142 Origin of Life: Puzzles and Solutions in Astrobiology

rJwkswavepgroup2018NE.jpg(Photo by Nerissa Escanlar)

I joined ELSI as a research scientist nearly a half year ago, in December 2017, so many things are still fairly new to me. This May, a workshop named "Puzzles and Solutions in Astrobiology" was held (link here: http://old.elsi.jp/ja/research/activities/2018/05/20180514_18_ws_news.html) in ELSI from the 14th to the 18th, and the organizers invited many scientists from a broad range of research fields, including chemists, biologists, biochemists, geobiologists, astronomers, and planetary scientists. All of these provided me a great chance to gain knowledge about the main topics in the field of astrobiology. Actually, I have participated in ELSI symposiums or workshops several times in the past two years, and an overall impression for me is that the discussion in astrobiology conferences is usually open, free, and full of debates and controversies.

This time, the title of the workshop piqued my curiosity about what will be the new questions in this field and how we could solve them. Indeed, the topics were quite broad reflecting the different perspectives from various fields, and their methods to solve the same question are presented and discussed at different temporal/spatial scales and microscopic/macroscopic levels. I was excited to discover that the discussions and debates were very common and pertinent, exactly was the organizers had aimed to achieve. Time was limited, but it set the stage for future work. During each section, there was coffee break for further conversation or discussion. I felt it can refresh people and provide great atmosphere for discussing the problems in more detail. Maybe in the future I will try to participate in organizing a similar workshop, therefore the experience this time was valuable experience for me.

As I am a chemist working on mineral-catalyzed organic synthesis related with the prebiotic chemical evolution, the prebiotic chemistry section on Tuesday was the easiest for me to understand. I saw more links between chemists and biologists in the question on "Replicator-first"or "Metablism-first"origin. Over a whole day, the overall discussions were focused on the following questions (summarized by Carlos Briones of the SOC): 1) Did life emerge through a continuous or sharp transition from geochemistry to biochemistry? 2) Did the primitive life own a heterotrophic or autotrophic origin? 3) Replicator-first"or"Metablism-first"? 4) What is the timing and sequence of replication (autonomy), compartmentalisation (individuality) and self-replication (heredity) processes? One of the speakers, Ullrich Muller from the University of California San Diego, claimed that the debate on "Genome/Replicator first" and "Metabolism first" is unnecessary if you define that metabolism is a character within life. I noticed that there was no talk given on autotrophic proto-metabolism evolution (CO2 fixation as one of the core processes, currently studied by Norio Kitadai in ELSI), therefore this section missed some aspect of autotrophic origin. Dougal J. Ritson, from the University of Cambridge, presented his recent work on the synthesis of RNA, protein and lipid precursors in a common cyanosulfidic chemical system, which is interesting for me because it demonstrated that a group of key and complex biological macromolecules can be synthesized within the same geochemical setting by following a totally non-biological pathway. This work shares similarities with another ELSI researcher, Albert Fahrenbach, using photochemistry. Though currently I prefer to believe that life emerged through a continuous transition from geochemistry to biochemistry, a totally different geochemical pathway sustaining proto-metabolism could also be possible if the first life is heterotrophic. Despite the importance of organic synthesis, I felt that metabolism cannot be simply reduced to the synthesis of several molecules which is the same to replication. I should definitely think more about my own research on prebiotic synthesis.

For the first time, I began to understand the importance of plate tectonics and emergence of land on the origin of life, that is to induce subducted plate formation, volcanic outgassing and metamorphism processes. Through these processes, continents which are exposed to the atmosphere and undergo weathering processes can supply important nutrients (such as P, Mg) for sustaining life. Emerged land above the sea level seems important also because several prebiotic synthesis (such as amino acid polymerization to synthesize peptide) was found to be promoted by water-dry cycle based on the experimental results. As another evidence, the distribution of chlorophyll on the modern Earth is found to enrich nearby the continental edge, suggesting continents are supplying the source of nutrients today. Therefore the timing of land emerging and the size of the emerged land are critical (Hervé Martin) for understanding the origin and evolution of life.

Furthermore, this question has inspired the discussion on the possible place for life's emergence. Supposing that not much nutrients can be supplied from continental land ~3.8 Gy ago, then we have to explore other places, such as the deep sea hydrothermal vent environment as the first birth place. These two different environments will lead to the discussion on the primitive metabolism pathway too (phototrophic or chemotrophic), that is which metabolism pathway dominated in the primitive evolutionary stage? Deep sea hydrothermal vents are an environment where no solar energy is available, however, there are indeed abundant chemicals originated from the hydrothermal fluids (such as H2S, Fe2+, H2, etc.), and in reality there are many organisms living in that environment in the modern ocean sustained by chemotrophic metabolism.

Ocean chemistry is another topic which caught my great attention. Since life as we know it cannot live without water, the physicochemical feature of the ocean is of primary importance for understanding the evolution of both primitive and modern life forms. Francis Albarède claimed that the ocean chemistry changed remarkably after the Great Oxygenation Event (GOE). The timing of expansion of continents above sea is just later than the GOE (supposedly at 2.4 Ga). This process has been considered to control the ocean and atmosphere chemistry. The pre-GOE ocean chemistry could have been controlled by hydrothermal inputs dominated by Fe2+ and HS-. The current ocean, on the other hand, includes more of Ca2+, Mg2+, SO42-, NO3- and Fe3+ which determines the alkalinity, and the water column character is largely mediated by microbial activities. That is to say, the current hydrosphere is partially shaped by the biosphere, which is different from the case in the Hadean time. During the Hadean, when the size of the organism community was still small, ocean chemistry was expected to largely constrain the available organism types.

Another basic question is that what is the difference between an abiotic and a biotic system? In this workshop, several approaches were proposed to understand the nature of life at different organization levels, including the microscopic molecular level (fidelity transition by Daisuke Kiga, diversity transition by Kunihiko Kaneko in the replicating system), and cellular/organelle level (host-parasite interaction by Eugene Koonin). The evolution of LUCA (Last Universal Common Ancestor) can be inferred from the evolution of LECA (last Eukaryal Common Ancestor) by comparative genomic and phylogenetic methods (Maureen O'Malley). This question can be also pursued by modeling the function of living forms using physicochemical theory. Addy Pross proposed a different type of stability (dynamic kinetic stability (DKS)) to describe the biological replicating system, which is different from the thermodynamic stability associated with regular non-living chemical system. In summary, these modeling studies show that the effort to reduce biology into chemistry and physics could be feasible, given that a careful consideration on the difference between animate and inanimate system is added, but it remains a very tall order.

There were also many interesting presentations given by planetary scientists, describing the "Small Mars Problem", "Meteorites", "Origin of elements", "bombardment" and their relations with the astrobiology problem, although this part is much more difficult for me to fully understand. Also from geobiologists, there is a section on "Biosignatures", which is a field to find biological evidence in the old rocks. Many problems were raised in this field. Researchers have found that the mineral biomorphs are easily produced in the lab, and the organisms fossilized in the lab do not always generate morphology with large difference from the abiotic one, especially in the presence of organics. For solving these problems, several possible solutions were proposed, including obtaining fresh samples, doing multiple analysis (test syngenecity, get more integrated isotope evidence of metabolism such as C, S and Fe, get more statistic data), and also using nanoprobes (such as TEM).

Finally, I like the big picture introduced by Guillaume Avice when he considers the astrobiology problem. Habitability is determined by all the characters in atmosphere (P, T, redox, gases), hydrosphere (pH, Eh, water chemistry), and lithosphere (nutrients, volatiles, redox). This is a big picture that I should bear in mind to do origin of life research.