Abiogenesis

Abiogenesis

Overview
Abiogenesis (ˌeɪbaɪ.ɵˈdʒɛnɨsɪs ) or biopoesis is the study of how biological life arises from inorganic matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 through natural processes, and the method by which life on Earth
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

 arose. Most amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s, often called "the building blocks of life", can form via natural chemical reactions unrelated to life, as demonstrated in the Miller–Urey experiment and similar experiments that involved simulating some of the hypothetical conditions of the early Earth in a laboratory.
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Encyclopedia
Abiogenesis (ˌeɪbaɪ.ɵˈdʒɛnɨsɪs ) or biopoesis is the study of how biological life arises from inorganic matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 through natural processes, and the method by which life on Earth
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

 arose. Most amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s, often called "the building blocks of life", can form via natural chemical reactions unrelated to life, as demonstrated in the Miller–Urey experiment and similar experiments that involved simulating some of the hypothetical conditions of the early Earth in a laboratory. In all living things, these amino acids are organized into protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s, and the construction of these proteins is mediated by nucleic acid
Nucleic acid
Nucleic acids are biological molecules essential for life, and include DNA and RNA . Together with proteins, nucleic acids make up the most important macromolecules; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information...

s, that are themselves synthesized through biochemical pathways catalysed by proteins. Which of these organic molecules first arose and how they formed the first life is the focus of abiogenesis.

In any theory of abiogenesis, two aspects of life have to be accounted for: replication
Self-replication
Self-replication is any behavior of a dynamical system that yields construction of an identical copy of that dynamical system. Biological cells, given suitable environments, reproduce by cell division. During cell division, DNA is replicated and can be transmitted to offspring during reproduction...

 and metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

. The question of which came first gave rise to different types of theories. In the beginning, metabolism-first theories (Oparin coacervate
Coacervate
A coacervate is a tiny spherical droplet of assorted organic molecules which is held together by hydrophobic forces from a surrounding liquid....

) were proposed, and only later thinking gave rise to the modern, replication-first approach.

The first living things on Earth are thought to be single cell prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...

s (which lack a cell nucleus), perhaps evolved from protobiont
Protobiont
Protobionts are systems that are considered to have possibly been the precursors to prokaryotic cells. If RNA is trapped inside, the system can use the RNA or select for it....

s (organic molecules surrounded by a membrane-like structure). The oldest ancient fossil microbe-like objects are dated to be 3.5 Ga (billion years old), approximately one billion years after the formation of the Earth itself, with reliable fossil evidence of the first life found in rocks 3.4 Gyr old. By 2.4 Ga, the ratio of stable isotopes of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

, iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 shows the action of living things on inorganic minerals and sediments and molecular biomarkers indicate photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, demonstrating that life on Earth was widespread by this time.

The sequence of chemical events that led to the first nucleic acids is not known. Several hypotheses about early life have been proposed, most notably the iron-sulfur world theory
Iron-sulfur world theory
The iron-sulfur world theory is a set of proposals for the origin of life and the early evolution of life advanced by Günter Wächtershäuser, a Munich patent lawyer with a degree in chemistry who had been encouraged and supported by philosopher Karl R. Popper to publish his ideas. The theory...

 (metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 without genetics
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

) and the RNA world hypothesis
RNA world hypothesis
The RNA world hypothesis proposes that life based on ribonucleic acid pre-dates the current world of life based on deoxyribonucleic acid , RNA and proteins. RNA is able both to store genetic information, like DNA, and to catalyze chemical reactions, like an enzyme protein...

 (RNA life-forms).

Spontaneous generation


Until the early 19th century, people generally believed in the ongoing spontaneous generation
Spontaneous generation
Spontaneous generation or Equivocal generation is an obsolete principle regarding the origin of life from inanimate matter, which held that this process was a commonplace and everyday occurrence, as distinguished from univocal generation, or reproduction from parent...

 of certain forms of life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

 from non-living matter. This was paired with the belief in heterogenesis, e.g. that one form of life derived from a different form (e.g. bees from flowers). Classical notions of abiogenesis, now more precisely known as spontaneous generation, held that certain complex, living organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

s are generated by decaying organic substances. According to Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

, it was a readily observable truth that aphid
Aphid
Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies or whiteflies, are small sap sucking insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions...

s arise from the dew which falls on plants, flies
Fly
True flies are insects of the order Diptera . They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax...

 from putrid matter, mice from dirty hay, crocodiles from rotting logs at the bottom of bodies of water, and so on.

In the 17th century, such assumptions started to be questioned; for example, in 1646, Sir Thomas Browne published his Pseudodoxia Epidemica
Pseudodoxia Epidemica
Pseudodoxia Epidemica or Enquries into very many received tenets and commonly presumed truths, also known simply as Pseudodoxia Epidemica or Vulgar Errors, is a work by Thomas Browne refuting the common errors and superstitions of his age. It first appeared in 1646 and went through five subsequent...

(subtitled Enquiries into Very many Received Tenets, and Commonly Presumed Truths), which was an attack on false beliefs and "vulgar errors." His conclusions were not widely accepted. For example, his contemporary, Alexander Ross
Alexander Ross (writer)
Alexander Ross was a prolific Scottish writer and controversialist. He was Chaplain-in-Ordinary to Charles I.-Life:He was born in Aberdeen, and entered King's College, Aberdeen, in 1604. About 1616 he succeeded Thomas Parker in the mastership of the free school at Southampton, an appointment which...

 wrote: "To question this (i.e., spontaneous generation) is to question reason, sense and experience. If he doubts of this let him go to Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...

, and there he will find the fields swarming with mice, begot of the mud of Nylus
Nile
The Nile is a major north-flowing river in North Africa, generally regarded as the longest river in the world. It is long. It runs through the ten countries of Sudan, South Sudan, Burundi, Rwanda, Democratic Republic of the Congo, Tanzania, Kenya, Ethiopia, Uganda and Egypt.The Nile has two major...

, to the great calamity of the inhabitants."

