Alternative Pathways in Astrobiology: Reviewing and Synthesizing Contingency and Non-Biomolecular Origins of Terrestrial and Extraterrestrial Life
Abstract
:1. Introduction
“…any replay of the tape (of life) would lead evolution down a pathway radically different from the road actually taken…”—Stephen Jay Gould, 1989
2. The N = 1 Problem and Its Relevance in the Search for Extraterrestrial Life
- (a)
- A “messy” chemical state is the default state of any prebiotic chemical space on a given planetary body of astrobiological interest (including primitive Earth). Such a messy prebiotic chemical space provides the starting material for prebiotic chemistry to happen, is likely different from planet to planet, is impossible to simulate completely precisely in the laboratory, and may not even be necessarily reproducible on the same planet.
- (b)
- Non-biological chemicals could have played a role during the OoL, which may have produced life with a molecular makeup different from life on Earth.
- (c)
- The role of contingency could shape life differently from life as we know it on Earth due to differing planetary conditions and “messy” prebiotic chemical spaces.
3. The “Messy” State of Prebiotic Chemistry
4. Non-Biological Chemicals and Their Prebiotic Chemistry During the OoL
5. The Role of Contingencies during the OoL (on and off Earth) Linking Non-Biomolecular Compounds
- Black circle = arbitrary LUCA on Earth, eventually leading to the terrestrial life tree (orange box).
- Black square = arbitrary “LUCA” on another planetary body, which eventually follows its own unique “phylogenic” path towards ET life (yellow box).
- Blue circles = deterministic chemical states (CS) inspired by and leading to modern biochemistry, such as an RNA world, a metabolism first world, etc., which assumes a direct history between prebiotic chemistry and modern biology.
- White circles = the diverse potential CS that may not follow a deterministic path toward life. These CS are composed of a blend of biomolecules and non-biomolecules, likely dominated in abundance by non-biomolecules originating from the abiotic chemical space, encompassing plausible abiotic chemicals that existed on primitive Earth or other early-planetary bodies. Contingency (by geochemical “selection”) propels CS further towards different areas of the “life-likeness” spectrum (i.e., non-life, somewhat life-like, and life) towards a more “life-like” state (defined from a terrestrial perspective). More advanced “life-like” CS can move forward along this spectrum by manipulating their environment, potentially metabolizing and replicating through the use of novel molecules not found in the abiotic chemical space (i.e., autotrophy), all while still undergoing “selection”, advancing their status to become arbitrary “LUCA”. At any given point, any CS can reach a dead-end, meaning such a CS cannot progress to any form of life.
- White dashed arrows = the process for a CS to emerge, which is derived directly from a particular abiotic chemical space, i.e., all components within the CS are solely chemicals within the abiotic chemical space of any geological niches on early Earth or other planetary bodies.
- Double-headed solid black arrows = a transition from a CS to another CS. These black arrows are double-headed, which suggests that a CS can return to a former state through, for example, the degradation of certain components.
- Dashed black arrows = the mixing of two or more CS (the two or more circles at the origin of the arrows) to form a novel CS composed of a mixture of components of the initial CS (the one circle at the destination of the arrows). These arrows are single-headed, as de-mixing may not result in recovery of identical initial CS as before; however, de-mixing (not shown) could result in one CS splitting into two novel CS. The mixing can result from the mixing of two or more biomolecular CS (blue circles), some “non-biomolecular” CS (white circle) with some biomolecular CS, or two or more non-biomolecular CS. For example, mixing of a non-biomolecular CS with a biomolecular CS can result in a new CS where the components of the initial non-biomolecular CS drive some type of novel reaction amongst the components of the initial biomolecular CS, ultimately leading to the emergence of a more “life-like” biomolecular CS that is “scaffolded” by some non-biomolecular process or chemical(s).
- Orange and yellow arrows = the progression of a CS to a LUCA. These arrows are single-headed, indicating the fact that once LUCA emerges, which is defined as “life”, it cannot reversibly convert back to a “non-life” CS state (as “life” by definition must either be able to sustain its own life, or undergo extinction (see below for further explanation).
- Note 1: The arbitrary term LUCA was used as a point depicting the beginning of Darwinian-like evolution, but the concept of LUCA remains unclear in the field; for example, LUCA could either be a living entity that acquired the full ability of transcription-translation mechanisms [94] or one derived from progenotes [95,96]. The latter is not depicted in Figure 2 for simplicity. Similarly, the definition of terms we use to describe life in this paper, such as ‘life as we know it’ and ‘life as we don’t know it’, is not discussed here, as it is well covered elsewhere [97].
- Note 2: Though not explicitly shown in the model, the arrival of unique elements or organic compounds via by means of comets and meteorites [98] could have occurred during meteoritic bombardment periods, which were was common on Early Earth [99]. These additions might have influenced the available abiotic chemical space, potentially affecting the progression of “life”, and the existence of more “life-like” CSs at different times. This could also be applied to ET life cases. In this case, more emergent CSs could have emerged from the altered abiotic chemical space, while more “life-like” CSs could incorporate some aspects of the modified abiotic chemical space to access novel chemicals, chemistries, reactions, and functions. This concept simplifies to a larger abiotic chemical space, which can be seen as encompassing components from both the Earth’s abiotic chemical space and that introduced by comets and meteorites.
