Abiogenesis

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"Origin of life" redirects here. Abiogenesis_sentence_0

For non-scientific views on the origins of life, see Creation myth. Abiogenesis_sentence_1

Not to be confused with Biogenesis. Abiogenesis_sentence_2

For the oldest life forms, see Earliest known life forms. Abiogenesis_sentence_3

In evolutionary biology, abiogenesis, or informally the origin of life (OoL), is the natural process by which life has arisen from non-living matter, such as simple organic compounds. Abiogenesis_sentence_4

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. Abiogenesis_sentence_5

Although the occurrence of abiogenesis is uncontroversial among scientists, its possible mechanisms are poorly understood. Abiogenesis_sentence_6

There are several principles and hypotheses for how abiogenesis could have occurred. Abiogenesis_sentence_7

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. Abiogenesis_sentence_8

It primarily uses tools from biology, chemistry, and geophysics, with more recent approaches attempting a synthesis of all three: more specifically, astrobiology, biochemistry, biophysics, geochemistry, molecular biology, oceanography and paleontology. Abiogenesis_sentence_9

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). Abiogenesis_sentence_10

Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules. Abiogenesis_sentence_11

Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Abiogenesis_sentence_12

Researchers generally think that current life descends from an RNA world, although other self-replicating molecules may have preceded RNA. Abiogenesis_sentence_13

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. Abiogenesis_sentence_14

Scientists have proposed various external sources of energy that may have triggered these reactions, including lightning and radiation. Abiogenesis_sentence_15

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. Abiogenesis_sentence_16

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. Abiogenesis_sentence_17

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. Abiogenesis_sentence_18

Earth remains the only place in the universe known to harbour life, and fossil evidence from the Earth informs most studies of abiogenesis. Abiogenesis_sentence_19

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). Abiogenesis_sentence_20

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. Abiogenesis_sentence_21

However, a number of discoveries suggest that life may have appeared on Earth even earlier. Abiogenesis_sentence_22

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. Abiogenesis_sentence_23

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. Abiogenesis_sentence_24

Emphasis must continue to map the chemical landscape of potential primordial informational polymers. Abiogenesis_sentence_25

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. Abiogenesis_sentence_26

Thermodynamics, self-organization, and information: Physics Abiogenesis_section_0

Thermodynamics principles: Energy and entropy Abiogenesis_section_1

In antiquity it was commonly thought, for instance by Empedocles and Aristotle, that the life of the individuals of some species, and more generally, life itself, could start with high temperature, i.e. implicitly by thermal cycling. Abiogenesis_sentence_27

Similarly, it was realized early on that life requires a loss of entropy, or disorder, when molecules organize themselves into living matter. Abiogenesis_sentence_28

This Second Law of thermodynamics needs to be considered when self-organization of matter to higher complexity happens. Abiogenesis_sentence_29

Because living organisms are machines, the Second Law applies to life as well. Abiogenesis_sentence_30

Thus, contrary to a naive view of the Second Law, the emergence of life and increased complexity does not contradict this law: First, a living organism creates order in some places (e.g. its living body or dwelling) on the expense of an increase of entropy elsewhere (e.g. heat and waste production). Abiogenesis_sentence_31

Second, the Second Law of thermodynamics actually predicts an increase in complexity and in correlations between a system and its surrounding, when undergoing interaction - with memory and genetic adaptation being examples of such correlations between a living organism and its environment. Abiogenesis_sentence_32

Obtaining free energy Abiogenesis_section_2

Bernal said on the Miller–Urey experiment that Abiogenesis_sentence_33

Multiple sources of energy were available for chemical reactions on the early Earth. Abiogenesis_sentence_34

For example, heat (such as from geothermal processes) is a standard energy source for chemistry. Abiogenesis_sentence_35

Other examples include sunlight and electrical discharges (lightning), among others. Abiogenesis_sentence_36

In fact, lightning is a plausible energy source for the origin of life, given that just in the tropics lightning strikes about 100 million times a year. Abiogenesis_sentence_37

Computer simulations also suggest that cavitation in primordial water reservoirs such as breaking sea waves, streams and oceans can potentially lead to the synthesis of biogenic compounds. Abiogenesis_sentence_38

Unfavourable reactions can also be driven by highly favourable ones, as in the case of iron-sulfur chemistry. Abiogenesis_sentence_39

For example, this was probably important for carbon fixation (the conversion of carbon from its inorganic form to an organic one). Abiogenesis_sentence_40

Carbon fixation via iron-sulfur chemistry is highly favourable, and occurs at neutral pH and 100C. Abiogenesis_sentence_41

Iron-sulfur surfaces, which are abundant near hydrothermal vents, are also capable of producing small amounts of amino acids and other biological metabolites. Abiogenesis_sentence_42

Self-organization Abiogenesis_section_3

The discipline of synergetics studies self-organization in physical systems. Abiogenesis_sentence_43

In his book Synergetics Hermann Haken has pointed out that different physical systems can be treated in a similar way. Abiogenesis_sentence_44

He gives as examples of self-organization several types of lasers, instabilities in fluid dynamics, including convection, and chemical and biochemical oscillations. Abiogenesis_sentence_45

In his preface he mentions the origin of life, but only in general terms: Abiogenesis_sentence_46

Multiple dissipative structures Abiogenesis_section_4

This theory postulates that the hallmark of the origin and evolution of life is the microscopic dissipative structuring of organic pigments and their proliferation over the entire Earth surface. Abiogenesis_sentence_47

Present day life augments the entropy production of Earth in its solar environment by dissipating ultraviolet and visible photons into heat through organic pigments in water. Abiogenesis_sentence_48

This heat then catalyzes a host of secondary dissipative processes such as the water cycle, ocean and wind currents, hurricanes, etc. Abiogenesis_sentence_49

Selforganization by dissipative structures Abiogenesis_section_5

The 19th-century physicist Ludwig Boltzmann first recognized that the struggle for existence of living organisms was neither over raw material nor energy, but instead had to do with entropy production derived from the conversion of the solar spectrum into heat by these systems. Abiogenesis_sentence_50

Boltzmann thus realized that living systems, like all irreversible processes, were dependent on the dissipation of a generalized chemical potential for their existence. Abiogenesis_sentence_51

In his book "What is Life", the 20th-century physicist Erwin Schrödinger emphasized the importance of Boltzmann's deep insight into the irreversible thermodynamic nature of living systems, suggesting that this was the physics and chemistry behind the origin and evolution of life. Abiogenesis_sentence_52

However, irreversible processes, and much less living systems, could not be conveniently analyzed under this perspective until Lars Onsager, and later Ilya Prigogine, developed an elegant mathematical formalism for treating the "self-organization" of material under a generalized chemical potential. Abiogenesis_sentence_53

This formalism became known as Classical Irreversible Thermodynamics and Prigogine was awarded the Nobel Prize in Chemistry in 1977 "for his contributions to non-equilibrium thermodynamics, particularly the theory of dissipative structures". Abiogenesis_sentence_54

The analysis by Prigogine showed that if a system were left to evolve under an imposed external potential, material could spontaneously organize (lower its entropy) forming what he called "dissipative structures" which would increase the dissipation of the externally imposed potential (augment the global entropy production). Abiogenesis_sentence_55

Non-equilibrium thermodynamics has since been successfully applied to the analysis of living systems, from the biochemical production of ATP to optimizing bacterial metabolic pathways to complete ecosystems. Abiogenesis_sentence_56

Current life, the result of abiogenesis: biology Abiogenesis_section_6

Definition of life Abiogenesis_section_7

As many as 123 definitions of life have been compiled. Abiogenesis_sentence_57

When discussing the origin of life, a definition of life itself is fundamental. Abiogenesis_sentence_58

The definition is somewhat disagreed upon (although follows the same basic principles) because different biology textbooks define life differently. Abiogenesis_sentence_59

James Gould: Abiogenesis_sentence_60

whereas according to Neil Campbell and Jane Reece Abiogenesis_sentence_61

This difference can also be found in books on the origin of life. Abiogenesis_sentence_62

John Casti gives a single sentence: Abiogenesis_sentence_63

Dirk Schulze-Makuch and Louis Irwin spend in contrast the whole first chapter of their book on this subject. Abiogenesis_sentence_64

Fermentation Abiogenesis_section_8

Albert Lehninger has stated around 1970 that fermentation, including glycolysis, is a suitable primitive energy source for the origin of life. Abiogenesis_sentence_65

Fermentation involves glycolysis, which, rather inefficiently, transduces the chemical energy of sugar into the chemical energy of ATP. Abiogenesis_sentence_66

Chemiosmosis Abiogenesis_section_9

As Fermentation had around 1970 been elucidated, whereas the mechanism of oxidative phosphorylation had not and some controversies still existed, fermentation may have looked too complex for investigators of the origin of life at that time. Abiogenesis_sentence_67

Peter Mitchell's Chemiosmosis is now however generally accepted as correct. Abiogenesis_sentence_68

Even Peter Mitchell himself assumed that fermentation preceded chemiosmosis. Abiogenesis_sentence_69

Chemiosmosis is however ubiquitous in life. Abiogenesis_sentence_70

A model for the origin of life has been presented in terms of chemiosmosis. Abiogenesis_sentence_71

Both respiration by mitochondria and photosynthesis in chloroplasts make use of chemiosmosis to generate most of their ATP. Abiogenesis_sentence_72

Today the energy source of all life can be linked to photosynthesis, and one speaks of primary production by sunlight. Abiogenesis_sentence_73

The oxygen used for oxidizing reducing compounds by organisms at hydrothermal vents at the bottom of the ocean is the result of photosynthesis at the Oceans' surface. Abiogenesis_sentence_74

ATP synthase Abiogenesis_section_10

The mechanism of ATP synthesis is complex and involves a closed membrane in which the ATP synthase is embedded. Abiogenesis_sentence_75

The ATP is synthesized by the F1 subunit of ATP synthase by the binding change mechanism discovered by Paul Boyer. Abiogenesis_sentence_76

The energy required to release formed strongly-bound ATP has its origin in protons that move across the membrane. Abiogenesis_sentence_77

These protons have been set across the membrane during respiration or photosynthesis. Abiogenesis_sentence_78

RNA world Abiogenesis_section_11

The RNA world hypothesis describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins. Abiogenesis_sentence_79

It is widely accepted that current life on Earth descends from an RNA world, although RNA-based life may not have been the first life to exist. Abiogenesis_sentence_80

This conclusion is drawn from many independent lines of evidence, such as the observations that RNA is central to the translation process and that small RNAs can catalyze all of the chemical groups and information transfers required for life. Abiogenesis_sentence_81

The structure of the ribosome has been called the "smoking gun," as it showed that the ribosome is a ribozyme, with a central core of RNA and no amino acid side chains within 18 angstroms of the active site where peptide bond formation is catalyzed. Abiogenesis_sentence_82

The concept of the RNA world was first proposed in 1962 by Alexander Rich, and the term was coined by Walter Gilbert in 1986. Abiogenesis_sentence_83

In March 2020, astronomer Tomonori Totani presented a statistical approach for explaining how an initial active RNA molecule might have been produced randomly in the universe sometime since the Big Bang. Abiogenesis_sentence_84

Phylogeny and LUCA Abiogenesis_section_12

The most commonly accepted location of the root of the tree of life is between a monophyletic domain Bacteria and a clade formed by Archaea and Eukaryota of what is referred to as the "traditional tree of life" based on several molecular studies starting with Carl Woese. Abiogenesis_sentence_85

A very small minority of studies have concluded differently, namely that the root is in the domain Bacteria, either in the phylum Firmicutes or that the phylum Chloroflexi is basal to a clade with Archaea+Eukaryotes and the rest of Bacteria as proposed by Thomas Cavalier-Smith. Abiogenesis_sentence_86

More recently, Peter Ward has proposed an alternative view which is rooted in abiotic RNA synthesis which becomes enclosed within a capsule and then creates RNA ribozyme replicates. Abiogenesis_sentence_87

It is proposed that this then bifurcates between Dominion Ribosa (RNA life), and after the loss of ribozymes RNA viruses as Domain Viorea, and Dominion Terroa, which after creating a large cell within a lipid wall, creating DNA the 20 based amino acids and the triplet code, is established as the last universal common ancestor or LUCA, of earlier phylogenic trees. Abiogenesis_sentence_88

In 2016, a set of 355 genes likely present in the Last Universal Common Ancestor (LUCA) of all organisms living on Earth was identified. Abiogenesis_sentence_89

A total of 6.1 million prokaryotic protein coding genes from various phylogenic trees were sequenced, identifying 355 protein clusters from amongst 286,514 protein clusters that were probably common to LUCA. Abiogenesis_sentence_90

The results Abiogenesis_sentence_91

The results depict methanogenic clostridia as a basal clade in the 355 phylogenies examined, and suggest that LUCA inhabited an anaerobic hydrothermal vent setting in a geochemically active environment rich in H2, CO2 and iron. Abiogenesis_sentence_92

A study at the University of Düsseldorf created phylogenic trees based upon 6 million genes from bacteria and archaea, and identified 355 protein families that were probably present in the LUCA. Abiogenesis_sentence_93

