Chemical evolution is the gradual formation of complex molecules from simple ones, leading to the origin of life. Learn about this idea here.
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Introduction
Chemical evolution is the sequence of chemical changes in originally nonliving matter that give rise to life. The phrase “chemical evolution” is also used, in astronomy and cosmology, to describe the changing makeup of the Universe’s stock of chemical elements through deep time since the Big Bang, from hydrogen and helium immediately after the Big Bang to the full array of elements observed today.
Basic Concept
The first known living things on Earth were prokaryotes, a type of cell similar to present-day bacteria. Prokaryote fossils have been found in 3.4-million-year-old rock in the southern part of Africa, and in even older rocks in Australia, including some that appear to be photosynthetic. All forms of life are theorized to have evolved from the original prokaryotes, probably 3.5-4.0 billion years ago.The chemical and physical conditions of the primitive Earth are invoked to explain the origin of life, which was preceded by chemical evolution of organic chemicals. Astronomers believe that about 14 billion years ago, all matter was concentrated in a single mass, and that it blew apart with a “big bang.” In time, as the Universe expanded and temperatures became low enough to permit the existence of atoms, stars and galaxies formed out of the primordial hydrogen and helium produced by the big bang.
The nuclear processes in the hearts of these stars and in the explosions of some of them in novae produced heavier elements such as carbon, iron, and oxygen. Eventually, in part of the Milky Way galaxy, a disk-shaped cloud of dust condensed and formed the Sun and its planets, including the Earth. Heat produced by compaction kept the Earth liquid as it formed. Then, as the planet cooled, Earth’s layers formed. The first atmosphere was made up of hot hydrogen gas, too light to be held by Earth’s gravity. Water vapor, carbon monoxide, carbon dioxide, nitrogen, and methane replaced the hydrogen atmosphere. As Earth cooled, water vapor condensed and torrential rains began cooling its surface, rising up as steam; eventually the Earth’s surface cooled enough for the fallen rain to exist as water on the surface, whereupon it began to fill up the low parts of the surface, forming the seas. Also present were lightning, volcanic activity, and ultraviolet radiation. It was in this setting that life began.
According to prevalent theory, chemical evolution occurred in four stages. In the first stage of chemical evolution, molecules in the primitive environment formed simple organic substances, such as amino acids. This concept was first proposed in 1936 in a book entitled, “The Origin of Life on Earth,” by the Russian scientist, Aleksandr Ivanovich Oparin (1894– 1980). He considered hydrogen, ammonia, water vapor, and methane to be components in the early atmosphere. Oxygen was lacking in this chemically-reducing environment. Oparin stated that ultraviolet radiation from the Sun provided the energy for the transformation of these substances into organic molecules. Scientists today state that such spontaneous synthesis occurred only in the primitive environment. Abiogenesis became impossible when photosynthetic cells added oxygen to the atmosphere. The oxygen in the atmosphere gave rise to the ozone layer, which then shielded Earth from ultraviolet radiation. Newer versions of this hypothesis contend that the primitive atmosphere also contained carbon monoxide, carbon dioxide, nitrogen, hydrogen sulfide, and hydrogen. Present-day volcanoes emit these substances.
In 1957, Stanley Miller (1930–) and Harold Urey (1893–1981) provided laboratory evidence that first-stage chemical evolution as described by Oparin could well have occurred. Miller and Urey created an apparatus that simulated the primitive environment. They used a warmed flask of water for the ocean and an atmosphere of water, hydrogen, ammonia, and methane. Sparks discharged into the artificial atmosphere represented lightning. A condenser cooled the atmosphere, causing rain that returned water and dissolved compounds back to the simulated sea. When Miller and Urey analyzed the components of the solution after a week, they found various organic compounds had formed. These included some of the amino acids that compose the proteins of living things. Their results gave credence to the idea that simple substances in the warm primordial seas gave rise to the chemical building blocks of organisms.
In the second stage of chemical evolution, the simple organic molecules (such as amino acids) that formed and accumulated joined together into larger structures (such as proteins). The units linked to each other by the process of dehydration synthesis to form polymers. A problem with this part of the hypothesis was that the abiotic synthesis of polymers had to occur without the assistance of enzymes. In addition, these reactions give off water and would, and would therefore not occur spontaneously in a watery environment. Sydney Fox of the University of Miami suggested that waves or rain in the primitive environment splashed organic monomers on fresh lava or hot rocks, which would have allowed polymers to form abiotically. When he tried to do this in his laboratory, Fox produced proteinoids—abiotically synthesized polypeptides.
In the third step in chemical evolution, it is suggested, polymers interacted with each other and organized into aggregates known as protobionts. Protobionts are not capable of reproducing, but had other properties of living things. Scientists have successfully produced protobionts from organic.
Darwinian evolution requires a mechanism for generation of diversity in a population, and selective differences between individuals that influence reproduction. In biology, diversity is generated by mutations and selective differences arise because of the encoded functions of the sequences (e.g., ribozymes or proteins). Here, I draw attention to a process that I will call chemical evolution, in which the diversity is generated by random chemical synthesis instead of (or in addition to) mutation, and selection acts on physicochemical properties, such as hydrolysis, photolysis, solubility, or surface binding. Chemical evolution applies to short oligonucleotides that can be generated by random polymerization, as well as by template-directed replication, and which may be too short to encode a specific function. Chemical evolution is an important stage on the pathway to life, between the stage of “just chemistry” and the stage of full biological evolution. A mathematical model is presented here that illustrates the differences between these three stages. Chemical evolution leads to much larger differences in molecular concentrations than can be achieved by selection without replication. However, chemical evolution is not open-ended, unlike biological evolution. The ability to undergo Darwinian evolution is often considered to be a defining feature of life. Here, I argue that chemical evolution, although Darwinian, does not quite constitute life, and that a good place to put the conceptual boundary between non-life and life is between chemical and biological evolution.
Chemical Evolution
Chemical evolution and the origins of life is a topic that spans and transcends many domains: all disciplines of science, eras, cultures, and even space. It is a topic that enduringly captures the imagination of scientists and the general public alike.Understanding how life arose on the Earth and elsewhere is a historical and reconstructive endeavor, but it is also very much a contemporary and ever-refreshing scientific undertaking.These efforts have rich histories and have been driven by competing ideas ranging from protein-, RNA-, metabolism-, and lipid-world hypothesis to “far-out” ideas such as panspermia. From the perspective of chemists this pursuit is focused on understanding how elements and molecules that accumulate on a young planet can transform—under the abiotic geochemical constraints—into self-assembling, self-sustaining interactive systems with emerging patterns and behavior, and begin to evolve into what could be considered as living entities.
