characterized as a debris field that overlaps with a WLP. We assume the ponds per area during the Hadean Eon is the same as today, and that all continental crust remains above sea level; however, we vary the ponds per area by plus or minus one order of magnitude to obtain error bars. Ninety-five percent of WLP depositions in each model occur before the corresponding dotted vertical line; 40 m to 80 m is the optimal range of carbonaceous meteoroid diameters to reach terminal velocity, while still landing a substantial fraction of mass within a WLP area; and 1 m to 10 m is the optimal range of WLP radii to avoid complete evaporation within a few months, while also allowing nonnegligible nucleobase concentrations to be reached upon meteorite deposition. The numbers of carbonaceous meteoroid impactors in the 20- to 40-m-radius range are based on the mass delivery rate, the main-belt size-frequency distribution, and the fraction of impactors that are Mighei type, Renazzo type, or Ivuna type (6, 21, 22) (see SI Text).

11328 | www.pnas.org/cgi/doi/10.1073/pnas.1710339114 Pearce et al. Downloaded by guest on October 2, 2021 Downloaded by guest on October 2, 2021 nyatvtdwe h odi e,adU htdsoito sol ciae hntepn sdry. are is seepage pond al. and the et hydrolysis when Pearce activated Nucleobase only to seepage. is up and photodissociation carry photodissociation, UV which UV and meteorites, wet, hydrolysis, is and include pond IDPs sinks the carbonaceous Nucleobase when include nucleobase. activated sources only each Nucleobase of seepage. respectively, and mg, evaporation ∼3 include sinks Water precipitation. 2. Fig. by offset dissociation is and (26), WLPs seepage in (28), nucleobases hydrolysis of to buildup due losses the by 2, Fig. in shown As WLPs in Evolution Nucleobase of existence cycles. the wet–dry establish 20 seasonal clearly to results up fill These only respectively. environment) 10%, (dry year a and of environment) quarters (intermediate three half approximately environ- of wet we states the dry from these, in range levels and of water WLPs The each intermediate, details). ment for For dry, 1 Table . environments: (see wet early different warm three and examine hot (65 for temperatures logues different two precipita- select ground). are the WLPs in of pores (through cycles seepage wet–dry and the evaporation, of tion, thus temperatures and causes level abundances (33) water envi- time greater this dry 50 from at around and flow sources wet radiogenic heat of seasonal increased The creating ronments. WLPs (24), for sinusoidally conditions Eon, varies physical Hadean of the variation during the illustrates 2 Fig. WLPs of Cycles Life Ga 3.82 before respectively. occurred for Ga, depositions mod- 3.77 models of and bombardment majority optimal minimum the less and suggest els, LHB The the Ga. depositions, 4.17 WLP before depo- occurred of sitions majority the model—suggests bombardment maximum epeetterslso hs acltos We calculations. these of results the present we 3A, Fig. In nilsrto ftesucsadsnso odwtradncebssi u oe fioae Lso al at.Teol ae oreis source water only The Earth. early on WLPs isolated of model our in nucleobases and water pond of sinks and sources the of illustration An ◦ o80 to C ◦ 3) h aiu atr htcnrlthe control that factors various The (34). C 0t 100% to ∼60 . at . a nulrainfall Annual Ga. 3.7 to Ga ∼4.5 ul Lsexperiencing WLPs full. ◦ n 20 and C ◦ )a ana- as C) % and oatnaeU aito by radiation UV a attenuate takes to radiation. only UV it attenuates also show WLPs, Studies of radi- base UV the absorb at collects can which water pond to of up column WLP ation 1-m during of a afforded absence as be the phases, would in wet protection Earth Some early 36). on (27, incident ozone was that radiation UV ae,adpeiiaindrn e eid.I n environment destruction any In and periods. input wet of during balance tops, precipitation drying, and flat rates, to spikes, response of in combination tration 3B Fig. The in troughs sources. and IDP environ- only pond from identical maintain sites. Earth to early Earth warm our early to analogue hot ments rainier a choose on we sites therefore, quickly; more evaporate (5 warm matching (50 in hot WLPs in evolution nucleobases the slow. model of very we sensitivity, be temperature would this