“Primary broth” as an environment for the emergence of life has its drawbacks. In the aqueous environment, proteins, RNA and DNA are unstable. These long molecules eventually break up into separate links – amino acids or nucleotides. The chemical combination of amino acids in a protein or nucleotides in RNA occurs with the release of water. Therefore, when there are many around it, the balance of this reaction is shifted towards the breakdown of the protein or RNA (such a decay involving water is called “hydrolysis”). The first to draw attention to this problem in the context of the origin of life John Bernal back in 1949.
Cells build long molecules using activated nucleotides and amino acids. In the construction of RNA and DNA, nucleotide triphosphates are used, their combination in a chain is accompanied by the release of pyrophosphoric acid, rather than water. A similar trick allows the cells to collect proteins. However, Miller’s apparatus produces normal, not activated amino acids. Without complex cellular activation systems, it is impossible to obtain proteins from them in the aqueous medium.
Therefore, on a lifeless planet, to unite amino acids and nucleotides into chains, one must somehow get rid of water. For this purpose, for example, bays and lagoons on the shores of the ocean, which fill with water at high tide and dry out in low tide, are well suited. Similar conditions of frequent drying are possible in volcanic areas due to regular discharges of hot water from geysers.
Experiments have shown that baking dry mixtures of nucleotides at a temperature of 120 оС leads to the formation of short chains of RNA from 3-10 nucleotides. To combine amino acids in proteins, these conditions are too stringent, they lead to the decomposition of the amino acids themselves. However, as it turned out, complete drying and not required: it is enough that in the solution there was no “chemically free” water. In a strong brine (100 grams or more NaCl per liter, as in the Dead Sea), all water molecules are firmly bound to sodium and chlorine ions and are not chemically free. Therefore, in brine at temperatures of 60-80 оС, the equilibrium is shifted toward bonding, and the amino acids combine in short chains (this reaction is called “salt peptide synthesis” and will be described in more detail in the following chapters). So, although water is necessary for life, it should not be too much.
Another problem of the “primary broth” theory is related to the fact that it has no boundaries. In cells, thousands of genes “play in the same team” thanks to the cell membrane separating them from the external environment. In order to create the simplest cooperation between different RNAs, some of which are engaged in copying, while others, for example, prepare nucleotide details for this, these RNAs should somehow separate their close company from the rest of the world. Then the nucleotides will go on copying only those RNAs that made them, and the copying RNA will copy only itself and its neighbors that supply it with nucleotides. In the boundless broth, these nucleotides will blur and reach all neighbors, regardless of their contribution to the common cause. Copying RNA, which tries to replicate not only itself, but also other molecules, in conditions of the broth can not distinguish allies from hangers-on and eventually lose in competition.
Scientists have proposed two different solutions to the problem of borders, which by analogy with the “primary broth” were named “primary pizza” and “primary mayonnaise”. In the “primary pizza” model, set out in the book “The Major Transitions in Evolution” (John Maynard Smith, Eörs Szathmáry, 1995), life was born as a thin layer of organic molecules on the clay surface (for example, kaolinite or smectite); whose particles consist of thin aluminosilicate layers. Clay can swell, because water molecules, like other cations and anions, penetrate between these layers, increasing the distances between them. Experiments have shown that clay quite firmly binds amino acids, nucleotides, proteins, RNA and other biological molecules. On the surface of clay crystals and between its layers they accumulate in high concentration even from a very dilute solution. Adsorbing properties of clay are also used in everyday life: you probably saw smectite called “mineral filler for cat litter”. On clay nucleotides spontaneously line up exactly as they need to be “stitched” into the RNA chain. Long RNA molecules are very rarely separated from the clay completely, but can move slowly along its surface, detaching itself from one end or the other. Mathematical modeling of the interaction of different RNAs on a flat mineral surface (Czaran T., Szathmary E., 2000) showed that under such conditions groups of different molecules linked by mutual help are easily formed, and the multiplication of parasites is limited and does not lead to the extinction of cooperating RNA.
So, although there are no clear boundaries on the mineral surface, it sufficiently restricts the mobility of RNA and nucleotides, so that close self-help groups can appear.
Another contribution of the mineral substrate to the emergence of life is that it can work as a catalyst, i.e., accelerate chemical reactions. This function comes to the fore in those variants of the “primary pizza” model, in which non-clay is offered as a mineral base, and sulfide minerals are pyrite (FeS2), sphalerite (ZnS), alabandine (MnS). Further in this chapter, we will consider in more detail the relationship of sulfide minerals with biochemistry.
The model of “primary mayonnaise” was proposed by Harold Morowitz in the book “Mayonnaise and The Origin of Life: Thoughts of Minds and Molecules”. It suggests that primitive analogues of cell membranes existed from ancient times, even before the appearance of self-replicating RNAs. In other words, the whole world of RNA existed inside the protocells – small fatty vesicles. The theory of “primary mayonnaise” has fewer supporters than the theory of “primary pizza”, because there is a nutritional problem for the rosette: nucleotides pass very badly through the membranes. In modern cells, there are special transport proteins for this, but no adequate solution for the uptake of nucleotides by primitive protocells has been found so far. But in the “primary mayonnaise” model, very effective separation of RNA molecules into cooperative groups is achieved, so scientists are not in a hurry to reject it. Moreover, there are ways of combining theories of “primary pizza” and “primary mayonnaise”: clay particles, as it turns out, help the formation of membrane vesicles, with the resulting bubble surrounding the clay particle from all sides.