From the viewpoint of astrochemistry, prebiotic-chemistry, and biochemistry, this thematic issue covers our current understanding of a spectrum of topics associated with the chemical origins of life. Along with their compilations of impressive advances, each contribution also acknowledges remaining unsolved problems and challenges that are to be faced as we aspire to address the grand question “Can the origins of life be demonstrated or understood experimentally?” While answering that question in a historically accurate context may be not possible, it is indeed within the grasp of chemists (with guidance from observations in astrochemistry, geological constraints of early Earth and its atmosphere, prebiotic organic chemistry and extant biochemistry) to demonstrate in the laboratory the transformations of molecules—by chemical reactions—to systems that approximate the behavior/phenomenon observed in biology. Although this may fall short of “recreating life in a test tube”, it surely would be a more modest—but still a powerful—substantiation of the emergence and evolution of chemical processes that can lead to the origins of life.
This thematic issue begins with Sandford and colleagues describing the Prebiotic Astrochemistry and the Formation of Molecules of Astrobiological Interest in Interstellar Clouds and Protostellar Disks, where planetary systems form, thus laying the foundation for chemical species that are available for processes leading to the origins of life on planets. If this process is indeed “universal” then questions arise about the possibility of extraterrestrial life elsewhere in the universe, and Sandford and colleagues suggest that life may be common where local conditions favor these chemical processes. With Earth being our only current example of a planet harboring life to guide our search for life elsewhere, there must be a set of criteria for life detection missions using our understanding of the essence and uniqueness of biology and its processes. One such singularity of biology on the Earth is homochirality, including the use of l-amino acids and d-sugars of single-handedness. However, using chiral asymmetry as a definitive biosignature is complicated by nonbiological processes that can interfere. Glavin et al. review the enantiomeric and isotopic compositions and distributions of molecules found in meteorites and propose a set of criteria for The Search for Chiral Asymmetry as a Potential Biosignature in our Solar System. These authors also present arguments for “sample return missions” that may represent our best chance to establish firmly the origins of chiral asymmetry as a potential biosignature of life elsewhere in our solar system.
Once source molecules, such as amino acids, nucleobases, and sugar building blocks have been deposited (or formed in situ) on the early planet (e.g., Earth), the ensuing interactions and reactions between them leading to more complex molecules are hypothesized to be a natural part of the processes under geochemical constraints. For example, phosphorylation, a central reaction in the metabolic, structural, and replicative processes in biology, is also widely assumed to have played an important role in jumpstarting chemical evolution toward life on the early Earth. As Pasek points out, the Thermodynamics of Prebiotic Phosphorylation can be used to evaluate the likelihood of various sources, processes, and geochemical environments that enabled the conversion of inorganic phosphorus compounds to organic phosphates for further processing. Amino acids are prebiotically plausible compounds of the early Earth and are the building blocks of peptides and proteins, which are the catalytic engines of extant biology. Prior to the emergence of coded protein enzymes, noncovalent interactions of prebiotic peptides could have played a key role in the emerging interconnected molecular networks. Ashkenasy, Leman and colleagues summarize mechanisms of how amino acids (and coexisting molecules) were transformed into peptides and protopeptides and discuss the early roles of Prebiotic Peptides as Molecular Hubs in the Origin of Life. Furthermore, they review the plausible interactions of prebiotic peptides (and protopeptides) with other classes of molecules, thereby emphasizing a systems chemistry approach for synergistic interactions in chemical evolution.
Another crucial class of molecules in life are nucleotides, the monomers of RNA and DNA, whose prebiotic availability is more challenging when compared to amino acids.
Nevertheless, the central role played by RNA in extant biology, especially of the ribosome in peptide synthesis, has forced the reckoning with an RNA world scenario on early Earth. This powerful impetus has resulted in intensive efforts to fashion plausible prebiotic synthesis of canonical nucleotides. Krishnamurthy and colleagues review the current status the Chemistry of Abiotic Nucleotide Synthesis in a prebiotic context and point to the remaining challenges in elucidating pathways to the building blocks of RNA and DNA that are universally acceptable as prebiotically plausible in an early Earth geochemical context. The plethora of noncanonical products that are formed in many of the reactions that produce the canonical sugars (ribose) and nucleobases (A, U, G, and C), raises the question: Were alternative nucleosides and nucleotides involved in chemical evolution and the origins of life?Hud et al. point out in their review, Prebiotic Syntheses of Noncanonical Nucleosides and Nucleotides, that alternative nucleobases, sugars, backbone linkers, and nucleosides/-tides would have been formed by similar or competing chemistries. More than the four canonical nucleotides for RNA, these alternative building blocks have the potential to give rise to self-assembling/functional alternative proto-RNA or pre-RNA candidates, whose roles need to be given serious consideration and evaluated in the chemical evolutionary processes and transitions leading to extant biopolymers on the early Earth.
In all of the above scenarios, the chirality of the building blocks must be considered, especially as one proceeds toward polymeric and supramolecular assemblies. How homochirality or one-handedness manifested in biology as d-sugars and l-amino acids is an extremely important, but far from understood, phenomenon.Blackmond tackles this question by considering Autocatalytic Models for the Origin of Homochirality and discusses the Soai reaction (the only known example of an amplifying autocatalytic reaction) in depth as a model to examine whether a prebiotically plausible variant would be feasible. Increasingly, it appears that the emergence of homochirality is not determined by a single event but rather by a series of synergistical and sustained chemical and physical processes.
While the historical details are still unclear, at some point evolution, chemical or biological, sorted out all that was necessary to make ribozymes, catalytic molecules encoded in RNA—as epitomized by the ribosome, which is responsible for coded-protein synthesis in all living systems.Williams and colleagues, in their review, Root of the Tree: The Significance, Evolution, and Origins of the Ribosome, argue that ribosome complexity is a molecular fossil that has implied information about ancient biological processes, which allows for a reconstruction of how the proteins and RNA coevolved—as the ribosome became a ribozyme possessing the exquisite specific activity of peptide synthesis. One might surmise that the earliest ribozymes would have been crude constructs possessing not-so specific activities, molecules with rather promiscuous activities, catalyzing a range of reactions. However, Chen et al. examine the evidence and discuss the Promiscuous Ribozymes and Their Proposed Role in Prebiotic Evolution. Their review suggests that the de novo ribozymes are not more promiscuous than their evolved counterparts pointing to the advantage of being promiscuous under selective pressures for prebiotic chemical evolution. In the final article, Maurel and associates consider the function of ribozymes under high pressure, examining the structure–function relationship of the self-cleaving hairpin and hammerhead ribozymes. What they describe in Ribozyme Chemistry: To Be or Not To Be under High Pressure suggests that such activities of these ribozymes are slowed down by increase in pressure. They examine plausible mechanisms to account for this behavior, which they discuss in the context of the extreme conditions that can occur in deep-sea vents or hydrothermal surfaces.