In 1665, Robert Hooke
Robert Hooke
Robert Hooke FRS was an English natural philosopher, architect and polymath.His adult life comprised three distinct periods: as a scientific inquirer lacking money; achieving great wealth and standing through his reputation for hard work and scrupulous honesty following the great fire of 1666, but...

 published the first drawings of a microorganism. Hooke was followed in 1676 by Anton van Leeuwenhoek
Anton van Leeuwenhoek
Antonie Philips van Leeuwenhoek was a Dutch tradesman and scientist from Delft, Netherlands. He is commonly known as "the Father of Microbiology", and considered to be the first microbiologist...

, who drew and described microorganisms that are now thought to have been protozoa
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

 and bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

. Many felt the existence of microorganisms was evidence in support of spontaneous generation, since microorganisms seemed too simplistic for sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...

, and asexual reproduction
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...

 through cell division
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

 had not yet been observed.

The first solid evidence against spontaneous generation came in 1668 from Francesco Redi
Francesco Redi
Francesco Redi was an Italian physician, naturalist, and poet.-Biography:The son of Gregorio Redi and Cecilia de Ghinci was born in Arezzo on February 18, 1626. After schooling with the Jesuits, he attended the University of Pisa...

, who proved that no maggot
Maggot
In everyday speech the word maggot means the larva of a fly ; it is applied in particular to the larvae of Brachyceran flies, such as houseflies, cheese flies, and blowflies, rather than larvae of the Nematocera, such as mosquitoes and Crane flies...

s appeared in meat when flies were prevented from laying eggs. 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. The alternative seemed to be biogenesis
Biogenesis
Biogenesis is the law that living things come only from other living things, e.g. a spider lays eggs, which develop into spiders. It may also refer to biochemical processes of production in living organisms.-Spontaneous generation:...

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

In 1768, Lazzaro Spallanzani
Lazzaro Spallanzani
Lazzaro Spallanzani was an Italian Catholic priest, biologist and physiologist who made important contributions to the experimental study of bodily functions, animal reproduction, and essentially discovered echolocation...

 demonstrated that microbes were present in the air, and could be killed by boiling. In 1861, Louis Pasteur
Louis Pasteur
Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

 performed a series of experiments which demonstrated that organisms such as bacteria and fungi do not spontaneously appear in sterile, nutrient-rich media.

Pasteur and Darwin



By the middle of the 19th century, the theory of biogenesis
Biogenesis
Biogenesis is the law that living things come only from other living things, e.g. a spider lays eggs, which develop into spiders. It may also refer to biochemical processes of production in living organisms.-Spontaneous generation:...

 had accumulated so much evidential support, due to the work of Louis Pasteur
Louis Pasteur
Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

 and others, that the alternative theory of spontaneous generation had been effectively disproven. Pasteur himself remarked, after a definitive finding in 1864, "Never will the doctrine of spontaneous generation recover from the mortal blow struck by this simple experiment."
The collapse of spontaneous generation, however, left a vacuum of scientific thought on the question of how life had first arisen.

In a letter to Joseph Dalton Hooker
Joseph Dalton Hooker
Sir Joseph Dalton Hooker OM, GCSI, CB, MD, FRS was one of the greatest British botanists and explorers of the 19th century. Hooker was a founder of geographical botany, and Charles Darwin's closest friend...

 on February 1, 1871, Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

 addressed the question, suggesting that the original spark of life may have begun in a "warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, so that a protein compound was chemically formed ready to undergo still more complex changes". He 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." In other words, the presence of life itself makes the search for the origin of life dependent on the sterile conditions of the laboratory.

"Primordial soup" theory





No new notable research or theory on the subject appeared until 1924, when Alexander Oparin reasoned that atmospheric oxygen prevents the synthesis of certain organic compounds that are necessary building blocks for the evolution of life. In his The Origin of Life, Oparin proposed that the "spontaneous generation of life" that had been attacked by Louis Pasteur
Louis Pasteur
Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

 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. Oparin argued that a "primeval soup" of organic molecules could be created in an oxygenless atmosphere through the action of sunlight. These would combine in evermore complex ways until they formed coacervate
Coacervate
A coacervate is a tiny spherical droplet of assorted organic molecules which is held together by hydrophobic forces from a surrounding liquid....

 droplets. These droplets would "grow
Cell growth
The term cell growth is used in the contexts of cell development and cell division . When used in the context of cell division, it refers to growth of cell populations, where one cell grows and divides to produce two "daughter cells"...

" by fusion with other droplets, and "reproduce
Biological reproduction
Reproduction is the biological process by which new "offspring" individual organisms are produced from their "parents". Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction...

" through fission into daughter droplets, and so have a primitive metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 in which those factors which promote "cell integrity" survive, and those that do not become extinct. Many modern theories of the origin of life still take Oparin's ideas as a starting point.

Around the same time, J. B. S. Haldane
J. B. S. Haldane
John Burdon Sanderson Haldane FRS , known as Jack , was a British-born geneticist and evolutionary biologist. A staunch Marxist, he was critical of Britain's role in the Suez Crisis, and chose to leave Oxford and moved to India and became an Indian citizen...

 suggested that the Earth's prebiotic oceans—different from their modern counterparts—would have formed a "hot dilute soup" in which organic compounds could have formed. This idea was called biopoiesis or biopoesis, the process of living matter evolving from self-replicating but nonliving molecules.

In 1952, in the Miller-Urey experiment
Miller-Urey experiment
The Miller and Urey experiment was an experiment that simulated hypothetical conditions thought at the time to be present on the early Earth, and tested for the occurrence of chemical origins of life. Specifically, the experiment tested Alexander Oparin's and J. B. S...

, a mixture of water, hydrogen, methane, and ammonia was cycled through an apparatus that delivered electrical sparks to the mixture. After one week, it was found that about 10% to 15% of the carbon in the system was now in the form of organic compounds, including amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s, which are the building blocks of protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s.

