Chemical evolution as a source of life

Any scientific discussion on the origin of life on Earth immediately raises difficult questions: What was the chemical composition of the Earth's surface in the pre-biological period, before life was created? What are the conditions for the formation of life? What are the chances of an event of "beginning of life" occurring exclusively on Earth? Did life begin in one place or at the same time in several places?

From: Almost, 2000, booklet, 13 Winter. 1997
1.1.1998
By: Raphael Aiken
Raphael Aiken, Department of Organic Chemistry, The Hebrew University of Jerusalem.

Any scientific discussion on the origin of life on Earth immediately raises difficult questions: What was the chemical composition of the Earth's surface in the pre-biological period, before life was created? What are the conditions for the formation of life? What are the chances of an event of "beginning of life" occurring exclusively on Earth? Did life begin in one place or at the same time in several places?

Many questions are still unanswered, but science today provides several answers and some reasonable hypotheses. The new theories on the origin of life are mostly based on the assumptions of two scientists: the Russian biochemist Alexander Oparin, they were the first to come up with Haldane, and the British biochemist John Holden
One separately (Operin in 1924 and Holden in 1929), hypothesized about the conditions that prevailed in the early atmosphere, and which according to their claim made possible the creation of the organic compounds, such as proteins and carbohydrates, which are the basis of life. According to them, the ancient atmosphere was a recirculating atmosphere, i.e. rich in hydrogen-rich compounds. It is assumed that at the stage before the appearance of life on Earth, the atmosphere consisted mainly of oxygen, nitrogen and water and small amounts of phosphine, it had no free oxygen at all, as there is (S(2)H) and hydrogen sulfide ((3)PH) in the atmosphere today. According to the currently accepted hypothesis, simple chemical compounds were added in the oceans and humid atmosphere over millions of years. The compounds were exposed to the effects of energy sources such as lightning, heat and ultraviolet radiation from the sun, electrical breakdowns, radioactivity in the earth's crust, heat originating from volcanoes and energy from shock waves (without oxygen, the ozone layer, which dampens radiation, was also absent.) Sources Energy of this kind may result in the creation of new bonds between molecules, and these bonds resulted in the formation of more complex and organized organic molecules, such as amino acids, sugars, purines and more.

The scientists call this primitive state with the picturesque nickname "the primordial soup" (or the "prebiotic soup", before there was life). It is estimated that the concentration of organic substances in the "broth" was about XNUMX gram per liter of liquid. In the course of time, the concentration of the substances in the organic soup increased, as a result of the evaporation of the water, and a process of connecting small molecules to each other took place later. Such a process is called polymerization (formation of polymers). Examples of this are proteins which were formed from many units of amino acids and nucleotides, which connected to each other and formed nucleic acids (DNA and RNA). Ofrin and Helden saw these stages as the necessary condition for the formation of life in our world.

Laboratory experiments

It was only after the Second World War that scientists began to try to imitate in the laboratory the formation of organic substances of the ancient world based on the ideas of Oprin and Holden. (Miller) One of the most famous experiments was the one carried out by the American Stanley Miller from the University of San Diego in California in 1953. Miller prepared in his laboratory equipment (see Figure 1, almost 2000 No. 13, p. 21) a mixture of ammonia gas, propellant gas, and water (These are simple molecules common in the atmosphere). He exposed the mixture to rejuvenating conditions, that is, he added hydrogen, such as the situation that probably prevailed in the ancient world. Through this system he transmitted an electric spark (as an imitation of lightning). A week later he checked the system, and to his surprise he discovered a variety of organic substances in it. The most important of them were four amino acids - glycine, alanine, aspartic acid and glutamic acid, which are among the building blocks of proteins. By the way, the amino acids that were created in Miller's experiment are the most common among those that make up animal proteins today. In light of his laboratory findings and other considerations, Miller supported the assumption that life began in marine systems, where one of the sources of energy for the formation of organic molecules is the sun. Some researchers hypothesize that the origin of life is extraterrestrial and that extraterrestrial molecules "caught a ride" on meteorites or solar dust and "settled" on the surface of the earth.

