Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O 2 ). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain.
89-644: The Oparin/Urey Medal honours important contributions to the field of origins of life . The medal is awarded by the International Society for the Study of the Origin of Life ( ISSOL ). The award was originally named for Alexander Ivanovich Oparin , one of the pioneers in researching the origins of life. In 1993, the Society decided to alternate the name of the award so as to also honour the memory of Harold C. Urey , one of
178-572: A biogenic origin. Parts of the Dresser formation preserve hot springs on land, but other regions seem to have been shallow seas. A molecular clock analysis suggests the LUCA emerged prior to the Late Heavy Bombardment (3.9 Gya). All chemical elements except for hydrogen and helium derive from stellar nucleosynthesis. The basic chemical ingredients of life – the carbon-hydrogen molecule (CH),
267-443: A boundary is needed to separate ordered life processes from chaotic non-living matter. Irene Chen and Jack W. Szostak suggest that elementary protocells can give rise to cellular behaviors including primitive forms of differential reproduction, competition, and energy storage. Competition for membrane molecules would favor stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids and even
356-410: A cell's primary system of energy conversion. The mechanism, now ubiquitous in living cells, powers energy conversion in micro-organisms and in the mitochondria of eukaryotes, making it a likely candidate for early life. Mitochondria produce adenosine triphosphate (ATP), the energy currency of the cell used to drive cellular processes such as chemical syntheses. The mechanism of ATP synthesis involves
445-468: A closed membrane in which the ATP synthase enzyme is embedded. The energy required to release strongly bound ATP has its origin in protons that move across the membrane. In modern cells, those proton movements are caused by the pumping of ions across the membrane, maintaining an electrochemical gradient. In the first organisms, the gradient could have been provided by the difference in chemical composition between
534-525: A diverse array of amino acids. Later work has focused on two other potential reducing environments: outer space and deep-sea hydrothermal vents. 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. These elements gradually accreted and began orbiting in disks of gas and dust. Gravitational accretion of material at
623-461: A habitable world is formed with a supply of minerals and liquid water. Prebiotic synthesis creates a range of simple organic compounds, which are assembled into polymers such as proteins and RNA. On the other side, the process after the LUCA is readily understood: biological evolution caused the development of a wide range of species with varied forms and biochemical capabilities. However, the derivation of living things such as LUCA from simple components
712-410: A laboratory setting. Self-assembled vesicles are essential components of primitive cells. The theory of classical irreversible thermodynamics treats self-assembly under a generalized chemical potential within the framework of dissipative systems . The second law of thermodynamics requires that overall entropy increases, yet life is distinguished by its great degree of organization. Therefore,
801-463: A likely constituent of Earth's primordial sea. PAHs have been detected in nebulae , and in the interstellar medium , in comets, and in meteorites. The PAH world hypothesis posits PAHs as precursors to the RNA world. A star, HH 46-IR, resembling the sun early in its life, is surrounded by a disk of material which contains molecules including cyanide compounds, hydrocarbons , and carbon monoxide. PAHs in
890-584: A living organism creates order in some places (e.g. its living body) at the expense of an increase of entropy elsewhere (e.g. heat and waste production). Multiple sources of energy were available for chemical reactions on the early Earth. Heat from geothermal processes is a standard energy source for chemistry. Other examples include sunlight, lightning, atmospheric entries of micro-meteorites, and implosion of bubbles in sea and ocean waves. This has been confirmed by experiments and simulations. Unfavorable reactions can be driven by highly favorable ones, as in
979-439: A membrane site and a specific compound trapped in the vesicle. Such site/compound pairs are transmissible to the daughter vesicles leading to the emergence of distinct lineages of vesicles, which would have allowed natural selection . A protocell is a self-organized, self-ordered, spherical collection of lipids proposed as a stepping-stone to the origin of life. A functional protocell has (as of 2014) not yet been achieved in