- Note 3: Although not mentioned explicitly, emergent polymers from the initial abiotic chemical space with early catalytic abilities could have altered the reaction topography within its own chemical space, leading to the formation of new CSs. These new CSs either enhance or hinder the life-like properties of a particular CS. Additionally, the formation of these new CSs could also be maintained by auto-catalytic reactions, potentially giving rise to a where new functional chemistries and reducing the dependence on input from other CSs for persistence.
- Note 4: Terrestrial life may be an outlier, and existence of ET life remains improbable (but not impossible), as we have not observed a single instance of ET life. Several factors contribute to the absence of signs of ET life, possibly because such life never developed. Firstly, chemistries that lead to the OoL (as well as the chemistries of life) must be viable for a long period of time from the onset of planetary formation due to the timescales of non-catalyzed chemical reactions [100]. Secondly, early OoL events (and life itself) occurring on a particular planet must remain persistent, i.e., keeping pace and changing to survive in the ever-changing environmental and geochemical settings by developing replication and fine-tuning metabolism and protection to avoid extinction [101]. Life on Earth has adapted and survived for ~3–3.5 billion years despite numerous extinction events. Hence, if any of the requirements for OoL off Earth are not met, we will never observe extant ET life, either due to early extinction of early life or failure to fend off its last planetary extinction threat. In either case, the planet would remain barren of life.
6. Caveats of the Contingency and Non-Biomolecular OoL Model
- (a)
- We did not specify the specific chemical, geological, and physical constraints necessary for conditions leading to life in the model because such constraints are not necessarily fixed and are bound to be updated with new data and modeling. Nevertheless, the emergence of any CS, like any prebiotic system, is driven and governed by constraints via its primitive environment parameters (pH, temperature, etc.) [51]. Early CSs are constrained by the rules of chemistry, which are governed by thermodynamics (before kinetic control) and also geology, i.e., geological niches present on early Earth (e.g., hydrothermal vents, evaporating ponds, etc.). Such constraints act as “selective” pressures on CSs and can fluctuate from time to time, either in short or longer durations. As CSs evolve to become more complex and functional, resembling life forms (as illustrated in Figure 2), the imperative to adapt to such constraints for survival intensifies. Sudden changing chemical and environmental constraints due to ambient geology, for example, temperature rising because of increasing atmospheric CO2 from volcanic events, may be too harsh on advanced CSs made up of macromolecules and could lead to thermal degradation. For example, RNA and polyesters within a particular CS in water are prone to accelerated hydrolysis when heat is added [105,106]. Alternatively, oceans getting cooler due to tectonic events, i.e., the movement of Earth’s primordial continental plates [107] allowing movement of water between them, could slow down the chemical reactions that make certain CSs. How these constraints act on a particular CS is of particular interest and helps evaluate their plausibility for survival and innovation (which may come when an (advanced) CS has the ability to create solutions for a problem likely via some replication system).
- (b)
- The presence and composition of the actual chemicals available within any abiotic chemical space is contingent upon the prevailing local and global geochemistry of a specific celestial body of astrobiological significance, e.g., early Earth or Noachian Mars. This likely will change from time to time (e.g., [108,109,110]) during the prebiotic epoch and across geological timescales, and hence primitive chemical systems will not be provided with continuous supplies of all organic chemicals all of the time; the supply of critical organic compounds would have implications on both the ability to generate emergent CS and on the identity of the emergent CS (it may be that no single emergent CS is identical to any other one). Global or local perturbating geological conditions provide further screening on the heterogeneous emergent CS [51], for example, through degradation or seclusion, that can eliminate some or all emergent CS from existence. This can lead to the preservation of the “fittest” CS, in terms of the persistence, while “functional” CS—those with abilities important for progressing towards a more “life-like” state, such as autocatalytic properties—may be expelled during such perturbations if they are not fit for persistence. A “functional” CS may be lost altogether at some point in time, although the function may be deemed useful towards achieving more “life-like” states. Hence, how would the system recover such a loss? Can the same lost “functional” CS be produced again if the chemical and geological conditions revert to the original state (e.g., the reversal of a snowball Earth-like event to a warm Earth-like one [111]) that produced the lost “functional” CS in the first place (it may never revert in the end)? Or can the system produce another CS that can perform the same (or a similar) function as the lost “functional” CS, simply with different components? This would, of course, depend on the function, and whether that function must necessarily be achieved in one way, or whether there are multiple ways to achieve that function. As the introduced model consists of a multitude of emergent CSs, which theoretically are constantly being produced (from the abiotic chemical space) and degraded, what are the probabilities of recovery of a lost “functional” CS if emergent CS may never be identical to any other one (and if more than one CS is required to lead to another resulting CS)? This is where we anticipate determinism coming into play, where the lost functionality can be possibly re-gained through a different pathway towards the formation and/or composition of a CS.