Among the many researchers who dealt with the problem of the origin of life or evolution and its team. They (Ponnamperuma) pre-biologically, let's remember the researcher Ponnamperuma used a laboratory facility with two pollens (see photo, almost 2 no. 2000 p. 13), where the upper one represents the "atmosphere" and the lower pollen the "ocean" and there is a flow of materials among them. The chemical reactions took place in the space (HCN) through electrical breakdown. Thus it was found that interstellar hydrogen cyanide (HCHO) forms bases such as adenine, while formaldehyde leads to the formation of carbohydrates. As mentioned, polymeric biological molecules (biopolymers), such as proteins and nucleic acids, consist of small building units (monomers). The transition from monomers to polymers occurs through adhesion reactions and removal of a water molecule (dehydration). Such reactions could have occurred in ancient soil, when a mixture of organic substances from the "primitive soup" reached the shores of the oceans and adsorbed on the surface of clays that were exposed to the heat of the sun. The possibility that clays have a catalytic effect (an effect of speeding up processes) regarding organic substances was tested in the laboratory, and proved to be possible.

"Biological markers" - what are they?

In geochemical studies, which examine the content of organic molecules in geological systems, it became clear that tiny amounts of organic substances originating from animals and plants were preserved in fossils and ancient sediments, whose age was estimated to be millions of years, without significant structural change. Organic compounds whose molecular structure indicates that they were formed in biological processes and that they are resistant to chemical changes (bioligical markers) are called "biological markers", so for example it was discovered that certain cyanide found in sea lilies today have a structural resemblance to the cyanide found in sea lilies. Fossils, whose age is estimated at about 170 million years.

In picture 3 (Ref. 2000 No. 13, p. 29), you can see structures of biological markers called bio-lipids and geo-lipids. The age of such hydrocarbons is estimated at about 700 million years. The importance of the biological markers is in the information they provide regarding the type of organisms that contained them (usually with a slight structural change) in ancient days. A comparative study of the organic components in ancient and young rocks indicates the great similarity between the contemporary and ancient biochemical processes. There is difficulty in making a complete comparison, because many species of ancient living things have become extinct and do not exist today.

Not much is known yet about primary biological processes. A theory dealing with the development of life and survival (Darwin) Charles Darwin's evolution of species, but does not outline the ways of their formation. In recent years, the field of molecular biology has developed, which deals with the study of the molecular basis of life processes. However, the link describing the origin of life (the origin of the primary living cell) on Earth is still missing. In spite of the accumulated knowledge about the chemical nature of the ancient world, the connection to the formation of life systems in the sea and on land has not yet been found. Finding this connecting link is also a fascinating challenge for researchers involved in the study of space and the origin of life.

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For possible prebiotic binding agents see almost 2000 No. 13, p. 21. And here and Steinman (Calvin) are examples of the action of binding agents: the Calvin researchers irradiated the amino acids glycine and leucine with ultraviolet radiation (Steinman) in the presence of the binding agent cyanamide, and received the substances diglycine, triglycine, leucine-glycine and glycyl-leucine. 24 hours later, polypeptides were formed in the presence of dicyanamide. It turns out that about 15% of the ancient soup consisted of hydrogen cyanide, so its tetramer, diaminomelanitrile, was involved in polymerization reactions in the formation of the purine base adenine, which is a component of nucleic acids and coenzymes. It is possible that the gas phosphine (which is also found in Jupiter's atmosphere) was present in the ancient atmosphere, became phosphate in ancient oceans and thereby assisted in the attachment reactions.

For further reading: Evolution - Collected. D. Frankel, L. Peled, H. Bernholtz, Chapter 1995, The Center for Science Teaching, . XNUMX