# Chemical evolution

As chemical evolution or abiogenesis is known to not fully known mechanism of development of living organisms from inorganic and organic substances. It began in the Hadean (until about 4 billion years ago), the first section of the Precambrian . The evolution of cellular organisms began in the Eoarchean , the second period of the Precambrian . Prokaryotes arose . Since then, life has been formed from life ( biogenesis ). The unknown mixture of inorganic substances which facilitated the emergence of life is often a primordial soup ( English primordial soup ), Urschleim or primordial ooze referred to, but the reality of this idea is controversial.

The characteristic of chemical evolution is the spontaneous structure formation through autocatalysis , including the emergence of homochirality . The prerequisite is steady state equilibria far from thermodynamic equilibrium . Thermal and chemical gradients of hot springs in the sea floor are likely to be the driving force.

There are various hypotheses about the course of chemical evolution. They are mainly supported by experiments based on geological knowledge about the chemical composition of the earth's atmosphere , the hydrosphere and the lithosphere as well as climatic conditions at that time. The chemical formation of complex molecules that are necessary for biological processes has already been observed, but no living system has yet been formed. The experiments are currently insufficient to formulate a closed theory that can explain how life came about.

It seems certain that only one form of life, namely that based on nucleic acids ( RNA and DNA ), has prevailed (if there should have been more or are even possible). Essential indications for this theory are the equality of the building blocks of the two most essential macromolecules typical of life in all known forms of life (the five nucleotides as building blocks of nucleic acids and the 21 amino acids as building blocks of proteins ) and the universally valid genetic code .

## Preliminary considerations

Chemical evolution hypotheses have to explain various aspects:

1. The abiogenic origin of biomolecules, i.e. their development from non-living or non-organic precursors ( cosmochemistry ).
2. The emergence of self-replicating and varying chemical information systems, i.e. the emergence of cells (presumably in co- evolution with viruses ).
3. The emergence of the mutual dependence of function ( enzymes ) and information (RNA, DNA).
4. The environmental conditions of the earth 4.5 to 3.5 billion years ago (or possibly on comparable other celestial bodies in our or other planetary systems - such as moons, exoplanets , exo-moons or even planemos , now or earlier).

Older ideas about spontaneous generation were refuted in 1860 by experiments by Louis Pasteur for microbiology too . The new sciences of cell biology , virology and biochemistry (molecular biology) were able to substantiate the theory of evolution, but they also shed light on the enormous complexity of life processes, so that an answer to the question of the beginning seemed hopeless and was initially largely ignored.

A unified model for chemical evolution is still pending, possibly because basic principles have not yet been discovered.

### Biomolecules

Origin and function of biomolecules

The prebiotic formation of the complex organic molecules can be divided into three steps:

1. Formation of simple organic molecules ( alcohols , acids , heterocycles such as purines and pyrimidines ) from inorganic substances.
2. Formation of the basic building blocks ( simple sugars , amino acids , pyrroles , fatty acids , nucleotides ) of complex organic molecules from simple organic molecules.
3. Creation of the complex organic molecules from the basic building blocks.

The elemental analysis of these molecules leads to the question of which inorganic compounds were necessary for their formation.

Composition of the biomolecules
C. H O N S. P
carbohydrates X X X
Lipids X X X X X
Proteins X X X X X
Nucleotides X X X X X
Porphyrins X X X X
possible inorganic source of the elements
reduced oxidized
Carbon (C) Methane (CH 4 ) Carbon dioxide (CO 2 ), carbon monoxide (CO)
Hydrogen (H) Hydrogen (H 2 ) Water (H 2 O)
Oxygen (O) Water (H 2 O) Oxygen (O 2 )
Nitrogen (N) Ammonia (NH 3 ) Nitrates (NO 3 - )
Sulfur (S) Hydrogen sulfide (H 2 S) Sulphates (SO 4 2− )
Phosphorus (P) Phosphine (PH 3 ) Phosphates (PO 4 3− )

All hypotheses assume that, in addition to water and phosphate, only the reduced forms of today's chemical compounds were initially available in sufficient quantities, since the primordial atmosphere hardly contained any molecular oxygen.

