Thermodynamic Jump from Prebiotic Microsystems to Primary Living Cells

Abstract

It is proposed that the primary living cells (“probionts”) cannot emerge of organic substance simply by continuous chemical complication of prebiotic macromolecules and microsystems. The complication must be accompanied by the radical thermodynamic transformation (“jump”) of prebiotic microsystems that resulted in the acquired ability to extract free energy from the environment and export entropy. This transformation is called “the thermodynamic inversion” The inversion may occur by means of the efficient (intensified) response of the microsystems on the oscillations of physic-chemical parameters in hydrothermal environment. In this case the surplus available free energy within a microsystem, when combined with the informational modality, facilitates its conversion into a new microsystem—a living probiont. It is shown the schematic representation of an oscillating prebiotic microsystem that is transforming into a living probiont. A new kind of laboratory and computational experiments on prebiotic chemistry under oscillating conditions is offered to verify the inversion concept.

1. Introduction

In the middle of the last century, E. Schrödinger published his famous work “What is life?” [1]. His conclusion was as follows: there are two levels of order—order from chaos (“primary”) and order from order (“secondary”). It is the second order that corresponds to life. However, Schrödinger did not explain how the secondary order may arise from the primary one. Accordingly, he did not ask himself how life arises. At present time, some other researchers consider the gap between non-living and living systems as “Mysterious Evolution Jump”, or the “Missing Link”, that should be examined in framework of the origin of life problem [2,3]. S. Jheeta also defines the origin of life as a transition of self-organization (physico-chemical process) to instructed chemistry (this is a biological process already) [4]. By now nature of this gap is not satisfactory explained in the framework of the three existing approaches to evolution of cells: compositional (or metabolism-first), RNA-world (or genes-first) and cellular (last universal common ancestor). This paper briefly discusses the thermodynamic approach to understanding nature of the Evolution Jump based on the author’s inversion concept [5,6], and how this scenario can be linked to the provisions of Schrödinger.

2. Thermodynamic Difference between Living and Non-Living Systems

Entropy, free energy, and information are basic thermodynamic notions. Owing to their universality, they can be used for a fundamental comparison of living and nonliving systems. It is well known that biological evolution paradoxically proceeds with accumulation of free energy (that is, its contribution prevails over the contribution of entropy). In contrast, non-biological systems evolve with the tendency of entropy growth, in accordance with the 2nd law of thermodynamics (in particular, stars or magmatic systems continuously dissipate free energy into the environment, i.e., lose it). There is no generally accepted explanation for this difference. But it is obvious that the emergence of the first living microorganisms from non-living organic microsystems implies a reversal of the thermodynamic tendency (as on the Earth as on another planet). The tendency to increase of entropy (in non-living prebiotic systems) must be replaced with the opposite tendency to increase of free energy (in the simplest living cells) through some intermediate state (neutral or zero “point”, where the contributions of free energy and entropy are approximately equal). The same turn must occur with the tendencies to rise of informational entropy (in prebiotic systems) and to concentration of information (in biological systems). The author calls this turn ‘thermodynamic inversion’ [5].

3. Moment of the Origin of Life

According to the developed approach, the conversion of prebiotic microsystems into the simplest forms of life occurs under conditions far from equilibrium in the presence of multilevel oscillations of physico-chemical parameters in the maternal hydrothermal environment. Those nonequilibrium conditions sustain self-organization events investigated by Ilya Prigogine and his colleagues [7,8]. An essence of such physico-chemical (i.e., not biological) self-organization considered by them can be correlated with the primary “order of chaos” proposed by Schrödinger [1]. Self-organization under far from equilibrium conditions is considered by the article’s author as a necessary preliminary stage for the transition to life. The actual transition to life is carried out through the following thermodynamic inversion in the prebiotic organic microsystems, when they perform effective (enhanced) response to external oscillating influences; the concepts of other authors imply that the transition to life is simply due to the consistent chemical complication of organic molecules and microsystems. The inversion radically changes macro-state of a prebiotic microsystem. At the inversion moment the transforming microsystem efficiently extracts free energy and information from the environment and exports entropy. As a result, the total contribution of free energy and information into the microsystem become prevalent over the total contribution of entropy. At this moment of the actual origin of life, “over-entropic” free energy and information arise. In this way, biochemical and bioinformatic processes are triggered in a transformed microsystem. It is exceptionally important to express an essence of life processes in the language of behavior instructions that a living system uses to obtain external resources and survival [9]. Some thoughts related with arising of instructed chemistry in the course of thermodynamic inversion were published previously [5, Chapters 6,7].