The underlying hypothesis held by Oparin and Haldane was that conditions on the primeval Earth favored chemical reactions that synthesized organic compounds from inorganic precursors. A recent reanalysis of the saved vials containing the original extracts that resulted in the Miller and Urey experiments, using current and more advanced analytical equipment and technology, has uncovered more biochemicals than originally discovered in the 1950s. One of the more important findings was 23 amino acids, far more than five originally discovered.

Complex biological molecules and protocells


Sidney W. Fox
Sidney W. Fox
Sidney Walter Fox was a Los Angeles-born biochemist responsible for unique discoveries in the autosynthesis of protocells.-Professor:In 1943 Fox was granted his first academic position at Iowa State College....

 also experimented with abiogenesis and the primordial soup theory. In one of his experiments, he allowed amino acids to dry out as if in puddled in a warm, dry spot many millions of years ago. The "miracle" was that, as they dried, the amino acids formed long, thread-like, submicroscopic now named "proteinoids".

Another experiment using a similar method involved simply setting the right conditions for life to form. Dr. Fox collected volcanic material from a cinder cone in Hawaii, but discovered that the temperature was over 100 degrees celsius just four inches beneath the surface of the cinder cone. Might this have been the enivronment in which life was created - molecules could have formed and then been washed through the loose volcanic ash and into the sea by water? Dr. Fox certainly thought so. He placed hunks of lava over amino acids derived from methane, ammonia and water. With everything sterilised, he baked the lava over the amino acids for a few hours in a glass oven. A brown, sticky substance formed over the surface and when the lava was drenched in sterilised water it formed a thick, brown liquid. Although it looked unpromising, it was actually one of the most incredible discoveries in abiogenesis to this day. The amino acids combined to form proteinoids, but the proteinoids had combined to form small, cell-like spheres. He called these "microspheres". They were not true cells, although they clumped together in chains as do blue-green algae, they reproduced asexually and could form double membranes through which diffusion of molecules and osmosis could occur, yet they had no DNA. Professor Colin S. Pittendrigh stated in December 1967 that "laboratories will be creating a living cell within ten years."

Early conditions



Morse and MacKenzie have suggested that oceans
Origin of water on Earth
The question of the origin of water on Earth, or the question of why there is clearly more water on the Earth than on the other planets of the Solar System, has not been clarified...

  may have appeared first
Cool Early Earth
The Cool Early Earth theory posits that the early planet Earth had a calm influx of bolides and a cool climate allowing fluid water, after the planetary accretion but before the occurrence of the Late Heavy Bombardment in the Hadean geological eon. The Cool Early Earth is believed to have been the...

 in the Hadean
Hadean
The Hadean is the geologic eon before the Archean. It started with the formation of the Earth about 4.7 Ga and ended roughly 3.8 Ga, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld", referring to the "hellish"...

 eon, as soon as two hundred million years (200 Ma) after the Earth was formed, in a hot 100 °C (212 °F) reducing
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 environment, and that the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 of about 5.8 rose rapidly towards neutral. This has been supported by Wilde who has pushed the date of the zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...

 crystals found in the metamorphosed quartzite
Quartzite
Quartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...

 of Mount Narryer
Narryer Gneiss Terrane
The Narryer Gneiss Terrane is a geological complex in Western Australia that is composed of a tectonically interleaved and polydeformed mixture of granite, mafic intrusions and metasedimentary rocks in excess of 3.3 billion years old, with the majority of the Narryer Gneiss Terrane in excess of 3.6...

 in Western Australia, previously thought to be 4.1–4.2 Ga, to 4.404 Ga. This means that oceans and continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...

 existed within 150 Ma of Earth's formation.

Despite this, the Hadean
Hadean
The Hadean is the geologic eon before the Archean. It started with the formation of the Earth about 4.7 Ga and ended roughly 3.8 Ga, though the latter date varies according to different sources. The name "Hadean" derives from Hades, Greek for "Underworld", referring to the "hellish"...

 environment was one highly hazardous to life. Frequent collisions with large objects, up to 500 kilometres (310.7 mi) in diameter, would have been sufficient to vaporise the ocean within a few months of impact, with hot steam mixed with rock vapour leading to high altitude clouds completely covering the planet. 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. After that, it would have begun to rain at low altitude. 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 years after the impact event.

Between 3.8 and 4.1 Ga, changes in the orbits of the gaseous giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

 planets may have caused a late heavy bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...

 that pockmarked the moon and other inner planets (Mercury, Mars, and presumably Earth and Venus). This would likely have sterilized the planet, had life appeared before that time.

By examining the time interval between such devastating environmental events, the time interval when life might first have come into existence can be found for different early environments. The study by Maher and Stevenson shows that if the deep marine hydrothermal setting provides a suitable site for the origin of life, abiogenesis could have happened as early as 4.0 to 4.2 Ga, whereas if it occurred at the surface of the earth abiogenesis could only have occurred between 3.7 and 4.0 Ga.

Other research suggests a colder start to life. Work by Leslie Orgel
Leslie Orgel
Leslie Eleazer Orgel FRS was a British chemist.Born in London, England, Orgel received his B.A. in chemistry with first class honours from Oxford University in 1949...

 and colleagues on the synthesis of purines has shown that freezing temperatures are advantageous, due to the concentrating effect for key precursors such as hydrogen cyanide.
Research by Stanley Miller
Stanley Miller
Stanley Lloyd Miller was an American chemist and biologist who is known for his studies into the origin of life, particularly the Miller–Urey experiment which demonstrated that organic compounds can be created by fairly simple physical processes from inorganic substances...

 and colleagues suggested that while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures. Based on this research, Miller suggested a beginning of life involving freezing conditions and exploding meteorites. An article in Discover Magazine
Discover (magazine)
Discover is an American science magazine that publishes articles about science for a general audience. The monthly magazine was launched in October 1980 by Time Inc. It was sold to Family Media, the owners of Health, in 1987. Walt Disney Company bought the magazine when Family Media went out of...

 points to research by the Miller group indicating the formation of seven different amino acids and 11 types of nucleobases in ice when ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 and cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....

 were left in a freezer from 1972–1997. This article also describes research by Christof Biebricher showing the formation of RNA molecules 400 bases long under freezing conditions using an RNA template, a single-strand chain of RNA that guides the formation of a new strand of RNA. As that new RNA strand grows, it adheres to the template. The explanation given for the unusual speed of these reactions at such a low temperature is eutectic freezing
Eutectic point
A eutectic system is a mixture of chemical compounds or elements that has a single chemical composition that solidifies at a lower temperature than any other composition. This composition is known as the eutectic composition and the temperature is known as the eutectic temperature...

