Building blocks that fall from the sky
How did life on Earth begin? Scientists from the “Heidelberg Initiative for the Origin of Life” have set about answering this truly existential question. Indeed, they are going one step further and examining the conditions under which life can emerge. The initiative was founded by Thomas Henning, Director at the Max Planck Institute for Astronomy in Heidelberg, and brings together researchers from chemistry, physics and the geological and biological sciences.
18 MaxPlanckResearch 3 | 18 Photo: DLR biologists andchemistsforstudyinglife. sible forstudyingtheuniverseandEarth, physicists and geophysicists were respon- there wasacleardivisionoftasks:astro- pects ofthisquestion.Foralongtime, angles, scientists can answer different as- proaching thesepuzzlesfromvarious alone inthevastnessofspace?Byap- Does lifeexistanywhereelse,orarewe T TEXT THOMAS BUEHRKE BUEHRKE THOMAS Earth formandlifebegin? universe evolve,andhowdid nate us the most: how did the tence aretheonesthatfasci- he greatquestions ofourexis- adenine, guanine and uracil – to Earth, thereby providing the ingredients for the formation formation RNAofmolecules. the for ingredients the providing thereby Earth, to – uracil and guanine adenine, nucleobases the as such – chemicals key brought have may meteorites cargo: Precious ilton, Canada. as theMcMasterUniversityinHam- with internationalinstitutionssuch ciplines, but also cooperates closely porates researchers fromdifferentdis name isabbreviated,notonlyincor- as Henning.HIFOL,theinitiative’s founded threeyearsago,”saysThom- for theOriginsofLife,whichwas to dowiththeHeidelbergInitiative disciplines. “That’s whatwe’retrying specialization andcombinedifferent forcing researchers to break down this However, recentdevelopmentsare - the scope of the Heidelberg Initiative: it the scopeofHeidelbergInitiative:it precisely thisrealizationthatbroadens emergence oflifeasweknowit.Itis ronmental conditionsthatfavorthe some ofwhichpresumablyofferenvi- is hometobillionsofrockyplanets, ning. Accordingly, our Milky Way alone giants weidentifiedinitially,” saysHen commonplace thantheJupiter-likegas terrestrial planetsofthiskindaremore er thantheSun.“We nowknowthat rocky planetsorbitingaroundstarsoth ofanevergreaternumber covery The initiativewastriggeredbythedis- FOCUS_The OriginofLife 3 | MaxPlanckResearch 18MaxPlanckResearch 19 - - 20 traditional origins-of-lifescientists, who icism,” saysHenning. and praise,italsobrought us somecrit life onEarth.“Aswellasrecognition posed a scenario for the emergence of with apublicationinwhichtheypro- Master University, causedquitea stir Ben Pearce and Ralph Pudritz from Mc- Semenov,colleague Dmitry aswell as Late lastyear, ThomasHenningandhis AN EXCELLENTSTUDY PRAISE ANDCRITICISMFOR ing onextrasolarplanets. something likethistohappen–includ erally, what conditions are needed for emerged onEarthbutalso,moregen- must asknotonlyhowlifecouldhave (right). far so system its in identified been have that planets rocky Earth-like Trappist- star relatively seven dwarf the red and the producing for responsible also is It space. in locations many at place take to and Nebulae Eagle and Omega the areas: such three shows above Sharpless image the The dust. and gas of clouds in form planets and stars nurseries: Cosmic MaxPlanckResearch 3 MaxPlanckResearch The criticismcamefromthe more 2-54 complex (from left). This process of formation not only gave rise to our solar system some some system solar our to rise gave only not formation of process This left). (from complex | 18 - - Gilbert, aNobelPrizewinner inChem- proposed some30yearsago byWalter widely used“RNAworld”hypothesis Henning andhiscolleaguesapplied the came about. cursors wereformed–andofhowthis the firstmoleculesoflifeortheirpre- conditions potentiallyexistedwhen indeed contribute to questions ofwhat originality”. Infact, astronomers can “outstanding scientificexcellenceand cation the2017CozzarelliPrizeforits view, however, andawardedthepubli- Academy ofSciencetookadifferent TheNational edge ofastrochemistry? ecules –eveniftheyhaveadeepknowl astronomers understandaboutbiomol- mers following the thought: what do challenged thestudyofastrono- As astartingpointforthe study, - istry. Thisstatesthatthefirsteverter- RNA moleculesorrelatedbiopolymers? DNA