The first proto-L.U.C.A. was probably not a typical membrane-bound cell, but rather a coordinated consortium of replicating genetic elements (maybe some virus-like particles) that might dwell in networks of inorganic compartments, as described in the first part of the article.
F) Inorganic zinc sulfides were likely good candidates for the early formation of such primeval enclosed compartments. Moreover, zinc sulfide was also a very powerful photo-catalyst that could reduce CO2 to formate (precursor of other organic compounds including intermediates of the Krebs cycle) and drive various transformations of nitrogen containing substrates. Posteriorly, single-chain fatty acids and prenyls (reported both in meteorites), could spontaneously self-assemble into leaky vesicles, allowing exchanges of metabolites. The self-organization of lipid compartments from a chaotic mixture of amphipathic monomers (due to thermodynamic reasons) was a nodal event in the evolution of cell-life, these double-layer membranes could in fact undergo spontaneous growth and division and at the same time they provided a protective and discriminative shell permeable to small substrates as nucleotides and aminoacids.
G) Stanley Miller and his colleagues showed that simple building blocks such as amino acids, nitrogenous bases and carbohydrates, could be produced from inorganic compounds under conditions imitating the primordial atmosphere, as long as energy was provided in the form of electric discharges or UV light. In addition, typical primordial compounds like carbonyl sulfides were catalysts of peptide bond formation (as some ribozymes and dipeptides).
H) With the evolution of the first proto-cells, division was probably either mechanical by shearing forces, due to an abundance of amphiphilics in the medium, or simply due to the evaporation of water, which could favor division by mechanical stress. Furthermore, RNA encapsulated in vesicles could exert an osmotic pressure on the vesicle membrane, driving the uptake of additional membrane components from some vesicles at the expense of other, shrinking ones. Thus, more efficient RNA replication could cause a faster cellular growth, leading to the emergence of Darwinian evolution at a proto-cellular level.
I) Eventually, the interactions between RNA and peptides would have brought to the appearance of the first genetic code (possibly only a two letters code) and the first ribosomal, RNA-catalyzed protein synthesis (probably with a reduced amino acid alphabet).
J) The replication of the content of the vesicles (for instance ribozymes and peptides) soon became coupled with the growth and division of the proto-cell, due probably to the evolution of very versatile proteinaceous-enzymes, that could have taken over RNA’s role in assisting genetic copying and in other biochemical processes.
K) At a later stage, the organisms “learned” to make DNA, gaining the advantage of possessing a more robust carrier of genetic information. The “RNA world” became therefore the “DNA world” and, most likely at that point, lipid biosynthesis became self-made by the cell.
L) With the evolution of bulky polar lipids and so tighter membranes able to maintain ion gradients, the first cells could escape the geothermal fields and invade terrestrial water basins with low potassium/sodium (K+/Na+) ratio like rivers and oceans (first free-living L.U.C.A.).
At that moment, coordination between the inner content and the surface of the cell was already present, and the membrane was as of now a system of transportation and channeling containing some integral and peripheral membrane proteins. Unfortunately it has not yet been clarified how this coordination is linked to the evolution of membrane proteins themselves. Nevertheless, since the organization of the living system emerged in a way that its own survival was inevitably intertwined with the medium, membrane proteins evolved coupling probably from the beginning the inner cell biochemical processes with the external environment (primeval cognition). This structural/functional integration is a historical, dialogic product of interactions that allow to maintain the autonomy of living systems through regulated exchanges of matter and energy with the outside. Nonetheless, drastic (predictable or unpredictable) perturbations of the plastic, but fragile, whole-equilibrium could affect the rate of compensation and adaptation of living organisms, bringing about complex, evolutionary changes, which could end (in a broader prospect) in species rearrangements of the planetary ecosystem.
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