. As an ice crystal forms, it stays pure: only molecules of water join the growing crystal, while impurities like salt or cyanide are excluded. These impurities become crowded in microscopic pockets of liquid within the ice, and this crowding causes the molecules to collide more often.

Evidence of the early appearance of life comes from the Isua supercrustal belt in Western Greenland and from similar formations in the nearby Akilia Island
Akilia island
Akilia Island is an island in southwestern Greenland, about 22 kilometers south of Nuuk , at . Akilia is the location of a rock formation that has been proposed to contain the oldest known sedimentary rocks on Earth,...

s. Carbon entering into rock formations has a ratio of Carbon-13
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...

 (13C) to Carbon-12
Carbon-12
Carbon-12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons....

 (12C) of about −5.5 (in units of δ13C), where because of a preferential biotic uptake of 12C, biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....

 has a δ13C of between −20 and −30. These isotopic fingerprints are preserved in the sediments, and Mojzis has used this technique to suggest that life existed on the planet already by 3.85 billion years ago. Lazcano and Miller (1994) suggest that the rapidity of the evolution of life is dictated by the rate of recirculating water through mid-ocean submarine vents. Complete recirculation takes 10 million years, thus any organic compounds produced by then would be altered or destroyed by temperatures exceeding 300 °C (572 °F). They estimate that the development of a 100 kilobase genome of a DNA/protein primitive heterotroph
Heterotroph
A heterotroph is an organism that cannot fix carbon and uses organic carbon for growth. This contrasts with autotrophs, such as plants and algae, which can use energy from sunlight or inorganic compounds to produce organic compounds such as carbohydrates, fats, and proteins from inorganic carbon...

 into a 7000 gene filamentous cyanobacterium would have required only 7 Ma. The Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...

 winning chemist, Christian de Duve
Christian de Duve
Christian René, viscount de Duve is a Nobel Prize-winning cytologist and biochemist. De Duve was born in Thames Ditton, Surrey, Great Britain, as a son of Belgian refugees. They returned to Belgium in 1920...

, argues that the determination of chemistry means that "life has to emerge quickly... Chemical reactions happen quickly or not at all; if any reaction takes a millennium to complete then the chances are all the reagents will simply dissipate or breakdown in the meantime, unless they are replenished by other faster reactions".

Current models


There is no truly "standard model" of the origin of life. Most currently accepted models draw at least some elements from the framework laid out by the Oparin-Haldane hypothesis. Under that umbrella, however, are a wide array of disparate discoveries and conjectures such as the following, listed in a rough order of postulated emergence:
  • Some theorists suggest that the atmosphere of the early Earth may have been chemically reducing
    Redox
    Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

     in nature, composed primarily of methane
    Methane
    Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

     (CH4), ammonia
    Ammonia
    Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

     (NH3), water
    Water
    Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

     (H2O), hydrogen sulfide
    Hydrogen sulfide
    Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

     (H2S), carbon dioxide
    Carbon dioxide
    Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

     (CO2) or carbon monoxide
    Carbon monoxide
    Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

     (CO), and phosphate
    Phosphate
    A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...

     (PO43-), with molecular oxygen
    Oxygen
    Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

     (O2) and ozone
    Ozone
    Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

     (O3) either rare or absent.
  • In such a reducing atmosphere, electrical activity can catalyze the creation of certain basic small molecule
    Molecule
    A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

    s (monomer
    Monomer
    A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...

    s) of life, such as amino acids. This was demonstrated in the Miller–Urey experiment by Stanley L. Miller and Harold C. Urey in 1953.
  • Phospholipid
    Phospholipid
    Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...

    s (of an appropriate length) can form lipid bilayer
    Lipid bilayer
    The lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...

    s, a basic component of the cell membrane
    Cell membrane
    The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

    .
  • A fundamental question is about the nature of the first self-replicating molecule. Since replication is accomplished in modern cells through the cooperative action of proteins and nucleic acids, the major schools of thought about how the process originated can be broadly classified as "proteins first" and "nucleic acids first".
  • The principal thrust of the "nucleic acids first" argument is as follows:
  1. The polymer
    Polymer
    A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

    ization of nucleotide
    Nucleotide
    Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...

    s into random RNA
    RNA
    Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

     molecules might have resulted in self-replicating ribozyme
    Ribozyme
    A ribozyme is an RNA molecule with a well defined tertiary structure that enables it to catalyze a chemical reaction. Ribozyme means ribonucleic acid enzyme. It may also be called an RNA enzyme or catalytic RNA. Many natural ribozymes catalyze either the hydrolysis of one of their own...

    s (RNA world hypothesis
    RNA world hypothesis
    The RNA world hypothesis proposes that life based on ribonucleic acid pre-dates the current world of life based on deoxyribonucleic acid , RNA and proteins. RNA is able both to store genetic information, like DNA, and to catalyze chemical reactions, like an enzyme protein...

    )
  2. Selection
    Selection
    In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...

     pressures for catalytic efficiency and diversity might have resulted in ribozymes which catalyse peptidyl transfer (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. The first ribosome
    Ribosome
    A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....

     might have been created by such a process, resulting in more prevalent protein synthesis.
  3. Synthesized proteins might then outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer, relegating nucleic acids to their modern use, predominantly as a carrier of genomic information.


No one has synthesized a "protocell" using basic components which would have the necessary properties of life (the so-called "bottom-up-approach"). Without such a proof-of-principle, explanations have tended to be short on specifics. However, some researchers are working in this field, notably Steen Rasmussen
Steen Rasmussen
Steen Rasmussen was born in Elsinore, Denmark, in 1955. He is an Artificial Life scientist who has published numerous reviews and reports in the Journal, Artificial Life. He coined the term, complex systems dogma, which alludes to the presupposition that simple lower-level elements can give rise to...

 at Los Alamos National Laboratory
Los Alamos National Laboratory
Los Alamos National Laboratory is a United States Department of Energy national laboratory, managed and operated by Los Alamos National Security , located in Los Alamos, New Mexico...

 and Jack Szostak at Harvard University
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...