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#17330858476331068-837: A number of possible routes. Some center on high temperature/concentration conditions in which condensation becomes energetically favorable, while others focus on the availability of plausible prebiotic condensing agents. Experimental evidence for the formation of peptides in uniquely concentrated environments is bolstered by work suggesting that wet-dry cycles and the presence of specific salts can greatly increase spontaneous condensation of glycine into poly-glycine chains. Other work suggests that while mineral surfaces, such as those of pyrite, calcite, and rutile catalyze peptide condensation, they also catalyze their hydrolysis. The authors suggest that additional chemical activation or coupling would be necessary to produce peptides at sufficient concentrations. Thus, mineral surface catalysis, while important,
1157-464: A proton gradient) across a membrane. This results in an electrical potential or ion concentration difference across the membrane. The reduced chemical compounds are oxidized by a series of respiratory integral membrane proteins with sequentially increasing reduction potentials , with the final electron acceptor being oxygen (in aerobic respiration ) or another chemical substance (in anaerobic respiration). A proton motive force drives protons down
1246-423: A replicator molecule. Possible precursors to protein synthesis include the synthesis of short peptide cofactors or the self-catalysing duplication of RNA. It is likely that the ancestral ribosome was composed entirely of RNA, although some roles have since been taken over by proteins. Major remaining questions on this topic include identifying the selective force for the evolution of the ribosome and determining how
1335-473: A sapphire substrate with a web of thin cracks under a heat flow, similar to the environment of deep-ocean vents , as a mechanism to separate and concentrate prebiotically relevant building blocks from a dilute mixture, purifying their concentration by up to three orders of magnitude. The authors propose this as a plausible model for the origin of complex biopolymers. This presents another physical process that allows for concentrated peptide precursors to combine in
1424-418: A sugar molecule and RNA precursor, has been detected in regions of space including around protostars and on meteorites. As early as the 1860s, experiments demonstrated that biologically relevant molecules can be produced from interaction of simple carbon sources with abundant inorganic catalysts. The spontaneous formation of complex polymers from abiotically generated monomers under the conditions posited by
1513-826: A synthesis of many sciences. Life functions through the specialized chemistry of carbon and water, and builds largely upon four key families of chemicals: lipids for cell membranes, carbohydrates such as sugars, amino acids for protein metabolism, and nucleic acid DNA and RNA for the mechanisms of heredity. Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules. Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Researchers generally think that current life descends from an RNA world , although other self-replicating and self-catalyzing molecules may have preceded RNA. Other approaches ( "metabolism-first" hypotheses ) focus on understanding how catalysis in chemical systems on
1602-470: A turbulent atmosphere, and a hydrosphere subject to intense ultraviolet light from a T Tauri stage Sun , from cosmic radiation , and from continued asteroid and comet impacts. Despite all this, niche environments likely existed conducive to life on Earth in the Late-Hadean to Early-Archaean. The Late Heavy Bombardment hypothesis posits that a period of intense impact occurred at ~3.9 Gya during
1691-410: A urea solution to freeze-thaw cycles under a reductive atmosphere, with spark discharges as an energy source. The explanation given for the unusual speed of these reactions at such a low temperature is eutectic freezing , which crowds impurities in microscopic pockets of liquid within the ice, causing the molecules to collide more often. Prebiotic peptide synthesis is proposed to have occurred through
1780-410: Is methanogenesis , a form of carbon-dioxide respiration, that is used to produce methane gas by anaerobic digestion . Biogenic methane is a sustainable alternative to fossil fuels. However, uncontrolled methanogenesis in landfill sites releases large amounts of methane into the atmosphere, acting as a potent greenhouse gas . Sulfate respiration produces hydrogen sulfide , which is responsible for
1869-606: Is a critical component of the global nitrogen , iron , sulfur , and carbon cycles through the reduction of the oxyanions of nitrogen, sulfur, and carbon to more-reduced compounds. The biogeochemical cycling of these compounds, which depends upon anaerobic respiration, significantly impacts the carbon cycle and global warming . Anaerobic respiration occurs in many environments, including freshwater and marine sediments, soil, subsurface aquifers, deep subsurface environments, and biofilms. Even environments that contain oxygen, such as soil, have micro-environments that lack oxygen due to
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#17330858476331958-456: Is available, an explanation is needed for why the set used is so small. Formamide is attractive as a medium that potentially provided a source of amino acid derivatives from simple aldehyde and nitrile feedstocks. Alexander Butlerov showed in 1861 that the formose reaction created sugars including tetroses, pentoses, and hexoses when formaldehyde is heated under basic conditions with divalent metal ions like calcium. R. Breslow proposed that