therefore evolve from simpler um in viruses. Did the more complex to DNA,onlyactingasastorage medi- formation carrier in a subordinate role ganisms, however, as an in- RNA serves perform catalyticfunctions.Inmostor communicate geneticinformationand ble-stranded structureofDNA. ly single-stranded–unlikethedou of thymine.Inaddition,RNAistypical- contains abaseknownasuracilinstead guanine andcytosine;however, RNA four organicbases,includingadenine, er of life today: both are made up of resembles DNA, the information carri- cleic acids(RNA).Structurally, RNA restrial lifeformswerebasedonribonu 1 , which is located located is which , Likewise, moleculesofRNAcanalso 4.6 billion years ago, but also continues continues also but ago, years billion 40 light years from Earth, Earth, from years light
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Photo: ESO FOCUS_The Origin of Life
In 2009, a chemical experiment by Brit- Indeed, the nucleobases adenine, gua- There are almost no remnants of the ish researchers was considered a major nine and uracil, as well as amino acids, very early phase in which Earth cooled breakthrough, for it showed that RNA have been detected inside meteorites. from a glowing ball into a rocky plan- building blocks can form if certain mol- These bases are formed from simple et. Only the presence of tiny zircon ecules are present and react with one molecules of hydrogen cyanide, carbon crystals, which could be as much as 4.4 another under very specific conditions. monoxide and ammonia in the pres- billion years old, suggests that Earth But where were the most favorable con- ence of water. formed a solid crust at quite an early ditions present in nature? stage. At that time, Earth was exposed For a long time, it has been suspect- ZIRCON CRYSTALS to a much heavier bombardment of ed that life emerged at hydrothermal INDICATE A SOLID CRUST meteorites than it is today, as is demon- vents – so-called black and white smok- strated by crater statistics for the Moon, ers – on the deep seafloor. However, it Meteorites have also been found to con- which received the same onslaught. is unclear whether the nitrogen need- tain the mineral Schreibersite, which re- The cosmic projectiles presumably ed for synthesis exists at a sufficient leases phosphorus capable of forming brought both water and organic mole- concentration in that environment. phosphorylated molecules in water – cules to our planet. Moreover, the substances are diluted by these too are required for RNA synthe- Just as little is known about the dis- a steady flow of water, which prevents sis. Astronomical observations have tribution of land and water in primeval complex chemical reactions from tak- shown that all of the necessary precur- times as it is about the temperature, for ing place. sor materials to form RNA are present in example, which is a crucial factor in “This is where we come into play,” the dust discs in which planets form, so chemical reactions. For this reason, the says Thomas Henning. “We asked our- they must also have been present in the astronomers calculated models in which selves what other geochemical condi- solar nebula that gave birth to our solar they varied key parameters of the devel- tions might have been present to allow system 4.6 billion years ago. But how oping crust over a wide range of values. this RNA synthesis to take place.” The and when did the organic building Earth at the time undoubtedly had wa- idea is that the most important build- blocks arrive on Earth? And what did ter reservoirs with a wide range of siz-
Photo: ESO Photo: M. Kornmesser/ESO ing blocks came to Earth from space. Earth look like at the time? es, just as it does now. >
3 | 18 MaxPlanckResearch 21 Weak Heavy rainfall rainfall
UV light Meteorites 0.4 Wm-2 Meteorites ~ 2-20 cm ~ 2-20 cm
Evaporation
Photodissociation 1-10 m Hydrolysis
1-10 m
The cradle of life: Dmitry Semenov (left) Percolation and Thomas Henning from the Max Planck Institute for Astronomy have outlined a scenario in which various Dry Wet reactions once occurred in small, warm pools. These reactions led to the formation of the first self-replicating Seasonal cycle RNA molecules. The diagram on the left shows the numerous influences that affected chemical compounds in these small bodies of water.
