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Building Blocks That Fall from the Sky

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Building Blocks That Fall from the Sky 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 FOCUS_The Origin of Life TEXT THOMAS BUEHRKE he great questions of our exis- However, recent developments are The initiative was triggered by the dis- tence are the ones that fasci- forcing researchers to break down this covery of an ever greater number of nate us the most: how did the specialization and combine different rocky planets orbiting around stars oth- universe evolve, and how did disciplines. “That’s what we’re trying er than the Sun. “We now know that Earth form and life begin? to do with the Heidelberg Initiative terrestrial planets of this kind are more DoesT life exist anywhere else, or are we for the Origins of Life, which was commonplace than the Jupiter-like gas alone in the vastness of space? By ap- founded three years ago,” says Thom- giants we identified initially,” says Hen- proaching these puzzles from various as Henning. HIFOL, as the initiative’s ning. Accordingly, our Milky Way alone angles, scientists can answer different as- name is abbreviated, not only incor- is home to billions of rocky planets, pects of this question. For a long time, porates researchers from different dis- some of which presumably offer envi- there was a clear division of tasks: astro- ciplines, but also cooperates closely ronmental conditions that favor the physicists and geophysicists were respon- with international institutions such emergence of life as we know it. It is sible for studying the universe and Earth, as the McMaster University in Ham- precisely this realization that broadens biologists and chemists for studying life. ilton, Canada. the scope of the Heidelberg Initiative: it Precious cargo: meteorites may have brought key chemicals – such as the nucleobases adenine, guanine and uracil – to Earth, thereby providing the ingredients for the formation of RNA molecules. Photo: DLR 3 | 18 MaxPlanckResearch 19 Cosmic nurseries: stars and planets form in clouds of gas and dust. The image above shows three such areas: the Omega and Eagle Nebulae and the Sharpless 2-54 complex (from left). This process of formation not only gave rise to our solar system some 4.6 billion years ago, but also continues to take place at many locations in space. It is also responsible for producing the red dwarf star Trappist-1, which is located 40 light years from Earth, and the seven relatively Earth-like rocky planets that have been identified in its system so far (right). must ask not only how life could have challenged the study of the astrono- istry. This states that the first ever ter- emerged on Earth but also, more gen- mers following the thought: what do restrial life forms were based on ribonu- erally, what conditions are needed for astronomers understand about biomol- cleic acids (RNA). Structurally, RNA something like this to happen – includ- ecules – even if they have a deep knowl- resembles DNA, the information carri- ing on extrasolar planets. edge of astrochemistry? The National er of life today: both are made up of Academy of Science took a different four organic bases, including adenine, PRAISE AND CRITICISM FOR view, however, and awarded the publi- guanine and cytosine; however, RNA AN EXCELLENT STUDY cation the 2017 Cozzarelli Prize for its contains a base known as uracil instead “outstanding scientific excellence and of thymine. In addition, RNA is typical- Late last year, Thomas Henning and his originality”. In fact, astronomers can ly single-stranded – unlike the dou- colleague Dmitry Semenov, as well as indeed contribute to questions of what ble-stranded structure of DNA. Ben Pearce and Ralph Pudritz from Mc- conditions potentially existed when Likewise, molecules of RNA can also Master University, caused quite a stir the first molecules of life or their pre- communicate genetic information and with a publication in which they pro- cursors were formed – and of how this perform catalytic functions. In most or- posed a scenario for the emergence of came about. ganisms, however, RNA serves as an in- life on Earth. “As well as recognition As a starting point for the study, formation carrier in a subordinate role and praise, it also brought us some crit- Henning and his colleagues applied the to DNA, only acting as a storage medi- icism,” says Henning. widely used “RNA world” hypothesis um in viruses. Did the more complex The criticism came from the more proposed some 30 years ago by Walter DNA therefore evolve from simpler traditional origins-of-life scientists, who Gilbert, a Nobel Prize winner in Chem- RNA molecules or related biopolymers? Photo: ESO 20 MaxPlanckResearch 3 | 18 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.
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