2047-493: 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. Polycyclic aromatic hydrocarbons (PAH) are the most common and abundant polyatomic molecules in the observable universe , and are a major store of carbon. They seem to have formed shortly after the Big Bang, and are associated with new stars and exoplanets . They are
2136-451: Is far from understood. Although Earth remains the only place where life is known, the science of astrobiology seeks evidence of life on other planets. The 2015 NASA strategy on the origin of life aimed to solve the puzzle by identifying interactions, intermediary structures and functions, energy sources, and environmental factors that contributed to the diversity, selection, and replication of evolvable macromolecular systems, and mapping
2225-405: Is not sufficient alone for peptide synthesis. Many prebiotically plausible condensing/activating agents have been identified, including the following: cyanamide, dicyanamide, dicyandiamide, diaminomaleonitrile, urea, trimetaphosphate, NaCl, CuCl 2, (Ni,Fe)S, CO, carbonyl sulfide (COS), carbon disulfide (CS 2 ) , SO 2, and diammonium phosphate (DAP). An experiment reported in 2024 used
2314-714: Is poisonous only to aerobic organisms ( eukaryotes and aerobic bacteria), which did not yet exist. It can play roles in other chemical processes such as the synthesis of the amino acid glycine. DNA and RNA components including uracil, cytosine and thymine can be synthesized under outer space conditions, using starting chemicals such as pyrimidine found in meteorites. Pyrimidine may have been formed in red giant stars or in interstellar dust and gas clouds. All four RNA-bases may be synthesized from formamide in high-energy density events like extraterrestrial impacts. Other pathways for synthesizing bases from inorganic materials have been reported. Freezing temperatures are advantageous for
2403-460: Is released. Therefore, anaerobic respiration is less efficient than aerobic. Anaerobic cellular respiration and fermentation generate ATP in very different ways, and the terms should not be treated as synonyms. Cellular respiration (both aerobic and anaerobic) uses highly reduced chemical compounds such as NADH and FADH 2 (for example produced during glycolysis and the citric acid cycle ) to establish an electrochemical gradient (often
2492-399: Is that life is not required to have formed on each planet it occurs on, but rather in a more limited set of locations, or even a single location, and then spread about the galaxy to other star systems via cometary or meteorite impact. Panspermia did not get much scientific support because it was largely used to deflect the need of an answer instead of explaining observable phenomena. Although
2581-462: Is to explain how such a complex and tightly interlinked system could develop by evolutionary steps, as at first sight all its parts are necessary to enable it to function. For example, a cell, whether the LUCA or in a modern organism, copies its DNA with the DNA polymerase enzyme, which is in turn produced by translating the DNA polymerase gene in the DNA. Neither the enzyme nor the DNA can be produced without
2670-465: Is universal today. That in turn implies a suite of cellular machinery including messenger RNA , transfer RNA , and ribosomes to translate the code into proteins . Those proteins included enzymes to operate its anaerobic respiration via the Wood–Ljungdahl metabolic pathway , and a DNA polymerase to replicate its genetic material. The challenge for abiogenesis (origin of life) researchers
2759-503: Is unknown. Minimum age estimates are based on evidence from the geologic rock record . The earliest physical evidence of life so far found consists of microbialites in the Nuvvuagittuq Greenstone Belt of Northern Quebec, in banded iron formation rocks at least 3.77 and possibly as old as 4.32 Gya. The micro-organisms lived within hydrothermal vent precipitates, soon after the 4.4 Gya formation of oceans during
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2848-589: The Earth is 4.54 Gya as found by radiometric dating of calcium-aluminium-rich inclusions in carbonaceous chrondrite meteorites, the oldest material in the Solar System. The Hadean Earth (from its formation until 4 Gya) was at first inhospitable to any living organisms. During its formation, the Earth lost a significant part of its initial mass, and consequentially lacked the gravity to hold molecular hydrogen and
2937-409: The Earth's crust , pressure cycling leads to the periodic formation of vesicles. Under the same conditions, random peptide chains are being formed, which are being continuously selected for their ability to integrate into the vesicle membrane. A further selection of the vesicles for their stability potentially leads to the development of functional peptide structures, associated with an increase in
