Evolution And The Origin Of Life
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Others favor the same kind of lipidic vesicle-based model but an heterotrophic one as they hypothesize that the vesicle could have acquired complex nutrients such as nucleotides from the environment [ 20 ]. However, the problem of the abiotic synthesis of nutrients as complex as nucleotides remains unresolved. Lipidic vesicle models require specific conditions allowing the vesicles to emerge and persist long enough, including a continuous flow of matter and energy and the possibility for the waste products to be diluted in an open milieu so that the system is not hindered by their increasing concentration [ 9 ].
The energy and the matter are provided by the vent fluids and the open milieu is the vast ocean.
7 Theories on the Origin of Life
There are good arguments supporting the idea that many hydrothermal vents already existed on early Earth [ 22 , 23 ], some of which were deep-sea [ 21 , 24 ]. These deep-sea hydrothermal sites would have offered protective settings from the intensive solar UV radiations and the frequent massive meteorite impacts suffered by early Earth [ 22 ].
In addition, these fluids are enriched in calcium. Hence, mixing of these fluids with seawater results in the precipitation of large carbonate chimneys [ 27 ]. Actively venting chimneys and flanges are highly porous as they show fine anastomosing networks of carbonate lined with brucite, with tiny interconnected pores on the micrometer scale, indicating the mixing of seawater and hydrothermal fluids within the interior walls [ 21 ]. Fluids percolate through the carbonate anastomosing networks. It has already been recognized that vents systems were chemically reactive environments that constituted suitable conditions for sustained prebiotic syntheses [ 28 ].
At high temperatures, lipid compounds can be produced by aqueous Fischer-Tropsch-type FTT synthesis [ 29 ]. Such abiogenic production of short-chain hydrocarbons has been recently found at LCHF [ 25 ].
The millimolar concentrations of abiogenic CH 4 present in the LCHF effluent could be at the origin of the carbon reduction in such hydrothermal systems [ 30 ]. Actually, fluids collected from the Rainbow and the Lost City hydrothermal fields were clearly enriched in organic compounds with a dominance of aliphatic hydrocarbons C9-C14 , aromatic compounds C6-C16 and carboxylic acids C8-C18 even though a mixed origin, i.
Moreover experiments simulating molecular transport in elongated hydrothermal pore systems showed extreme accumulation of molecules in a wide variety of plugged pores [ 33 ]. New experiments demonstrated that thermal gradients across narrow channels can provide the energy necessary to concentrate dilute molecular solutions and thus allow the self-assembly of lipidic vesicles from an initially dilute solution [ 34 ].
Vesicles with membranes composed of bi-layers from mixtures of amphiphilic and hydrophobic molecules could have formed from the organic compounds present locally at high concentrations. The stability of bilayer lipidic membranes at high pressure and temperature is nevertheless still debated. However primitive membranes would have been composed of a diverse mixture of amphiphiles. This mixt character may have imparted essential stability to primitive membranes [ 36 , 37 ]. Furthermore, polycyclic aromatic hydrocarbon PAH may have contributed to stabilizing them as cholesterol stabilizes cell membranes of extant organisms today [ 35 , 31 ].
Recent experiments show that monoglycerides are synthesized under hydrothermal conditions by simple condensation reactions which represent a plausible step in the self-assembly of protocellular structures toward boundary membranes that would be stable over a range of pH values in the salty seas of the prebiotic environment.
Actually, in these experiments, no salts were present either during the synthesis or during the formation of bilayers, which occurs at pH 8. The authors justified their conclusions by the fact that monoglycerides are virtually immune to the effect of pH and divalent cations, because they do not have ionic head groups that can interact with cations in solution [ 38 ].
Finally, this membrane stability problem would have been a strong selection factor among all the possible sorts of vesicles with heterogeneous membranes. The hypothesis that lipidic vesicles were produced on early Earth 4, million years ago at sites such as LCHF makes deep-sea hydrothermal sites the best candidates to be at the origin of evolution and sets new constraints.
Heterotrophic scenarios that call for abiotic synthesis of nucleotides at hydrothermal sites require a source of phosphorus, whereas mid-ocean ridges are not a source, but a sink of phosphorus [ 39 ]. This is a major constraint, and scenarios based on acetyl phosphate raise the same issue [ 40 ]. The answer may be that, at that remote time, arsenic was used instead of phosphorus, as claimed recently. However the interpretation by the authors of the data from the GFAJ-1 bacterial strain, isolated from Mono Lake, California [ 41 ] is very much debated [ 42 , 43 ] and abiotic synthesis of nutrients as complex as nucleotides remains an unresolved question, be it phosphorus or arsenic-based.