. Others have argued that a "top-down approach" is more feasible. One such approach, successfully attempted by Craig Venter
Craig Venter
John Craig Venter is an American biologist and entrepreneur, most famous for his role in being one of the first to sequence the human genome and for his role in creating the first cell with a synthetic genome in 2010. Venter founded Celera Genomics, The Institute for Genomic Research and the J...

 and others at The Institute for Genomic Research
The Institute for Genomic Research
The Institute for Genomic Research was a non-profit genomics research institute founded in 1992 by Craig Venter in Rockville, Maryland, United States. It is now a part of the J. Craig Venter Institute.-History:...

, involves engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life were reached.
The biologist John Desmond Bernal coined the term biopoesis for this process, and suggested that there were a number of clearly defined "stages" that could be recognised in explaining the origin of life.
  • Stage 1: The origin of biological monomers
  • Stage 2: The origin of biological polymers
  • Stage 3: The evolution from molecules to cell


Bernal suggested that evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 may have commenced early, some time between Stage 1 and 2.

Origin of organic molecules


There are two possible sources of organic molecules on the early Earth:
  1. Terrestrial origins – organic synthesis driven by impact shocks or by other energy sources (such as ultraviolet light or electrical discharges) (e.g. Miller's experiments)
  2. Extraterrestrial origins – delivery by objects (e.g. carbonaceous chondrites) or gravitational attraction of organic molecules or primitive life-forms from space


Recently, estimates of these sources suggest that the heavy bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...

 before 3.5 Ga within the early atmosphere made available quantities of organics comparable to those produced by other energy sources.

"Soup" theory today: Miller's experiment and subsequent work


Biochemist Robert Shapiro
Robert Shapiro (chemist)
Robert Shapiro was professor emeritus of chemistry at New York University. He is best known for his work on the origin of life, having written two books on the topic: Origins, a Skeptic’s Guide to the Creation of Life on Earth and Planetary Dreams...

 has summarized the "primordial soup" theory of Oparin and Haldane in its "mature form" as follows:
  1. The early Earth had a chemically reducing atmosphere.
  2. This atmosphere, exposed to energy in various forms, produced simple organic compounds ("monomer
    Monomer
    A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...

    s").
  3. These compounds accumulated in a "soup", which may have been concentrated at various locations (shorelines, oceanic vents etc.).
  4. By further transformation, more complex organic polymer
    Polymer
    A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

    s – and ultimately life – developed in the soup.

Reducing atmosphere

Whether the mixture of gases used in the Miller–Urey experiment truly reflects the atmospheric content of early Earth
Early Earth
The "Early Earth" is a term usually defined as Earth's first billion years, or gigayear. On the geologic time scale, the "early Earth" comprises all of the Hadean eon , as well as the Eoarchean and part of the Paleoarchean eras of the Archean eon.This period of Earth's history, being its earliest,...

 is a controversial topic. Other less reducing gases produce a lower yield and variety. It was once thought that appreciable amounts of molecular oxygen were present in the prebiotic atmosphere , which would have essentially prevented the formation of organic molecules; however, the current scientific consensus is that such was not the case. (See Oxygen catastrophe
Oxygen Catastrophe
The Great Oxygenation Event , also called the Oxygen Catastrophe or Oxygen Crisis or Great Oxidation, was the biologically induced appearance of free oxygen in Earth's atmosphere. This major environmental change happened around 2.4 billion years ago.Photosynthesis was producing oxygen both before...

).
Monomer formation


One of the most important pieces of experimental support for the "soup" theory came in 1953. A graduate student, Stanley Miller
Stanley Miller
Stanley Lloyd Miller was an American chemist and biologist who is known for his studies into the origin of life, particularly the Miller–Urey experiment which demonstrated that organic compounds can be created by fairly simple physical processes from inorganic substances...

, and his professor, Harold Urey
Harold Urey
Harold Clayton Urey was an American physical chemist whose pioneering work on isotopes earned him the Nobel Prize in Chemistry in 1934...

, 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. The now-famous "Miller–Urey experiment" used a highly reduced mixture of gases—methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

, ammonia and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

—to form basic organic monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...

s, such as amino acids. 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.

Apart from the Miller–Urey experiment, the next most important step in research on prebiotic organic synthesis was the demonstration by Joan Oró
Joan Oró
Joan Oró i Florensa was a biochemist from Catalonia , whose research has been of importance in understanding the origin of life. He participated...

 that the nucleic acid purine base, adenine, was formed by heating aqueous ammonium cyanide
Ammonium cyanide
Ammonium cyanide is an unstable inorganic compound with the formula NH4CN.-Uses:Ammonium cyanide is generally used in organic synthesis. Being unstable, it is not shipped or sold commercially.-Preparation:...

 solutions. In support of abiogenesis in eutectic ice, more recent work demonstrated the formation of s-triazine
Triazine
A triazine is one of three organic chemicals, isomeric with each other, whose molecular formula is 333 and whose empirical formula is CHN.- Structure :...

s (alternative nucleobase
Nucleobase
Nucleobases are a group of nitrogen-based molecules that are required to form nucleotides, the basic building blocks of DNA and RNA. Nucleobases provide the molecular structure necessary for the hydrogen bonding of complementary DNA and RNA strands, and are key components in the formation of stable...

s), pyrimidine
Pyrimidine
Pyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring...

s (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).
Regarding monomer accumulation

The "soup" theory relies on the assumption proposed by Darwin that in an environment with no pre-existing life, organic molecules may have accumulated and provided an environment for chemical evolution.
Regarding further transformation

The spontaneous formation of complex polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s from abiotically generated monomers under the conditions posited by the "soup" theory is not at all a straightforward process. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were formed in high concentration during the Miller–Urey and Oró experiments. The Miller experiment, for example, produces many substances that would undergo cross-reactions with the amino acids or terminate the peptide chain.