UV rays and lightning are used as energy sources . According to some very noteworthy recent theories, the energy required for the creation of bio-molecules would alternatively come from anaerobic redox processes between reduced volcanic gases and sulfidic minerals such as pyrite ( FeS 2 ).

### Role of the earth's atmosphere

The earth's atmosphere was cool at the time of abiogenesis, see the so-called paradox of the weak young sun . It had a reducing character, so it was largely free of molecular oxygen and without an ozone layer . Water vapor collected in the atmosphere at hot temperatures. This condensed and water collected on the surface of the earth.

### The role of water in the evolution of life

H 2 O is a chemical compound that occurs naturally on earth in all three physical states.

Life as we know (or define) it needs water as a universal solvent. It has properties that, according to accepted scientific understanding, enable life to arise (see also anthropic principle ). Possibly life can arise and exist independently of water, but many scientists assume that the presence of liquid water (in a certain area or on a certain planet like Mars ) not only enables our kind of life, but is very likely to arise power.

The following properties of water are relevant for the origin of life:

• Water is liquid in a temperature range in which organic molecules are stable.
• Water is particularly suitable as a polar medium for chemical reactions , as it enables homogeneous mixing, can provide protons for catalysis and has a high heat capacity and thus absorbs excess reaction heat .
• The anomaly of the water prevents water from freezing from the bottom and maintains an area of ​​uniform temperature.
• Water, are dissolved in the materials, such as sea water, forms at freezing regions of different concentrations, which are surrounded by Eismembranen. According to the controversial sea ice hypothesis of the physicist Hauke ​​Trinks , not only biomolecules but also life came into being.

## Hypotheses and experiments on chemical evolution

### Oparin-Haldane hypothesis

In the 1920s, the British scientist JBS Haldane and the Soviet biochemist Aleksandr Oparin independently published one of the most famous hypotheses on evolution . The theory says that even then the conditions of the earth favored certain chemical reactions. Both researchers were of the opinion that organic molecules can be formed from abiogenic materials under the action of an external energy source (e.g. the very intense ultraviolet radiation ) and that the primordial atmosphere, which contained ammonia , water vapor and a small proportion of free oxygen, could have had a reducing effect. Both also suspected that the first life forms appeared in the warm primordial ocean and were not autotrophic but heterotrophic .

Oparin believed that life evolved from coacervates - spontaneously formed, spherical aggregates of lipid molecules - held together by electrostatic forces and which may have been the precursors of cells. Oparin's work with coacervates confirmed that enzymes, which are fundamental to biochemical reactions in metabolism , worked more efficiently when enclosed in a membrane envelope than when free-swimming in aqueous solutions. Haldane, who was unfamiliar with Oparin's coacervates, believed that simple organic molecules formed first, which became increasingly complex when exposed to ultraviolet light, until cells eventually formed. Haldane and Oparin's ideas formed the basis for much of the research that dealt with abiogenesis over the next few decades.

### The Miller-Urey experiment

The experimental setup of the Miller-Urey experiment

In 1953 the chemist Stanley Miller and Harold C. Urey tested this hypothesis using the primordial soup experiment. In the experiment they showed that in an environment - similar to the assumed prebiotic conditions - more complex organic compounds such as amino acids and lower carboxylic and fatty acids are formed from inorganic compounds (water, ammonia and hydrogen) and methane can. In later, mostly more complexly structured primordial soup experiments, all the essential building blocks of living beings [amino acids, lipids , purines (nucleotide bases) and sugars], as well as the complex organic compounds porphyrins and isoprenes could be produced.