3026-506: The Hadean . Life consists of reproduction with (heritable) variations. NASA defines life as "a self-sustaining chemical system capable of Darwinian [i.e., biological] evolution ." Such a system is complex; the last universal common ancestor (LUCA), presumably a single-celled organism which lived some 4 billion years ago, already had hundreds of genes encoded in the DNA genetic code that
3115-606: The Isua supracrustal belt in southwestern Greenland, dating to 3.7 Gya, have shown biogenic carbon isotopes . 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. In the Pilbara region of Western Australia, compelling evidence of early life
3204-467: 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 twice the mass of the sun. As these stars reached the end of their lifecycles , they ejected these heavier elements, among them carbon and oxygen, throughout the universe. These heavier elements allowed for the formation of new objects, including rocky planets and other bodies. According to
3293-402: The last universal common ancestor of all modern organisms (LUCA) is thought to have been quite different from the origin of life, investigations into LUCA can guide research into early universal characteristics. A genomics approach has sought to characterise LUCA by identifying the genes shared by Archaea and Bacteria , members of the two major branches of life (with Eukaryotes included in
3382-507: 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 . 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. The age of
3471-453: The phospholipids of today. Such micro-encapsulation would allow for metabolism within the membrane and the exchange of small molecules, while retaining large biomolecules inside. Such a membrane is needed for a cell to create its own electrochemical gradient to store energy by pumping ions across the membrane. Fatty acid vesicles in conditions relevant to alkaline hydrothermal vents can be stabilized by isoprenoids which are synthesized by
3560-471: The "soup" theory is not straightforward. 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. Biology uses essentially 20 amino acids for its coded protein enzymes, representing a very small subset of the structurally possible products. Since life tends to use whatever
3649-472: The Earth may have been a predominantly water world between 4.4 and 4.3 Gya. It is debated whether or not crust was exposed above this ocean due to uncertainties of what early plate tectonics looked like. For early life to have developed, it is generally thought that a land setting is required, so this question is essential to determining when in Earth's history life evolved. The post-Moon-forming impact Earth likely existed with little if any continental crust,
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3738-551: The Hadean. A cataclysmic impact event would have had the potential to sterilise all life on Earth by volatilising liquid oceans and blocking the Sun needed for photosynthesising primary producers, pushing back the earliest possible emergence of life to after Late Heavy Bombardment. Recent research questions both the intensity of the Late Heavy Bombardment as well as its potential for sterilisation. Uncertainties as to whether Late Heavy Bombardment
3827-408: The Hadean. The microbes resembled modern hydrothermal vent bacteria, supporting the view that abiogenesis began in such an environment. Biogenic graphite has been found in 3.7 Gya metasedimentary rocks from southwestern Greenland and in microbial mat fossils from 3.49 Gya cherts in the Pilbara region of Western Australia . Evidence of early life in rocks from Akilia Island, near
3916-529: The Miller–Urey experiment that "it is not enough to explain the formation of such molecules, what is necessary, is a physical-chemical explanation of the origins of these molecules that suggests the presence of suitable sources and sinks for free energy." However, current scientific consensus describes the primitive atmosphere as weakly reducing or neutral, diminishing the amount and variety of amino acids that could be produced. The addition of iron and carbonate minerals, present in early oceans, however, produces
4005-570: The Moon-forming impact. This scenario has found support from the dating of 4.404 Gya zircon crystals with high δ O values from metamorphosed quartzite of Mount Narryer in Western Australia. The Hadean atmosphere has been characterized as a "gigantic, productive outdoor chemical laboratory," similar to volcanic gases today which still support some abiotic chemistry. Despite the likely increased volcanism from early plate tectonics,
4094-543: The Murchison meteorite suggest that the RNA component uracil and related molecules, including xanthine , were formed extraterrestrially. NASA studies of meteorites suggest that all four DNA nucleobases (adenine, guanine and related organic molecules) have been formed in outer space. The cosmic dust permeating the universe contains complex organics ("amorphous organic solids with a mixed aromatic – aliphatic structure") that could be created rapidly by stars. Glycolaldehyde ,