More fundamentally, it can be argued that the most crucial challenge unanswered by this theory is how the relatively simple organic building blocks polymerise and form more complex structures, interacting in consistent ways to form a protocell. For example, in an aqueous environment hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 of oligomers/polymers into their constituent monomers would be favored over the condensation of individual monomers into polymers.

The deep sea vent theory


The deep sea vent, or hydrothermal vent
Hydrothermal vent
A hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots. Hydrothermal vents exist because the earth is both...

, theory for the origin of life on Earth posits that life may have begun at submarine hydrothermal vents, where hydrogen-rich fluids emerge from below the sea floor and interface with carbon dioxide-rich ocean water. Sustained chemical energy in such systems is derived from redox reactions, in which electron donors, such as molecular hydrogen, react with electron acceptors, such as carbon dioxide (see iron-sulfur world theory
Iron-sulfur world theory
The iron-sulfur world theory is a set of proposals for the origin of life and the early evolution of life advanced by Günter Wächtershäuser, a Munich patent lawyer with a degree in chemistry who had been encouraged and supported by philosopher Karl R. Popper to publish his ideas. The theory...

).

Mike Russel demonstrated that alkaline vents created a chemical gradient, in which conditions are ideal for an abiogenic hatchery for life. Their microscopic compartments "provide a natural means of concentrating organic molecules, composed of iron-sulphur minerals such as mackinawite
Mackinawite
Mackinawite is an iron nickel sulfide mineral with formula 1 + xS . It crystallizes in the tetragonal crystal system and occurs as opaque bronze to grey-white tabular crystals and anhedral masses. It has a Mohs hardness of 2.5 and a specific gravity of 4.17.-Occurrence:It occurs in serpentinized...

, endowered these mineral cells with the catalytic properties envisaged by Günter Wächtershäuser
Günter Wächtershäuser
Günter Wächtershäuser , a German chemist turned patent lawyer, is mainly known for his work on the origin of life, and in particular his iron-sulfur world theory, a theory that life on Earth had hydrothermal origins...

.

Fox's experiments


In the 1950s and 1960s, Sidney W. Fox
Sidney W. Fox
Sidney Walter Fox was a Los Angeles-born biochemist responsible for unique discoveries in the autosynthesis of protocells.-Professor:In 1943 Fox was granted his first academic position at Iowa State College....

 studied the spontaneous formation of peptide
Peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...

 structures under conditions that might plausibly have existed early in Earth's history. He demonstrated that amino acids could spontaneously form small peptides. These amino acids and small peptides could be encouraged to form closed spherical membranes, called proteinoid
Proteinoid
Proteinoids, or thermal proteins, are protein-like molecules formed inorganically from amino acids. Some theories of abiogenesis propose that proteinoids were a precursor to the first living cells....

 microspheres, which show many of the basic characteristics of 'life'.

Eigen's hypothesis


In the early 1970s, the problem of the origin of life was approached by Manfred Eigen
Manfred Eigen
Manfred Eigen is a German biophysical chemist who won the 1967 Nobel Prize in Chemistry for work on measuring fast chemical reactions.-Career:...

 and Peter Schuster
Peter Schuster
Peter K. Schuster is a renowned theoretical chemist, known for his work with the German Nobel Laureate Manfred Eigen in developing the quasispecies model...

 of the Max Planck Institute for Biophysical Chemistry
Max Planck Institute for Biophysical Chemistry
The Max Planck Institute for Biophysical Chemistry in Göttingen is a research institute of the Max Planck Society. Currently, 812 people work at the Institute, 353 of them are scientists....

. They examined the transient stages between the molecular chaos and a self-replicating hypercycle
Hypercycle (chemistry)
A Hypercycle is a new level of organization whereby self-replicative units are connected in a cyclic, autocatalytic manner. The self-replicative units are themselves catalytic cycles. The Hypercycle is a specific model of the chemical origin of life, pioneered by Eigen and Schuster...

 in a prebiotic soup.

In a hypercycle, the information storing system
Information
Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...

 (possibly RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

) produces an enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

, 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. Mathematically treated, hypercycles could create quasispecies
Quasispecies model
The quasispecies model is a description of the process of the Darwinian evolution of certain self-replicating entities within the framework of physical chemistry...

, which through natural selection entered into a form of Darwinian evolution. A boost to hypercycle theory was the discovery that RNA, in certain circumstances, forms itself into ribozyme
Ribozyme
A ribozyme is an RNA molecule with a well defined tertiary structure that enables it to catalyze a chemical reaction. Ribozyme means ribonucleic acid enzyme. It may also be called an RNA enzyme or catalytic RNA. Many natural ribozymes catalyze either the hydrolysis of one of their own...

s, capable of catalyzing their own chemical reactions. However, these reactions are limited to self-excisions (in which a longer RNA molecule becomes shorter), and much rarer small additions that are incapable of coding for any useful protein. The hypercycle theory is further degraded since the hypothetical RNA would require the existence of complex biochemicals such as nucleotides which are not formed under the conditions proposed by the Miller–Urey experiment.

Hoffmann's contributions


Geoffrey W. Hoffmann
Geoffrey W. Hoffmann
Geoffrey W. Hoffmann, is an Australian-Canadian theoretical biologist. Hoffmann was a faculty member in the Department of Physics at the University of British Columbia and is currently chairman and chief scientist at Network Immunology Inc. in Vancouver, Canada...

, a student of Eigen, contributed to the concept of life involving both replication and metabolism emerging from catalytic noise. His contributions included showing that an early sloppy translation machinery can be stable against an error catastrophe of the type that had been envisaged as problematical by Leslie Orgel
Leslie Orgel
Leslie Eleazer Orgel FRS was a British chemist.Born in London, England, Orgel received his B.A. in chemistry with first class honours from Oxford University in 1949...