Although this showed the basic possibility of the natural formation of organic molecules, the significance of this result for the actual course of the development of earthly life is now often viewed critically. In the primordial soup experiment, it was assumed that the earthly atmosphere had a chemically reducing character, which corresponded to the state of knowledge at the time. Today, on the other hand, it is often assumed that the atmosphere was only slightly reducing or even neutral at this time, although the question has not yet been finally clarified and local chemical inhomogeneities of the atmospheric conditions are also being discussed, for example in the vicinity of volcanoes. Later experiments showed that organic molecules are also formed under such changed atmospheric conditions; even those that did not arise in the original experiment, but the yield is greatly reduced. It is therefore often argued that other possibilities for the origin of organic molecules must have at least played an additional role. Mostly the formation of organic molecules in space and their transfer to earth by meteorites or the formation in the vicinity of so-called black smokers are mentioned .

As a further argument against the origin of the biological organic molecules according to the primordial soup experiment, it is often cited that a racemate , i.e. a mixture of L- amino acids and D- amino acids , arose in this experiment . However, many amino acids occurring in living organisms are L -configured (see biological chirality ). So there should be a natural process that prefers to select chiral molecules with a certain handedness. From astrobiologists should be noted that this is easier to explain the universe as photochemical processes are with circularly polarized radiation as, for example, produced by pulsars able to destroy only certain chiral molecules handedness. In fact, chiral organic molecules have been found in meteorites, in which the abundance of the L -form predominated by up to 9%. However, in 2001 it was shown that self-replicating peptide systems are also able to effectively amplify homochiral products from an original racemate, which, according to these researchers, supports the view of the earthly origin of the handedness of biological molecules.

Günter Wächtershäuser expresses fundamental doubts about the conditions of the primordial soup experiment .

### More reactions

From the intermediate products formaldehyde (CH 2 O) and hydrogen cyanide (HCN) occurring in the Miller-Urey experiment , further biomolecules can be produced under the simulated conditions of the earth 4.5 billion years ago. This is how Juan Oro succeeded in synthesizing adenine in 1961:

 Educts Products 2 CH 2 O HCN H 2 O Serine 5 CH 2 O Ribose 5 HCN Adenine

He also demonstrated the formation of adenine and guanine by the thermal polymerization of ammonium cyanide in aqueous solution. Adenosine triphosphate (ATP) is formed from ribose, adenine and triphosphate , which is used in organisms as a universal energy carrier and as a building block (as monophosphate) of ribonucleic acids (RNA).

### Involvement of minerals and rocks

• The organic molecules are protected from UV radiation in tiny cavities in the rock.
• Crystal surfaces can serve as a matrix for growing macromolecules. The crystal surfaces can prefer certain molecular shapes. L - and D - amino acids are deposited on a calcite crystal in different places.
• Aharon Katchalssky ( Weizmann Institute Israel) was able to produce proteins with a chain length of more than 50 amino acids in almost 100 percent yield in an aqueous solution with the help of the clay mineral montmorillonite .
• Metal ions can act as catalysts or electron donors or be incorporated into biomolecules.
• Clay minerals often have an electrical charge and can thus attract and hold charged organic molecules.

### Iron-Sulfur World (ESW)

A particularly intensive form of participation of minerals and rocks in the prebiotic synthesis of organic molecules can affect the surface of iron sulfide have played -Mineralen ( "Ur-sandwich", English: primordial sandwich ). The Miller-Urey theory has serious limitations, especially with regard to the lack of an explanation for the polymerization of the monomeric building blocks formed in the biomolecules.

An alternative scenario for the early evolution of life has therefore been developed by Günter Wächtershäuser since the early 1980s , who quickly won the support of the philosopher Karl Popper for his alternative theory . According to this, life on earth would have originated on the surface of iron-sulfur minerals, i.e. sulfides that are still formed today through geological processes in deep-sea volcanoes, occurred much more frequently in the early days of the earth and probably also on many exoplanets , exos -Moons and planemos must be available. In our solar system, too, some of the larger moons of the gas planets are suspected to be an extraterrestrial ocean under the ice sheet .