They were based upon an anaerobic metabolism fixing carbon dioxide and nitrogen. Abiogenesis_sentence_94

It suggests that the LUCA evolved in an environment rich in hydrogen, carbon dioxide and iron. Abiogenesis_sentence_95

Key issues in abiogenesis Abiogenesis_section_13

What came first: protein or nucleic acids? Abiogenesis_section_14

Possible precursors for the evolution of protein synthesis include a mechanism to synthesize short peptide cofactors or form a mechanism for the duplication of RNA. Abiogenesis_sentence_96

It is likely that the ancestral ribosome was composed entirely of RNA, although some roles have since been taken over by proteins. Abiogenesis_sentence_97

Major remaining questions on this topic include identifying the selective force for the evolution of the ribosome and determining how the genetic code arose. Abiogenesis_sentence_98

Eugene Koonin said, Abiogenesis_sentence_99

Emergence of the genetic code Abiogenesis_section_15

See: Genetic code. Abiogenesis_sentence_100

Error in translation catastrophe Abiogenesis_section_16

Hoffmann has shown that an early error-prone translation machinery can be stable against an error catastrophe of the type that had been envisaged as problematical for the origin of life, and was known as "Orgel's paradox". Abiogenesis_sentence_101

Homochirality Abiogenesis_section_17

Main article: Homochirality Abiogenesis_sentence_102

Homochirality refers to a geometric uniformity of some materials composed of chiral units. Abiogenesis_sentence_103

Chiral refers to nonsuperimposable 3D forms that are mirror images of one another, as are left and right hands. Abiogenesis_sentence_104

Living organisms use molecules that have the same chirality ("handedness"): with almost no exceptions, amino acids are left-handed while nucleotides and sugars are right-handed. Abiogenesis_sentence_105

Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst, they are formed in a 50/50 mixture of both enantiomers (called a racemic mixture). Abiogenesis_sentence_106

Known mechanisms for the production of non-racemic mixtures from racemic starting materials include: asymmetric physical laws, such as the electroweak interaction; asymmetric environments, such as those caused by circularly polarized light, quartz crystals, or the Earth's rotation, statistical fluctuations during racemic synthesis, and spontaneous symmetry breaking. Abiogenesis_sentence_107

Once established, chirality would be selected for. Abiogenesis_sentence_108

A small bias (enantiomeric excess) in the population can be amplified into a large one by asymmetric autocatalysis, such as in the Soai reaction. Abiogenesis_sentence_109

In asymmetric autocatalysis, the catalyst is a chiral molecule, which means that a chiral molecule is catalyzing its own production. Abiogenesis_sentence_110

An initial enantiomeric excess, such as can be produced by polarized light, then allows the more abundant enantiomer to outcompete the other. Abiogenesis_sentence_111

Clark has suggested that homochirality may have started in outer space, as the studies of the amino acids on the Murchison meteorite showed that L-alanine is more than twice as frequent as its D form, and L-glutamic acid was more than three times prevalent than its D counterpart. Abiogenesis_sentence_112

Various chiral crystal surfaces can also act as sites for possible concentration and assembly of chiral monomer units into macromolecules. Abiogenesis_sentence_113

Compounds found on meteorites suggest that the chirality of life derives from abiogenic synthesis, since amino acids from meteorites show a left-handed bias, whereas sugars show a predominantly right-handed bias, the same as found in living organisms. Abiogenesis_sentence_114

Early universe and Earth: astronomy and geology Abiogenesis_section_18

Main article: Timeline of the evolutionary history of life Abiogenesis_sentence_115

See also: Chronology of the Universe Abiogenesis_sentence_116

See also: Geological history of Earth Abiogenesis_sentence_117

Early universe with first stars Abiogenesis_section_19

Soon after the Big Bang, which occurred roughly 14 Gya, the only chemical elements present in the universe were hydrogen, helium, and lithium, the three lightest atoms in the periodic table. Abiogenesis_sentence_118

These elements gradually came together to form stars. Abiogenesis_sentence_119

These early stars were massive and short-lived, producing heavier elements through stellar nucleosynthesis. Abiogenesis_sentence_120

Carbon, currently the fourth most abundant chemical element in the universe (after hydrogen, helium and oxygen), was formed mainly in white dwarf stars, particularly those bigger than two solar masses. Abiogenesis_sentence_121

As these stars reached the end of their lifecycles, they ejected these heavier elements, among them carbon and oxygen, throughout the universe. Abiogenesis_sentence_122

These heavier elements allowed for the formation of new objects, including rocky planets and other bodies. Abiogenesis_sentence_123

Emergence of the Solar System Abiogenesis_section_20

According to the nebular hypothesis, the formation and evolution of the Solar System began 4.6 Gya with the gravitational collapse of a small part of a giant molecular cloud. Abiogenesis_sentence_124

Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed. Abiogenesis_sentence_125

Emergence of Earth Abiogenesis_section_21

The Earth, formed 4.5 Gya, was at first inhospitable to any living organisms. Abiogenesis_sentence_126

Based on numerous observations and studies of the geological time-scale, the Hadean Earth is thought to have had a secondary atmosphere, formed through degassing of the rocks that accumulated from planetesimal impactors. Abiogenesis_sentence_127

At first, it was thought that the Earth's atmosphere consisted of hydrogen compounds—methane, ammonia and Water vapor—and that life began under such reducing conditions, which are conducive to the formation of organic molecules. Abiogenesis_sentence_128

According to later models, suggested by studying ancient minerals, the atmosphere in the late Hadean period consisted largely of water vapor, nitrogen and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen, and sulfur compounds. Abiogenesis_sentence_129

During its formation, the Earth lost a significant part of its initial mass, with a nucleus of the heavier rocky elements of the protoplanetary disk remaining. Abiogenesis_sentence_130

As a consequence, Earth lacked the gravity to hold any molecular hydrogen in its atmosphere, and rapidly lost it during the Hadean period, along with the bulk of the original inert gases. Abiogenesis_sentence_131

The solution of carbon dioxide in water is thought to have made the seas slightly acidic, giving them a pH of about 5.5. Abiogenesis_sentence_132

The atmosphere at the time has been characterized as a "gigantic, productive outdoor chemical laboratory." Abiogenesis_sentence_133

It may have been similar to the mixture of gases released today by volcanoes, which still support some abiotic chemistry. Abiogenesis_sentence_134

Emergence of the ocean Abiogenesis_section_22

Oceans may have appeared first in the Hadean Eon, as soon as 200 My after the Earth formed, in a hot, 100 C, reducing environment, and the pH of about 5.8 rose rapidly towards neutral. Abiogenesis_sentence_135

This scenario has found support from the dating of 4.404  Gyo zircon crystals from metamorphosed quartzite of Mount Narryer in the Western Australia Jack Hills of the Pilbara, which provide evidence that oceans and continental crust existed within 150 Ma of Earth's formation. Abiogenesis_sentence_136

Despite the likely increased volcanism and existence of many smaller tectonic "platelets," it has been suggested that between 4.4-4.3 Gyo, the Earth was a water world, with little if any continental crust, an extremely turbulent atmosphere and a hydrosphere subject to intense ultraviolet (UV) light, from a T Tauri stage Sun, cosmic radiation and continued bolide impacts. Abiogenesis_sentence_137

Late heavy bombardment Abiogenesis_section_23

The Hadean environment would have been highly hazardous to modern life. Abiogenesis_sentence_138

Frequent collisions with large objects, up to 500 km in diameter, would have been sufficient to sterilize the planet and vaporize the oceans within a few months of impact, with hot steam mixed with rock vapor becoming high altitude clouds that would completely cover the planet. Abiogenesis_sentence_139

After a few months, the height of these clouds would have begun to decrease but the cloud base would still have been elevated for about the next thousand years. Abiogenesis_sentence_140

After that, it would have begun to rain at low altitude. Abiogenesis_sentence_141

For another two thousand years, rains would slowly have drawn down the height of the clouds, returning the oceans to their original depth only 3,000 y after the impact event. Abiogenesis_sentence_142

Traditionally it was thought that during the period between 4.28 and 3.8 Gya, changes in the orbits of the giant planets may have caused a heavy bombardment by asteroids and comets that pockmarked the Moon and the other inner planets (Mercury, Mars, and presumably Earth and Venus). Abiogenesis_sentence_143

This would likely have repeatedly sterilized the planet, had life appeared before that time. Abiogenesis_sentence_144

Geologically, the Hadean Earth would have been far more active than at any other time in its history. Abiogenesis_sentence_145

Studies of meteorites suggests that radioactive isotopes such as aluminium-26 with a half-life of 7.17 ky, and potassium-40 with a half-life of 1.25 Gy, isotopes mainly produced in supernovae, were much more common. Abiogenesis_sentence_146

Internal heating as a result of gravitational sorting between the core and the mantle would have caused a great deal of mantle convection, with the probable result of many more smaller and more active tectonic plates than now exist. Abiogenesis_sentence_147

The time periods between such devastating environmental events give time windows for the possible origin of life in the early environments. Abiogenesis_sentence_148

If the deep marine hydrothermal setting was the site for the origin of life, then abiogenesis could have happened as early as 4.0-4.2 Gya. Abiogenesis_sentence_149

If the site was at the surface of the Earth, abiogenesis could only have occurred between 3.7-4.0 Gya. Abiogenesis_sentence_150

Estimates of the production of organics from these sources suggest that the Late Heavy Bombardment before 3.5 Ga within the early atmosphere made available quantities of organics comparable to those produced by terrestrial sources. Abiogenesis_sentence_151

It has been estimated that the Late Heavy Bombardment may also have effectively sterilized the Earth's surface to a depth of tens of meters. Abiogenesis_sentence_152

If life evolved deeper than this, it would have also been shielded from the early high levels of ultraviolet radiation from the T Tauri stage of the Sun's evolution. Abiogenesis_sentence_153

Simulations of geothermically heated oceanic crust yield far more organics than those found in the Miller–Urey experiments. Abiogenesis_sentence_154

In the deep hydrothermal vents, Everett Shock has found "there is an enormous thermodynamic drive to form organic compounds, as seawater and hydrothermal fluids, which are far from equilibrium, mix and move towards a more stable state." Abiogenesis_sentence_155

Shock has found that the available energy is maximized at around 100–150 C, precisely the temperatures at which the hyperthermophilic bacteria and thermoacidophilic archaea have been found, at the base of the phylogenetic tree of life closest to the Last Universal Common Ancestor (LUCA). Abiogenesis_sentence_156

Earliest evidence of life: palaeontology Abiogenesis_section_24

Main article: Earliest known life forms Abiogenesis_sentence_157

The earliest life on Earth existed more than 3.5 Gya (billion years ago), during the Eoarchean Era when sufficient crust had solidified following the molten Hadean Eon. Abiogenesis_sentence_158

The earliest physical evidence so far found consists of microfossils in the Nuvvuagittuq Greenstone Belt of Northern Quebec, in banded iron formation rocks at least 3.77 and possibly 4.28 Gyo. Abiogenesis_sentence_159

This finding suggested life developed very soon after oceans formed. Abiogenesis_sentence_160

The structure of the microbes was noted to be similar to bacteria found near hydrothermal vents in the modern era, and provided support for the hypothesis that abiogenesis began near hydrothermal vents. Abiogenesis_sentence_161

Biogenic graphite has been found in 3.7 Gyo metasedimentary rocks from southwestern Greenland and microbial mat fossils found in 3.48 Gyo sandstone from Western Australia. Abiogenesis_sentence_162

Evidence of early life in rocks from Akilia Island, near the Isua supracrustal belt in southwestern Greenland, dating to 3.7 Gya have shown biogenic carbon isotopes. Abiogenesis_sentence_163

In other parts of the Isua supracrustal belt, graphite inclusions trapped within garnet crystals are connected to the other elements of life: oxygen, nitrogen, and possibly phosphorus in the form of phosphate, providing further evidence for life 3.7 Gya. Abiogenesis_sentence_164

At Strelley Pool, in the Pilbara region of Western Australia, compelling evidence of early life was found in pyrite-bearing sandstone in a fossilized beach, that showed rounded tubular cells that oxidized sulfur by photosynthesis in the absence of oxygen. Abiogenesis_sentence_165

Further research on zircons from Western Australia in 2015 suggested that life likely existed on Earth at least 4.1 Gya. Abiogenesis_sentence_166

Conceptual history until the 1960s: biology Abiogenesis_section_25

Panspermia Abiogenesis_section_26

Main article: Panspermia Abiogenesis_sentence_167

Panspermia is the hypothesis that life exists throughout the universe, distributed by meteoroids, asteroids, comets and planetoids. Abiogenesis_sentence_168

The panspermia hypothesis does not attempt to explain how life first originated but merely shifts the origin to another planet or a comet. Abiogenesis_sentence_169

The advantage of an extraterrestrial origin of primitive life is that life is not required to have formed on each planet it occurs on, but rather in a single location, and then spread about the galaxy to other star systems via cometary and/or meteorite impact. Abiogenesis_sentence_170