Celestial bodies considered as candidates for the emergence of evolution should plausibly have a very long-standing process of plate tectonics in order to allow the occurrence and maintenance of deep-sea hydrothermal sites such as LCHF. These constraints would add to classical requirements, e.
The requirement of the existence of plate tectonics would suggest Mars as a possible cradle of evolution. Mars is very likely to be in a stagnant-lid regime now, and has been for much of its history. However, if surface water was present in its early history, it may have been in the active-lid regime at that time [ 45 , 46 ]. In addition, lines of evidence suggest that hydrothermal conditions might have existed on Mars: although an igneous origin cannot be excluded, the formation of the carbonate-rich outcrops recently identified by the Spirit Rover could be due to aqueous processes under hydrothermal conditions in the planets first billion years [ 47 ].
Moons like Europa could be questioned too [ 45 ]. Finally, within such a shift of paradigm in the approach of the primordial ancestor, there would be further consequences. In particular the search for biosignatures would be affected, not when biosignatures are supposed to be related to relatively high-evolving microorganisms, but in the search for the primordial ancestor. In the presented lipidic vesicle-based model, the only differences are the membrane sites and their ligands but of course they are not yet identified. However, the model implies the enanthioselectivity of the synthesized compounds which represents a specific signature.
There is no point in attempting to define life because of the irreducible metaphysical aspect of the concept. Instead it seems more appropriate to focus on the process of evolution, as the source of the primordial ancestor on Earth and presumably similar systems elsewhere. The consensus to be reached in the quest for the primordial ancestor should be in explaining the minimal processes that allowed evolution to emerge and persist. A process is proposed here that requires only three conditions without necessarily involving biopolymers.
I am grateful to Robert Berman, Ilektra Kouranti, Arnaud Lucien, and Romain Tessera for reviewing the text and fruitful discussions with them and I thank the reviewers for their very useful and relevant comments. National Center for Biotechnology Information , U. Int J Mol Sci. Published online Jun 1. Marc Tessera.
Origin of Life on Earth | Biology Biological Principles
Author information Article notes Copyright and License information Disclaimer. This article has been cited by other articles in PMC. Abstract The question of the primordial ancestor must be approached through the search for the origin of evolution, not through the search for the origin of life. Keywords: origin of life, origin of evolution, open far-from-equilibrium systems, lipidic vesicles, heredity, hydrothermal vents. The Primordial Ancestor and the Conditions for Evolution 2.
What Is the Question? Three Conditions for Evolution Within the paradigm of open, far from equilibrium systems that should maintain their level of organization, it is possible to only envisage three conditions that would permit the systems to evolve: 1 Local conditions that allow the emergence of open non-equilibrium structural systems, organized on a macroscopic level, generated by a flow of matter and energy that is continuously supplied.
Two Schools of Thought There are presently two schools of thought regarding the above matter. Open in a separate window. Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Consequences of the Model: Homochirality and Polymers The catalyzing effects of the sites would have crucial consequences by leading to enantioselectivity. Next Experiments Experiments that prove the concept of a complex membrane catalyzing the synthesis of compounds that stabilize its local arrangements are yet to be developed. Need for Lost City-Like Hydrothermal Vents Lipidic vesicle models require specific conditions allowing the vesicles to emerge and persist long enough, including a continuous flow of matter and energy and the possibility for the waste products to be diluted in an open milieu so that the system is not hindered by their increasing concentration [ 9 ].
Synthesis of Lipid Compounds It has already been recognized that vents systems were chemically reactive environments that constituted suitable conditions for sustained prebiotic syntheses [ 28 ]. The Phosphorus Issue The hypothesis that lipidic vesicles were produced on early Earth 4, million years ago at sites such as LCHF makes deep-sea hydrothermal sites the best candidates to be at the origin of evolution and sets new constraints.
Need for a Tectonic Plate Dynamic Regime? Origin of Evolution versus Origin of Life: Consequences Finally, within such a shift of paradigm in the approach of the primordial ancestor, there would be further consequences. Conclusions There is no point in attempting to define life because of the irreducible metaphysical aspect of the concept. Acknowledgments I am grateful to Robert Berman, Ilektra Kouranti, Arnaud Lucien, and Romain Tessera for reviewing the text and fruitful discussions with them and I thank the reviewers for their very useful and relevant comments.
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