The great advantage of this concept over all other theories is that it is the first time that it is possible to link the formation of complex bio-molecules to a continuously available and reliable energy supply. The energy comes from the reduction of sulfur in iron-sulfur minerals such as pyrite (FeS 2 ) with elemental hydrogen (H 2 ) ( reaction scheme: FeS 2  + H 2  ⇌ FeS + H 2 S) and provides enough energy to produce a prebiotic ammonia synthesis and also to drive endergonic synthesis reactions for monomeric building blocks of biomolecules and for their polymerization. Similar to iron ions, other heavy metal ions also form insoluble sulfides with hydrogen sulfide (see hydrogen sulfide group ).

In addition, pyrite and other iron-sulfur minerals offer positively charged surfaces on which the predominantly negatively charged biomolecules (organic acids, phosphate esters, thiolates) can accumulate (often through complex formation reactions ), concentrate and react with one another. The substances required for this, such as hydrogen sulfide, carbon monoxide and iron (II) salts, also get directly from the solution to the surface of this "iron-sulfur world" (ESW). For his theory, Wächtershäuser draws on the basic metabolic mechanisms that still exist today and derives from these a consistent scenario of the synthesis of complex organic molecules and biomolecules (organic acids, amino acids, sugars, nucleobases, lipids) from simple inorganic precursor molecules, the can be found in volcanic gases (NH 3 , H 2 , CO, CO 2 , CH 4 , H 2 S).

In contrast to the Miller-Urey hypothesis, no external force in the form of lightning or UV radiation is required; In addition, the first simple chemical reactions take place much faster at elevated temperatures without being hindered by them (such as enzyme-catalyzed biochemical reactions). As temperatures of up to 350 ° C can be reached in deep sea volcanoes, one can easily imagine the origin of life at these high temperatures. Only later, after the development of temperature-sensitive catalysts (vitamins, proteins), must further evolution have taken place in cooler niches.

Anaerobic bacteria, whose archaic metabolism includes iron and sulfur, still exist today - metabolic product: iron sulfide .

The Wächershäuser scenario therefore fits well with the observed conditions in black smokers in the deep sea, because in these structures, due to steep temperature gradients from the inside to the outside, such a narrowing appears to be easily possible. The currently living microorganisms, which are considered to be the most primitive, are at the same time the most thermophilic with a (previous) temperature maximum for growth at +113 ° C. In addition, iron-sulfur centers have an important function in many current enzymes, e.g. B. the respiratory chain . This could point to the original involvement of Fe-S minerals in the evolution of life, especially since these are metabolic products of anaerobic bacteria even today.

The thesis of the chemo- autotrophic origin of life in black smokers is represented in particular by the researchers William Martin and Michael Russell. Incidentally, because of the extensive independence from the light of a central star, the emergence of life is also conceivable at a greater distance from it (or completely without it), outside the classic habitable zone .

### Chemical evolution in the continental crust

Geologist Ulrich Schreiber and physical chemist Christian Mayer from the University of Duisburg-Essen pursue a completely different approach to chemical evolution and the origin of life with a model that focuses on the earth's crust as the reaction environment. They thus include a reaction space that has not yet been considered: water- and gas-bearing fracture zones (tectonic fault zones) within the first continental crustal complexes (cratons). The continental crust is thicker and cooler than the oceanic crust and has a much more heterogeneous structure. Tensions within the crust lead to fault zones reaching into the earth's mantle, through which the mantle gases rise. In open fault zones, all of the raw materials required for the organic chemistry of life are found in abundance. Phosphate from dissolved apatites, CO, H 2 , CO 2 , N 2 , NH 3 , HCN, sulfur and boron compounds, all alkali and alkaline earth metals, iron sulfides and all metallic catalysts. Reactions of CO and H 2 , comparable to the Fischer / Tropsch synthesis, lead to long-chain molecules in all deep floors with different pH values, pressure and temperature conditions, which form the basis for cell components. Furthermore, hydrothermal chemistry provides amino acids, organic bases and sugars.

