Amino acids are one of the most fundamental building blocks of life. The first step towards the formation of life is the synthesis of amino acids. To solve the puzzle of origin of life on Earth, many scientists and researchers work on the explanation of the very first amino acid creation on our planet. In Zita Martins and her team’s study (Martins et al., 2013), the researchers design experiments to verify whether amino acids could be synthesized when an impact shock is applied to a typical cometary ice mixture, a process known as shock synthesis. They shoot ice mixtures with similar composition to a comet with a steel projectile in a light gas gun to simulate the conditions when a rocky object clashes with icy surface, such as comets, or when an icy object collides with rock surface, such as Jovian and Saturnian satellites.
The result is a detectable amount of several amino acids. This work has large implications on the existence of life across the whole solar system. Also, this paper provides a novel mechanism in which the first amino acids on Earth might have been synthesized as opposed to the delivery hypothesis and Earth-pool hypothesis, which would be further explained. Finally, these seemingly contradictory theories in fact complement each other to form a panorama of this very inquiry of amino acids and cooperate to answer an even larger puzzle: when did life first emerge on Earth? Martin’s study is pretty valuable in the context that many organic substances were detected by Cassini-Huygens. According to Ostro and his group (Ostro et al., 2006), data from radar albedos (percentage of reflection detected by radar) show contamination of near-surface water ice in Saturnian satellites, which could be caused by compounds like ammonia, silicates, and metallic oxides. That is to say, the chemical condition on the surface of these satellites could resemble that of the ice mixture in Martins’ experiments (Martins et al.
, 2013). It is then possible that amino acids were created when rocky objects collide with these satellites. It is well supported by astronomical observations that there are a great amount of impacts between asteroids and meteorites with planetary or satellite surfaces every year, and one impact in the right place at the right time would create amino acids thanks to the mechanism of shock synthesis. Therefore, it is highly possible that apart from Earth, some other planets or satellites in the Solar System harbor life.The same point is alluded to in media’s portray of this inquiry (Alan Boyle and Science Editor, 2013), which says that the discovery of shock synthesis increases the possibility that life is widespread across the Solar System. With the help of the media’s interpretation, the public would gain an insight in the origin of very first building blocks of the solar system and be more likely to support further aerospace exploration especially when huge governmental funding is needed. They would also gain an appreciation of the diversity of our solar system now that Earth might not be the only life-harboring planet. The study of shock synthesis also offers a new mechanism through which the first amino acids found their way to Earth.
Organic compounds are common within the parent body of carbonaceous (containing compounds of carbon) meteorites, and these compounds could have reacted under the high temperature and high pressure during an impact, following the pathway provided by Martins’ shock synthesis experiments (Martins et al., 2013). There were questions concerning whether the amino acid products could endure such extreme physical conditions, but it is mostly resolved by E. Pierazzo and C. F.
Chyba’s study (Pierazzo and Chyba, 1999) on amino acid survival in large cometary impacts. They argue that small comets would explode in the atmosphere following Tunguska projectile so that not all organics would be destroyed during the impacts. Also, part of the amino acids could endure the shock heating of large cometary impacts so if they are created by shock synthesis, some would remain intact and make further polymerization (combining of amino acids to form proteins) possible. Further, in grazing impacts where the angle of entry into atmosphere is oblique, the peak temperature and pressure during impacts would be lower, allowing potentially formed amino acids to survive.Interestingly, in the larger field, there are quite different theories explaining how amino acids came into being on Earth. In N.R. Lerner’s study (Lerner et al.
, 1993), the authors propose the Strecker synthesis as a source of amino acids in carbonaceous chondrites to explain their deuterium (hydrogen-2) enrichment (also as D-enrichment, a process where the ratio of hydrogen-2 to hydrogen-1 atoms becomes higher). D-enrichment indicates components from interstellar precursors, so meteorites and comets are probably the carriers of these D-enriched amino acids that in turn result in the D-enrichment of chondrites. Lerner’s experiment shows that the retention of deuterium in amino acids produced by Strecker’s reaction is as high as the retention rate in those chondrites (as much as 50%), making itself a plausible pathway for amino acid synthesis in meteorite parent bodies. This mechanism is different from that proposed by Martins and her team (Martins et al.
, 2013) since in this case, chemical reactions happen inside the meteorites and comets, and amino acids are carried by these astronomical objects to the Earth rather than being created from organic precursors on the meteorites and comets during the very impacts. In addition, unlabeled amino acids (those not deuterium-enriched) are also formed under the conditions of Strecker reaction, indicating multiple pathways for the synthesis of amino acids in meteorites and comets. Furthermore, in José Aponte’s study (Aponte et al., 2017), the authors explore the synthetic route of amino acids that leads to their occurrence inside meteorites, including both interstellar and parent body phase. Their analysis suggests that glycine and methylamine could have formed in meteoritic parent bodies, which is backed by carbon-13 (carbon atoms with 7 neutrons) isotopic analysis.