 ("Orgel's paradox") and calculations regarding the occurrence of a set of required catalytic activities together with the exclusion of catalytic activities that would be disruptive. This is called the stochastic theory of the origin of life.

Wächtershäuser's hypothesis


Another possible answer to this polymerization conundrum was provided in 1980s by the German chemist Günter Wächtershäuser
Günter Wächtershäuser
Günter Wächtershäuser , a German chemist turned patent lawyer, is mainly known for his work on the origin of life, and in particular his iron-sulfur world theory, a theory that life on Earth had hydrothermal origins...

, encouraged and supported by Karl R. Popper, in his iron–sulfur world theory. In this theory, he postulated the evolution of (bio)chemical pathways as fundamentals of the evolution of life. Moreover, he presented a consistent system of tracing today's biochemistry back to ancestral reactions that provide alternative pathways to the synthesis of organic building blocks from simple gaseous compounds.

In contrast to the classical Miller experiments, which depend on external sources of energy (such as simulated lightning or ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 irradiation), "Wächtershäuser systems" come with a built-in source of energy, sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...

s of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 and other minerals (e.g. pyrite). The energy released from redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 reactions of these metal sulfides is not only available for the synthesis of organic molecules, but also for the formation of oligomer
Oligomer
In chemistry, an oligomer is a molecule that consists of a few monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Dimers, trimers, and tetramers are oligomers. Many oils are oligomeric, such as liquid paraffin...

s and polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s. It is therefore hypothesized that such systems may be able to evolve into autocatalytic set
Autocatalytic set
An autocatalytic set is a collection of entities, each of which can be created catalytically by other entities within the set, such that as a whole, the set is able to catalyze its own production. In this way the set as a whole is said to be autocatalytic...

s of self-replicating, metabolically active entities that would predate the life forms known today.

The experiment produced a relatively small yield of dipeptides (0.4% to 12.4%) and a smaller yield of tripeptide
Tripeptide
A tripeptide is a peptide consisting of three amino acids joined by peptide bonds.Examples of tripeptides are:*Eisenin is a peptide with immunological activity that is isolated from the Japanese marine alga, Eisenia bicyclis, which more commonly is known as, Arame*GHK-Cu is a human copper binding...

s (0.10%) but the authors also noted that: "under these same conditions dipeptides hydrolysed rapidly."

Radioactive beach hypothesis


Zachary Adam at the University of Washington
University of Washington
University of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...

, Seattle, claims that stronger tidal processes from a much closer moon may have concentrated grains of uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

 and other radioactive elements at the high water mark on primordial beaches where they may have been responsible for generating life's building blocks. According to computer models reported in Astrobiology, a deposit of such radioactive materials could show the same self-sustaining nuclear reaction as that found in the Oklo
Oklo
Oklo is a region near the town of Franceville, in the Haut-Ogooué province of the Central African state of Gabon. Several natural nuclear fission reactors were discovered in the uranium mines in the region in 1972.-History:...

 uranium ore seam in Gabon
Gabon
Gabon , officially the Gabonese Republic is a state in west central Africa sharing borders with Equatorial Guinea to the northwest, Cameroon to the north, and with the Republic of the Congo curving around the east and south. The Gulf of Guinea, an arm of the Atlantic Ocean is to the west...

. Such radioactive beach sand provides sufficient energy to generate organic molecules, such as amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s and sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

s from acetonitrile
Acetonitrile
Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...

 in water. Radioactive monazite
Monazite
Monazite is a reddish-brown phosphate mineral containing rare earth metals. It occurs usually in small isolated crystals. There are actually at least four different kinds of monazite, depending on relative elemental composition of the mineral:...

 also releases soluble phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...

 into regions between sand-grains, making it biologically "accessible". Thus amino acids, sugars and soluble phosphates can all be simultaneously produced, according to Adam. Radioactive actinide
Actinide
The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...

s, then in greater concentrations, could have formed part of organo-metallic complexes. These complexes could have been important early catalysts to living processes.

John Parnell of the University of Aberdeen
University of Aberdeen
The University of Aberdeen, an ancient university founded in 1495, in Aberdeen, Scotland, is a British university. It is the third oldest university in Scotland, and the fifth oldest in the United Kingdom and wider English-speaking world...

 suggests that such a process could provide part of the "crucible of life" on any early wet rocky planet, so long as the planet is large enough to have generated a system of plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

 which brings radioactive minerals to the surface. As the early Earth is believed to have had many smaller "platelets" it would provide a suitable environment for such processes.

Thermodynamic origin of life: ultraviolet and temperature-assisted replication (UVTAR) model


Karo Michaelian of the National Autonomous University of Mexico
National Autonomous University of Mexico
The Universidad Nacional Autónoma de México is a university in Mexico. UNAM was founded on 22 September 1910 by Justo Sierra as a liberal alternative to the Roman Catholic-sponsored Royal and Pontifical University of Mexico The Universidad Nacional Autónoma de México (UNAM) (National Autonomous...

 points out that any model for the origin of life must take into account the fact that life is an irreversible thermodynamic process which arises and persists to produce entropy. Entropy production is not incidental to the process of life, but rather the fundamental reason for its existence. Present day life augments the entropy production of Earth by catalysing the water cycle through evapotranspiration
Evapotranspiration
Evapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...

.
Michaelian argues that if the thermodynamic function of life today is to produce entropy through coupling with the water cycle, then this probably was its function at its very beginnings. 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 200–300 nm wavelength range, just that high energy part of the sun's spectrum that could have penetrated the dense prebiotic atmosphere. Cnossen et al.
have shown that the amount of ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 (UV) light reaching the Earth's surface in the Archean could have been up to 31 orders of magnitude larger than it is today at 260 nm where RNA and DNA absorb most strongly. Absorption and dissipation of UV light by these organic molecules at the Archean ocean surface would have increased significantly the temperature of the surface skin layer leading to enhanced evaporation and thus augmenting the primitive water cycle. Since absorption and dissipation of high energy photons is an entropy producing process, Michaelian argues that non-equilbrium abiogenic synthesis of RNA and DNA utilizing UV light would have been thermodynamically favored.