The described thermodynamic transition is demonstrated on the scheme (Figure 1). It is shown the intermediate state of the system between its non-living (left) and living (right) states. Production (contribution) of free energy by some chemical reactions (red arrows) and entropy by other reactions (blue arrows) are similar. Shift of this thermodynamic balance in the direction to prevalence of the free energy (and information) contribution means the conversion of oscillating prebiotic microsystem into the primary form of life. After the inversion, random sequences of amino acids and nucleotides are reorganized into primary functional sequences, which allows for the accumulation of bioinformation [5,10]. It is the thermodynamic upheaval that can be compared with the secondary “order from order” by Schrödinger [1]. In the long run, a separate organism tends to die. That occurs when its organic components are transformed to a high disorder state, and its spatially ordered energy becomes chaotic [11].

4. Approaches to Experimental Verification of the Proposed Concept

By now almost all experiments on prebiotic chemistry have been carried out under stable conditions. However, some rare simulations were conducted under simple oscillating environments [2,3,12,13,14]. Although a response of organic macromolecules and microsystems to external influences was not explored during these experiments, they have helped demonstrate the importance of oscillating conditions for the advancement of prebiotic evolution. General results of the mentioned simulations consist of the increased yield of polycondensation products, in comparison to the experiments under stable (non-oscillating) conditions. For instance, the synthesis and oligomerization of glycine take place in rather stable hydrothermal conditions, but the synthesized molecules display the tendency to decompose [15]. However, under oscillations of temperature, these molecules can be reorganized into increasingly complex structures. Some laboratory experiments have demonstrated advancements of prebiotic chemical evolution under fluctuations of temperature. For instance, the amplification of DNA under temperature oscillations between 64 °C and 92 °C accelerated the division of giant vesicles [3]. The computational modeling of thermal cycling also demonstrates the acceleration of the pace of prebiotic chemical evolution [16].

Outlining the optimal thermodynamic corridor for chemical scenarios for the origin of life would be very desirable step of laboratory simulations in this field. The proposed experiments can give us a chance to check the theoretically elaborated notions. A goal of such modified laboratory experiments would consist of the exploration of the intermediate state between non-life and life in various prebiotic models (microsystems) that are of highly fluctuating environments. The arising of the response to external changes and its development within the organic matrix can be verified. The intermediate area between organic chemistry and primary biochemistry (i.e., the “transitional” chemistry) is very vast. For instance, it is possible to detect the enhancement of active transport which is responsible for molecules/atoms transferring against the gradients of concentration (energy). The rise of homochirality degree during the proposed experiments could also indicate the initial sparks of biochemical processes. It is expected that the continuous self-assembly of prebiotic microsystems under oscillating conditions should facilitate the incessant recombination of (macro) molecules.

A general scheme of the suggesting laboratory simulations is given in [6]. Various combinations of two constituents should be examined in the experimental chamber: different kinds of prebiotic models (RNA, Lipid, and Proteinoide Worlds) and regimes of physic-chemical oscillations (varying amplitudes, frequencies, and periods). It is supposed that the continuous “pumping” of the models by external oscillations generate a continuous response in them, including induced chemical reactions directed to compensation for the influences. The oscillating parameters can be different: pressure, temperature, electric potential, pH, redox potential, etc. The first step in this experimental method would be finding the most promising combination of prebiotic models and characteristic regimes of oscillations which provide obvious signs of an intermediate state between non-life and life. According to the preliminary evaluation, the regular constituent of the oscillations should be within the interval between split second and 30 min [6].

5. Conclusions

According to the approach developed by the author, thermodynamic inversion is a natural mechanism that draws a separating line between non-living (conditionally, “physicochemical world”) and living (“biological world”) systems. Since the moment of inversion, the formed primary microorganisms and their communities exist in concordance with the biological laws already. The author proposes that the “thermodynamic inversion” explains the transition from the primary “order from chaos” to the secondary “order from order” stated by Schrödinger.