Evidence for the panspermia hypothesis is scant, but it finds some support in studies of Martian meteorites found in Antarctica and in studies of extremophile microbes' survival in outer space tests. Abiogenesis_sentence_171

In August 2020, scientists reported that bacteria from Earth, particularly Deinococcus radiodurans, which is highly resistant to environmental hazards, were found to survive for three years in outer space, based on studies conducted on the International Space Station. Abiogenesis_sentence_172

Origin of life posited directly after the Big Bang and have spread over the entire Universe Abiogenesis_section_27

An extreme speculation is that the biochemistry of life could have begun as early as 17 My (million years) after the Big Bang, during a habitable epoch, and that life may exist throughout the universe. Abiogenesis_sentence_173

Panspermia by life brought from Mars to Earth Abiogenesis_section_28

Carl Zimmer has speculated that the chemical conditions, including the presence of boron, molybdenum and oxygen needed for the initial production of RNA, may have been better on early Mars than on early Earth. Abiogenesis_sentence_174

If so, life-suitable molecules originating on Mars may have later migrated to Earth via meteor ejections. Abiogenesis_sentence_175

Spontaneous generation Abiogenesis_section_29

General acceptance of spontaneous generation until the 19th century Abiogenesis_section_30

Main article: Spontaneous generation Abiogenesis_sentence_176

Traditional religion attributed the origin of life to supernatural deities who created the natural world. Abiogenesis_sentence_177

Spontaneous generation, the first naturalistic theory of life arising from non-life, goes back to Aristotle and ancient Greek philosophy, and continued to have support in Western scholarship until the 19th century. Abiogenesis_sentence_178

Classical notions of spontaneous generation held that certain "lower" or "vermin" animals are generated by decaying organic substances. Abiogenesis_sentence_179

According to Aristotle, it was readily observable that aphids arise from dew on plants, flies from putrid matter, mice from dirty hay, crocodiles from rotting sunken logs, and so on. Abiogenesis_sentence_180

A related theory was heterogenesis: that some forms of life could arise from different forms (e.g. bees from flowers). Abiogenesis_sentence_181

The modern scientist John Bernal said that the basic idea of such theories was that life was continuously created as a result of chance events. Abiogenesis_sentence_182

In the 17th century, people began to question such assumptions. Abiogenesis_sentence_183

In 1646, Thomas Browne published his Pseudodoxia Epidemica (subtitled Enquiries into Very many Received Tenets, and commonly Presumed Truths), which was an attack on false beliefs and "vulgar errors." Abiogenesis_sentence_184

His contemporary, Alexander Ross, erroneously refuted him, stating: Abiogenesis_sentence_185

In 1665, Robert Hooke published the first drawings of a microorganism. Abiogenesis_sentence_186

Hooke was followed in 1676 by Antonie van Leeuwenhoek, who drew and described microorganisms that are now thought to have been protozoa and bacteria. Abiogenesis_sentence_187

Many felt the existence of microorganisms was evidence in support of spontaneous generation, since microorganisms seemed too simplistic for sexual reproduction, and asexual reproduction through cell division had not yet been observed. Abiogenesis_sentence_188

Van Leeuwenhoek took issue with the ideas common at the time that fleas and lice could spontaneously result from putrefaction, and that frogs could likewise arise from slime. Abiogenesis_sentence_189

Using a broad range of experiments ranging from sealed and open meat incubation and the close study of insect reproduction he became, by the 1680s, convinced that spontaneous generation was incorrect. Abiogenesis_sentence_190

The first experimental evidence against spontaneous generation came in 1668 when Francesco Redi showed that no maggots appeared in meat when flies were prevented from laying eggs. Abiogenesis_sentence_191

It was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous sentiment regarding spontaneous generation was false. Abiogenesis_sentence_192

The alternative hypothesis was biogenesis: that every living thing came from a pre-existing living thing (omne vivum ex ovo, Latin for "every living thing from an egg"). Abiogenesis_sentence_193

In 1768, Lazzaro Spallanzani demonstrated that microbes were present in the air, and could be killed by boiling. Abiogenesis_sentence_194

In 1861, Louis Pasteur performed a series of experiments that demonstrated that organisms such as bacteria and fungi do not spontaneously appear in sterile, nutrient-rich media, but could only appear by invasion from without. Abiogenesis_sentence_195

Spontaneous generation considered disproven in the 19th century Abiogenesis_section_31

By the middle of the 19th century, biogenesis had accumulated so much evidence in support that the alternative theory of spontaneous generation had been effectively disproven. Abiogenesis_sentence_196

Pasteur remarked, about a finding of his in 1864 which he considered definitive, Abiogenesis_sentence_197

gave a mechanism by which life diversified from a few simple organisms to a variety of to complex forms. Abiogenesis_sentence_198

Today, scientists agree that all current life descends from earlier life, which has become progressively more complex and diverse through Charles Darwin's mechanism of evolution by natural selection. Abiogenesis_sentence_199

Darwin wrote to Hooker in 1863 stating that, Abiogenesis_sentence_200

In On the Origin of Species, he had referred to life having been "created", by which he "really meant 'appeared' by some wholly unknown process", but had soon regretted using the Old Testament term "creation". Abiogenesis_sentence_201

Etymology of biogenesis and abiogenesis Abiogenesis_section_32

Main article: Biogenesis Abiogenesis_sentence_202

The term biogenesis is usually credited to either Henry Bastian or to Thomas Huxley. Abiogenesis_sentence_203

Bastian used the term around 1869 in an unpublished exchange with John Tyndall to mean "life-origination or commencement". Abiogenesis_sentence_204

In 1870, Huxley, as new president of the British Association for the Advancement of Science, delivered an address entitled Biogenesis and Abiogenesis. Abiogenesis_sentence_205

In it he introduced the term biogenesis (with an opposite meaning to Bastian's) as well as abiogenesis: Abiogenesis_sentence_206

Abiogenesis_description_list_0

  • And thus the hypothesis that living matter always arises by the agency of pre-existing living matter, took definite shape; and had, henceforward, a right to be considered and a claim to be refuted, in each particular case, before the production of living matter in any other way could be admitted by careful reasoners. It will be necessary for me to refer to this hypothesis so frequently, that, to save circumlocution, I shall call it the hypothesis of Biogenesis; and I shall term the contrary doctrine—that living matter may be produced by not living matter—the hypothesis of Abiogenesis.Abiogenesis_item_0_0

Subsequently, in the preface to Bastian's 1871 book, The Modes of Origin of Lowest Organisms, Bastian referred to the possible confusion with Huxley's usage and explicitly renounced his own meaning: Abiogenesis_sentence_207

Abiogenesis_description_list_1

  • A word of explanation seems necessary with regard to the introduction of the new term Archebiosis. I had originally, in unpublished writings, adopted the word Biogenesis to express the same meaning—viz., life-origination or commencement. But in the meantime, the word Biogenesis has been made use of, quite independently, by a distinguished biologist [Huxley], who wished to make it bear a totally different meaning. He also introduced the word Abiogenesis. I have been informed, however, on the best authority, that neither of these words can—with any regard to the language from which they are derived—be supposed to bear the meanings which have of late been publicly assigned to them. Wishing to avoid all needless confusion, I therefore renounced the use of the word Biogenesis, and being, for the reason just given, unable to adopt the other term, I was compelled to introduce a new word, in order to designate the process by which living matter is supposed to come into being, independently of pre-existing living matter.Abiogenesis_item_1_1

Since the end of the nineteenth century, 'evolutive abiogenesis' means increasing complexity and evolution of matter from inert to living states. Abiogenesis_sentence_208

Oparin: Primordial soup hypothesis Abiogenesis_section_33

Main article: Primordial soup Abiogenesis_sentence_209

Further information: Miller–Urey experiment Abiogenesis_sentence_210

There is no single generally accepted model for the origin of life. Abiogenesis_sentence_211

Scientists have proposed several plausible hypotheses which share some common elements. Abiogenesis_sentence_212

While differing in details, these hypotheses are based on the framework laid out by Alexander Oparin (in 1924) and John Haldane (in 1925), that the first molecules constituting the earliest cells Abiogenesis_sentence_213

Oparin and Haldane suggested that the atmosphere of the early Earth may have been chemically reducing in nature, composed primarily of methane (CH4), ammonia (NH3), water (H2O), hydrogen sulfide (H2S), carbon dioxide (CO2) or carbon monoxide (CO), and phosphate (PO4), with molecular oxygen (O2) and ozone (O3) either rare or absent. Abiogenesis_sentence_214

According to later models, the atmosphere in the late Hadean period consisted largely of nitrogen (N2) and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen (H2), and sulfur compounds; while it did lack molecular oxygen and ozone, it was not as chemically reducing as Oparin and Haldane supposed. Abiogenesis_sentence_215

No new notable research or hypothesis on the subject appeared until 1924, when Oparin reasoned that atmospheric oxygen prevents the synthesis of certain organic compounds that are necessary building blocks for life. Abiogenesis_sentence_216

In his book The Origin of Life, he proposed (echoing Darwin) that the "spontaneous generation of life" that had been attacked by Pasteur did, in fact, occur once, but was now impossible because the conditions found on the early Earth had changed, and preexisting organisms would immediately consume any spontaneously generated organism. Abiogenesis_sentence_217

Oparin argued that a "primeval soup" of organic molecules could be created in an oxygenless atmosphere through the action of sunlight. Abiogenesis_sentence_218

These would combine in ever more complex ways until they formed coacervate droplets. Abiogenesis_sentence_219

These droplets would "grow" by fusion with other droplets, and "reproduce" through fission into daughter droplets, and so have a primitive metabolism in which factors that promote "cell integrity" survive, and those that do not become extinct. Abiogenesis_sentence_220

Many modern theories of the origin of life still take Oparin's ideas as a starting point. Abiogenesis_sentence_221

About this time, Haldane suggested that the Earth's prebiotic oceans (quite different from their modern counterparts) would have formed a "hot dilute soup" in which organic compounds could have formed. Abiogenesis_sentence_222

Bernal called this idea biopoiesis or biopoesis, the process of living matter evolving from self-replicating but non-living molecules, and proposed that biopoiesis passes through a number of intermediate stages. Abiogenesis_sentence_223

Robert Shapiro has summarized the "primordial soup" theory of Oparin and Haldane in its "mature form" as follows: Abiogenesis_sentence_224

Abiogenesis_ordered_list_2

  1. The early Earth had a chemically reducing atmosphere.Abiogenesis_item_2_2
  2. This atmosphere, exposed to energy in various forms, produced simple organic compounds ("monomers").Abiogenesis_item_2_3
  3. These compounds accumulated in a "soup" that may have concentrated at various locations (shorelines, oceanic vents etc.).Abiogenesis_item_2_4
  4. By further transformation, more complex organic polymers—and ultimately life—developed in the soup.Abiogenesis_item_2_5

John Bernal Abiogenesis_section_34

John Bernal showed that based upon this and subsequent work there is no difficulty in principle in forming most of the molecules we recognize as the necessary molecules for life from their inorganic precursors. Abiogenesis_sentence_225

The underlying hypothesis held by Oparin, Haldane, Bernal, Miller and Urey, for instance, was that multiple conditions on the primeval Earth favoured chemical reactions that synthesized the same set of complex organic compounds from such simple precursors. Abiogenesis_sentence_226

Bernal coined the term biopoiesis in 1949 to refer to the origin of life. Abiogenesis_sentence_227

In 1967, he suggested that it occurred in three "stages": Abiogenesis_sentence_228

Abiogenesis_ordered_list_3

  1. the origin of biological monomersAbiogenesis_item_3_6
  2. the origin of biological polymersAbiogenesis_item_3_7
  3. the evolution from molecules to cellsAbiogenesis_item_3_8

Bernal suggested that evolution commenced between stages 1 and 2. Abiogenesis_sentence_229

Bernal regarded the third stage, in which biological reactions were incorporated behind a cell's boundary, as the most difficult. Abiogenesis_sentence_230

Modern work on the way that cell membranes self-assemble, and the work on micropores in various substrates, may be a key step towards understanding the development of independent free-living cells. Abiogenesis_sentence_231

Miller–Urey experiment Abiogenesis_section_35

One of the most important pieces of experimental support for the "soup" theory came in 1952. Abiogenesis_sentence_232

Stanley Miller and Harold Urey performed an experiment that demonstrated how organic molecules could have spontaneously formed from inorganic precursors under conditions like those posited by the Oparin-Haldane hypothesis. Abiogenesis_sentence_233

The now-famous Miller–Urey experiment used a highly reducing mixture of gases—methane, ammonia, and hydrogen, as well as water vapor—to form simple organic monomers such as amino acids. Abiogenesis_sentence_234

The mixture of gases was cycled through an apparatus that delivered electrical sparks to the mixture. Abiogenesis_sentence_235

After one week, it was found that about 10% to 15% of the carbon in the system was then in the form of a racemic mixture of organic compounds, including amino acids, which are the building blocks of proteins. Abiogenesis_sentence_236