Large lakes and seas were presumably and then fill up with water again may strated in impressive fashion, meteorites unsuitable breeding sites for RNA, be- have favored the formation of longer of this order of magnitude do not make cause the precursors must be present in RNA chains,” says Henning. it to the ground unscathed – rather, they concentrated form in order to react break up into many small fragments and with one another. However, the model THE IDEAL RADIUS IS fall to Earth over a large area. This means showed that small pools with a diame- BETWEEN 40 AND 80 METERS it is possible for tiny pieces, measuring ter and depth of just a few meters were just a few centimeters across, to have ideal: they were large enough not to dry In the model simulations, the research- landed in the pools. Depending on their out too quickly, yet small enough to al- ers also varied the impact rate and size size, they would then have released the low high nucleobase concentrations to distribution of the meteorites. If these nucleobases over a period of several days accumulate rapidly. are too small, they burn up completely to months. After that, the nucleotides – At the same time, the biomolecules in the atmosphere; if they are too large, and the RNA molecules formed from were at the mercy of destructive influ- they hit the ground with too much ki- them – would have to be synthesized ences: in the water, they were endan- netic energy. “A radius of between 40 within a few years. gered by electrolysis and, in the open, and 80 meters is the optimum size to The simulations show that meteor- by the intensive UV radiation of the allow the meteorites to deliver their ites could have transported a sufficient sun. Some 95 per cent of UV radiation molecular payload to the ground,” ex- quantity of nucleobases, to small pools is absorbed by a layer of water just one plains Dmitry Semenov, an expert in on Earth and thereby triggered the for- meter thick. The optimum scenario chemical networks within protoplane- mation of RNA molecules in at least seems to be one where the pools expe- tary discs and co-author of the study. one such pool. The RNA world could rienced seasonal variations in fill level This range is two to four times the have emerged within 200 to 300 mil- due to rainfall and drying out through size of the meteorite that exploded over lion years from the point at which evaporation and percolation: “The cy- the Russian city of Chelyabinsk in Feb- Earth’s surface became habitable – that
cles in which shallow pools first dry out ruary 2013. As that incident demon- is, over four billion years ago. Graphic: McMaster University
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“Based on what we know about planet supplied by flashes of lightning. After has become a Max Planck Fellow: the formation and the chemistry of the so- some time, they were able to detect or- Max Planck Society supports him with lar system, we’ve proposed a consistent ganic molecules, including amino ac- research funding and finances part of scenario for the origins of life on Earth,” ids, using a chromatograph. his 16-person group. says Semenov. “Now, the experimental- Today, however, researchers assume “In our chemical experiments, we ists need to work out how life could ac- that Earth’s primordial atmosphere had reproduce the conditions stipulated to tually have emerged under these very a different composition to that as- us by the astrophysicists,” says Trapp. specific early conditions.” In fact, the sumed by Miller and Urey, containing The researchers therefore take real me- nucleobases are just the first step. Oth- less methane and instead higher levels teorite material and place it in a reac- er necessary processes include, for ex- of hydrogen, carbon dioxide, nitrogen tor, where its reactions then produce ample, the formation of complex RNA- and water. In these conditions, it was numerous organic molecules that can like molecules, cell membranes and probably more difficult to synthesize be analyzed using fast chromatograph- ultimately the DNA-protein world of the necessary building blocks for RNA. ic techniques. The results are surpris- today’s organisms. ing: the tiny meteorite particles on the It is impossible to talk about chem- GENUINE METEORITE MATERIAL nanometer scale act as catalysts for ical experiments relating to the origins REACTING IN A REACTOR these reactions. of life without mentioning the famous Interestingly, substances formed in Miller-Urey experiment in the 1950s. The job of investigating how this could the process then act as catalysts them- Stanley Miller and Harold Clayton Urey have been possible falls to Oliver Trapp, selves, speeding up the production of placed simple chemical substances who carried out research at the Univer- the same or even other substances. The within a hypothetical early Earth atmo- sity of Heidelberg before accepting a reaction enters a state of dynamic im- sphere inside a reaction vessel. They professorship at LMU Munich. In order balance: only the substances that form then exposed them to electrical dis- to maintain his productive collabora- catalytically the fastest are able to accu-
Graphic: McMaster University Photo: Axel M. Quetz/MPI for Astronomy charges in order to mimic the energy tion with the team in Heidelberg, he mulate in large enough quantities. “A