Through the various articles in this thematic issue on Chemical Evolution and the Origins of Life we hope the reader will see how this collective scientific endeavor is making meaningful progress on multiple fronts and offering plausible alternative solutions to long-standing problems for which once attractive and popular solutions have not held up to experimental scrutiny. The intent of this special issue is not to make the claims that specific problems have been solved, but to highlight the progress made so far and to emphasize that there are still problems that are (and may be) unsolved. This special issue should be construed as an open invitation and a call for more chemists to get involved in this fascinating and enigmatic journey—of developing a systematic understanding of the origins of life from a chemical perspective: the chemical-beginning, chemical-evolution, and chemical-life.
Abiogenesis
In evolutionary biology, abiogenesis, or informally the origin of life,is the natural process by which life has arisen from non-living matter, such as simple organic compounds.While the details of this process are still unknown, the prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but an evolutionary process of increasing complexity that involved molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes.Although the occurrence of abiogenesis is uncontroversial among scientists, its possible mechanisms are poorly understood. There are several principles and hypotheses for how abiogenesis could have occurred.
The study of abiogenesis aims to determine how pre-life chemical reactions gave rise to life under conditions strikingly different from those on Earth today. It primarily uses tools from biology, chemistry, and geophysics, with more recent approaches attempting a synthesis of all three:[15] more specifically, astrobiology, biochemistry, biophysics, geochemistry, molecular biology, oceanography and paleontology. Life functions through the specialized chemistry of carbon and water and builds largely upon four key families of chemicals: lipids (cell membranes), carbohydrates (sugars, cellulose), amino acids (protein metabolism), and nucleic acids (DNA and RNA). Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules. Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Researchers generally think that current life descends from an RNA world, although other self-replicating molecules may have preceded RNA.Miller–Urey experiment Synthesis of small organic molecules in a mixture of simple gases that is placed in a thermal gradient by heating (right) and cooling (left) the mixture at the same time, a mixture that is also subject to electrical discharges
The classic 1952 Miller–Urey experiment and similar research demonstrated that most amino acids, the chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. Scientists have proposed various external sources of energy that may have triggered these reactions, including lightning and radiation. Other approaches (“metabolism-first” hypotheses) focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication.
The alternative panspermia hypothesis speculates that microscopic life arose outside Earth by unknown mechanisms, and spread to the early Earth on space dust and meteoroids. It is known that complex organic molecules occur in the Solar System and in interstellar space, and these molecules may have provided starting material for the development of life on Earth. It has also been suggested that life arises in a Great Prebiotic Spot on a given world.
Earth remains the only place in the universe known to harbour life, and fossil evidence from the Earth informs most studies of abiogenesis. The age of the Earth is 4.54 Gy (billion year); the earliest undisputed evidence of life on Earth dates from at least 3.5 Gya (Gy ago), and possibly as early as the Eoarchean Era (3.6–4.0 Gya). In 2017 scientists found possible evidence of early life on land in 3.48 Gyo (Gy old) geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Pilbara Craton of Western Australia. However, a number of discoveries suggest that life may have appeared on Earth even earlier. As of 2017, microfossils (fossilised microorganisms) within hydrothermal-vent precipitates dated 3.77 to 4.28 Gya in rocks in Quebec may harbour the oldest record of life on Earth, suggesting life started soon after ocean formation 4.4 Gya during the Hadean Eon.
The NASA strategy on abiogenesis states that it is necessary to identify interactions, intermediary structures and functions, energy sources, and environmental factors that contributed to the diversity, selection, and replication of evolvable macromolecular systems.Emphasis must continue to map the chemical landscape of potential primordial informational polymers. The advent of polymers that could replicate, store genetic information, and exhibit properties subject to selection likely was a critical step in the emergence of prebiotic chemical evolution.
Astrochemistry
Astrochemistry is the study of the abundance and reactions of molecules in the Universe, and their interaction with radiation. The discipline is an overlap of astronomy and chemistry. The word “astrochemistry” may be applied to both the Solar System and the interstellar medium. The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites, is also called cosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds is of special interest, because it is from these clouds that solar systems form.
Observations of solar spectra as performed by Athanasius Kircher (1646), Jan Marek Marci (1648), Robert Boyle (1664), and Francesco Maria Grimaldi (1665) all predated Newton’s 1666 work which established the spectral nature of light and resulted in the first spectroscope. Spectroscopy was first used as an astronomical technique in 1802 with the experiments of William Hyde Wollaston, who built a spectrometer to observe the spectral lines present within solar radiation. These spectral lines were later quantified through the work of Joseph Von Fraunhofer.
Spectroscopy was first used to distinguish between different materials after the release of Charles Wheatstone‘s 1835 report that the sparks given off by different metals have distinct emission spectra. This observation was later built upon by Léon Foucault, who demonstrated in 1849 that identical absorption and emission lines result from the same material at different temperatures. An equivalent statement was independently postulated by Anders Jonas Ångström in his 1853 work Optiska Undersökningar, where it was theorized that luminous gases emit rays of light at the same frequencies as light which they may absorb.
This spectroscopic data began to take upon theoretical importance with Johann Balmer’s observation that the spectral lines exhibited by samples of hydrogen followed a simple empirical relationship which came to be known as the Balmer Series. This series, a special case of the more general Rydberg Formula developed by Johannes Rydberg in 1888, was created to describe the spectral lines observed for Hydrogen. Rydberg’s work expanded upon this formula by allowing for the calculation of spectral lines for multiple different chemical elements. The theoretical importance granted to these spectroscopic results was greatly expanded upon the development of quantum mechanics, as the theory allowed for these results to be compared to atomic and molecular emission spectra which had been calculated a priori.
While radio astronomy was developed in the 1930s, it was not until 1937 that any substantial evidence arose for the conclusive identification of an interstellar molecule – up until this point, the only chemical species known to exist in interstellar space were atomic. These findings were confirmed in 1940, when McKellar et al. identified and attributed spectroscopic lines in an as-of-then unidentified radio observation to CH and CN molecules in interstellar space. In the thirty years afterwards, a small selection of other molecules were discovered in interstellar space: the most important being OH, discovered in 1963 and significant as a source of interstellar oxygen, and H2CO (Formaldehyde), discovered in 1969 and significant for being the first observed organic, polyatomic molecule in interstellar space.