4183-450: The archaean branch in the two-domain system ). It appears there are 60 proteins common to all life and 355 prokaryotic genes that trace to LUCA; their functions imply that the LUCA was anaerobic with the Wood–Ljungdahl pathway , deriving energy by chemiosmosis , and maintaining its hereditary material with DNA, the genetic code , and ribosomes . Although the LUCA lived over 4 billion years ago (4 Gya ), researchers believe it
4272-480: The bulk of the original inert gases. Soon after initial accretion of Earth at 4.48 Ga, its collision with Theia , a hypothesised impactor, is thought to have created the ejected debris that would eventually form the Moon. This impact would have removed the Earth's primary atmosphere, leaving behind clouds of viscous silicates and carbon dioxide. This unstable atmosphere was short-lived and condensed shortly after to form
4361-401: The bulk silicate Earth, leaving behind an atmosphere largely consisting of water vapor, nitrogen , and carbon dioxide , with smaller amounts of carbon monoxide , hydrogen, and sulfur compounds. The solution of carbon dioxide in water is thought to have made the seas slightly acidic , with a pH of about 5.5. Condensation to form liquid oceans is theorised to have occurred as early as
4450-573: The carbon-hydrogen positive ion (CH+) and the carbon ion (C+) – were produced by ultraviolet light from stars. Complex molecules, including organic molecules, form naturally both in space and on planets. Organic molecules on the early Earth could have had either terrestrial origins, with organic molecule synthesis driven by impact shocks or by other energy sources, such as ultraviolet light, redox coupling, or electrical discharges; or extraterrestrial origins ( pseudo-panspermia ), with organic molecules formed in interstellar dust clouds raining down on to
4539-436: The case of iron-sulfur chemistry. For example, this was probably important for carbon fixation . Carbon fixation by reaction of CO 2 with H 2 S via iron-sulfur chemistry is favorable, and occurs at neutral pH and 100 °C. Iron-sulfur surfaces, which are abundant near hydrothermal vents, can drive the production of small amounts of amino acids and other biomolecules. In 1961, Peter Mitchell proposed chemiosmosis as
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#17330858476334628-1089: The characteristic 'rotten egg' smell of coastal wetlands and has the capacity to precipitate heavy metal ions from solution, leading to the deposition of sulfidic metal ores . Dissimilatory denitrification is widely used in the removal of nitrate and nitrite from municipal wastewater. An excess of nitrate can lead to eutrophication of waterways into which treated water is released. Elevated nitrite levels in drinking water can lead to problems due to its toxicity. Denitrification converts both compounds into harmless nitrogen gas. Specific types of anaerobic respiration are also critical in bioremediation , which uses microorganisms to convert toxic chemicals into less-harmful molecules to clean up contaminated beaches, aquifers, lakes, and oceans. For example, toxic arsenate or selenate can be reduced to less toxic compounds by various anaerobic bacteria via anaerobic respiration. The reduction of chlorinated chemical pollutants , such as vinyl chloride and carbon tetrachloride , also occurs through anaerobic respiration. Anaerobic respiration
4717-448: The chemical landscape of potential primordial informational polymers . The advent of polymers that could replicate, store genetic information, and exhibit properties subject to selection was, it suggested, most likely a critical step in the emergence of prebiotic chemical evolution. Those polymers derived, in turn, from simple organic compounds such as nucleobases , amino acids , and sugars that could have been formed by reactions in
4806-522: The early Earth might have provided the precursor molecules necessary for self-replication. The classic 1952 Miller–Urey experiment demonstrated that most amino acids, the chemical constituents of proteins , can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth . External sources of energy may have triggered these reactions, including lightning , radiation , atmospheric entries of micro-meteorites and implosion of bubbles in sea and ocean waves. While
4895-438: The early Earth, which were very different from those that prevail today. The structure of the ribosome has been called the "smoking gun", with a central core of RNA and no amino acid side chains within 18 Å of the active site that catalyzes peptide bond formation. The concept of the RNA world was proposed in 1962 by Alexander Rich , and the term was coined by Walter Gilbert in 1986. There were initial difficulties in
4984-505: The environment. A successful theory of the origin of life must explain how all these chemicals came into being. One ancient view of the origin of life, from Aristotle until the 19th century, is of spontaneous generation . This theory held that "lower" animals such as insects were generated by decaying organic substances, and that life arose by chance. This was questioned from the 17th century, in works like Thomas Browne 's Pseudodoxia Epidemica . In 1665, Robert Hooke published