This provided direct experimental support for the second point of the "soup" theory, and it is around the remaining two points of the theory that much of the debate now centers. Abiogenesis_sentence_237

A 2011 reanalysis of the saved vials containing the original extracts that resulted from the Miller and Urey experiments, using current and more advanced analytical equipment and technology, has uncovered more biochemicals than originally discovered in the 1950s. Abiogenesis_sentence_238

One of the more important findings was 23 amino acids, far more than the five originally found. Abiogenesis_sentence_239

In November 2020, a team of international scientists reported studies which suggest that the primeval atmosphere of the Earth was much different than the conditions used in the Miller-Urey studies. Abiogenesis_sentence_240

Primordial origin of biological molecules: Chemistry Abiogenesis_section_36

The chemical processes on the pre-biotic early Earth are called chemical evolution. Abiogenesis_sentence_241

The elements, except for hydrogen and helium, ultimately derive from stellar nucleosynthesis. Abiogenesis_sentence_242

In 2016, astronomers reported that the very basic chemical ingredients of life—the carbon-hydrogen molecule (CH, or methylidyne radical), the carbon-hydrogen positive ion (CH+) and the carbon ion (C+)—are largely the result of ultraviolet light from stars, rather than other forms of radiation from supernovae and young stars, as thought earlier. Abiogenesis_sentence_243

Complex molecules, including organic molecules, form naturally both in space and on planets. Abiogenesis_sentence_244

There are two possible sources of organic molecules on the early Earth: Abiogenesis_sentence_245

Abiogenesis_ordered_list_4

  1. Terrestrial origins – organic molecule synthesis driven by impact shocks or by other energy sources (such as UV light, redox coupling, or electrical discharges; e.g., Miller's experiments)Abiogenesis_item_4_9
  2. Extraterrestrial origins – formation of organic molecules in interstellar dust clouds, which rain down on planets. (See pseudo-panspermia)Abiogenesis_item_4_10

Observed extraterrestrial organic molecules Abiogenesis_section_37

See also: List of interstellar and circumstellar molecules and Panspermia § Pseudo-panspermia Abiogenesis_sentence_246

An organic compound is any member of a large class of gaseous, liquid, or solid chemicals whose molecules contain carbon. Abiogenesis_sentence_247

Carbon is the fourth most abundant element in the Universe by mass after hydrogen, helium, and oxygen. Abiogenesis_sentence_248

Carbon is abundant in the Sun, stars, comets, and in the atmospheres of most planets. Abiogenesis_sentence_249

Organic compounds are relatively common in space, formed by "factories of complex molecular synthesis" which occur in molecular clouds and circumstellar envelopes, and chemically evolve after reactions are initiated mostly by ionizing radiation. Abiogenesis_sentence_250

Based on computer model studies, the complex organic molecules necessary for life may have formed on dust grains in the protoplanetary disk surrounding the Sun before the formation of the Earth. Abiogenesis_sentence_251

According to the computer studies, this same process may also occur around other stars that acquire planets. Abiogenesis_sentence_252

Amino acids Abiogenesis_section_38

NASA announced in 2009 that scientists had identified another fundamental chemical building block of life in a comet for the first time, glycine, an amino acid, which was detected in material ejected from comet Wild 2 in 2004 and grabbed by NASA's Stardust probe. Abiogenesis_sentence_253

Glycine has been detected in meteorites before. Abiogenesis_sentence_254

Carl Pilcher, who leads the NASA Astrobiology Institute commented that Abiogenesis_sentence_255

Comets are encrusted with outer layers of dark material, thought to be a tar-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by ionizing radiation. Abiogenesis_sentence_256

It is possible that a rain of material from comets could have brought significant quantities of such complex organic molecules to Earth. Abiogenesis_sentence_257

Amino acids which were formed extraterrestrially may also have arrived on Earth via comets. Abiogenesis_sentence_258

It is estimated that during the Late Heavy Bombardment, meteorites may have delivered up to five million tons of organic prebiotic elements to Earth per year. Abiogenesis_sentence_259

PAH world hypothesis Abiogenesis_section_39

Main article: PAH world hypothesis Abiogenesis_sentence_260

Polycyclic aromatic hydrocarbons (PAH) are the most common and abundant of the known polyatomic molecules in the observable universe, and are considered a likely constituent of the primordial sea. Abiogenesis_sentence_261

In 2010, PAHs, have been detected in nebulae. Abiogenesis_sentence_262

Polycyclic aromatic hydrocarbons (PAH) are known to be abundant in the universe, including in the interstellar medium, in comets, and in meteorites, and are some of the most complex molecules so far found in space. Abiogenesis_sentence_263

Other sources of complex molecules have been postulated, including extraterrestrial stellar or interstellar origin. Abiogenesis_sentence_264

For example, from spectral analyses, organic molecules are known to be present in comets and meteorites. Abiogenesis_sentence_265

In 2004, a team detected traces of PAHs in a nebula. Abiogenesis_sentence_266

In 2010, another team also detected PAHs, along with fullerenes, in nebulae. Abiogenesis_sentence_267

The use of PAHs has also been proposed as a precursor to the RNA world in the PAH world hypothesis. Abiogenesis_sentence_268

The Spitzer Space Telescope has detected a star, HH 46-IR, which is forming by a process similar to that by which the Sun formed. Abiogenesis_sentence_269

In the disk of material surrounding the star, there is a very large range of molecules, including cyanide compounds, hydrocarbons, and carbon monoxide. Abiogenesis_sentence_270

In 2012, NASA scientists reported that PAHs, subjected to interstellar medium conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics—"a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively." Abiogenesis_sentence_271

Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks." Abiogenesis_sentence_272

NASA maintains a database for tracking PAHs in the universe. Abiogenesis_sentence_273

More than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. Abiogenesis_sentence_274

PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets. Abiogenesis_sentence_275

Nucleobases Abiogenesis_section_40

Observations suggest that the majority of organic compounds introduced on Earth by interstellar dust particles are considered principal agents in the formation of complex molecules, thanks to their peculiar surface-catalytic activities. Abiogenesis_sentence_276

Studies reported in 2008, based on C/C isotopic ratios of organic compounds found in the Murchison meteorite, suggested that the RNA component uracil and related molecules, including xanthine, were formed extraterrestrially. Abiogenesis_sentence_277

In 2011, a report based on NASA studies of meteorites found on Earth was published suggesting DNA components (adenine, guanine and related organic molecules) were made in outer space. Abiogenesis_sentence_278

Scientists also found that the cosmic dust permeating the universe contains complex organics ("amorphous organic solids with a mixed aromaticaliphatic structure") that could be created naturally, and rapidly, by stars. Abiogenesis_sentence_279

Sun Kwok of The University of Hong Kong suggested that these compounds may have been related to the development of life on Earth said that "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life." Abiogenesis_sentence_280

The sugar glycolaldehyde Abiogenesis_section_41

Glycolaldehyde, the first example of an interstellar sugar molecule, was detected in the star-forming region near the centre of our galaxy. Abiogenesis_sentence_281

It was discovered in 2000 by Jes Jørgensen and Jan Hollis. Abiogenesis_sentence_282

In 2012, Jørgensen's team reported the detection of glycolaldehyde in a distant star system. Abiogenesis_sentence_283

The molecule was found around the protostellar binary IRAS 16293-2422 400 light years from Earth. Abiogenesis_sentence_284

Glycolaldehyde is needed to form RNA, which is similar in function to DNA. Abiogenesis_sentence_285

These findings suggest that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation. Abiogenesis_sentence_286

Because sugars are associated with both metabolism and the genetic code, two of the most basic aspects of life, it is thought the discovery of extraterrestrial sugar increases the likelihood that life may exist elsewhere in our galaxy. Abiogenesis_sentence_287

Polyphosphates Abiogenesis_section_42

A problem in most scenarios of abiogenesis is that the thermodynamic equilibrium of amino acid versus peptides is in the direction of separate amino acids. Abiogenesis_sentence_288

What has been missing is some force that drives polymerization. Abiogenesis_sentence_289

The resolution of this problem may well be in the properties of polyphosphates. Abiogenesis_sentence_290

Polyphosphates are formed by polymerization of ordinary monophosphate ions PO4. Abiogenesis_sentence_291

Several mechanisms of organic molecule synthesis have been investigated. Abiogenesis_sentence_292

Polyphosphates cause polymerization of amino acids into peptides. Abiogenesis_sentence_293

They are also logical precursors in the synthesis of such key biochemical compounds as adenosine triphosphate (ATP). Abiogenesis_sentence_294

A key issue seems to be that calcium reacts with soluble phosphate to form insoluble calcium phosphate (apatite), so some plausible mechanism must be found to keep calcium ions from causing precipitation of phosphate. Abiogenesis_sentence_295

There has been much work on this topic over the years, but an interesting new idea is that meteorites may have introduced reactive phosphorus species on the early Earth. Abiogenesis_sentence_296

Based on recent computer model studies, the complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth. Abiogenesis_sentence_297

According to the computer studies, this same process may also occur around other stars that acquire planets. Abiogenesis_sentence_298

(Also see Extraterrestrial organic molecules). Abiogenesis_sentence_299

The accumulation and concentration of organic molecules on a planetary surface is also considered an essential early step for the origin of life. Abiogenesis_sentence_300

Identifying and understanding the mechanisms that led to the production of prebiotic molecules in various environments is critical for establishing the inventory of ingredients from which life originated on Earth, assuming that the abiotic production of molecules ultimately influenced the selection of molecules from which life emerged. Abiogenesis_sentence_301

In 2019, scientists reported detecting, for the first time, sugar molecules, including ribose, in meteorites, suggesting that chemical processes on asteroids can produce some fundamentally essential bio-ingredients important to life, and supporting the notion of an RNA world prior to a DNA-based origin of life on Earth, and possibly, as well, the notion of panspermia. Abiogenesis_sentence_302

Chemical synthesis in the laboratory Abiogenesis_section_43

As early as the 1860s, experiments have demonstrated that biologically relevant molecules can be produced from interaction of simple carbon sources with abundant inorganic catalysts. Abiogenesis_sentence_303

Fox proteinoids Abiogenesis_section_44

Main article: Proteinoid Abiogenesis_sentence_304

In trying to uncover the intermediate stages of abiogenesis mentioned by Bernal, Sidney Fox in the 1950s and 1960s studied the spontaneous formation of peptide structures (small chains of amino acids) under conditions that might plausibly have existed early in Earth's history. Abiogenesis_sentence_305

In one of his experiments, he allowed amino acids to dry out as if puddled in a warm, dry spot in prebiotic conditions: In an experiment to set suitable conditions for life to form, Fox collected volcanic material from a cinder cone in Hawaii. Abiogenesis_sentence_306

He discovered that the temperature was over 100 C just 4 inches (100 mm) beneath the surface of the cinder cone, and suggested that this might have been the environment in which life was created—molecules could have formed and then been washed through the loose volcanic ash into the sea. Abiogenesis_sentence_307

He placed lumps of lava over amino acids derived from methane, ammonia and water, sterilized all materials, and baked the lava over the amino acids for a few hours in a glass oven. Abiogenesis_sentence_308

A brown, sticky substance formed over the surface, and when the lava was drenched in sterilized water, a thick, brown liquid leached out. Abiogenesis_sentence_309

He found that, as they dried, the amino acids formed long, often cross-linked, thread-like, submicroscopic polypeptide molecules. Abiogenesis_sentence_310

Sugars Abiogenesis_section_45

In particular, experiments by Butlerov (the formose reaction) showed that tetroses, pentoses, and hexoses are produced when formaldehyde is heated under basic conditions with divalent metal ions like calcium. Abiogenesis_sentence_311

The reaction was scrutinized and subsequently proposed to be autocatalytic by Breslow in 1959. Abiogenesis_sentence_312

Nucleobases Abiogenesis_section_46

Similar experiments (see below) demonstrate that nucleobases like guanine and adenine could be synthesized from simple carbon and nitrogen sources like hydrogen cyanide and ammonia. Abiogenesis_sentence_313

Formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals. Abiogenesis_sentence_314

Formamide is ubiquitous in the Universe, produced by the reaction of water and hydrogen cyanide (HCN). Abiogenesis_sentence_315

It has several advantages as a biotic precursor, including the ability to easily become concentrated through the evaporation of water. Abiogenesis_sentence_316

Although HCN is poisonous, it only affects aerobic organisms (eukaryotes and aerobic bacteria), which did not yet exist. Abiogenesis_sentence_317

It can play roles in other chemical processes as well, such as the synthesis of the amino acid glycine. Abiogenesis_sentence_318

In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic compounds of life, including uracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, using starting chemicals, such as pyrimidine, found in meteorites. Abiogenesis_sentence_319

Pyrimidine, like PAHs, the most carbon-rich chemical found in the Universe, may have been formed in red giant stars or in interstellar dust and gas clouds. Abiogenesis_sentence_320

A group of Czech scientists reported that all four RNA-bases may be synthesized from formamide in the course of high-energy density events like extraterrestrial impacts. Abiogenesis_sentence_321