The discovery of interstellar formaldehyde – and later, other molecules with potential biological significance such as water or carbon monoxide – is seen by some as strong supporting evidence for abiogenetic theories of life: specifically, theories which hold that the basic molecular components of life came from extraterrestrial sources. This has prompted a still ongoing search for interstellar molecules which are either of direct biological importance – such as interstellar glycine, discovered in 2009– or which exhibit biologically relevant properties like Chirality – an example of which (propylene oxide) was discovered in 2016 – alongside more basic astrochemical research.
Cosmochemistry
Cosmochemistry or chemical cosmology is the study of the chemical composition of matter in the universe and the processes that led to those compositions. This is done primarily through the study of the chemical composition of meteorites and other physical samples. Given that the asteroid parent bodies of meteorites were some of the first solid material to condense from the early solar nebula, cosmochemists are generally, but not exclusively, concerned with the objects contained within the Solar System.
In 1938, Swiss mineralogist Victor Goldschmidt and his colleagues compiled a list of what they called “cosmic abundances” based on their analysis of several terrestrial and meteorite samples. Goldschmidt justified the inclusion of meteorite composition data into his table by claiming that terrestrial rocks were subjected to a significant amount of chemical change due to the inherent processes of the Earth and the atmosphere.
This meant that studying terrestrial rocks exclusively would not yield an accurate overall picture of the chemical composition of the cosmos. Therefore, Goldschmidt concluded that extraterrestrial material must also be included to produce more accurate and robust data. This research is considered to be the foundation of modern cosmochemistry.
During the 1950s and 1960s, cosmochemistry became more accepted as a science. Harold Urey, widely considered to be one of the fathers of cosmochemistry, engaged in research that eventually led to an understanding of the origin of the elements and the chemical abundance of stars. In 1956, Urey and his colleague, German scientist Hans Suess, published the first table of cosmic abundances to include isotopes based on meteorite analysis.
The continued refinement of analytical instrumentation throughout the 1960s, especially that of mass spectrometry, allowed cosmochemists to perform detailed analyses of the isotopic abundances of elements within meteorites. in 1960, John Reynolds determined, through the analysis of short-lived nuclides within meteorites, that the elements of the Solar System were formed before the Solar System itself which began to establish a timeline of the processes of the early Solar System.
Evolution of metal ions in biological systems
The Earth began as an iron aquatic world with low oxygen. The Great Oxygenation Event occurred approximately 2.4 Ga (billion years ago) as cyanobacteria and photosynthetic life induced the presence of dioxygen in the planet’s atmosphere. Iron became insoluble (as did other metals) and scarce while other metals became soluble. Sulfur was a very important element during this time. Once oxygen was released into the environment, sulfates made metals more soluble and released those metals into the environment; especially into the water. Incorporation of metals perhaps combatted oxidative stress.
The central chemistry of all these cells has to be reductive in order that the synthesis of the required chemicals, especially biopolymers, is possible. The different anaerobic, autocatalysed, reductive, metabolic pathways seen in the earliest known cells developed in separate energised vesicles, protocells, where they were produced cooperatively with certain bases of the nucleic acids.
Hypotheses proposed for how elements became essential is their relative quantity in the environment as life formed. This has produced research on the origin of life; for instance, Orgel and Crick hypothesized that life was extraterrestrial due to the alleged low abundance of molybdenum on early Earth (it is now suspected that there were larger quantities than previously thought. Another example is life forming around thermal vents based on the availability of zinc and sulfur. In conjunction with this theory is that life evolved as chemoautotrophs. Therefore, life occurred around metals and not in response to their presence. Some evidence for this theory is that inorganic matter has self-contained attributes that life adopted as shown by life’s compartmentalization. Other evidence includes the ready binding of metals by artificial proteins without evolutionary history.
The prebiotic chemistry of life had to be reductive in order to obtain, e.g. Carbon monoxide (CO) and Hydrogen cyanide (HCN) from existing CO2 and N2 in the atmosphere. CO and HCN were precursor molecules of the essential biomolecules, proteins, lipids, nucleotides and sugars. However, atmospheric oxygen levels increased considerably, and it was then necessary for cells to have control over the reduction and oxidation of such small molecules in order to build and break down cells when necessary, without the inevitable oxidation (breaking down) of everything. Transition metal ions, due to their multiple oxidation states, were the only elements capable of controlling the oxidation states of such molecules, and thus were selected for.
O-donors such as HPO2−
4 were abundant in the prebiotic atmosphere. Metal ion binding to such O-donors was required to build the biological polymers, since the bond is generally weak, it can catalyze the required reaction and dissociate after (i.e. Mg2+ in DNA synthesis).
Around 4 Ga, the acidic seawater contained high amounts of H2S and thus created a reducing environment with a potential of around −0.2 V.So any element that had a large negative value with respect to the reduction potential of the environment was available in its free ionic form and can subsequently be incorporated into cells, i.e. Mg2+ has a reduction potential of −2.372 V, and was available in its ionic form at that time.
Around 2 Ga, an increase in atmospheric oxygen levels took place, causing an oxidation of H2S in the surroundings, and an increase in the pH of the sea water. The resulting environment had become more oxidizing and thus allowed the later incorporation of the heavier metals such as copper and zinc.
Another factor affecting the availability of metal ions was their solubilities with H2S. Hydrogen sulfide was abundant in the early sea giving rise to H2S in the prebiotic acidic conditions and HS− in the neutral (pH = 7.0) conditions. In the series of metal sulfides, insolubility increases at neutral pH following the Irving–Williams series:
- Mn(II) < Fe(II) < Co(II) ≤ Ni(II) < Cu(II) > Zn(II)
So in high amounts of H2S, which was the prebiotic condition, only Fe was most prominently available in its ionic form due to its low insolubility with sulfides. The increasing oxidation of H2S into Sulphate ion leads to the later release of Co+2, Ni+2, Cu+2, and Zn+2 since all of their sulfates are soluble.
Magnesium
Magnesium is the eighth most abundant element on earth. It is the fourth most abundant element in vertebrates and the most abundant divalent cation within cells. The most available form of magnesium (Mg2+) for living organisms can be found in the hydrosphere. The concentration of Mg2+ in seawater is around 55 mM. Mg2+ is readily available to cells during early evolution due to its high solubility in water. Other transition metals like calcium precipitate from aqueous solutions at much lower concentrations than the corresponding Mg2+ salts.