5073-441: The evolution and preservation of polymers like RNA that store information. Only one or two types of amphiphiles have been studied which might have led to the development of vesicles. There is an enormous number of possible arrangements of lipid bilayer membranes, and those with the best reproductive characteristics would have converged toward a hypercycle reaction, a positive feedback composed of two mutual catalysts represented by
5162-513: The explanation of the abiotic synthesis of the nucleotides cytosine and uracil. 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. RNA replicase can function as both code and catalyst for further RNA replication, i.e. it can be autocatalytic. Jack Szostak has shown that certain catalytic RNAs can join smaller RNA sequences together, creating
5251-492: The first drawings of a microorganism . In 1676, Antonie van Leeuwenhoek drew and described microorganisms, probably protozoa and bacteria . Van Leeuwenhoek disagreed with spontaneous generation, and by the 1680s convinced himself, using experiments ranging from sealed and open meat incubation and the close study of insect reproduction, that the theory was incorrect. In 1668 Francesco Redi showed that no maggots appeared in meat when flies were prevented from laying eggs. By
5340-511: The first molecules constituting the earliest cells slowly self-organized from a primordial soup , and this theory is called the Oparin–Haldane hypothesis . Haldane suggested that the Earth's prebiotic oceans consisted of a "hot dilute soup" in which organic compounds could have formed. J. D. Bernal showed that such mechanisms could form most of the necessary molecules for life from inorganic precursors. In 1967, he suggested three "stages":
5429-417: The first to exist. Another model echoes Darwin's "warm little pond" with cycles of wetting and drying. RNA is central to the translation process. Small RNAs can catalyze all the chemical groups and information transfers required for life. RNA both expresses and maintains genetic information in modern organisms; and the chemical components of RNA are easily synthesized under the conditions that approximated
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#17330858476335518-430: The first to propose the study of cosmochemistry . The current list of medalists is shown below: Origins of life Abiogenesis is the natural process by which life arises from non-living matter , such as simple organic compounds . The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single event, but a process of increasing complexity involving
5607-521: The flow from a hydrothermal vent and the surrounding seawater, or perhaps meteoric quinones that were conducive to the development of chemiosmotic energy across lipid membranes if at a terrestrial origin. The RNA world hypothesis describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins. Many researchers concur that an RNA world must have preceded the DNA-based life that now dominates. However, RNA-based life may not have been
5696-609: The following generation, thus initiating the evolution of life. The lipid world theory postulates that the first self-replicating object was lipid -like. Phospholipids form lipid bilayers in water while under agitation—the same structure as in cell membranes. These molecules were not present on early Earth, but other amphiphilic long-chain molecules also form membranes. These bodies may expand by insertion of additional lipids, and may spontaneously split into two offspring of similar size and composition. Lipid bodies may have provided sheltering envelopes for information storage, allowing
5785-592: The formation of a habitable planet , the prebiotic synthesis of organic molecules, molecular self-replication , self-assembly , autocatalysis , and the emergence of cell membranes . The transition from non-life to life has never been observed experimentally, but many proposals have been made for different stages of the process. The study of abiogenesis aims to determine how pre-life chemical reactions gave rise to life under conditions strikingly different from those on Earth today. It primarily uses tools from biology and chemistry , with more recent approaches attempting
5874-405: The formose reaction; the advantages and disadvantages of isoprenoids incorporated within the lipid bilayer in different microenvironments might have led to the divergence of the membranes of archaea and bacteria. Laboratory experiments have shown that vesicles can undergo an evolutionary process under pressure cycling conditions. Simulating the systemic environment in tectonic fault zones within
5963-561: The genetic code arose. Anaerobic respiration In aerobic organisms undergoing respiration, electrons are shuttled to an electron transport chain , and the final electron acceptor is oxygen . Molecular oxygen is an excellent electron acceptor. Anaerobes instead use less-oxidizing substances such as nitrate ( NO 3 ), fumarate ( C 4 H 2 O 4 ), sulfate ( SO 4 ), or elemental sulfur (S). These terminal electron acceptors have smaller reduction potentials than O 2 . Less energy per oxidized molecule