Use of high temperature Abiogenesis_section_47

In 1961, it was shown that the nucleic acid purine base adenine can be formed by heating aqueous ammonium cyanide solutions. Abiogenesis_sentence_322

Use of low (freezing) temperature Abiogenesis_section_48

Other pathways for synthesizing bases from inorganic materials were also reported. Abiogenesis_sentence_323

Orgel and colleagues have shown that freezing temperatures are advantageous for the synthesis of purines, due to the concentrating effect for key precursors such as hydrogen cyanide. Abiogenesis_sentence_324

Research by Miller and colleagues suggested that while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures. Abiogenesis_sentence_325

Research by the Miller group notes the formation of seven different amino acids and 11 types of nucleobases in ice when ammonia and cyanide were left in a freezer from 1972 to 1997. Abiogenesis_sentence_326

Other work demonstrated the formation of s-triazines (alternative nucleobases), pyrimidines (including cytosine and uracil), and adenine from urea solutions subjected to freeze-thaw cycles under a reductive atmosphere (with spark discharges as an energy source). Abiogenesis_sentence_327

The explanation given for the unusual speed of these reactions at such a low temperature is eutectic freezing. Abiogenesis_sentence_328

As an ice crystal forms, it stays pure: only molecules of water join the growing crystal, while impurities like salt or cyanide are excluded. Abiogenesis_sentence_329

These impurities become crowded in microscopic pockets of liquid within the ice, and this crowding causes the molecules to collide more often. Abiogenesis_sentence_330

Mechanistic exploration using quantum chemical methods provide a more detailed understanding of some of the chemical processes involved in chemical evolution, and a partial answer to the fundamental question of molecular biogenesis. Abiogenesis_sentence_331

Use of less-reducing gas in Miller–Urey experiment Abiogenesis_section_49

At the time of the Miller–Urey experiment, scientific consensus was that the early Earth had a reducing atmosphere with compounds relatively rich in hydrogen and poor in oxygen (e.g., CH4 and NH3 as opposed to CO2 and nitrogen dioxide (NO2)). Abiogenesis_sentence_332

However, current scientific consensus describes the primitive atmosphere as either weakly reducing or neutral (see also Oxygen Catastrophe). Abiogenesis_sentence_333

Such an atmosphere would diminish both the amount and variety of amino acids that could be produced, although studies that include iron and carbonate minerals (thought present in early oceans) in the experimental conditions have again produced a diverse array of amino acids. Abiogenesis_sentence_334

Other scientific research has focused on two other potential reducing environments: outer space and deep-sea thermal vents. Abiogenesis_sentence_335

Synthesis based on hydrogen cyanide Abiogenesis_section_50

A research project completed in 2015 by John Sutherland and others found that a network of reactions beginning with hydrogen cyanide and hydrogen sulfide, in streams of water irradiated by UV light, could produce the chemical components of proteins and lipids, as well as those of RNA, while not producing a wide range of other compounds. Abiogenesis_sentence_336

The researchers used the term "cyanosulfidic" to describe this network of reactions. Abiogenesis_sentence_337

Issues during laboratory synthesis Abiogenesis_section_51

The spontaneous formation of complex polymers from abiotically generated monomers under the conditions posited by the "soup" theory is not at all a straightforward process. Abiogenesis_sentence_338

Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were also formed in high concentration during the Miller–Urey and Joan Oró experiments. Abiogenesis_sentence_339

The Miller–Urey experiment, for example, produces many substances that would react with the amino acids or terminate their coupling into peptide chains. Abiogenesis_sentence_340

Autocatalysis Abiogenesis_section_52

Main article: Autocatalysis Abiogenesis_sentence_341

Autocatalysts are substances that catalyze the production of themselves and therefore are "molecular replicators." Abiogenesis_sentence_342

The simplest self-replicating chemical systems are autocatalytic, and typically contain three components: a product molecule and two precursor molecules. Abiogenesis_sentence_343

The product molecule joins together the precursor molecules, which in turn produce more product molecules from more precursor molecules. Abiogenesis_sentence_344

The product molecule catalyzes the reaction by providing a complementary template that binds to the precursors, thus bringing them together. Abiogenesis_sentence_345

Such systems have been demonstrated both in biological macromolecules and in small organic molecules. Abiogenesis_sentence_346

Systems that do not proceed by template mechanisms, such as the self-reproduction of micelles and vesicles, have also been observed. Abiogenesis_sentence_347

It has been proposed that life initially arose as autocatalytic chemical networks. Abiogenesis_sentence_348

British ethologist Richard Dawkins wrote about autocatalysis as a potential explanation for the origin of life in his 2004 book The Ancestor's Tale. Abiogenesis_sentence_349

In his book, Dawkins cites experiments performed by Julius Rebek and his colleagues in which they combined amino adenosine and pentafluorophenyl esters with the autocatalyst amino adenosine triacid ester (AATE). Abiogenesis_sentence_350

One product was a variant of AATE, which catalyzed the synthesis of themselves. Abiogenesis_sentence_351

This experiment demonstrated the possibility that autocatalysts could exhibit competition within a population of entities with heredity, which could be interpreted as a rudimentary form of natural selection. Abiogenesis_sentence_352

Encapsulation: morphology Abiogenesis_section_53

See also: Evolution of cells Abiogenesis_sentence_353

Encapsulation without a membrane Abiogenesis_section_54

Oparin's coacervate Abiogenesis_section_55

Membraneless polyester droplets Abiogenesis_section_56

Researchers Tony Jia and Kuhan Chandru have proposed that membraneless polyesters droplets could have been significant in the Origins of Life. Abiogenesis_sentence_354

Given the "messy" nature of prebiotic chemistry, the spontaneous generation of these combinatorial droplets may have played a role in early cellularization before the innovation of lipid vesicles. Abiogenesis_sentence_355

Protein function within and RNA function in the presence of certain polyester droplets was shown to be preserved within the droplets. Abiogenesis_sentence_356

Additionally, the droplets have scaffolding ability, by allowing lipids to assemble around them that may have prevented leakage of genetic materials. Abiogenesis_sentence_357

Proteinoid microspheres Abiogenesis_section_57

Fox observed in the 1960s that the proteinoids that he had synthesized could form cell-like structures that have been named "proteinoid microspheres". Abiogenesis_sentence_358

The amino acids had combined to form proteinoids, and the proteinoids had combined to form small globules that Fox called "microspheres". Abiogenesis_sentence_359

His proteinoids were not cells, although they formed clumps and chains reminiscent of cyanobacteria, but they contained no functional nucleic acids or any encoded information. Abiogenesis_sentence_360

Based upon such experiments, Colin Pittendrigh stated in 1967 that "laboratories will be creating a living cell within ten years," a remark that reflected the typical contemporary naivety about the complexity of cell structures. Abiogenesis_sentence_361

Lipid world Abiogenesis_section_58

Main article: Gard model Abiogenesis_sentence_362

The lipid world theory postulates that the first self-replicating object was lipid-like. Abiogenesis_sentence_363

It is known that phospholipids form lipid bilayers in water while under agitation—the same structure as in cell membranes. Abiogenesis_sentence_364

These molecules were not present on early Earth, but other amphiphilic long-chain molecules also form membranes. Abiogenesis_sentence_365

Furthermore, these bodies may expand (by insertion of additional lipids), and under excessive expansion may undergo spontaneous splitting which preserves the same size and composition of lipids in the two progenies. Abiogenesis_sentence_366

The main idea in this theory is that the molecular composition of the lipid bodies is the preliminary way for information storage, and evolution led to the appearance of polymer entities such as RNA or DNA that may store information favourably. Abiogenesis_sentence_367

Studies on vesicles from potentially prebiotic amphiphiles have so far been limited to systems containing one or two types of amphiphiles. Abiogenesis_sentence_368

This in contrast to the output of simulated prebiotic chemical reactions, which typically produce very heterogeneous mixtures of compounds. Abiogenesis_sentence_369

Within the hypothesis of a lipid bilayer membrane composed of a mixture of various distinct amphiphilic compounds there is the opportunity of a huge number of theoretically possible combinations in the arrangements of these amphiphiles in the membrane. Abiogenesis_sentence_370

Among all these potential combinations, a specific local arrangement of the membrane would have favoured the constitution of a hypercycle, actually a positive feedback composed of two mutual catalysts represented by a membrane site and a specific compound trapped in the vesicle. Abiogenesis_sentence_371

Such site/compound pairs are transmissible to the daughter vesicles leading to the emergence of distinct lineages of vesicles which would have allowed Darwinian natural selection. Abiogenesis_sentence_372

Protocells Abiogenesis_section_59

Main article: Protocell Abiogenesis_sentence_373

A protocell is a self-organized, self-ordered, spherical collection of lipids proposed as a stepping-stone to the origin of life. Abiogenesis_sentence_374

A central question in evolution is how simple protocells first arose and differed in reproductive contribution to the following generation driving the evolution of life. Abiogenesis_sentence_375

Although a functional protocell has not yet been achieved in a laboratory setting, there are scientists who think the goal is well within reach. Abiogenesis_sentence_376

Self-assembled vesicles are essential components of primitive cells. Abiogenesis_sentence_377

The second law of thermodynamics requires that the universe move in a direction in which entropy increases, yet life is distinguished by its great degree of organization. Abiogenesis_sentence_378

Therefore, a boundary is needed to separate life processes from non-living matter. Abiogenesis_sentence_379

Researchers Irene Chen and Szostak amongst others, suggest that simple physicochemical properties of elementary protocells can give rise to essential cellular behaviours, including primitive forms of differential reproduction competition and energy storage. Abiogenesis_sentence_380

Such cooperative interactions between the membrane and its encapsulated contents could greatly simplify the transition from simple replicating molecules to true cells. Abiogenesis_sentence_381

Furthermore, competition for membrane molecules would favour stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids and even the phospholipids of today. Abiogenesis_sentence_382

Such micro-encapsulation would allow for metabolism within the membrane, the exchange of small molecules but the prevention of passage of large substances across it. Abiogenesis_sentence_383

The main advantages of encapsulation include the increased solubility of the contained cargo within the capsule and the storage of energy in the form of an electrochemical gradient. Abiogenesis_sentence_384

Another approach to the notion of a protocell concerns the term "chemoton" (short for 'chemical automaton') which refers to an abstract model for the fundamental unit of life introduced by Hungarian theoretical biologist Tibor Gánti. Abiogenesis_sentence_385

It is the oldest known computational abstract of a protocell. Abiogenesis_sentence_386

Gánti conceived the basic idea in 1952 and formulated the concept in 1971 in his book The Principles of Life (originally written in Hungarian, and translated to English only in 2003). Abiogenesis_sentence_387

He surmised the chemoton as the original ancestor of all organisms, or the last universal common ancestor. Abiogenesis_sentence_388

The basic assumption of the chemoton model is that life should fundamentally and essentially have three properties: metabolism, self-replication, and a bilipid membrane. Abiogenesis_sentence_389

The metabolic and replication functions together form an autocatalytic subsystem necessary for the basic functions of life, and a membrane encloses this subsystem to separate it from the surrounding environment. Abiogenesis_sentence_390

Therefore, any system having such properties may be regarded as alive, and it will be subjected to natural selection and contain a self-sustaining cellular information. Abiogenesis_sentence_391

Some consider this model a significant contribution to origin of life as it provides a philosophy of evolutionary units. Abiogenesis_sentence_392

Nonetheless, a 2012 study led by Mulkidjanian of the University of Osnabrück, suggests that inland pools of condensed and cooled geothermal vapor have the ideal characteristics for the origin of life. Abiogenesis_sentence_393

Scientists confirmed in 2002 that by adding a montmorillonite clay to a solution of fatty acid micelles (lipid spheres), the clay sped up the rate of vesicles formation 100-fold. Abiogenesis_sentence_394

Furthermore, recent studies have found that the repeated actions of dehydration and rehydration trapped biomolecules like RNA inside the lipid protocells found within hot springs and providing the necessary preconditions for evolution by natural selection. Abiogenesis_sentence_395

Lipid vesicles formation in fresh water Abiogenesis_section_60

Bruce Damer and David Deamer have come to the conclusion that cell membranes cannot be formed in salty seawater, and must therefore have originated in freshwater. Abiogenesis_sentence_396

Before the continents formed, the only dry land on Earth would be volcanic islands, where rainwater would form ponds where lipids could form the first stages towards cell membranes. Abiogenesis_sentence_397

These predecessors of true cells are assumed to have behaved more like a superorganism rather than individual structures, where the porous membranes would house molecules which would leak out and enter other protocells. Abiogenesis_sentence_398

Only when true cells had evolved would they gradually adapt to saltier environments and enter the ocean. Abiogenesis_sentence_399

Vesicles consisting of mixtures of RNA-like biochemicals Abiogenesis_section_61

Another protocell model is the Jeewanu. Abiogenesis_sentence_400

First synthesized in 1963 from simple minerals and basic organics while exposed to sunlight, it is still reported to have some metabolic capabilities, the presence of semipermeable membrane, amino acids, phospholipids, carbohydrates and RNA-like molecules. Abiogenesis_sentence_401