Since magnesium was readily available in early evolution, it can be found in every cell type living organism. Magnesium in anaerobic prokaryotes can be found in MgATP. Magnesium also has many functions in prokaryotes such as glycolysis, all kinases, NTP reaction, signalling, DNA/RNA structures and light capture. In aerobic eukaryotes, magnesium can be found in cytoplasm and chloroplasts. The reactions in these cell compartments are glycolysis, photophosphorylation and carbon assimilation.
ATP, the main source of energy in almost all living organisms, must bind with metal ions such as Mg2+ or Ca2+ to function. Examination of cells with limited magnesium supply has shown that a lack of magnesium can cause a decrease in ATP. Magnesium in ATP hydrolysis acts as a co-factor to stabilize the high negative charge transition state. MgATP can be found in both prokaryotes and eukaryotes cells. However, most of the ATP in cells is MgATP. Following the Irving–Williams series, magnesium has a higher binding constant than the Ca2+. Therefore, the dominant ATP in living organisms is MgATP. A greater binding constant also gives magnesium the advantage as a better catalyst over other competing transition metals.
Manganese
Magnesium Center in Cyanobacterial photosystem II. Incorporation of manganese sparked evolution of complex plant life.
Evidence suggests that manganese (Mn) was first incorporated into biological systems roughly 3.2–2.8 billion years ago, during the Archean Period. Together with calcium, it formed the manganese-calcium oxide complex (determined by X-ray diffraction) which consisted of a manganese cluster, essentially an inorganic cubane (cubical) structure. The incorporation of a manganese center in photosystem II was highly significant, as it allowed for photosynthetic oxygen evolution of plants. The oxygen-evolving complex (OEC) is a critical component of photosystem II contained in the thylakoid membranes of chloroplasts; it is responsible for terminal photooxidation of water during light reactions.
The incorporation of Mn in proteins allowed the complexes the ability to reduce reactive oxygen species in Mn-superoxide dismutase (MnSOD) and catalase, in electron transfer-dependent catalysis (for instance in certain class I ribonucleotide reductases) and in the oxidation of water by photosystem II (PSII), where the production of thiobarbituric acid-reactive substances is decreased. This is due to manganese’s ability to reduce superoxide anion and hydroxyl radicals as well as its chain-breaking capacity.
Iron
Iron (Fe) is the most abundant element in the Earth and the fourth most abundant element in the crust, approximately 5 percent by mass. Due to the abundance of iron and its role in biological systems, the transition and mineralogical stages of iron have played a key role in Earth surface systems. It played a larger role in the geological past in marine geochemistry, as evidenced by the deposits of Precambrian iron-rich sediments. The redox transformation of Fe(II) to Fe(III), or vice versa, is vital to a number of biological and element cycling processes. The reduction of Fe(III) is seen to oxidize sulfur (from H2S to SO4−2), which is a central process in marine sediments. Many of the first metalloproteins consisted of iron-sulphur complexes formed during photosynthesis.Iron is the main redox metal in biological systems. In proteins, it is found in a variety of sites and cofactors, including, for instance, haem groups, Fe–O–Fe sites, and iron–sulfur clusters.
The prevalence of iron is apparently due to the large availability of Fe(II) in the initial evolution of living organisms, before the rise of photosynthesis and an increase in atmospheric oxygen levels which resulted in the precipitation of iron in the environment as Fe(OH)3. It has flexible redox properties because such properties are sensitive to ligand coordination, including geometry. Iron can be also used in enzymes due to its Lewis acid properties, for example in nitrile hydratase. Iron is frequently found in mononuclear sites in the reduced Fe(II) form, and functions in dioxygen activation; this function is used as a major mechanism adopted by living organisms to avoid the kinetic barrier hindering the transformation of organic compounds by O2. Iron can be taken up selectively as ferredoxins, Fe-O-Fe (hemerythrin and ribonucleotide reductase), Fe (many oxidases), apart from iron porphyrin. Variation in the related proteins with any one of these chemical forms of iron has produced a wide range of enzymes. All of these arrangements are modified to function both in the sense of reactivity and the positioning of the protein in the cell. Iron can have various redox and spin states, and it can be held in many stereochemistries.Coenzyme F430 – Theorized as the first occurrence of nickel in biological systems
Nickel and cobalt
Coenzyme B12 – Theorized as the first occurrence of cobalt in a biological system
Around 4–3 Ga, anaerobic prokaryotes began developing metal and organic cofactors for light absorption. They ultimately ended up making chlorophyll from Mg(II), as is found in cyanobacteria and plants, leading to modern photosynthesis. However, chlorophyll synthesis requires numerous steps.
The process starts with uroporphyrin, a primitive precursor to the porphyrin ring which may be biotic or abiotic in origin, which is then modified in cells differently to make Mg, Fe, nickel (Ni), and cobalt (Co) complexes. The centers of these rings are not selective, thus allowing the variety of metal ions to be incorporated. Mg porphyrin gives rise to chlorophyll, Fe porphyrin to heme proteins, Ni porphyrin yields factor F-430, and Co porphyrin Coenzyme B12.
Copper
Before the Great Oxygenation Event, copper was not readily available for living organisms. Most early copper was Cu+ and Cu. This oxidation state of copper is not very soluble in water. One billion years ago, after the great oxidation event the oxygen pressure rose sufficiently to oxidise Cu+ to Cu2+, increasing its solubility in water. As a result, the copper became much more available for living organisms.
Most copper-containing proteins and enzymes can be found in eukaryotes. Only a handful of prokaryotes such as aerobic bacteria and cyanobacteria contain copper enzymes or proteins. Copper can be found in both prokaryotes and eukaryotes superoxide dismutase (SOD) enzyme. There are three distinct types of SOD, containing Mn, Fe and Cu respectively. Mn-SOD and Fe-SOD are found in most prokaryotes and mitochondria of the eukaryotic cell. Cu-SOD can be found in the cytoplasmic fraction of the eukaryotic cells. The three elements, copper, iron and manganese, can all catalyze superoxide to ordinary molecular oxygen or hydrogen peroxide. However, Cu-SOD is more efficient than Fe-SOD and Mn-SOD. Most prokaryotes only utilize Fe-SOD or Mn-SOD due to the lack of copper in the environment. Some organisms did not develop Cu-SOD due to the lack of a gene pool for the Cu-SOD adoption.
Zinc
Zinc (Zn) was incorporated into living cells in two waves. Four to three Ga, anaerobic prokaryotes arose, and the atmosphere was full of H2S and highly reductive. Thus most zinc was in the form of insoluble ZnS. However, because seawater at the time was slightly acidic, some Zn(II) was available in its ionic form and became part of early anaerobic prokaryotes’ external proteases, external nucleases, internal synthetases and dehydrogenases.