6052-418: The gradient (across the membrane) through the proton channel of ATP synthase . The resulting current drives ATP synthesis from ADP and inorganic phosphate. Fermentation , in contrast, does not use an electrochemical gradient but instead uses only substrate-level phosphorylation to produce ATP. The electron acceptor NAD is regenerated from NADH formed in oxidative steps of the fermentation pathway by
6141-407: The hot and dense centers of these protoplanetary disks formed stars by the fusion of hydrogen. Early stars were massive and short-lived, producing all the heavier elements through stellar nucleosynthesis . Element formation through stellar nucleosynthesis proceeds to its most stable element Iron- 56 . Heavier elements were formed during supernovae at the end of a stars lifecycle. Carbon , currently
6230-666: The interest in panspermia grew when the study of meteorites found traces of organic materials in them, it is currently accepted that life started locally on Earth. The idea that life originated from non-living matter in slow stages appeared in Herbert Spencer 's 1864–1867 book Principles of Biology , and in William Turner Thiselton-Dyer 's 1879 paper "On spontaneous generation and evolution". On 1 February 1871 Charles Darwin wrote about these publications to Joseph Hooker , and set out his own speculation, suggesting that
6319-450: The interstellar medium can be transformed through hydrogenation , oxygenation , and hydroxylation to more complex organic compounds used in living cells. The majority of organic compounds introduced on Earth by interstellar dust particles have helped to form complex molecules, thanks to their peculiar surface-catalytic activities. Studies of the C/ C isotopic ratios of organic compounds in
6408-433: The materials for DNA and RNA to form on the early Earth . The amino acid glycine was found in material ejected from comet Wild 2 ; it had earlier been detected in meteorites. Comets are encrusted with dark material, thought to be a tar -like organic substance formed from simple carbon compounds under ionizing radiation. A rain of material from comets could have brought such complex organic molecules to Earth. It
6497-440: The middle of the 19th century, spontaneous generation was considered disproven. Another ancient idea dating back to Anaxagoras in the 5th century BC is panspermia , the idea that life exists throughout the universe , distributed by meteoroids , asteroids , comets and planetoids . It does not attempt to explain how life originated in itself, but shifts the origin of life on Earth to another heavenly body. The advantage
6586-629: The origin of biological monomers ; the origin of biological polymers ; and the evolution from molecules to cells. In 1952, Stanley Miller and Harold Urey carried out a chemical experiment to demonstrate how organic molecules could have formed spontaneously from inorganic precursors under prebiotic conditions like those posited by the Oparin–Haldane hypothesis. It used a highly reducing (lacking oxygen) mixture of gases— methane , ammonia , and hydrogen , as well as water vapor —to form simple organic monomers such as amino acids . Bernal said of
6675-508: The original spark of life may have begun in a "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." Darwin went on to explain that "at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed." Alexander Oparin in 1924 and J. B. S. Haldane in 1929 proposed that
6764-442: The other. The evolutionary process could have involved molecular self-replication , self-assembly such as of cell membranes , and autocatalysis via RNA ribozymes . Nonetheless, the transition of non-life to life has never been observed experimentally, nor has there been a satisfactory chemical explanation. The preconditions to the development of a living cell like the LUCA are clear enough, though disputed in their details:
6853-588: The planet. An organic compound is a chemical whose molecules contain carbon. Carbon is abundant in the Sun, stars, comets, and in the atmospheres of most planets. 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 . Purine and pyrimidine nucleobases including guanine , adenine , cytosine , uracil , and thymine have been found in meteorites . These could have provided
6942-575: The potential for self-replication. 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 experimental conditions. If such conditions were present on early Earth, then natural selection would favor the proliferation of such autocatalytic sets , to which further functionalities could be added. Self-assembly of RNA may occur spontaneously in hydrothermal vents. A preliminary form of tRNA could have assembled into such
7031-469: The reaction was autocatalytic in 1959. Nucleobases, such as guanine and adenine, can be synthesized from simple carbon and nitrogen sources, such as hydrogen cyanide (HCN) and ammonia. Formamide produces all four ribonucleotides when warmed with terrestrial minerals. Formamide is ubiquitous in the Universe, produced by the reaction of water and HCN. It can be concentrated by the evaporation of water. HCN