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sort of chemical evolution takes place,” pered by a problem that has been hith- says Oliver Trapp. “The aim is to see erto unknown – one that affects planets whether this chemical evolution leads orbiting cool, low-luminosity stars to an RNA world.” In these experi- known as “M dwarfs”. The latest exam- ments, it has also become apparent that ples of such planets are Proxima Centau- a kind of motor is needed to drive the ri b and Trappist-1d. In these systems, reactions: the natural light/dark cycle the habitable zone lies much closer to of day and night. In other experiments, the star than is the case with our hotter Trapp’s group is studying fatty acids Sun. A potentially inhabited planet will and the question of how cell mem- therefore presumably exhibit bound ro- branes could have formed. tation, in which the same hemisphere The analysis of chemical reactions always faces the star, resulting in perpet- in different conditions requires the use ual day on one half of the planet and of high-throughput screening tech- perpetual night on the other. niques, which allow Trapp’s laboratory Computer simulations by a research to run and analyze chemical processes group led by Ludmila Carone from the in 64 minireactors, each with a capaci- Max Planck Institute for Astronomy in ty of just 1.5 milliliters. If certain con- Heidelberg show that a specific air cur- ditions prove especially promising at rent forms in the atmosphere of such
Evolution in the lab: working with Harold this stage, they are then studied in planets. This current causes the ozone Clayton Urey in the 1950s, Stanley Miller greater detail in two-liter reactors. “We to accumulate in the equatorial region, (pictured) succeeded in producing organic jokingly refer to this as our Urey-Miller while it is practically absent in all oth- molecules in a reaction vessel – including 2.0,” says Trapp. er regions. “If we can’t detect ozone on amino acids. This famous experiment is a distant planet, that doesn’t necessar- today inspiring researchers to carry out further tests. SEARCHING FOR BIOMARKERS ily mean there’s no oxygen there at all,” IN THE ATMOSPHERE Carone explains. “We might simply have looked in the wrong place – and The reactions are very complex, and the ozone is hiding elsewhere.” the researchers are only just beginning Nevertheless, astronomers are to address many of the questions. How- searching for a potential second Earth ever, Oliver Trapp believes that the in M dwarf systems too, especially as emergence of life is inevitable if the these are much more common than conditions are right. “I’m even con- stars that resemble our Sun. The Hei- vinced that the chemical structure of delberg Max Planck researchers have potential extraterrestrial life will close- also been on a very special planetary ly resemble that of life on Earth.” hunt for a good two years now. Work- This also raises the question of ing with colleagues from other Ger- whether we will be able to detect the man and Spanish institutes, they have activity of life on another planet. The built an instrument that is studying corresponding biomarkers are general- around 300 M dwarfs using the largest ly considered to be molecular oxygen, telescope at Calar Alto Observatory in ozone and methane. However, it is im- southern Spain and looking out for portant to bear in mind that the oxy- signs of rocky planets. gen concentration on Earth did not However, the James Webb Space reach its current value until approxi- Telescope is another source of great mately 300 million years ago. That is a hope for astronomers. Launching in relatively short time frame in terms of two years’ time, at the earliest, this suc- biological evolution. cessor to Hubble will travel millions of In addition, detecting these sub- kilometers from Earth on its mission to
stances on an exoplanet could be ham- explore the universe. One of the prima- Photo: Special Collections & Archives, UC San Diego Library
24 MaxPlanckResearch 3 | 18 A gigantic compound eye: the gold-sputtered main mirror of the James Webb Space Telescope has a diameter of six and a half meters and is made up of 18 segments. From May 2020, at the earliest, the instrument will scour the heavens and train its sights on distant exoplanets.
ry objectives is to study exoplanets. The SUMMARY
Max Planck Institute for Astronomy de- l Today’s life is based on the genetic information carrier DNA. As a forerunner veloped and built key components for to this, a world based on the simpler biomolecule RNA could have existed on one of the four measurement instru- the early Earth. ments, which goes by the name of l The building blocks for RNA might have arrived on Earth inside meteorites. MIRI, and so the Heidelberg astrono- Computer simulations suggest that the subsequent synthesis of RNA began mers will have the chance to make in pools measuring just a few meters in diameter. some of the first observations using the l Using high-throughput screening, it is possible to test a very large number super telescope. of chemical reactions in a short time and thus to narrow down the optimum conditions for the emergence of life. Like his colleague Oliver Trapp, Thomas Henning is also confident of the existence of life on faraway plan- ets: “Simply by virtue of the enormous GLOSSARY number of rocky planets in our Milky Electrolysis: Splitting of a chemical compound under the action of electrical current, Way – perhaps as many as a billion – resulting in the conversion of electrical energy into chemical energy. Electrolysis is the and the knowledge that life emerged reverse reaction to that taking place in a battery or fuel cell. very quickly on Earth, I think it’s very Exoplanet: A planet outside the gravitational field of our Sun but within the gravitation- likely that life exists on other planets.” al field of another star. Around 3,800 exoplanets are currently known to astronomers. Are we therefore heading for a second Nucleobases: Constituents of nucleic acids such as DNA or RNA. DNA contains the Copernican Revolution? “No,” replies nucleobases adenine, guanine, cytosine and thymine; in molecules of RNA, thymine is Henning, “we’re already in the midst substituted for uracil. They are called bases because they dissolve in water to form weakly basic solutions. Adenine and thymine, as well as guanine and cytosine, each form of it.” base pairs, which then combine with the sugar molecule deoxyribose and a phosphate group to produce the basic backbone of the DNA double helix. www.mpg.de/podcasts/
Photo: Special Collections & Archives, UC San Diego Library Photo: NASA/Desiree Stover ursprung-des-lebens (in German)
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