During the second wave, once the Great Oxygenation Event occurred, more Zn(II) ions were available in the seawater. This allowed its incorporation in the single-cell eukaryotes as they arose at this time. It is believed that the later addition of ions such as zinc and copper allowed them to displace iron and manganese from the enzyme superoxide dismutase (SOD). Fe and Mn complexes dissociate readily (Irving–Williams series) while Zn and Cu do not. This is why eukaryotic SOD contains Cu or Zn and its prokaryotic counterpart contains Fe or Mn.
Zn (II) doesn’t pose an oxidation threat to the cytoplasm. This allowed it to become a major cytoplasmic element in the eukaryotes. It became associated with a new group of transcription proteins, zinc fingers. This could only have occurred due to the long life of eukaryotes, which allowed time for zinc to exchange and hence become an internal messenger coordinating the action of other transcription factors during growth.
Molybdenum
Molybdenum (Mo) is the most abundant transition element in solution in the sea (mostly as dianionic molybdate ion) and in living organisms, its abundance in the Earth’s crust is quite low. Therefore, the use of Mo by living organisms seems surprising at first glance. Archaea, bacteria, fungi, plants, and animals, including humans, require molybdenum. It is also found in over 50 different enzymes. Its hydrolysis to water-soluble oxo-anionic species makes Mo readily accessible. Mo is found in the active sites of metalloenzymes that perform key transformations in the metabolism of carbon, nitrogen, arsenic, selenium, sulfur, and chlorine compounds.
The mononuclear Mo enzymes are widely distributed in the biosphere; they catalyze many significant reactions in the metabolism of nitrogen and sulfur-containing compounds as well as various carbonyl compounds (e.g., aldehydes, CO, and CO2). Nitrate reductases enzymes are important for the nitrogen cycle. They belong to a class of enzymes with a mononuclear Mo center and they catalyze the metabolism reaction of C, N, S, etc., in bacteria, plants, animals, and humans. Due to the oxidation of sulfides, The first considerable development was that of aerobic bacteria which could now utilize Mo. As oxygen began to accumulate in the atmosphere and oceans, the reaction of MoS2 to MoO4 also increased. This reaction made the highly soluble molybdate ion available for incorporation into critical metalloenzymes, and may have thus allowed life to thrive. It allowed organisms to occupy new ecological niches. Mo plays an important role in the reduction of dinitrogen to ammonia, which occurs in one type of nitrogenases. These enzymes are used by bacteria that usually live in a symbiotic relationship with plants; their role is nitrogen fixation, which is vital for sustaining life on earth. Mo enzymes also play important roles in sulfur metabolism of organisms ranging from bacteria to humans.
Tungsten
Tungsten is one of the oldest metal ions to be incorporated in biological systems, preceding the Great Oxygenation Event. Before the abundance of oxygen in Earth’s atmosphere, oceans teemed with sulfur and tungsten, while molybdenum, a metal that is highly similar chemically, was inaccessible in solid form. The abundance of tungsten and lack of free molybdenum likely explains why early marine organisms incorporated the former instead of the latter. However, as cyanobacteria began to fill the atmosphere with oxygen, molybdenum became available (molybdenum becomes soluble when exposed to oxygen) and molybdenum began to replace tungsten in the majority of metabolic processes, which is seen today, as tungsten is only present in the biological complexes of prokaryotes (methanogens, gram-positive bacteria, gram-negative aerobes and anaerobes), and is only obligated in hyperthermophilic archaea such as P. furiosus. Tungesten’s extremely high melting point (3,422 °C), partially explains its necessity in these archaea, found in extremely hot areas.
Although research into the specific enzyme complexes in which tungsten is incorporated is relatively recent (1970s), natural tungstoenzymes are abundantly found in a large number of prokaryotic microorganisms. These include formate dehydrogenase, formyl methanufuran dehydrogenase, acetylene hydratase, and a class of phylogenetically related oxidoreductases that catalyze the reversible oxidation of aldehydes. The first crystal structure of a tungsten- or pterin-containing enzyme, that of aldehyde ferredoxin oxidoreductase from P. furiosus, has revealed a catalytic site with one W atom coordinated to two pterin molecules which are themselves bridged by a magnesium ion.
Gas evolution reaction
A gas evolution reaction is a chemical reaction in which one of the end products is a gas such as oxygen or carbon dioxide.[1][2] Gas evolution reactions may be carried out in a fume chamber when the gases produced are poisonous when inhaled or explosive.
Molecular evolution
Molecular evolution is the process of change in the sequence composition of cellular molecules such as DNA, RNA, and proteins across generations. The field of molecular evolution uses principles of evolutionary biology and population genetics to explain patterns in these changes. Major topics in molecular evolution concern the rates and impacts of single nucleotide changes, neutral evolution vs. natural selection, origins of new genes, the genetic nature of complex traits, the genetic basis of speciation, evolution of development, and ways that evolutionary forces influence genomic and phenotypic changes.
The history of molecular evolution starts in the early 20th century with comparative biochemistry, and the use of “fingerprinting” methods such as immune assays, gel electrophoresis and paper chromatography in the 1950s to explore homologous proteins.[1][2] The field of molecular evolution came into its own in the 1960s and 1970s, following the rise of molecular biology. The advent of protein sequencing allowed molecular biologists to create phylogenies based on sequence comparison, and to use the differences between homologous sequences as a molecular clock to estimate the time since the last universal common ancestor. In the late 1960s, the neutral theory of molecular evolution provided a theoretical basis for the molecular clock,though both the clock and the neutral theory were controversial, since most evolutionary biologists held strongly to panselectionism, with natural selection as the only important cause of evolutionary change. After the 1970s, nucleic acid sequencing allowed molecular evolution to reach beyond proteins to highly conserved ribosomal RNA sequences, the foundation of a reconceptualization of the early history of life.
Evolution of proteins is studied by comparing the sequences and structures of proteins from many organisms representing distinct evolutionary clades. If the sequences/structures of two proteins are similar indicating that the proteins diverged from a common origin, these proteins are called homologous proteins. More specifically, homologous proteins that exist in two distinct species are called orthologs. Whereas, homologous proteins encoded by the genome of a single species are called paralogs.
The phylogenetic relationships of proteins are examined by multiple sequence comparisons. Phylogenetic trees of proteins can be established by the comparison of sequence identities among proteins. Such phylogenetic trees have established that the sequence similarities among proteins reflect closely the evolutionary relationships among organisms.