However, the nature and properties of the Jeewanu remains to be clarified. Abiogenesis_sentence_402

Electrostatic interactions induced by short, positively charged, hydrophobic peptides containing 7 amino acids in length or fewer, can attach RNA to a vesicle membrane, the basic cell membrane. Abiogenesis_sentence_403

Metal-sulfide precipitates Abiogenesis_section_62

William Martin and Michael Russell have suggested Abiogenesis_sentence_404

Pertinent geological environments Abiogenesis_section_63

Darwin's little pond Abiogenesis_section_64

An early concept, that life originated from non-living matter in slow stages, appeared in Herbert Spencer's 1864–1867 book Principles of Biology. Abiogenesis_sentence_405

In 1879 William Turner Thiselton-Dyer referred to this in a paper "On spontaneous generation and evolution". Abiogenesis_sentence_406

On 1 February 1871 Charles Darwin wrote about these publications to Joseph Hooker, and set out his own speculation, suggesting that the original spark of life may have begun in a Abiogenesis_sentence_407

He went on to explain that Abiogenesis_sentence_408

, p. : Abiogenesis_sentence_409

It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. Abiogenesis_sentence_410

But if (and oh! Abiogenesis_sentence_411

what a big if!) Abiogenesis_sentence_412

we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present, that a proteine compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed. Abiogenesis_sentence_413

— Darwin, 1 February 1871 Abiogenesis_sentence_414

More recent studies, in 2017, support the notion that life may have begun right after the Earth was formed as RNA molecules emerging from "warm little ponds". Abiogenesis_sentence_415

Volcanic hot springs and hydrothermal vents, shallow or deep Abiogenesis_section_65

For branching of Bacteria phyla, see Bacterial phyla. Abiogenesis_sentence_416

Martin Brazier has shown that early micro-fossils came from a hot world of gases such as methane, ammonia, carbon dioxide and hydrogen sulphide, which are toxic to much current life. Abiogenesis_sentence_417

Another analysis of the conventional threefold tree of life shows thermophilic and hyperthermophilic bacteria and archaea are closest to the root, suggesting that life may have evolved in a hot environment. Abiogenesis_sentence_418

Deep sea hydrothermal vents Abiogenesis_section_66

The deep sea vent, or alkaline hydrothermal vent, theory posits that life may have begun at submarine hydrothermal vents, Martin and Russell have suggested Abiogenesis_sentence_419

These form where hydrogen-rich fluids emerge from below the sea floor, as a result of serpentinization of ultra-mafic olivine with seawater and a pH interface with carbon dioxide-rich ocean water. Abiogenesis_sentence_420

The vents form a sustained chemical energy source derived from redox reactions, in which electron donors (molecular hydrogen) react with electron acceptors (carbon dioxide); see Iron–sulfur world theory. Abiogenesis_sentence_421

These are highly exothermic reactions. Abiogenesis_sentence_422

Russell demonstrated that alkaline vents created an abiogenic proton motive force (PMF) chemiosmotic gradient, in which conditions are ideal for an abiogenic hatchery for life. Abiogenesis_sentence_423

Their microscopic compartments "provide a natural means of concentrating organic molecules," composed of iron-sulfur minerals such as mackinawite, endowed these mineral cells with the catalytic properties envisaged by Günter Wächtershäuser. Abiogenesis_sentence_424

This movement of ions across the membrane depends on a combination of two factors: Abiogenesis_sentence_425

Abiogenesis_ordered_list_5

  1. Diffusion force caused by concentration gradient—all particles including ions tend to diffuse from higher concentration to lower.Abiogenesis_item_5_11
  2. Electrostatic force caused by electrical potential gradient—cations like protons H tend to diffuse down the electrical potential, anions in the opposite direction.Abiogenesis_item_5_12

These two gradients taken together can be expressed as an electrochemical gradient, providing energy for abiogenic synthesis. Abiogenesis_sentence_426

The proton motive force can be described as the measure of the potential energy stored as a combination of proton and voltage gradients across a membrane (differences in proton concentration and electrical potential). Abiogenesis_sentence_427

Szostak suggested that geothermal activity provides greater opportunities for the origination of life in open lakes where there is a buildup of minerals. Abiogenesis_sentence_428

In 2010, based on spectral analysis of sea and hot mineral water, Ignat Ignatov and Oleg Mosin demonstrated that life may have predominantly originated in hot mineral water. Abiogenesis_sentence_429

The hot mineral water that contains bicarbonate and calcium ions has the most optimal range. Abiogenesis_sentence_430

This case is similar to the origin of life in hydrothermal vents, but with bicarbonate and calcium ions in hot water. Abiogenesis_sentence_431

This water has a pH of 9–11 and is possible to have the reactions in seawater. Abiogenesis_sentence_432

According to Melvin Calvin, certain reactions of condensation-dehydration of amino acids and nucleotides in individual blocks of peptides and nucleic acids can take place in the primary hydrosphere with pH 9–11 at a later evolutionary stage. Abiogenesis_sentence_433

Some of these compounds like hydrocyanic acid (HCN) have been proven in the experiments of Miller. Abiogenesis_sentence_434

This is the environment in which the stromatolites have been created. Abiogenesis_sentence_435

David Ward of Montana State University described the formation of stromatolites in hot mineral water at the Yellowstone National Park. Abiogenesis_sentence_436

Stromatolites survive in hot mineral water and in proximity to areas with volcanic activity. Abiogenesis_sentence_437

Processes have evolved in the sea near geysers of hot mineral water. Abiogenesis_sentence_438

In 2011, Tadashi Sugawara from the University of Tokyo created a protocell in hot water. Abiogenesis_sentence_439

Experimental research and computer modelling suggest that the surfaces of mineral particles inside hydrothermal vents have catalytic properties similar to those of enzymes and are able to create simple organic molecules, such as methanol (CH3OH) and formic, acetic and pyruvic acid out of the dissolved CO2 in the water. Abiogenesis_sentence_440

The research reported above by Martin in 2016 supports the thesis that life arose at hydrothermal vents, that spontaneous chemistry in the Earth's crust driven by rock–water interactions at disequilibrium thermodynamically underpinned life's origin and that the founding lineages of the archaea and bacteria were H2-dependent autotrophs that used CO2 as their terminal acceptor in energy metabolism. Abiogenesis_sentence_441

Martin suggests, based upon this evidence that LUCA "may have depended heavily on the geothermal energy of the vent to survive". Abiogenesis_sentence_442

Fluctuating hydrothermal pools on volcanic islands or proto-continents Abiogenesis_section_67

Mulkidjanian and co-authors think that the marine environments did not provide the ionic balance and composition universally found in cells, as well as of ions required by essential proteins and ribozymes found in virtually all living organisms, especially with respect to K/Na ratio, Mn, Zn and phosphate concentrations. Abiogenesis_sentence_443

The only known environments that mimic the needed conditions on Earth are found in terrestrial hydrothermal pools fed by steam vents. Abiogenesis_sentence_444

Additionally, mineral deposits in these environments under an anoxic atmosphere would have suitable pH (as opposed to current pools in an oxygenated atmosphere), contain precipitates of sulfide minerals that block harmful UV radiation, have wetting/drying cycles that concentrate substrate solutions to concentrations amenable to spontaneous formation of polymers of nucleic acids, polyesters and depsipeptides, both by chemical reactions in the hydrothermal environment, as well as by exposure to UV light during transport from vents to adjacent pools. Abiogenesis_sentence_445

Their hypothesized pre-biotic environments are similar to the deep-oceanic vent environments most commonly hypothesized, but add additional components that help explain peculiarities found in reconstructions of the Last Universal Common Ancestor (LUCA) of all living organisms. Abiogenesis_sentence_446

Colín-García et al. Abiogenesis_sentence_447

(2016) discuss the advantages and disadvantages of hydrothermal vents as primitive environments. Abiogenesis_sentence_448

They mention the exergonic reactions in such systems could have been a source of free energy that promoted chemical reactions, additional to their high mineralogical diversity which implies the induction of important chemical gradients, thus favoring the interaction between electron donors and acceptors. Abiogenesis_sentence_449

Colín-García et al. Abiogenesis_sentence_450

(2016) also summarize a set of experiments proposed to test the role of hydrothermal vents in prebiotic synthesis. Abiogenesis_sentence_451

Volcanic ash in the ocean Abiogenesis_section_68

Geoffrey W. Hoffmann has argued that a complex nucleation event as the origin of life involving both polypeptides and nucleic acid is compatible with the time and space available in the primitive oceans of Earth Hoffmann suggests that volcanic ash may provide the many random shapes needed in the postulated complex nucleation event. Abiogenesis_sentence_452

This aspect of the theory can be tested experimentally. Abiogenesis_sentence_453

Gold's deep-hot biosphere Abiogenesis_section_69

In the 1970s, Thomas Gold proposed the theory that life first developed not on the surface of the Earth, but several kilometers below the surface. Abiogenesis_sentence_454

It is claimed that the discovery of microbial life below the surface of another body in our Solar System would lend significant credence to this theory. Abiogenesis_sentence_455

Gold also asserted that a trickle of food from a deep, unreachable, source is needed for survival because life arising in a puddle of organic material is likely to consume all of its food and become extinct. Abiogenesis_sentence_456

Gold's theory is that the flow of such food is due to out-gassing of primordial methane from the Earth's mantle; more conventional explanations of the food supply of deep microbes (away from sedimentary carbon compounds) is that the organisms subsist on hydrogen released by an interaction between water and (reduced) iron compounds in rocks. Abiogenesis_sentence_457

Radioactive beach hypothesis Abiogenesis_section_70

Zachary Adam claims that tidal processes that occurred during a time when the Moon was much closer may have concentrated grains of uranium and other radioactive elements at the high-water mark on primordial beaches, where they may have been responsible for generating life's building blocks. Abiogenesis_sentence_458

According to computer models, a deposit of such radioactive materials could show the same self-sustaining nuclear reaction as that found in the Oklo uranium ore seam in Gabon. Abiogenesis_sentence_459

Such radioactive beach sand might have provided sufficient energy to generate organic molecules, such as amino acids and sugars from acetonitrile in water. Abiogenesis_sentence_460

Radioactive monazite material also has released soluble phosphate into the regions between sand-grains, making it biologically "accessible." Abiogenesis_sentence_461

Thus amino acids, sugars, and soluble phosphates might have been produced simultaneously, according to Adam. Abiogenesis_sentence_462

Radioactive actinides, left behind in some concentration by the reaction, might have formed part of organometallic complexes. Abiogenesis_sentence_463

These complexes could have been important early catalysts to living processes. Abiogenesis_sentence_464

John Parnell has suggested that such a process could provide part of the "crucible of life" in the early stages of any early wet rocky planet, so long as the planet is large enough to have generated a system of plate tectonics which brings radioactive minerals to the surface. Abiogenesis_sentence_465

As the early Earth is thought to have had many smaller plates, it might have provided a suitable environment for such processes. Abiogenesis_sentence_466

Origin of metabolism: physiology Abiogenesis_section_71

Different forms of life with variable origin processes may have appeared quasi-simultaneously in the early history of Earth. Abiogenesis_sentence_467

The other forms may be extinct (having left distinctive fossils through their different biochemistry—e.g., hypothetical types of biochemistry). Abiogenesis_sentence_468

It has been proposed that: Abiogenesis_sentence_469

Metabolism-like reactions could have occurred naturally in early oceans, before the first organisms evolved. Abiogenesis_sentence_470

Metabolism may predate the origin of life, which may have evolved from the chemical conditions in the earliest oceans. Abiogenesis_sentence_471

Reconstructions in laboratories show that some of these reactions can produce RNA, and some others resemble two essential reaction cascades of metabolism: glycolysis and the pentose phosphate pathway, that provide essential precursors for nucleic acids, amino acids and lipids. Abiogenesis_sentence_472

Clay hypothesis Abiogenesis_section_72

Montmorillonite, an abundant clay, is a catalyst for the polymerization of RNA and for the formation of membranes from lipids. Abiogenesis_sentence_473

A model for the origin of life using clay was forwarded by Alexander Cairns-Smith in 1985 and explored as a plausible mechanism by several scientists. Abiogenesis_sentence_474

The clay hypothesis postulates that complex organic molecules arose gradually on pre-existing, non-organic replication surfaces of silicate crystals in solution. Abiogenesis_sentence_475

At the Rensselaer Polytechnic Institute, James Ferris' studies have also confirmed that montmorillonite clay minerals catalyze the formation of RNA in aqueous solution, by joining nucleotides to form longer chains. Abiogenesis_sentence_476

In 2007, Bart Kahr from the University of Washington and colleagues reported their experiments that tested the idea that crystals can act as a source of transferable information, using crystals of potassium hydrogen phthalate. Abiogenesis_sentence_477

"Mother" crystals with imperfections were cleaved and used as seeds to grow "daughter" crystals from solution. Abiogenesis_sentence_478

They then examined the distribution of imperfections in the new crystals and found that the imperfections in the mother crystals were reproduced in the daughters, but the daughter crystals also had many additional imperfections. Abiogenesis_sentence_479