7120-648: The reduction of oxidized compounds. These oxidized compounds are often formed during the fermentation pathway itself, but may also be external. For example, in homofermentative lactic acid bacteria, NADH formed during the oxidation of glyceraldehyde-3-phosphate is oxidized back to NAD by the reduction of pyruvate to lactic acid at a later stage in the pathway. In yeast , acetaldehyde is reduced to ethanol to regenerate NAD . There are two important anaerobic microbial methane formation pathways, through carbon dioxide / bicarbonate ( HCO 3 ) reduction (respiration) or acetate fermentation. Anaerobic respiration
7209-472: The right conditions. A similar role of increasing amino acid concentration has been suggested for clays as well. While all of these scenarios involve the condensation of amino acids, the prebiotic synthesis of peptides from simpler molecules such as CO, NH 3 and C, skipping the step of amino acid formation, is very efficient. The largest unanswered question in evolution is how simple protocells first arose and differed in reproductive contribution to
7298-461: The rock record both before and after the 3.9 Ga marker, suggesting that the early Earth was subject to continuous impacts that would not have had as great an impact on extinction as previously thought. If the Late Heavy Bombardment did not take place, this allows for the emergence of life to have taken place far before 3.9 Ga. If life evolved in the ocean at depths of more than ten meters, it would have been shielded both from late impacts and
7387-559: The slow diffusion characteristics of oxygen gas. An example of the ecological importance of anaerobic respiration is the use of nitrate as a terminal electron acceptor , or dissimilatory denitrification , which is the main route by which fixed nitrogen is returned to the atmosphere as molecular nitrogen gas. The denitrification process is also very important in host-microbe interactions. Like mitochondria in oxygen-respiring microorganisms, some single-cellular anaerobic ciliates use denitrifying endosymbionts to gain energy. Another example
7476-437: The survival rate of the vesicles. Life requires a loss of entropy, or disorder, as molecules organize themselves into living matter. At the same time, the emergence of life is associated with the formation of structures beyond a certain threshold of complexity . The emergence of life with increasing order and complexity does not contradict the second law of thermodynamics, which states that overall entropy never decreases, since
7565-495: The synthesis of purines, due to the concentrating effect for key precursors such as hydrogen cyanide. However, while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures. Seven amino acids and eleven types of nucleobases formed in ice when ammonia and cyanide were left in a freezer for 25 years. S- triazines (alternative nucleobases), pyrimidines including cytosine and uracil, and adenine can be synthesized by subjecting
7654-475: The then high levels of ultraviolet radiation from the sun. Geothermically heated oceanic crust could have yielded far more organic compounds through deep hydrothermal vents than the Miller–Urey experiments indicated. The available energy is maximized at 100–150 °C, the temperatures at which hyperthermophilic bacteria and thermoacidophilic archaea live. The exact timing at which life emerged on Earth
7743-697: Was far from the first form of life. Earlier cells might have had a leaky membrane and been powered by a naturally occurring proton gradient near a deep-sea white smoker hydrothermal vent . Earth remains the only place in the universe known to harbor life. Geochemical and fossil evidence from the Earth informs most studies of abiogenesis. The Earth was formed at 4.54 Gya, and the earliest evidence of life on Earth dates from at least 3.8 Gya from Western Australia . Some studies have suggested that fossil micro-organisms may have lived within hydrothermal vent precipitates dated 3.77 to 4.28 Gya from Quebec , soon after ocean formation 4.4 Gya during
7832-746: Was found in pyrite -bearing sandstone in a fossilized beach, with rounded tubular cells that oxidized sulfur by photosynthesis in the absence of oxygen. Carbon isotope ratios on graphite inclusions from the Jack Hills zircons suggest that life could have existed on Earth from 4.1 Gya. The Pilbara region of Western Australia contains the Dresser Formation with rocks 3.48 Gya, including layered structures called stromatolites . Their modern counterparts are created by photosynthetic micro-organisms including cyanobacteria . These lie within undeformed hydrothermal-sedimentary strata; their texture indicates
7921-575: Was one giant impact or a period of greater impact rates greatly changed the implication of its destructive power. The 3.9 Ga date arises from dating of Apollo mission sample returns collected mostly near the Imbrium Basin , biasing the age of recorded impacts. Impact modelling of the lunar surface reveals that rather than a cataclysmic event at 3.9 Ga, multiple small-scale, short-lived periods of bombardment likely occurred. Terrestrial data backs this idea by showing multiple periods of ejecta in
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