Protein evolution describes the changes over time in protein shape, function, and composition. Through quantitative analysis and experimentation, scientists have strived to understand the rate and causes of protein evolution. Using the amino acid sequences of hemoglobin and cytochrome c from multiple species, scientists were able to derive estimations of protein evolution rates. What they found was that the rates were not the same among proteins.Each protein has its own rate, and that rate is constant across phylogenies (i.e., hemoglobin does not evolve at the same rate as cytochrome c, but hemoglobins from humans, mice, etc. do have comparable rates of evolution.). Not all regions within a protein mutate at the same rate; functionally important areas mutate more slowly and amino acid substitutions involving similar amino acids occurs more often than dissimilar substitutions.Overall, the level of polymorphisms in proteins seems to be fairly constant. Several species (including humans, fruit flies, and mice) have similar levels of protein polymorphism.
In his Dublin 1943 lectures, “What Is Life?”, Erwin Schrodinger proposed that we could progress in answering this question by using statistical mechanics and partition functions, but not quantum mechanics and his wave equation. He described an “aperiodic crystal” which could carry genetic information, a description credited by Francis Crick and James D. Watson with having inspired their discovery of the double helical structure of DNA.Twenty fractals were discovered in solvent associated surface areas of > 5000 protein segments.The existence of these fractals proves that proteins function near critical points of second-order phase transitions, realizing Schrodinger’s conjecture. It opens a new biophysics field of accurate thermodynamic analysis of protein evolution based primarily on amino acid sequences.
Relation to nucleic acid evolution
Protein evolution is inescapably tied to changes and selection of DNA polymorphisms and mutations because protein sequences change in response to alterations in the DNA sequence. Amino acid sequences and nucleic acid sequences do not mutate at the same rate. Due to the degenerate nature of DNA, bases can change without affecting the amino acid sequence. For example, there are six codons that code for leucine. Thus, despite the difference in mutation rates, it is essential to incorporate nucleic acid evolution into the discussion of protein evolution. At the end of the 1960s, two groups of scientists—Kimura (1968) and King and Jukes (1969)—independently proposed that a majority of the evolutionary changes observed in proteins were neutral.Since then, the neutral theory has been expanded upon and debated.
Depending on the relative importance assigned to the various forces of evolution, three perspectives provide evolutionary explanations for molecular evolution.
Selectionist hypotheses argue that selection is the driving force of molecular evolution. While acknowledging that many mutations are neutral, selectionists attribute changes in the frequencies of neutral alleles to linkage disequilibrium with other loci that are under selection, rather than to random genetic drift.Biases in codon usage are usually explained with reference to the ability of even weak selection to shape molecular evolution
Neutralist hypotheses emphasize the importance of mutation, purifying selection, and random genetic drift.The introduction of the neutral theory by Kimura, quickly followed by King and Jukes‘ own findings,led to a fierce debate about the relevance of neodarwinism at the molecular level. The Neutral theory of molecular evolution proposes that most mutations in DNA are at locations not important to function or fitness. These neutral changes drift towards fixation within a population. Positive changes will be very rare, and so will not greatly contribute to DNA polymorphisms.Deleterious mutations do not contribute much to DNA diversity because they negatively affect fitness and so are removed from the gene pool before long.This theory provides a framework for the molecular clock.The fate of neutral mutations are governed by genetic drift, and contribute to both nucleotide polymorphism and fixed differences between species.
In the strictest sense, the neutral theory is not accurate. Subtle changes in DNA very often have effects, but sometimes these effects are too small for natural selection to act on.Even synonymous mutations are not necessarily neutral because there is not a uniform amount of each codon. The nearly neutral theory expanded the neutralist perspective, suggesting that several mutations are nearly neutral, which means both random drift and natural selection is relevant to their dynamics. The main difference between the neutral theory and nearly neutral theory is that the latter focuses on weak selection, not strictly neutral.
Mutationists hypotheses emphasize random drift and biases in mutation patterns.Sueoka was the first to propose a modern mutationist view. He proposed that the variation in GC content was not the result of positive selection, but a consequence of the GC mutational pressure.
Oxygen evolution
Oxygen evolution is the process of generating molecular oxygen (O2) by a chemical reaction, usually from water. Oxygen evolution from water is effected by oxygenic photosynthesis, electrolysis of water, and thermal decomposition of various oxides. The biological process supports aerobic life. When relatively pure oxygen is required industrially, it is isolated by distillation of liquified air.
Photosynthetic oxygen evolution is the fundamental process by which oxygen is generated in earth’s biosphere. The reaction is part of the light-dependent reactions of photosynthesis in cyanobacteria and the chloroplasts of green algae and plants. It utilizes the energy of light to split a water molecule into its protons and electrons for photosynthesis. Free oxygen, generated as a by-product of this reaction, is released into the atmosphere.
Water oxidation is catalyzed by a manganese-containing cofactor contained in photosystem II known as the oxygen-evolving complex (OEC) or water-splitting complex. Manganese is an important cofactor, and calcium and chloride are also required for the reaction to occur.The stoichiometry this reaction follows:2H2O ⟶ 4e− + 4H+ + O2
The protons are released into the thylakoid lumen, thus contributing to the generation of a proton gradient across the thylakoid membrane. This proton gradient is the driving force for ATP synthesis via photophosphorylation and coupling the absorption of light energy and oxidation of water to the creation of chemical energy during photosynthesis.
It was not until the end of the 18th century that Joseph Priestley discovered by accident the ability of plants to “restore” air that had been “injured” by the burning of a candle. He followed up on the experiment by showing that air “restored” by vegetation was “not at all inconvenient to a mouse.” He was later awarded a medal for his discoveries that: “…no vegetable grows in vain… but cleanses and purifies our atmosphere.” Priestley’s experiments were followed up by Jan Ingenhousz, a Dutch physician, who showed that “restoration” of air only worked in the presence of light and green plant parts.
It was not until the end of the 18th century that Joseph Priestley discovered by accident the ability of plants to “restore” air that had been “injured” by the burning of a candle. He followed up on the experiment by showing that air “restored” by vegetation was “not at all inconvenient to a mouse.” He was later awarded a medal for his discoveries that: “…no vegetable grows in vain… but cleanses and purifies our atmosphere.” Priestley’s experiments were followed up by Jan Ingenhousz, a Dutch physician, who showed that “restoration” of air only worked in the presence of light and green plant parts.
