113 - A Probable Origin of Life
26 July 2017GeneralFrançois Roddier
[The text below is the French translation of a research proposal I submitted, to study the origin of life using the DECLIC experiment on board the space station]
First attempts at study
According to Maynard Smith and Eörs Szathmary (1), the first serious study of the origin of life is due to AI Oparin (1924) and JBS Haldane (1929). Their argument was that if the primitive atmosphere lacked free oxygen, a wide variety of organic compounds could have been synthesized using energy provided by ultraviolet light and flash discharges.
In 1953, on the advice of Harold Urey, Stanley Miller tested this hypothesis by causing electrical discharges through an enclosure containing water, methane and ammonia. It produced a wide variety of organic compounds, including nucleotides with RNA and DNA.
However, essential molecules were absent or were obtained only in very low concentrations. Above all, the reactions produced lacked specificity, making it difficult to understand how polymers with very specific chemical bonds could be formed.
In a series of articles published between 1988 and 1992, Günter Wächtershäuser suggested that reactions could occur between ions fixed on a charged surface. The attraction between charges of opposite signs causes the ions in solution to attach to charged surfaces. They can move slowly on the surface, while maintaining the same orientation, which greatly increases both the speed and the specificity of the chemical reactions.
Researchers have recently shown that the confinement of molecules in small drops of liquid significantly improves the rate of reactions, suggesting applications in prebiotic chemistry (2). These results confirm hydrothermal sources as a possible origin of life, but no mention is made of the critical point of water (3).Self-organization and criticality
During these 50 last years, evidence has accumulated that the processes of self-organization take place when forces of attraction balance repulsive forces. They are of the same nature as the continuous phase transitions observed in fluids in a state of critical opalescence at the so-called critical temperature. This analogy was first recognized by Per Bak et al. (4), in relation to the omnipresence of the noise in 1 / f. They called this process "self-organized criticality".
A typical example is the formation of stars in astrophysics. The instability of Jeans which allows the stars to form is indeed of the same nature as that which causes the critical opalescence. In both cases, density fluctuations follow a power law (1 / f noise), as shown by the distribution of the initial masses of the new stars.
In his book "The Self-Organizing Universe" Erich Jantsh (5) showed that the whole universe self-organizes itself following similar sequences of events. A slow "macroevolution" during which large structures condense alternate with a rapid "microevolution" during which new elementary constituents are formed. Figure 1 summarizes this process. Following this pattern, the formation of stars is part of the macroevolution. It triggers the formation of new atoms such as those of helium which are heavier than those of hydrogen. The formation of helium is part of microevolution.
Fig. 1. The self-organization of the universe after Eric Jantsch (1980)
Following Per Bak, the Jantsch macroevolution can be considered as a continuous phase transition and its microevolution as an abrupt phase transition, in other words the evolution of the whole universe can be seen as a process oscillating around a " A "critical point" (see Figure 2).Self-organization and energy dissipation
Ilya Prigogine has shown that self-organization is a categorization of dissipative structures, that is, structures that appear spontaneously in the presence of a permanent flow of energy. Living beings or Bardard's cells are dissipative structures.
The dissipative structures behave like thermal machines: they use temperature differences to produce mechanical work. According to the second principle of thermodynamics known as the Carnot principle, this is possible only following cycles of transformations. The first thermal machines used the liquid-vapor transition of water to obtain large variations in volume.
Automotive engines are more efficient because they use much larger temperature differences to produce the same volume variations. However, much lower temperature variations are sufficient to produce natural thermal machines such as Bénard cells. This is particularly true near the critical point where very low temperature differences produce very large variations in volume.The critical point of water
The critical pressure of the water is 220 bars and its critical temperature 374 ° C. In saline water as in the ocean, the critical point is a little more than 2.200 m deep, while at hydrothermal sources the temperature readily exceeds 374 ° C.
Consider the water of a hydrothermal source located below 2.200m and whose temperature is somewhat higher than 374 ° C. Its density being less than that of the surrounding water, it forms a convective pen. During his ascent, his pressure goes down. Its temperature remains a moment superior to that of its environment until, when it becomes colder, it descends towards the source, closing the convective loop. At some point, the water reaches the condensation zone. Fine droplets form. The liquid water is then converted slowly and continuously into steam water without ever forming bubbles.
Fig. 2. The above surface shows the state of the water around the critical point.
The gray area is the condensation zone.
Figure 2 shows the state of water in a convective feather when describing a circle around the critical point, as indicated by the arrow. While the transition from liquid to gaseous state is continuous, the transition from gaseous state to liquid state is abrupt. Periodically, water condenses to form fine droplets of liquid water that grow until the water becomes completely liquid. It then sinks towards the hydrothermal source where it is heated above the critical temperature. It is then continuously transformed into vapor, without ever forming gaseous bubbles.
Condensation of the gas in liquid near the critical point is called "critical opalescence". There are very large fluctuations in density, a condition favorable to the formation of microdroplets. In the ocean other molecules can also condense. The polar molecules will retain the same orientation with respect to the surface of the droplet, thus promoting polar bonds. These conditions are particularly favorable to the formation of complex organic molecules.
An opportunity to test the origin of life
Although the conditions described above are suitable for the formation of complex organic molecules, the likelihood of such reactions occurring remains low unless the same situation occurs over a very long period of time.
It can be roughly estimated that the water circulation time in a convective feather is of the order of the day, while the lifetime of an active submarine volcano is of the order of one million d years. The same conditions have thus been able to reproduce several hundred thousand times. It is clear that if we want to repeat this process in the laboratory, it must be considerably accelerated.
The DECLIC experience offers such an opportunity. DECLIC is an experiment aboard the International Space Station. One of the versions aims at the study of chemical reactions in the vicinity of the critical point of water. Its weightless environment makes it possible to produce the critical conditions uniformly over its entire volume with an accuracy of three decimal places. It should be possible to adjust these conditions so as to describe circles around the critical point in a few seconds instead of a few days. Compared to the conditions at the origin of life, this would accelerate the process by at least 5 orders of magnitude, probably more as the conditions of the experiment would be constantly kept very close to the critical point.
If it is possible to monitor the chemical composition of the reaction chamber as a function of time, one should be able to reproduce within a few months and observe chemical reactions that took millions of years to occur. We strongly suggest that such an experiment be put on the DECLIC program.
1John Maynard Smith and Eörs Szathmary, The Origins of Life, Oxford (1999).
2 Ali Fallah-Araghi et al. Enhanced Chemical Synthesis at Soft Interfaces: A Universal Reaction-Adsorption Mechanism in Microcompartments.
3K. Ruiz-Mirazo, C. Briones, and A. de la Escosura, Prebiotic Systems Chemistry: New Perspectives of the Origins of Life, Chem. Rev. 114, 285 (2013).
4 Per Bak, Chao Tang and Kurt Wiesenfeld, Self-Organized Criticality: An Explanation of 1 / f Noise, Phys. Rev. Letters 4, vol. 59 (1987)
5 Erich Jantsch, The Self-Organizing Universe, Pergamon (1980).
[This proposal is supported by Roger Bonnet, former ESA Scientific Director].