In addition to all the necessary raw materials, there are other decisive advantages for the place where the continental crust is formed. It was protected from UV radiation and the particle stream of the solar wind that hit it in the early phase. Late meteorite impacts never affected the entire continental crust. The gases that still escape from mofettes (mineral wells, cold water geysers) on the surface of the earth are supercritical from a depth of 800 - 1000 meters (depending on the pressure and composition). This means that they represent an intermediate state between a liquid and a gas. It is crucial that carbon dioxide, for example, acts like an organic solvent in this phase state, in which hydrophobic organic substances are dissolved and with each other (as well as at the interface with water with hydrophilic components ) can react. The fault surfaces have innumerable corners and protrusions where the supercritical gases are trapped. This creates countless micro-autoclaves in which reactions can take place that are not possible on the surface, and that with a lifetime of the fault zones of millions of years.

The most interesting zone is found at a depth of about 1000 meters, where the phase transition from supercritical to subcritical carbon dioxide can be expected. This is where non-polar organic substances concentrate because the carbon dioxide loses its solvent properties. These organic substances could be detected in quartz crystals that grow in this environment. Due to pressure fluctuations (earth tides or CO 2 -controlled cold water geysers) there are also periodic phase transitions which cause the build-up and disintegration of vesicles. The vesicles enclose organic components in high concentration and accumulate amphiphilic substances (for example lipid precursors or amphiphilic peptides) in their membranes. It could also be shown that under these conditions an efficient molecular evolution is possible.

The boundaries of the fault areas are very heterogeneous. In addition to newly formed clay minerals and fresh, quake-induced microcracks in all mineral types, wallpapers made of iron sulfide minerals and heavy metals appear. Thus, the above-mentioned models, for example that of the iron-sulfur world, can be integrated or linked without any problems.

### The formation of macromolecules

Biomacromolecules are proteins and nucleic acids . The lengthening of the molecular chains ( polymerization ) requires energy and takes place with elimination of water ( condensation reaction ). Conversely, the splitting of the macromolecules ( hydrolysis ) provides energy. Since the chemical equilibrium is so far on the side of the monomers that these reactions are thermodynamically irreversible in the direction of polymer hydrolysis, there can be no polymer synthesis without an activated energy-supplying system. Theoretical auxiliary constructs, such as evaporation of water, addition of salt (removes water) or precipitation of the products, only changes this problem insignificantly. The creation of life is therefore very likely linked to the coupling to a reliable source of energy that can be used for polymer synthesis.

${\ displaystyle \ mathrm {[Monomers] _ {n} + (n-1) \, H_ {2} O \ rightarrow n \, Monomers \, + W {\ ddot {a}} rme}}$

but

${\ displaystyle \ mathrm {Energy + n \, Monomers \, \ rightarrow [Monomers] _ {n} + (n-1) \, H_ {2} O}}$

ATP is primarily used as an energy source in biochemistry today, but its formation requires the presence of enzymes. On the other hand, under the conditions of the proto-earth, one can also imagine the energization of polymer synthesis by hydrolytic cleavage of polyphosphate, which is still used today by some enzymes instead of ATP. However, even with polyphosphates it is difficult to imagine that these were available in the necessary quantities, since they can be formed spontaneously when phosphate-containing solutions are evaporated, but also hydrolyze spontaneously again relatively quickly when they come into solution again. Based on these considerations, one would have to demand a shallow sea bay as the place of origin of life, which regularly dries up and is flooded again. This would also interrupt all water-dependent processes again and again and at least greatly delay the emergence of life. Finally, one can also imagine a completely different system in which both the synthesis of the building blocks and the energy-dependent formation of polymers take place as a continuous process in uninterrupted coupling to a reliable energy source, namely anaerobic redox reactions with metal sulfides. These are still released in large quantities at the bottom of the oceans due to volcanic activity, where they form structures such as black smokers , which are densely populated by various microorganisms and higher animals.