formation However, Considering timescales. temperatures, such thousand-to-million-year at nucleotides for Earth, survive postsnowball can a or years few latitudes 5 a more-temperate to of (>80 days temperatures hot several At in too (40). hydrolyze is will it WLP temper- within high a nucleotides over If conditions warm (40). in atures favored is hand, 100 other the at 65 hours at of weeks slower couple few a is a laboratory taking temperatures, the low in at of Phosphorylation ture. efcso h dnn ocnrtosi WLPs in concentrations adenine the on focus we 3B, Fig. In tempera- to sensitive are stability and formation Nucleotide ◦ o80 to C ∼95% PNAS ◦ 3) ti fpriua neetta sediment, that interest particular of is It (37). )eryErhevrnet.Hte ponds Hotter environments. Earth early C) | eet h aitoso dnn concen- adenine of variations the reflects coe 4 2017 24, October ◦ 3) h tblt fncetdson nucleotides of stability The (39). C ◦ .-mlyro aatcsediment basaltic of layer ∼0.6-mm o35 to C >99.99% ◦ htete characterize either that C | (38). ◦ o.114 vol. ) n el formed newly any C), ◦ ◦ oprdwith compared C o35 to C | o 43 no. gand pg ∼1 ◦ )and C) | 11329

EARTH, ATMOSPHERIC, AND SEE COMMENTARY PLANETARY SCIENCES AB

Fig. 3. Histories of pond water and adenine concentration from IDPs. (A) The change in water level over time in our fiducial dry, intermediate, and wet environment WLPs due to evaporation, seepage, and precipitation. Precipitation rates from a variety of locations on Earth today are used in the models, and represent two classes of matching early Earth analogues: hot (Columbia, Indonesia, Cameroon) and warm (Thailand, Brazil, and Mexico) (for details, see Table 1). All models begin with an empty pond, and stabilize within 2 y. (B) The red and black-dotted curves represent the adenine concentrations over time from carbonaceous IDPs in our fiducial WLPs. The degenerate intermediate WLP environment used in this calculation is for a hot early Earth at 65 ◦C and a warm early Earth at 20 ◦C. The blue curve represents the corresponding water level in the WLP, with initial empty and full states labeled vertically. Three features are present: (1) the maximum adenine concentration at the onset of the dry phase, (2) a flat-top equilibrium between incoming adenine from IDPs and adenine destruction by UV irradiation, and (3) the minimum adenine concentration just before the pond water reaches its highest level. BRA, Brazil; CAM, Cameroon; COL, Columbia; Env, environment; IDN, Indonesia; MEX, Mexico; THA, Thailand.

and at any modeled temperature, the maximum adenine con- (Fig. S4). Only the adenine that has already flowed out of the centration from only IDP sources remains below 2 × 10 −7 ppb fragments gets rapidly photodestroyed; therefore, adenine within (Fig. S1). This is two orders of magnitude below current detec- larger fragments can survive up to ∼7 y. tion limits (14), making subsequent reactions negligible. The Thus, even though the carbon delivery rates from IDPs to early nucleobase mass fraction curves are practically independent Earth vastly exceed those from meteorites, it is the meteoritic of pond size (1 m < rp ≤ 10 m) once a stable, seasonal pat- material that is the dominant source of nucleobases for RNA tern is reached (<35 y). This is because, although larger ponds synthesis. have a larger collecting area for IDPs, they have an equivalent larger area for collecting precipitation and seeping nucleobases. Nucleotide and RNA Synthesis To form nucleotides in WLPs, ribose and a reduced P source Dominant Source of Surviving Nucleobases must be available. Ribose may have formed and stabilized in In Fig. 4, we assemble all of these results and compare carbona- borate-rich WLPs via the formose reaction (polymerization of ceous IDPs to meteorites as sources of adenine to our fiducial H2CO) (41). Additionally, phosphite has been detected in a pris- WLPs. Small meteorite fragments (1 cm in radius) are compared tine geothermal pool representative of early Earth, suggesting with IDPs in Fig. 4A. The effects of larger meteorite fragments the potential availability of reduced P to WLP environments (5 cm and 10 cm) on adenine concentration are displayed in Fig. 4B. Table 1. Precipitation models matching dry, intermediate, and The maximum adenine concentration