For gene-like behavior to be observed, the quantity of inheritance of these imperfections should have exceeded that of the mutations in the successive generations, but it did not. Abiogenesis_sentence_480

Thus Kahr concluded that the crystals "were not faithful enough to store and transfer information from one generation to the next." Abiogenesis_sentence_481

Iron–sulfur world Abiogenesis_section_73

Main article: Iron–sulfur world theory Abiogenesis_sentence_482

In the 1980s, Günter Wächtershäuser, encouraged and supported by Karl Popper, postulated his iron–sulfur world, a theory of the evolution of pre-biotic chemical pathways as the starting point in the evolution of life. Abiogenesis_sentence_483

It systematically traces today's biochemistry to primordial reactions which provide alternative pathways to the synthesis of organic building blocks from simple gaseous compounds. Abiogenesis_sentence_484

In contrast to the classical Miller experiments, which depend on external sources of energy (simulated lightning, ultraviolet irradiation), "Wächtershäuser systems" come with a built-in source of energy: sulfides of iron (iron pyrite) and other minerals. Abiogenesis_sentence_485

The energy released from redox reactions of these metal sulfides is available for the synthesis of organic molecules, and such systems may have evolved into autocatalytic sets constituting self-replicating, metabolically active entities predating the life forms known today. Abiogenesis_sentence_486

Experiments with such sulfides in an aqueous environment at 100 °C produced a relatively small yield of dipeptides (0.4% to 12.4%) and a smaller yield of tripeptides (0.10%) although under the same conditions, dipeptides were quickly broken down. Abiogenesis_sentence_487

Several models reject the self-replication of a "naked-gene", postulating instead the emergence of a primitive metabolism providing a safe environment for the later emergence of RNA replication. Abiogenesis_sentence_488

The centrality of the Krebs cycle (citric acid cycle) to energy production in aerobic organisms, and in drawing in carbon dioxide and hydrogen ions in biosynthesis of complex organic chemicals, suggests that it was one of the first parts of the metabolism to evolve. Abiogenesis_sentence_489

Concordantly, geochemist Russell has proposed that "the purpose of life is to hydrogenate carbon dioxide" (as part of a "metabolism-first," rather than a "genetics-first," scenario). Abiogenesis_sentence_490

Physicist Jeremy England has proposed that life was inevitable from general thermodynamic considerations: Abiogenesis_sentence_491

One of the earliest incarnations of this idea was put forward in 1924 with Oparin's notion of primitive self-replicating vesicles which predated the discovery of the structure of DNA. Abiogenesis_sentence_492

Variants in the 1980s and 1990s include Wächtershäuser's iron–sulfur world theory and models introduced by Christian de Duve based on the chemistry of thioesters. Abiogenesis_sentence_493

More abstract and theoretical arguments for the plausibility of the emergence of metabolism without the presence of genes include a mathematical model introduced by Freeman Dyson in the early 1980s and Stuart Kauffman's notion of collectively autocatalytic sets, discussed later that decade. Abiogenesis_sentence_494

Orgel summarized his analysis by stating, Abiogenesis_sentence_495

It is possible that another type of metabolic pathway was used at the beginning of life. Abiogenesis_sentence_496

For example, instead of the reductive citric acid cycle, the "open" acetyl-CoA pathway (another one of the five recognized ways of carbon dioxide fixation in nature today) would be compatible with the idea of self-organization on a metal sulfide surface. Abiogenesis_sentence_497

The key enzyme of this pathway, carbon monoxide dehydrogenase/acetyl-CoA synthase, harbors mixed nickel-iron-sulfur clusters in its reaction centers and catalyzes the formation of acetyl-CoA (similar to acetyl-thiol) in a single step. Abiogenesis_sentence_498

There are increasing concerns, however, that prebiotic thiolated and thioester compounds are thermodynamically and kinetically unfavorable to accumulate in presumed prebiotic conditions (i.e. hydrothermal vents). Abiogenesis_sentence_499

It has also been proposed that cysteine and homocysteine may have reacted with nitriles resulting from the Stecker reaction, readily forming catalytic thiol-reach poplypeptides. Abiogenesis_sentence_500

Zinc-world hypothesis Abiogenesis_section_74

The zinc world (Zn-world) theory of Mulkidjanian is an extension of Wächtershäuser's pyrite hypothesis. Abiogenesis_sentence_501

Wächtershäuser based his theory of the initial chemical processes leading to informational molecules (RNA, peptides) on a regular mesh of electric charges at the surface of pyrite that may have facilitated the primeval polymerization by attracting reactants and arranging them appropriately relative to each other. Abiogenesis_sentence_502

The Zn-world theory specifies and differentiates further. Abiogenesis_sentence_503

Hydrothermal fluids rich in H2S interacting with cold primordial ocean (or Darwin's "warm little pond") water leads to the precipitation of metal sulfide particles. Abiogenesis_sentence_504

Oceanic vent systems and other hydrothermal systems have a zonal structure reflected in ancient volcanogenic massive sulfide deposits (VMS) of hydrothermal origin. Abiogenesis_sentence_505

They reach many kilometers in diameter and date back to the Archean Eon. Abiogenesis_sentence_506

Most abundant are pyrite (FeS2), chalcopyrite (CuFeS2), and sphalerite (ZnS), with additions of galena (PbS) and alabandite (MnS). Abiogenesis_sentence_507

ZnS and MnS have a unique ability to store radiation energy, e.g. from UV light. Abiogenesis_sentence_508

During the relevant time window of the origins of replicating molecules, the primordial atmospheric pressure was high enough (>100 bar, about 100 atmospheres) to precipitate near the Earth's surface, and UV irradiation was 10 to 100 times more intense than now; hence the unique photosynthetic properties mediated by ZnS provided just the right energy conditions to energize the synthesis of informational and metabolic molecules and the selection of photostable nucleobases. Abiogenesis_sentence_509

The Zn-world theory has been further filled out with experimental and theoretical evidence for the ionic constitution of the interior of the first proto-cells before archaea, bacteria and proto-eukaryotes evolved. Abiogenesis_sentence_510

Archibald Macallum noted the resemblance of body fluids such as blood and lymph to seawater; however, the inorganic composition of all cells differ from that of modern seawater, which led Mulkidjanian and colleagues to reconstruct the "hatcheries" of the first cells combining geochemical analysis with phylogenomic scrutiny of the inorganic ion requirements of universal components of modern cells. Abiogenesis_sentence_511

The authors conclude that ubiquitous, and by inference primordial, proteins and functional systems show affinity to and functional requirement for K, Zn, Mn, and [PO 4] . Abiogenesis_sentence_512

Geochemical reconstruction shows that the ionic composition conducive to the origin of cells could not have existed in what we today call marine settings but is compatible with emissions of vapor-dominated zones of what we today call inland geothermal systems. Abiogenesis_sentence_513

Under the oxygen depleted, CO2-dominated primordial atmosphere, the chemistry of water condensates and exhalations near geothermal fields would resemble the internal milieu of modern cells. Abiogenesis_sentence_514

Therefore, the precellular stages of evolution may have taken place in shallow "Darwin ponds" lined with porous silicate minerals mixed with metal sulfides and enriched in K, Zn, and phosphorus compounds. Abiogenesis_sentence_515

Other abiogenesis scenarios Abiogenesis_section_75

We define a scenario as a set of related concepts pertinent to the origin of life that is or has been investigated. Abiogenesis_sentence_516

The concepts related to the Iron-Sulfur world can be considered as a scenario. Abiogenesis_sentence_517

We consider some other scenarios that may partially overlap with scenarios discussed above or with each other. Abiogenesis_sentence_518

Chemical pathways described by computer Abiogenesis_section_76

In September 2020, chemists described, for the first time, possible chemical pathways from nonliving prebiotic chemicals to complex biochemicals that could give rise to living organisms, based on a new computer program named ALLCHEMY. Abiogenesis_sentence_519

The hypercycle Abiogenesis_section_77

In the early 1970s, Manfred Eigen and Peter Schuster examined the transient stages between the molecular chaos and a self-replicating hypercycle in a prebiotic soup. Abiogenesis_sentence_520

In a hypercycle, the information storing system (possibly RNA) produces an enzyme, which catalyzes the formation of another information system, in sequence until the product of the last aids in the formation of the first information system. Abiogenesis_sentence_521

Mathematically treated, hypercycles could create quasispecies, which through natural selection entered into a form of Darwinian evolution. Abiogenesis_sentence_522

A boost to hypercycle theory was the discovery of ribozymes capable of catalyzing their own chemical reactions. Abiogenesis_sentence_523

The hypercycle theory requires the existence of complex biochemicals, such as nucleotides, which do not form under the conditions proposed by the Miller–Urey experiment. Abiogenesis_sentence_524

Organic pigments in dissipative structures Abiogenesis_section_78

In his "Thermodynamic Dissipation Theory of the Origin and Evolution of Life", Karo Michaelian has taken the insight of Boltzmann and the work of Prigogine to its ultimate consequences regarding the origin of life. Abiogenesis_sentence_525

This theory postulates that the hallmark of the origin and evolution of life is the microscopic dissipative structuring of organic pigments and their proliferation over the entire Earth surface. Abiogenesis_sentence_526

Present day life augments the entropy production of Earth in its solar environment by dissipating ultraviolet and visible photons into heat through organic pigments in water. Abiogenesis_sentence_527

This heat then catalyzes a host of secondary dissipative processes such as the water cycle, ocean and wind currents, hurricanes, etc. Michaelian argues that if the thermodynamic function of life today is to produce entropy through photon dissipation in organic pigments, then this probably was its function at its very beginnings. Abiogenesis_sentence_528

It turns out that both RNA and DNA when in water solution are very strong absorbers and extremely rapid dissipaters of ultraviolet light within the 230–290 nm wavelength (UV-C) region, which is a part of the Sun's spectrum that could have penetrated the prebiotic atmosphere. Abiogenesis_sentence_529

In fact, not only RNA and DNA, but many fundamental molecules of life (those common to all three domains of life) are also pigments that absorb in the UV-C, and many of these also have a chemical affinity to RNA and DNA. Abiogenesis_sentence_530

Nucleic acids may thus have acted as acceptor molecules to the UV-C photon antenna pigment donor molecules by providing an ultrafast channel for dissipation. Abiogenesis_sentence_531

Michaelian has shown using the formalism of non-linear irreversible thermodynamics that there would have existed during the Archean a thermodynamic imperative to the abiogenic UV-C photochemical synthesis and proliferation of these pigments over the entire Earth surface if they acted as catalysts to augment the dissipation of the solar photons. Abiogenesis_sentence_532

By the end of the Archean, with life-induced ozone dissipating UV-C light in the Earth's upper atmosphere, it would have become ever more improbable for a completely new life to emerge that did not rely on the complex metabolic pathways already existing since now the free energy in the photons arriving at Earth's surface would have been insufficient for direct breaking and remaking of covalent bonds. Abiogenesis_sentence_533

It has been suggested, however, that such changes in the surface flux of ultraviolet radiation due to geophysical events affecting the atmosphere could have been what promoted the development of complexity in life based on existing metabolic pathways, for example during the Cambrian explosion Abiogenesis_sentence_534

Some of the most difficult problems concerning the origin of life, such as enzyme-less replication of RNA and DNA, homochirality of the fundamental molecules, and the origin of information encoding in RNA and DNA, also find an explanation within the same dissipative thermodynamic framework by considering the probable existence of a relation between primordial replication and UV-C photon dissipation. Abiogenesis_sentence_535

Michaelian suggests that it is erroneous to expect to describe the emergence, proliferation, or even evolution, of life without overwhelming reference to entropy production through the dissipation of a generalized thermodynamic potential, in particular, the prevailing solar photon flux. Abiogenesis_sentence_536

Protein amyloid Abiogenesis_section_79

A new origin-of-life theory based on self-replicating beta-sheet structures has been put forward by Maury in 2009. Abiogenesis_sentence_537

The theory suggest that self-replicating and self-assembling catalytic amyloids were the first informational polymers in a primitive pre-RNA world. Abiogenesis_sentence_538

The main arguments for the amyloid hypothesis is based on the structural stability, autocatalytic and catalytic properties, and evolvability of beta-sheet based informational systems. Abiogenesis_sentence_539

Such systems are also error correcting and chiroselective. Abiogenesis_sentence_540

Fluctuating salinity: dilute and dry-down Abiogenesis_section_80

Theories of abiogenesis seldom address the caveat raised by Harold Blum: if the key informational elements of life – proto-nucleic acid chains – spontaneously form duplex structures, then there is no way to dissociate them. Abiogenesis_sentence_541

The Oparin–Haldane conjecture addresses the formation, but not the dissociation, of nucleic acid polymers and duplexes. Abiogenesis_sentence_542

However, nucleic acids are unusual because, in the absence of counterions (low salt) to neutralize the high charges on opposing phosphate groups, the nucleic acid duplex dissociates into single chains. Abiogenesis_sentence_543