The equilibrium of the polymer synthesis is shifted by increasing the concentration of the building blocks (monomers) and by dewatering the products in favor of the formation of the polymers. The prerequisite for this is compartmentalization, i.e. the delimitation of reaction spaces from one another, which are only in a limited exchange of substances with the environment. In conventional theory, this was settled in shallow, small bodies of water (ponds) with a high rate of evaporation, a basic idea that goes back to Charles Darwin. However, even today in volcanic areas of the deep sea, precipitated metal sulfides in the form of small caverns are observed in the large structures of black smokers , which also represent an attractive scenario of an environment in which all reactions from the monomer synthesis to the concentration and polymerization of the monomers with a "built-in." “Energy conservation system can expire.

Other approaches have been considered, but all have severe limitations and do not balance well with conditions on early Earth. In most cases, water exclusion is required for one or more steps, which is easy to achieve in the chemistry laboratory, but more difficult on the proto-earth. One of these systems is the polymerization of carbodiimides (R – N = C = N – R) or dicyan (N≡C – C≡N) in an anhydrous medium. Here the condensation of the building blocks is coupled with the reaction of carbodiimide, whereby the necessary energy is generated:

${\ displaystyle \ mathrm {[HX-OH] + [HX-OH] + energy \ \ leftrightharpoons \ [XX] + H_ {2} O}}$

(HX-OH = monomer, for example amino acid or ribose)

${\ displaystyle \ mathrm {[RN = C = NR] + H_ {2} O \ \ rightarrow \ [R-NH-CO-NH-R] + energy}}$

(if R = H urea is produced here )

Dicyan is formed from hydrogen cyanide under the action of UV , but the volatile molecule would also be lost in drying pools.

If a dry mixture of amino acids is heated to 130 ° C for a few hours, protein-like macromolecules are formed. If polyphosphates are present, 60 ° C is sufficient. These conditions can arise when water containing dissolved amino acids comes into contact with hot volcanic ash.

If a nucleotide mixture is heated to 55 ° C in the presence of polyphosphates, polynucleotides are formed , but the linkage is more likely to take place via the 5 'and 2' carbon atoms of the ribose , as it occurs more easily than the 5 'present in all organisms. -3 'linkage. Double helices are formed from both types of polynucleotides (compare structure of DNA ). However, the 5'-3 'double helix is ​​more stable than the 5'-2' helix.

If the hydroxyl group is missing on the 2'-carbon atom of the ribose , deoxyribose is present. Now only 5'-3 'links can form, as is typical for DNA.

### Formation of prebiotic structures (cell precursors)

Cells maintain their function by forming several reaction spaces (compartments) in which the metabolic processes take place separately from each other and undesired reactions can be avoided; Concentration gradients can be built up at the same time. There are several hypotheses that have been used to deduce that such structures could develop before cells were formed.

#### Coacervates

Above all, Alexander Iwanowitsch Oparin (1894–1980) dealt with the possibility of metabolism in coacervates (from Latin coacervatio : accumulation). He was able to show that delimited spaces with a simple metabolism can in principle arise through self-organization, provided that catalysts with specific properties are present. Since the substances used come from the repertoire of today's living organisms, Oparin's coacervates are not to be seen as precursors of cells, but as analogy models for the development of precursor cells.

If salt is added to colloidal solutions of biomacromolecules, small droplets with a diameter between 1 and 500 µm are formed, which contain the polymers in high concentrations.

Oparin examined mixtures of proteins ( histone and albumin ), proteins and carbohydrates (histone and gum arabic ) and proteins and polynucleotides (histone or clupein and DNA or RNA).

If droplets of histone and gum arabic contain the enzyme phosphorylase , then these droplets can absorb glucose-1-phosphate from the environment, convert it to starch and store it. The released phosphate diffuses outwards. The strength of the droplet increases until it breaks down into smaller droplets that can again contain phosphorylase, albeit less than the original droplet. This also slows down the metabolism. Here it becomes clear that to maintain the properties of a cell, the regeneration of the enzyme equipment after division is necessary.

If the starch-degrading enzyme amylase is also added, the result is coacervates with a two-stage metabolism:

1. Uptake of glucose-1-phosphate → build-up of starch.
2. Release of phosphate → breakdown of starch and release of maltose .

