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ON EVOLUTION OF PRIMEVAL LIFE (1st part)

In Allegretti on June 18, 2013 at 8:49 AM

Dealing with “life” and its origins from an epistemological and empirical point of view implies a phenomenological, historical description, dependent on the experience domain of the observer. Primordial, compartmentalized (membrane-delimited) life appears as a dissipative (operating far from thermodynamic equilibrium), open kind of organization originated between 3.5 and 3.8 billions of years ago, i.e. when the first fossils of prokaryotes are dated. The starting point was probably some inorganic matter, which spontaneously self-assembled and underwent a “molecular evolution” in the contingent environmental conditions with the constraints imposed by physical and chemical laws. Both experimental (simplification of existing prokaryotes and synthesis of proto-living systems from plausible pre-biotic molecules) and theoretical (nucleotide dynamics and cellular behavior simulations) approaches are actively involved in understanding how the first living prokaryotes emerged, trying to obtain in vitro the main properties of a living entity: self-reproduction, self-maintained organization and possibility to evolve. In addition, every kind of transition from chemistry to biology has to consider the support of an energy flow and metabolites from the external environment. Obviously, the number of hypothetical, contingent scenarios for the origin of life is virtually unlimited, but it is easy to imagine that the pathway was complex, discontinuous and full of trials. The fashion of this kind of research (a small section of the broad field of synthetic biology/artificial life) comes also from this relative freedom and creativeness in the experimental setting. First living cellular systems, in a systemic conceptual framework, were born up-taking from the environment chemical precursors and transforming them in such a way that the product of these transformations was ultimately a “close identity” (despite a variety of perturbations and the continuous changes of its components) consisting of a network of functionally related biochemical processes topologically distinct by a semi-permeable boundary (later self-made). The evolution of such persistent metabolic pathways in the first living systems is therefore indissolubly linked and dependent from the external environment, in fact, cell metabolism is the result of a dynamic interaction with the medium, which feeds the cell and accepts expelled byproducts through the boundary. The cellular system and the environment trigger changes and adaptations one with the other in a congruent way (primeval cognition), and, in this co-emergent dependency, the living system autonomy is surprisingly maintained through generations thanks to a complex multi-regulated and multi-pattern framework. Let’s try in the following lines to trace an approximate and not sequential pathway that brought to the origin of proto-living entities trying to emphasize both the molecular and the system evolution.

A) Several groups reported the formation and oligomerization of cyclic ribonuclotides in formamide solutions under UV radiation in the presence of phosphorous compounds in anoxic geothermal fields. These photostable, cyclic ribonucleotides could represent the monomers and the energy source for the abiotic formation of RNA replicators/ribozymes around 4 billions of years ago. The formation of phosphodiester bonds was favored by the ability of potassium and zinc ions (high quantity in geothermal fields) to catalyze transphosphorilation reactions.

B) Mineral/clay and lipid surfaces could serve as templates/catalyst for the abiotic synthesis of short/long organic molecules/polymers and their replication.

C) Geothermal fields were also full of water, silica, metal sulfides and amphiphilic molecules that could produce honeycomb porous reactors. Such porous membranes could naturally favor horizontal gene mixing and sharing of metabolites among the first proto-life forms (co-development). Some of these primitive compartments could have the possibility to encase a large amount of short/long RNA-replicators, which could remain viable if connected via metabolic networks.

D) The process of RNA replication could be favored in volcanic regions on the cold surface of the early earth where the temperature differences would cause convection currents, so that nucleic acids possibly encapsulated in proto-compartments would be often exposed to a burst of heat as passing near the hot rocks. This event would cause a double helix to separate into single strands, but they would almost instantly cool down again (as the heated water came across the bulk of cold one) forming new double strands, copies (with some mutations) of the original ones. In addition, mixtures of RNA fragments, after self-assemblage into self-replicators, spontaneously could form catalytic cycles. Such networks can grow very fast indicating an intrinsic ability of RNA populations to evolve through cooperation giving birth to hyper-cycles (different self-replicative informational cycles which interact and cooperate one with the other).

The replicating moieties of one inorganic bubble at a hydrothermal vent, sharing a common pool of metabolites and genes, could resemble a distinct evolutionary unit able of Darwinian selection and, eventually separable as an enclosed system. The emergence of L.U.C.A. (last universal common ancestor) was at that moment not so far.

Matteo Allegretti 

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