Early tides, driven by a close moon, could have generated rapid cycles of dilution (high tide, low salt) and concentration (dry-down at low tide, high salt) that exclusively promoted the replication of nucleic acids through a process dubbed tidal chain reaction (TCR). Abiogenesis_sentence_544

This theory has been criticized on the grounds that early tides may not have been so rapid, although regression from current values requires an Earth–Moon juxtaposition at around two Ga, for which there is no evidence, and early tides may have been approximately every seven hours. Abiogenesis_sentence_545

Another critique is that only 2–3% of the Earth's crust may have been exposed above the sea until late in terrestrial evolution. Abiogenesis_sentence_546

The TCR (tidal chain reaction) theory has mechanistic advantages over thermal association/dissociation at deep-sea vents because TCR requires that chain assembly (template-driven polymerization) takes place during the dry-down phase, when precursors are most concentrated, whereas thermal cycling needs polymerization to take place during the cold phase, when the rate of chain assembly is lowest and precursors are likely to be more dilute. Abiogenesis_sentence_547

A first protein that condenses substrates during thermal cycling: thermosynthesis Abiogenesis_section_81

Emergence of chemiosmotic machinery Today's bioenergetic process of fermentation is carried out by either the aforementioned citric acid cycle or the Acetyl-CoA pathway, both of which have been connected to the primordial Iron–sulfur world. Abiogenesis_sentence_548

In a different approach, the thermosynthesis hypothesis considers the bioenergetic process of chemiosmosis, which plays an essential role in cellular respiration and photosynthesis, more basal than fermentation: the ATP synthase enzyme, which sustains chemiosmosis, is proposed as the currently extant enzyme most closely related to the first metabolic process. Abiogenesis_sentence_549

First life needed an energy source to bring about the condensation reaction that yielded the peptide bonds of proteins and the phosphodiester bonds of RNA. Abiogenesis_sentence_550

In a generalization and thermal variation of the binding change mechanism of today's ATP synthase, the "first protein" would have bound substrates (peptides, phosphate, nucleosides, RNA 'monomers') and condensed them to a reaction product that remained bound until it was released after a temperature change by a thermal unfolding. Abiogenesis_sentence_551

The primordial first protein would therefore have strongly resembled the beta subunits of the ATP synthase alpha/beta subunits of today's F1 moiety in the FoF1 ATP synthase. Abiogenesis_sentence_552

Note however that today's enzymes function during isothermal conditions, whereas the hypothetical first protein worked on and during thermal cycling. Abiogenesis_sentence_553

The energy source under the thermosynthesis hypothesis was thermal cycling, the result of suspension of protocells in a convection current, as is plausible in a volcanic hot spring; the convection accounts for the self-organization and dissipative structure required in any origin of life model. Abiogenesis_sentence_554

The still ubiquitous role of thermal cycling in germination and cell division is considered a relic of primordial thermosynthesis. Abiogenesis_sentence_555

By phosphorylating cell membrane lipids, this first protein gave a selective advantage to the lipid protocell that contained the protein. Abiogenesis_sentence_556

This protein also synthesized a library of many proteins, of which only a minute fraction had thermosynthesis capabilities. Abiogenesis_sentence_557

As proposed by Dyson, it propagated functionally: it made daughters with similar capabilities, but it did not copy itself. Abiogenesis_sentence_558

Functioning daughters consisted of different amino acid sequences. Abiogenesis_sentence_559

Whereas the iron–sulfur world identifies a circular pathway as the most simple, the thermosynthesis hypothesis does not even invoke a pathway: ATP synthase's binding change mechanism resembles a physical adsorption process that yields free energy, rather than a regular enzyme's mechanism, which decreases the free energy. Abiogenesis_sentence_560

The described first protein may be simple in the sense that is requires only a short sequence of conserved amino acid residues, a sequent sufficient for the appropriate catalytic cleft. Abiogenesis_sentence_561

In contrast, it has been claimed that the emergence of cyclic systems of protein catalysts such as required by fermentation is implausible because of the length of many required sequences. Abiogenesis_sentence_562

Pre-RNA world: The ribose issue and its bypass Abiogenesis_section_82

It is possible that a different type of nucleic acid, such as peptide nucleic acid, threose nucleic acid or glycol nucleic acid, was the first to emerge as a self-reproducing molecule, only later replaced by RNA. Abiogenesis_sentence_563

Larralde et al., say that Abiogenesis_sentence_564

and they conclude that their Abiogenesis_sentence_565

The ester linkage of ribose and phosphoric acid in RNA is known to be prone to hydrolysis. Abiogenesis_sentence_566

Pyrimidine ribonucleosides and their respective nucleotides have been prebiotically synthesized by a sequence of reactions which by-pass the free sugars, and are assembled in a stepwise fashion by using nitrogenous or oxygenous chemistries. Abiogenesis_sentence_567

Sutherland has demonstrated high yielding routes to cytidine and uridine ribonucleotides built from small 2 and 3 carbon fragments such as glycolaldehyde, glyceraldehyde or glyceraldehyde-3-phosphate, cyanamide and cyanoacetylene. Abiogenesis_sentence_568

One of the steps in this sequence allows the isolation of enantiopure ribose aminooxazoline if the enantiomeric excess of glyceraldehyde is 60% or greater. Abiogenesis_sentence_569

This can be viewed as a prebiotic purification step, where the said compound spontaneously crystallized out from a mixture of the other pentose aminooxazolines. Abiogenesis_sentence_570

Ribose aminooxazoline can then react with cyanoacetylene in a mild and highly efficient manner to give the alpha cytidine ribonucleotide. Abiogenesis_sentence_571

Photoanomerization with UV light allows for inversion about the 1' anomeric centre to give the correct beta stereochemistry. Abiogenesis_sentence_572

In 2009 they showed that the same simple building blocks allow access, via phosphate controlled nucleobase elaboration, to 2',3'-cyclic pyrimidine nucleotides directly, which are known to be able to polymerize into RNA. Abiogenesis_sentence_573

This paper also highlights the possibility for the photo-sanitization of the pyrimidine-2',3'-cyclic phosphates. Abiogenesis_sentence_574

RNA structures Abiogenesis_section_83

While features of self-organization and self-replication are often considered the hallmark of living systems, there are many instances of abiotic molecules exhibiting such characteristics under proper conditions. Abiogenesis_sentence_575

Stan Palasek suggested based on a theoretical model that self-assembly of ribonucleic acid (RNA) molecules can occur spontaneously due to physical factors in hydrothermal vents. Abiogenesis_sentence_576

Virus self-assembly within host cells has implications for the study of the origin of life, as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules. Abiogenesis_sentence_577

Viral origin Abiogenesis_section_84

Recent evidence for a "virus first" hypothesis, which may support theories of the RNA world, has been suggested. Abiogenesis_sentence_578

One of the difficulties for the study of the origins of viruses is their high rate of mutation; this is particularly the case in RNA retroviruses like HIV. Abiogenesis_sentence_579

A 2015 study compared protein fold structures across different branches of the tree of life, where researchers can reconstruct the evolutionary histories of the folds and of the organisms whose genomes code for those folds. Abiogenesis_sentence_580

They argue that protein folds are better markers of ancient events as their three-dimensional structures can be maintained even as the sequences that code for those begin to change. Abiogenesis_sentence_581

Thus, the viral protein repertoire retain traces of ancient evolutionary history that can be recovered using advanced bioinformatics approaches. Abiogenesis_sentence_582

Those researchers think that "the prolonged pressure of genome and particle size reduction eventually reduced virocells into modern viruses (identified by the complete loss of cellular makeup), meanwhile other coexisting cellular lineages diversified into modern cells." Abiogenesis_sentence_583

The data suggest that viruses originated from ancient cells that co-existed with the ancestors of modern cells. Abiogenesis_sentence_584

These ancient cells likely contained segmented RNA genomes. Abiogenesis_sentence_585

A computational model (2015) has shown that virus capsids may have originated in the RNA world and that they served as a means of horizontal transfer between replicator communities since these communities could not survive if the number of gene parasites increased, with certain genes being responsible for the formation of these structures and those that favored the survival of self-replicating communities. Abiogenesis_sentence_586

The displacement of these ancestral genes between cellular organisms could favor the appearance of new viruses during evolution. Abiogenesis_sentence_587

Viruses retain a replication module inherited from the prebiotic stage since it is absent in cells. Abiogenesis_sentence_588

So this is evidence that viruses could originate from the RNA world and could also emerge several times in evolution through genetic escape in cells. Abiogenesis_sentence_589

RNA world Abiogenesis_section_85

Main article: RNA world Abiogenesis_sentence_590

A number of hypotheses of formation of RNA have been put forward. Abiogenesis_sentence_591

As of 1994, there were difficulties in the explanation of the abiotic synthesis of the nucleotides cytosine and uracil. Abiogenesis_sentence_592

Subsequent research has shown possible routes of synthesis; for example, formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals. Abiogenesis_sentence_593

Early cell membranes could have formed spontaneously from proteinoids, which are protein-like molecules produced when amino acid solutions are heated while in the correct concentration of aqueous solution. Abiogenesis_sentence_594

These are seen to form micro-spheres which are observed to behave similarly to membrane-enclosed compartments. Abiogenesis_sentence_595

Other possible means of producing more complicated organic molecules include chemical reactions that take place on clay substrates or on the surface of the mineral pyrite. Abiogenesis_sentence_596

Factors supporting an important role for RNA in early life include its ability to act both to store information and to catalyze chemical reactions (as a ribozyme); its many important roles as an intermediate in the expression of and maintenance of the genetic information (in the form of DNA) in modern organisms; and the ease of chemical synthesis of at least the components of the RNA molecule under the conditions that approximated the early Earth. Abiogenesis_sentence_597

Relatively short RNA molecules have been synthesized, capable of replication. Abiogenesis_sentence_598

Such replicase RNA, which functions as both code and catalyst provides its own template upon which copying can occur. Abiogenesis_sentence_599

Szostak has shown that certain catalytic RNAs can join smaller RNA sequences together, creating the potential for self-replication. Abiogenesis_sentence_600

If these conditions were present, Darwinian natural selection would favour the proliferation of such autocatalytic sets, to which further functionalities could be added. Abiogenesis_sentence_601

Such autocatalytic systems of RNA capable of self-sustained replication have been identified. Abiogenesis_sentence_602

The RNA replication systems, which include two ribozymes that catalyze each other's synthesis, showed a doubling time of the product of about one hour, and were subject to natural selection under the conditions that existed in the experiment. Abiogenesis_sentence_603

In evolutionary competition experiments, this led to the emergence of new systems which replicated more efficiently. Abiogenesis_sentence_604

This was the first demonstration of evolutionary adaptation occurring in a molecular genetic system. Abiogenesis_sentence_605

Depending on the definition, life started when RNA chains began to self-replicate, initiating the three mechanisms of Darwinian selection: heritability, variation of type, and differential reproductive output. Abiogenesis_sentence_606

The fitness of an RNA replicator (its per capita rate of increase) would likely be a function of its intrinsic adaptive capacities, determined by its nucleotide sequence, and the availability of resources. Abiogenesis_sentence_607

The three primary adaptive capacities may have been: (1) replication with moderate fidelity, giving rise to both heritability while allowing variation of type, (2) resistance to decay, and (3) acquisition of process resources. Abiogenesis_sentence_608

These capacities would have functioned by means of the folded configurations of the RNA replicators resulting from their nucleotide sequences. Abiogenesis_sentence_609

Experiments on the origin of life Abiogenesis_section_86

Both Eigen and Sol Spiegelman demonstrated that evolution, including replication, variation, and natural selection, can occur in populations of molecules as well as in organisms. Abiogenesis_sentence_610

Following on from chemical evolution came the initiation of biological evolution, which led to the first cells. Abiogenesis_sentence_611

No one has yet synthesized a "protocell" using simple components with the necessary properties of life (the so-called "bottom-up-approach"). Abiogenesis_sentence_612

Without such a proof-of-principle, explanations have tended to focus on chemosynthesis. Abiogenesis_sentence_613

However, some researchers work in this field, notably Steen Rasmussen and Szostak. Abiogenesis_sentence_614

Others have argued that a "top-down approach" is more feasible, starting with simple forms of current life. Abiogenesis_sentence_615

Spiegelman took advantage of natural selection to synthesize the Spiegelman Monster, which had a genome with just 218 nucleotide bases, having deconstructively evolved from a 4500-base bacterial RNA. Abiogenesis_sentence_616

Eigen built on Spiegelman's work and produced a similar system further degraded to just 48 or 54 nucleotides—the minimum required for the binding of the replication enzyme. Abiogenesis_sentence_617

Craig Venter and others at J. Abiogenesis_sentence_618 Craig Venter Institute engineered existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life are reached. Abiogenesis_sentence_619

In October 2018, researchers at McMaster University announced the development of a new technology, called a Planet Simulator, to help study the origin of life on planet Earth and beyond. Abiogenesis_sentence_620

It consists of a sophisticated climate chamber to study how the building blocks of life were assembled and how these prebiotic molecules transitioned into self-replicating RNA molecules. Abiogenesis_sentence_621

See also Abiogenesis_section_87

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Abiogenesis.