Therefore, meteorites inside which the amino acids are already synthesized could have brought the very first glycine and methylamine to Earth, right consistent with the delivery hypothesis that suggests meteorites are carriers of building blocks of life. Some studies also suggest that amino acids could be synthesized with the ingredients on prebiotic Earth regardless of extraterrestrial influences. Gheorghe Surpateanu indicates in his study (Surpateanu, 2018) that the first proteinogenic amino acids and their corresponding polypeptides (short chains of amino acids linked together) could have been formed starting with three syntones: methylene (CH2), nitrene (NH), and carbon monoxide (CO). These syntones go through a series of multi-step reactions with components from primordial atmosphere of Earth and yield amino acids and polypeptides.
The author argues that with the presence of these three precursors (CH2, NH, and CO) in an atmosphere with necessary components such as water, ammonia, and hydrogen sulfide, chemical reactions naturally take place and amino acids are yielded. Some of the intermediary products during this process would be analogous to the chemical composition of interstellar precursors, but are purely terrestrial, so the exogenous influence appears less important in the context of this paper. The synthesis of amino acids could be spontaneous given the primordial environment on Earth. Although many papers present diversely different theories, the mechanism of shock synthesis proposed by Martins and her group (Martins et al., 2013) is also backed by other works. In Yoshihiro Furukawa’s study (Furukawa et al.
, 2015), the authors indicate that although exogenous delivery of amino acids and nucleobases is the prevailing hypothesis, the variety and amounts of intact organics produced would be limited in that way. They contend that interplanetary dust particles and crater-forming objects (evidence of shock impact synthesis) account for a far higher mass flux of carbon to the ancient Earth than chondrites (evidence of delivery hypothesis). Thus, shock synthesis seems a more plausible hypothesis in this case, or at least it is proposed to be the dominant source of amino acids. Also, their experimental simulation results demonstrate the possibility of synthesis of amino acids and nucleobases under prebiotic Earth surface conditions, which agrees with Martins’ results (Martins et al., 2013).
Both Furukawa’s and Martins’ simulations demonstrate that shock impact is a possible source of amino acid synthesis, but Furukawa’s team goes further to argue that this would be a more dominant mechanism than extraterrestrial delivery of organics (Furukawa et al., 2015). We could see from this debate within the field that every paper has its own focus and point of view. It is common that their different approaches result in different conclusions about a single question: in this case, how amino acids first came to Earth. Some papers focus on the formation of amino acids in meteorites and comets themselves and conclude that these building blocks of life were previously well-prepared in those astronomical objects and then delivered to Earth. Some studies including the one by Martins et al.
concern about the creation of amino acids through the process of shock impacts, and thus conclude that shock impacts of meteorites and comets on Earth surface could be the source of amino acids. Other research discusses independent synthetic pathways in the primordial atmosphere and concludes that amino acids could have formed spontaneously on Earth. Indeed, it’s hard to say which paper is “wrong”, since every paper has its own basis and reasoning. It’s more appropriate to think of all these ideas as complementary to each other.
There could be more than one mechanism how amino acids found their way to Earth and perhaps all of these hypotheses are true: some were directly delivered by meteorites and comets, some created in the process of impacts, and others formed by terrestrial ingredients alone. These pathways are not really exclusive to each other, but probably form a whole picture about the very inquiry.Finally, these mechanisms provide insight into when life first emerged on Earth. Ben K.D. Pearce and his team (Pearce et al.
, 2018) explain in their paper how the information on early meteorite impacts is central to the inquiry of when life first appeared on Earth. The very mechanism of amino acid synthesis and retention could potentially answer a hard question: could life endure the Late Heavy Bombardment (LHB), a period of early Earth history where a huge number of asteroids collided with Earth? If shock impacts could create amino acids, these building blocks of life could have possibly endured harsh environment of primordial Earth where bombardments were so common and kept evolving into the earliest life. E. Pierazzo and C. F. Chyba (Pierazzo and Chyba, 1999) shows that a small portion of amino acids could survive during every impact, so probably during LHB, the earliest pre-life forms across the globe were largely decimated, but some would always endure. Raw materials were constantly refurnished by comet delivery (Aponte et al., 2017) or impact synthesis (Martins et al.
, 2013) so that a small group of organics could keep evolving without disturbance. And if so, the time taken for life to form on a planet would be significantly longer compared to the case where all previous lives were wiped out during the LHB period.In summary, Martins and her team’s study suggests it is possible that organic lives are spread across the Solar System and thus shows a future exploration is crucial. Further, the study provides a different point of view on how early life emerged on Earth and contributes to a larger picture. In the end, it turns out that these different theories could also work together to resolve a deeper question concerning the timescale of life evolution and have much further repercussions.?