- (c)
- Without a replication system, Darwinian-like evolution is not possible, and the long-term existence of any life-like CSs edging closer to life in the spectrum of “life-likeness” is unsustainable. This is because the formation and continuity of CSs depend on the constant availability of the same chemical sources that originally produced them. As mentioned before, the constant changes in geological conditions on early Earth could have caused chemical fluctuations that led to the rapid disappearance of a multitude of CSs, both emergent and those that are more “life-like”. If any CS indeed had a hand in the initial development of a living system, it would likely have required the ability to replicate informationally at some point. This replication would allow the CS (or a similar CS produced later) to persist long enough to transfer its structures, functions, products, or reactions to the primitive living system, even amidst fluctuations and the loss of chemical availability (i.e., it must be able to be replicate, assuming that there is only one deterministic pathway to the emergence of life). This replication could have been through a genetic sequence system [112], a composomic informational system [113], a constant supply of the required materials (see above) through some type of auto-catalytic system [114], or some other type of not-yet-known replication system.
- (d)
- We have shown the possibility that sequences of prebiological chemical states that eventually led to the emergence of arbitrary LUCA may have been “scaffolded” by non-biological compounds throughout history’s existence (Figure 2B). If the conclusion from [69] is true—that the inclusion of biological choice from chemical space of biomolecules bootstraps a CS towards producing biological products—then understanding when and how the processes involving non-biomolecules (if they were essential for the emergence of life) were removed from primitive biology is crucial and requires further investigation. For example, it has been shown that alpha hydroxy acid-charged-tRNAs can undergo in vitro translation that results in the synthesis of polyesters with modern biological translation machinery [115,116]. Does this mean that if alpha hydroxy acids and/or polyesters participated in primitive biology, that they were removed only after the emergence of the central dogma? Or, is this simply a biochemical coincidence? Were there other non-biological systems that participated in primitive biology that are now absent in modern biology? How do we know if there are?
7. Summary
Funding
Acknowledgments
Conflicts of Interest
References
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Term | Definition | Reference |
---|---|---|
Abiotic Chemistry | Any chemistry involving non-living or inanimate matter. | [24] |
Prebiotic Chemistry | A subset of abiotic chemistry that has led to life or chemistry that happened during the OoL. | [24,25] |
Biomolecules | Molecules that make up the structure and function of living organisms on Earth. | [26] |
Non-Biomolecules | Refers to prebiotic organic molecules, which are not directly employed in biochemistry or serve different roles to their biochemical functions. An example of non-biomolecules are alpha hydroxy acids. These compounds can polymerize prebiotically to become prebiotic polyesters. While some alpha hydroxy acids (e.g., lactic acid, malic acid, etc.) are considered to be biomolecular compounds in the broad sense, with roles in respiration, inflammation regulation, and molecular signaling [27,28], they are considered non-biomolecules in the prebiotic context. This is because, in prebiotic chemistry, these compounds are involved in polymerization [29], a function not used by living organisms for these compounds. Additionally, within the paper, it is important to distinguish between the terms ‘biomolecules’ and ‘non-biomolecules’ for clarity. Even if some molecules transition from prebiotic to biological roles, the term ‘non-biomolecules’ are not reclassified to ‘biomolecules’. However, outside this paper’s scope, biomolecules could include those that originally were non-biomolecules but became integral to life on Earth or elsewhere. | [30] and further updated in this paper |
Building Blocks | Refers to the fundamental chemical compounds, biomolecular and/or non-biomolecular, essential for the formation and function of life. This concept applies to terrestial life forms [31] and hypothetical or theoretical forms of extraterrestrial (ET) life. | [31] and this paper |
Terrestrial-biochemistries | Refers to the biochemical processes and molecular compositions associated with life as we know it on Earth. | [32] |
Xeno-biochemistries | Refers to “bio”chemical processes and ”bio”molecular compositions associated with ET life forms that could exist in environments different from those on Earth. In this paper, Xeno-biochemistry could, in principle, accommodate both biomolecules and/or non-biomolecules within its functionality and form. The word ‘Xeno-biochemistries’ carries the connotation of ‘xeno-biology’. However, other definitions of xeno-biology include the design and engineering of new life forms by humans using technology [33]. For the sake of simplicity, we will equate the ET definition mentioned above with xeno-biology. | This paper |
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Chandru, K.; Potiszil, C.; Jia, T.Z. Alternative Pathways in Astrobiology: Reviewing and Synthesizing Contingency and Non-Biomolecular Origins of Terrestrial and Extraterrestrial Life. Life 2024, 14, 1069. https://doi.org/10.3390/life14091069
Chandru K, Potiszil C, Jia TZ. Alternative Pathways in Astrobiology: Reviewing and Synthesizing Contingency and Non-Biomolecular Origins of Terrestrial and Extraterrestrial Life. Life. 2024; 14(9):1069. https://doi.org/10.3390/life14091069
Chicago/Turabian StyleChandru, Kuhan, Christian Potiszil, and Tony Z. Jia. 2024. "Alternative Pathways in Astrobiology: Reviewing and Synthesizing Contingency and Non-Biomolecular Origins of Terrestrial and Extraterrestrial Life" Life 14, no. 9: 1069. https://doi.org/10.3390/life14091069