in our model WLPs wet environments on a warm (5 ◦C to 35 ◦C) and hot (50 ◦C to from carbonaceous meteorites is 10 orders of magnitude higher 80 ◦C) early Earth than the maximum adenine concentration from carbonaceous −1 IDPs. The reason for this huge disparity is simply that car- Model Environment Analogue site P, m·y δp sp,y bonaceous meteoroid fragments—each carrying up to a few mil- Warm early Earth ligrams of adenine—are deposited into a WLP in a single event. (5 ◦C to 35 ◦C) Dry Mexico (MEX) 0.94 1.69 0.3 This allows adenine to reach parts per billion–parts per million- Intermediate Brazil (BRA) 1.8 0.50 0.85 level concentrations before seepage and UV photodissociation Wet Thailand (THA) 3.32 0.91 0.3 efficiently remove it from the WLP in one to a few wet–dry Hot early Earth cycles. The maximum guanine and uracil accumulated in our (50 to 80 ◦C) Dry Cameroon (CAM) 3.5 0.5 0.3 model WLPs from meteorites are also more than 10 orders of Intermediate Indonesia (IDN) 4.5 0.2 0.85 magnitude higher than those accumulated from IDPs (Figs. S2 Wet Columbia (COL) 6.0 0.5 0.3 and S3). A maximum adenine concentration of 2 ppm is still approximately one to two orders of magnitude lower than the Precipitation data from a variety of locations on Earth today (24, 35) initial adenine concentrations in aqueous experiments forming represent two classes of matching early Earth analogues: warm (Thailand, adenosine and AMP (2); however, these experiments only ran for Brazil, and Mexico) and hot (Columbia, Indonesia, Cameroon). For exam- ple, the conditions in Mexico on a warm early Earth match the conditions in an hour. Cameroon on a hot early Earth. P is the mean precipitation rate, δ is the sea- Adenine within larger fragments diffuses over several wet–dry p sonal precipitation amplitude, and sp is the phase shift. To obtain the rate of cycles, and, during the dry phase, no outflow occurs. For frag- the decrease in pond water for a given analogue site, table values are input ments 1, 5, and 10 cm in radius, 99% of the adenine is released into this equation: dL/dt = 0.83 + 0.06T − P [1 + δpsin (2π(t − sp)/τs)] (see into the pond water within ∼10 d, 8 mo, and 32 mo, respectively SI Text).

11330 | www.pnas.org/cgi/doi/10.1073/pnas.1710339114 Pearce et al. Downloaded by guest on October 2, 2021 Downloaded by guest on October 2, 2021 eree al. et Pearce to similar reasons for evolution and polymerization suit- RNA be for also able could cycles freeze–thaw impact-induced or seasonal S5). (Fig. negli- quadrillion)] still per (parts are ppq IDPs [.150 from gible concentrations turned is adenine seepage maximum if even off, fatty However, from (44). derived meteorites) bilayers within lipid acids of consist walls whose hydration, 0.5 size vesicles by of will encapsulated It (spheres are WLPs. nucleobases in if sinks reduced nucleobase drastically dominant be the of one is Seepage Conclusions 16 and to Discussion 1 just to them subjecting (8). by cycles possibly monomers wet–dry chains estimated 300 RNA than into the polymerize greater through Experiments can (43). nucleotides base seeping that WLP the show of at pores likelihood 400-µm-sized cycles, to wet–dry 0.001- few their a to reduce one within to occurring fast, tem- be the to would need of RNA also regardless into polymerization seepage, nucleotide to Therefore, perature. subject still are nucleobases, and adenosine form hour- to is 3). that sufficient (2, nucleobase This show AMP are experiments after sizes. timescales laboratory years fragment that to-week-long few given meteoroid time a on ample to depending half-year deposition, a within occurring UV and mm·d P, 5.1 reduced ribose, syn- involving experimentally step been (2). single has radiation a nucleotide in AMP previously thesized and the all increase destroys the Only rapidly to due radiation (42). concentration is UV highest shape its and U at fragments, The is outflow. large Adenine to from (1) continue present: outflow to are to pores features fragment ceases Three large environment. adenine radius). Env, within in drying, level. adenine cm Upon water remaining (10 in (2) the size decrease allows phase. in Rewetting large dry (3) only pond’s adenine. or 65 the released radius), at 65 of in at Earth onset cm Adenine Earth early (5 the meteoroid. early hot size at carbonaceous hot a in 40-m-radius a both medium a for only to radius), is from correspond in calculations originate and these compared and in are cm), used dry) (1 environment (never small 20 environments at are wet Earth fragments and early meteorite warm cycle) The (wet–dry WLPs. intermediate fiducial for our concentrations in meteorites and IDPs i.4. Fig. .Gl ,e l 21)Ncesd hshrlto ytemnrlschreibersite. mineral the by phosphorylation triphosphate (2015) adenosine al. of et M, Synthesis Gull (1963) 3. R Mariner C, world. Sagan RNA C, The life: Ponnamperuma of Origin 2. (1986) W Gilbert 1. AB lo ent htacl al at i tocre 3) with (30)] occurred it [if Earth early cold a that note we Also, meteorite-delivered using synthesized once Nucleotides, mm ·d [∼1.0 seepage of rate rapid the of Because 5:17198. conditions. Earth primitive possible under oprsno h cuuaino dnn rmcarbonaceous from adenine of accumulation the of comparison A (A) meteorites. and IDPs from concentrations adenine of histories Comparative −1 2),ncetd ytei ol edt efast, be to need would synthesis nucleotide (26)], ◦ o5 to µm h feto eert rgetszso dnn ocnrto.Tedgnrt nemdaeWLP intermediate degenerate The concentration. adenine on sizes fragment meteorite of effect The (B) 1). Table see details, (for C htfr pnaeul upon spontaneously form that µm Nature 199:222–226. 319:618. −1 ◦ c Rep Sci n ameryErha 20 at Earth early warm a and C to ainbtenteOiisIsiueadteHiebr ntaiefrthe for Initiative collabo- Heidelberg a the of part and Life. is Institute of research Origins Origins is NSERC This R.E.P. the Grant. an between Scholarship. Discovery by ration Graduate NSERC supported an Ontario was by during and B.K.D.P. supported support of Scholarship their research Graduate for Astron- The Canada Astrophysics for leave. Theoretical Institute sabbatical For- of Planck his Smith Institute Max the Michael the and (NSERC) thanks for omy R.E.P. Canada Supplement. Astronomy of Study Sciences Council for Natural eign a Research Institute by Engineering supported Planck abroad, and summer Max his during the hospitality their Rheinst thanks B.K.D.P. M. manuscript. thank We appeared have ACKNOWLEDGMENTS. could world RNA the RNA that the within in of implies formed emergence This have the would prefiguring world. time, polymers that RNA around first WLPs The Ga. 4.17 occurred before depositions meteorite of majority the that indicates tions (27, RNA and nucleotides, nucleobases, 47). for importance 46, critical protection of UV impact be as as would known Sedimentation also the impacts, (45). of large frustration emergence frequent the to likeli- for due the setbacks world process RNA reduce of rapid likelihood to This the seepage. cycles reduces to wet–dry lost also are few molecules a these a that to hood than one (less in quickly occurs Polymerization to polymerization. then source for nucleotides P create reduced years) a few could and ribose nucleobases RNA with Meteorite-delivered an react (39). of WLPs in emergence synthesis (50 the Earth early on hot A constraints world. strong place WLPs freez- and (8). thawing cycles cyclic wet–dry The resembles WLPs. ing for analyzed have we those .Nta P ent C redCL a rnnokM,Cia R(06 Rapid (2016) AR Chivas MJ, Kranendonk Van CRL, Friend VC, Bennett AP, Nutman 4. .Otm ,Kkgw ,Ihd ,Ngs ,Rsn T(04 vdnefrbiogenic for Evidence (2014) MT Rosing T, Nagase A, Ishida T, Kakegawa Y, Ohtomo 5. h asdlvr oe rvdn h otWPdeposi- WLP most the providing model delivery mass The of conditions chemical and physical the that conclude We mrec flf hw ydsoeyo ,0-ilo-erodmcoilstructures. microbial 3,700-million-year-old of discovery Nature by shown life of emergence rpiei al rhenIu eaeietr rocks. metasedimentary Isua Archaean early in graphite 0 o30mlinyasatrErhbcm habitable. became Earth after years million 300 to ∼200 537:535–538. PNAS ◦ etakterfre o hi huhflinsights. thoughtful their for referees the thank We de o rvdn aubecmet nthe on comments valuable providing for adter .Tefamnsaeete nysali ie( cm (1 size in small only either are fragments The C. ¨ | coe 4 2017 24, October ◦ o80 to C ◦ )fvr ai nucleotide rapid favors C) | o.114 vol. a Geosci Nat | o 43 no. 7:25–28. ◦ | n a and C 11331

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