#### Microspheres

In 1970, Sydney Fox was able to prove that the protein-like products ( proteinoids ) that arise when dry mixtures of amino acids are heated ( see above ) can also result in droplets that grow through self-aggregation , so-called microspheres . They are separated from the environment by a semipermeable membrane and take up further proteinaceous material from the environment. As a result, they continue to grow and break up again into smaller droplets. Fox also found that these systems have enzymatic properties, break down glucose or behave like esterases or peroxidases without any external enzymes being added.

#### Protocells

In 2008, Jack Szostak and coworkers at Massachusetts General Hospital, Boston, were able to show in model tests that so-called "protocells" (i.e. vesicles consisting of simple fatty acids , fatty alcohols and fatty acid glycerol esters ) show the following properties (in combination): and 100 ° C; they can include DNA and RNA polymers inside; they allow the denaturation (separation) of the polynucleotide strands at elevated temperature without loss of the individual strands from the protocell; they can take up non-specific (that is, without transmembrane transport systems, e.g. proteins) and very quickly charged molecules such as nucleotides .

Such vesicles also form spontaneously under the influence of periodic pressure fluctuations in the earth's crust. These processes could be reproduced in a pressure cell setting the natural conditions, with unilamellar and multilamellar membrane vesicles being formed. In the presence of amino acids, the periodic formation of vesicles leads to a molecular evolution of peptides, which stabilize the vesicles and thus enable them to survive for longer. Such vesicles could already develop simple functionalities that lead to primitive metabolism. Finally, they can reach the surface through the convection of the fluid phase and there are subject to a subsequent evolution under changed boundary conditions.

### The RNA world

The RNA world hypothesis is based on the results from the Miller-Urey experiment. It was first formulated in 1967 by Carl Woese . It states that earlier life was based solely on ribonucleic acids (RNA) for both information storage and catalysis of chemical reactions. Today, these functions are realized by the chemically more stable information storage medium DNA or functionally more flexible proteins. According to the RNA world hypothesis, the first cellular life forms were ribocytes . Ribosomes and the catalytically active ribosomal RNA , which could represent evolutionary remnants ( chemical or molecular fossils ) of this time, are regarded as an indication of the existence of the RNA world .

### Virus-first hypothesis

Based on the RNA world, it was affirmed in some places that viruses could represent the precursors to cellular life and develop before microbial cells ( virus first ). Alternative hypotheses on the origin of viruses state that they only emerged after the cells had formed from extremely reduced cells (virus reduction) or from RNA / DNA residues that had become independent (virus escape). Even if the virus-first hypothesis is controversial today, some astrobiologists suggest searching explicitly for viruses on other celestial bodies such as Mars if they actually develop before the much more complex cells.

### enrichment

The reaction equilibrium between monomers and dimers (of RNA or other organic molecules) is on the side of the monomers, even at the low concentrations in the free ocean. Autocatalysis and thus the creation of life is impossible there. A mechanism for enrichment is necessary. A combination of thermal convection and thermophoresis in porous minerals of hot springs has been proposed.

## Alternatively considered options

### Biomolecules from space

The earth is the bombardment of their existence since the dawn comets and meteorites exposed, especially in a large bombardment (Late Heavy Bombardment) designated first phase after the formation of the earth. Simple organic molecules, including amino acids and precursors of sugar molecules, have been detected in a number of meteorites , and mechanisms for their formation under space conditions have been proposed and simulated. The Swiss astrophysicist Kathrin Altwegg was able to detect amino acids on the Churyumov -Gerassimenko comet as part of the Rosetta sub-project ROSINA . However, the supply from space can only have contributed significantly to the concentration of such molecules if their lifespan in the biosphere is unusually long. That's not the case. Minerals in meteorites in particular are catalytically active and thus not only promote the build-up but also the breakdown of complex molecules on the time scale of hours.