Ingenhousz suggested in 1796 that CO2 (carbon dioxide) is split during photosynthesis to release oxygen, while the carbon combined with water to form carbohydrates. While this hypothesis was attractive and reasonable and thus widely accepted for a long time, it was later proven incorrect. Graduate student C.B. Van Niel at Stanford University found that purple sulfur bacteria reduce carbon to carbohydrates, but accumulate sulfur instead of releasing oxygen. He boldly proposed that, in analogy to the sulfur bacteria’s forming elemental sulfur from H2S (hydrogen sulfide), plants would form oxygen from H2O (water). In 1937, this hypothesis was corroborated by the discovery that plants are capable of producing oxygen in the absence of CO2. This discovery was made by Robin Hill, and subsequently the light-driven release of oxygen in the absence of CO2 was called the Hill reaction. Our current knowledge of the mechanism of oxygen evolution during photosynthesis was further established in experiments tracing isotopes of oxygen from water to oxygen gas.
Stellar nucleosynthesis
Stellar nucleosynthesis is the creation (nucleosynthesis) of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the original creation of hydrogen, helium and lithium during the Big Bang. As a predictive theory, it yields accurate estimates of the observed abundances of the elements.
It explains why the observed abundances of elements change over time and why some elements and their isotopes are much more abundant than others. The theory was initially proposed by Fred Hoyle in 1946,who later refined it in 1954.Further advances were made, especially to nucleosynthesis by neutron capture of the elements heavier than iron, by Margaret and Geoffrey Burbidge, William Alfred Fowler and Hoyle in their famous 1957 B2FH paper,which became one of the most heavily cited papers in astrophysics history.
Stars evolve because of changes in their composition (the abundance of their constituent elements) over their lifespans, first by burning hydrogen (main sequence star), then helium (horizontal branch star), and progressively burning higher elements. However, this does not by itself significantly alter the abundances of elements in the universe as the elements are contained within the star. Later in its life, a low-mass star will slowly eject its atmosphere via stellar wind, forming a planetary nebula, while a higher–mass star will eject mass via a sudden catastrophic event called a supernova. The term supernova nucleosynthesis is used to describe the creation of elements during the explosion of a massive star or white dwarf.
The advanced sequence of burning fuels is driven by gravitational collapse and its associated heating, resulting in the subsequent burning of carbon, oxygen and silicon. However, most of the nucleosynthesis in the mass range A = 28–56 (from silicon to nickel) is actually caused by the upper layers of the star collapsing onto the core, creating a compressional shock wave rebounding outward. The shock front briefly raises temperatures by roughly 50%, thereby causing furious burning for about a second. This final burning in massive stars, called explosive nucleosynthesis or supernova nucleosynthesis, is the final epoch of stellar nucleosynthesis.
A stimulus to the development of the theory of nucleosynthesis was the discovery of variations in the abundances of elements found in the universe. The need for a physical description was already inspired by the relative abundances of the chemical elements in the solar system. Those abundances, when plotted on a graph as a function of the atomic number of the element, have a jagged sawtooth shape that varies by factors of tens of millions (see history of nucleosynthesis theory).This suggested a natural process that is not random. A second stimulus to understanding the processes of stellar nucleosynthesis occurred during the 20th century, when it was realized that the energy released from nuclear fusion reactions accounted for the longevity of the Sun as a source of heat and light.
In 1920, Arthur Eddington, on the basis of the precise measurements of atomic masses by F.W. Aston and a preliminary suggestion by Jean Perrin, proposed that stars obtained their energy from nuclear fusion of hydrogen to form helium and raised the possibility that the heavier elements are produced in stars.
This was a preliminary step toward the idea of stellar nucleosynthesis. In 1928 George Gamow derived what is now called the Gamow factor, a quantum-mechanical formula yielding the probability for two contiguous nuclei to overcome the electrostatic Coulomb barrier between them and approach each other closely enough to undergo nuclear reaction due to the strong nuclear force which is effective only at very short distances. In the following decade the Gamow factor was used by Atkinson and Houtermans and later by Edward Teller and Gamow himself to derive the rate at which nuclear reactions would occur at the high temperatures believed to exist in stellar interiors.
In 1939, in a Nobel lecture entitled “Energy Production in Stars”, Hans Bethe analyzed the different possibilities for reactions by which hydrogen is fused into helium. He defined two processes that he believed to be the sources of energy in stars. The first one, the proton–proton chain reaction, is the dominant energy source in stars with masses up to about the mass of the Sun. The second process, the carbon–nitrogen–oxygen cycle, which was also considered by Carl Friedrich von Weizsäcker in 1938, is more important in more massive main-sequence stars.
These works concerned the energy generation capable of keeping stars hot. A clear physical description of the proton–proton chain and of the CNO cycle appears in a 1968 textbook. Bethe’s two papers did not address the creation of heavier nuclei, however. That theory was begun by Fred Hoyle in 1946 with his argument that a collection of very hot nuclei would assemble thermodynamically into iron.Hoyle followed that in 1954 with a paper describing how advanced fusion stages within massive stars would synthesize the elements from carbon to iron in mass.
Hoyle’s theory was extended to other processes, beginning with the publication of the 1957 review paper “Synthesis of the Elements in Stars”
by Burbidge, Burbidge, Fowler and Hoyle, more commonly referred to as the B2FH paper.This review paper collected and refined earlier research into a heavily cited picture that gave promise of accounting for the observed relative abundances of the elements; but it did not itself enlarge Hoyle’s 1954 picture for the origin of primary nuclei as much as many assumed, except in the understanding of nucleosynthesis of those elements heavier than iron by neutron capture. Significant improvements were made by Alastair G. W. Cameron and by Donald D. Clayton. In 1957 Cameron presented his own independent approach to nucleosynthesis, informed by Hoyle’s example, and introduced computers into time-dependent calculations of evolution of nuclear systems. Clayton calculated the first time-dependent models of the s-process in 1961 and of the r-process in 1965, as well as of the burning of silicon into the abundant alpha-particle nuclei and iron-group elements in 1968,and discovered radiogenic chronologies for determining the age of the elements.
Biblography
https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/chemical-evolution
https://pubmed.ncbi.nlm.nih.gov/28664404/
https://pubs.acs.org/doi/10.1021/acs.chemrev.0c00409
https://en.wikipedia.org/wiki/Abiogenesis
https://en.wikipedia.org/wiki/Astrochemistry
https://en.wikipedia.org/wiki/Evolution_of_metal_ions_in_biological_systems
https://en.wikipedia.org/wiki/Gas_evolution_reaction
https://en.wikipedia.org/wiki/Molecular_evolution
https://en.wikipedia.org/wiki/Oxygen_evolution
https://en.wikipedia.org/wiki/Stellar_nucleosynthesis