If one considers the homochirality of terrestrial biomolecules ( L- amino acids and D- sugar), one possible explanation would be that the amino acids come from space, since some of these meteoritic amino acids have an excess of the L -type of up to 9% has been. However, this distribution can also be explained by inorganic solid catalysts on earth.

### Life forms from space

The even more speculative panspermia hypothesis states that by “inoculating” the earth with lower, bacteria-like life forms from space, the first living beings came to earth. However, this only shifts the origin of life to another place and is no explanation for the origin of life per se. Panspermia could explain the comparatively short time between the creation of life and the high complexity of life. One argument against panspermia is the inhospitable conditions of space. However, there is evidence that bacteria can survive for longer periods in space conditions.

## Researcher in the field of chemical evolution

• Alexander Oparin : Coacervates ( see below )
• Harold C. Urey and Stanley L. Miller 1953: Formation of simple biomolecules in a simulated primordial atmosphere ( see below )
• Sidney W. Fox : Microspheres made of protenoids ( see below )
• Thomas R. Cech (University of Colorado) and Sidney Altman (Yale University New Haven Connecticut) 1981: autocatalytic RNA - Splicing : " Ribozymes " combine catalysis and information in one molecule. You can cut yourself out of a longer RNA chain and join the remaining ends back together.
• Walter Gilbert (Harvard University Cambridge) developed the idea of ​​the RNA world in 1986 ( see below )
• Hans Kuhn (Max Planck Institute for Biophysical Chemistry (Karl Friedrich Bonhoeffer Institute) in Göttingen) developed a model for the creation of a first, replicating oligomer in a very special place on the prebiotic earth under the condition of a very special cyclical change of temperature and many other special circumstances that happen to prevail in this place.
• In 1986 Günter von Kiedrowski ( Ruhr University Bochum ) published the first self-replicating system based on a hexanucleotide (DNA), an important contribution to understanding the growth functions of self-replicating systems.
• Manfred Eigen ( Max Planck Institute for Biophysical Chemistry Göttingen): Evolution of RNA-protein ensembles ( hypercycle ).
• Julius Rebek Jr. ( MIT Cambridge) makes an artificial molecule (aminoadenosine triazide ester) that replicates itself in chloroform solution. However, the copies are identical to the template, so that evolution is not possible for these molecules.
• John B. Corliss ( Goddard Space Flight Center of NASA): Hydrothermal vents of the oceans provide energy and chemicals that enable a chemical evolution largely undisturbed by meteorite impacts. Today they are still the habitat for archaebacteria ( Archaea ), which are very primitive in many ways .
• Günter Wächtershäuser (Munich): The first self-replicating structures with metabolism emerged on the surface of pyrite . The iron sulfide in pyrite provided the necessary energy for this. With the growing and disintegrating pyrite crystals, these systems could have grown and multiplied and the different populations were exposed to different environmental conditions ( selection conditions ).
• AG Cairns-Smith (University of Glasgow) and David C. Mauerzall (Rockefeller University New York) see clay minerals as a system that is itself initially subject to chemical evolution, resulting in many different, self-replicating crystals. Due to their electrical charge, these crystals attract organic molecules and catalyze the synthesis of complex biomolecules, the information content of the crystal structures initially serving as a template . These organic structures become more and more complex until they can multiply without the help of the clay minerals.
• Wolfgang Weigand , Mark Dörr et al. ( Friedrich Schiller University Jena ) show in 2003 that iron sulfide can catalyze the synthesis of ammonia from molecular nitrogen.
• Nick Lane (Department of Genetics, Evolution and Environment / University College, London) has generated simple organic compounds (formate, formaldehyde, ribose, deoxyribose) in a laboratory reactor under the conditions of alkaline hydrothermal vents on iron sulfide membranes.

## Individual evidence

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2. Abiogenesis. In: Lexicon of Biochemistry. Retrieved November 26, 2017 .
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4. Iris Frey in an interview with Chris Impey (ed.): Talking about Life: Conversations on Astrobiology . Cambridge 2010, ISBN 978-0-521-51492-7 , pp. 13-21, ( limited preview in Google book search).
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