A simple mechanism to explain the replication of RNA and DNA without the use of enzymes can also be given within the same thermodynamic framework by assuming that life arose when the temperature of the primitive seas had cooled to somewhat below the denaturing
Denaturation (biochemistry)
Denaturation is a process in which proteins or nucleic acids lose their tertiary structure and secondary structure by application of some external stress or compound, such as a strong acid or base, a concentrated inorganic salt, an organic solvent , or heat...

 temperature of RNA or DNA (based on the ratio of 18O/16O found in cherts of the Barberton greenstone belt of South Africa of about 3.5 to 3.2 Ga., surface temperatures are predicted to have been around 70±15 °C, similar to RNA or DNA denaturing temperatures). During the night, the surface water temperature would be below the denaturing temperature and single strand RNA/DNA could act as a template for the formation of double strand RNA/DNA. During the daylight hours, RNA and DNA would absorb UV light and convert this directly to heating of the ocean surface, raising the local temperature enough to allow for denaturing of RNA and DNA. The copying process would be repeated during the cool period overnight.
Such a temperature assisted mechanism of replication bears similarity to polymerase chain reaction
Polymerase chain reaction
The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....

 (PCR), a routine laboratory procedure to multiply DNA segments. Michaelian suggests that traditional origin of life research, expecting to describe the emergence of life from near-equilibrium conditions, is erroneous and that non-equilibrium conditions must be considered, in particular, the importance of entropy production to the emergence of life.

Since denaturation would be most probable in the late afternoon when the Archean sea surface temperature would be highest, and since late afternoon submarine sunlight is somewhat circularly polarized, the homochirality of the organic molecules of life can also be explained within the proposed thermodynamic framework.

Models to explain homochirality



Some process in chemical evolution must account for the origin of homochirality
Homochirality
Homochirality is a term used to refer to a group of molecules that possess the same sense of chirality. Molecules involved are not necessarily the same compound, but similar groups are arranged in the same way around a central atom. In biology homochirality is found in the chemical building blocks...

, i.e. all building blocks in living organisms having the same "handedness" (amino acids being left-handed, nucleic acid sugars (ribose
Ribose
Ribose is an organic compound with the formula C5H10O5; specifically, a monosaccharide with linear form H––4–H, which has all the hydroxyl groups on the same side in the Fischer projection....

 and deoxyribose
Deoxyribose
Deoxyribose, more, precisely 2-deoxyribose, is a monosaccharide with idealized formula H---3-H. Its name indicates that it is a deoxy sugar, meaning that it is derived from the sugar ribose by loss of an oxygen atom...

) being right-handed, and chiral phosphoglycerides
Glycerophospholipid
Glycerophospholipids or phosphoglycerides are glycerol-based phospholipids. They are the main component of biological membranes.-Structures:...

). 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. This is called a racemic
Racemic
In chemistry, a racemic mixture, or racemate , is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was "racemic acid", which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.- Nomenclature :A...

 mixture. Clark has suggested that homochirality may have started in space, as the studies of the amino acids on the Murchison meteorite
Murchison meteorite
The Murchison meteorite is named after Murchison, Victoria, in Australia. It is one of the most studied meteorites due to its large mass , the fact that it was an observed fall, and it belongs to a group of meteorites rich in organic compounds....

 showed L-alanine to be more than twice as frequent as its D form, and L-glutamic acid was more than 3 times prevalent than its D counterpart. It is suggested that polarised light has the power to destroy one enantiomer
Enantiomer
In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...

 within the proto-planetary disk. Noyes showed that beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...

 caused the breakdown of D-leucine
Leucine
Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins...

, in a racemic
Racemic
In chemistry, a racemic mixture, or racemate , is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was "racemic acid", which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.- Nomenclature :A...

 mixture, and that the presence of 14C
Carbon-14
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues , to date archaeological, geological, and hydrogeological...

, present in larger amounts in organic chemicals in the early Earth environment, could have been the cause. Robert M. Hazen reports upon experiments conducted in which various chiral crystal surfaces act as sites for possible concentration and assembly of chiral monomer units into macromolecules. Once established, chirality would be selected for. Work with organic compounds found on meteorites tends to suggest that chirality is a characteristic of abiogenic synthesis, as amino acids show a left-handed bias, whereas sugars show a predominantly right-handed bias.

Self-organization and replication


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. For example Martin and Russel show that physical compartmentation by cell membranes from the environment and self-organization of self-contained redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 reactions are the most conserved attributes of living things, and they argue therefore that inorganic matter with such attributes would be life's most likely last common ancestor.

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.

From organic molecules to protocells


The question "How do simple organic molecules form a protocell?" is largely unanswered but there are many hypotheses. Some of these postulate the early appearance of nucleic acids ("gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s-first") whereas others postulate the evolution of biochemical reactions and pathways first ("metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

-first"). Recently, trends are emerging to create hybrid models that combine aspects of both.

Researcher Martin Hanczyc supports the idea of a gradient between life and non-life (i.e. there is no simple line between the two). He thinks that building simple protocells, in the lab, is one of the first steps towards understanding more complex cells including those that may have later evolved into complex life. Hanczyc says that living cells often consist of somewhere around 1 000 000 types of molecules, whereas his labs are first aiming at creating life-like systems using around 10 molecules. His protocells display behaviors even simpler than those displayed by things like viruses (e.g. only basic motion, dividing and combining cell walls, and so on).

The RNA world


The RNA world hypothesis
RNA world hypothesis
The RNA world hypothesis proposes that life based on ribonucleic acid pre-dates the current world of life based on deoxyribonucleic acid , RNA and proteins. RNA is able both to store genetic information, like DNA, and to catalyze chemical reactions, like an enzyme protein...

 describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins. This has spurred scientists to try to determine if relatively short RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 molecules could have spontaneously formed that were capable of catalyzing their own continuing replication. A number of hypotheses of modes of formation have been put forward. Early cell membranes could have formed spontaneously from proteinoid
Proteinoid
Proteinoids, or thermal proteins, are protein-like molecules formed inorganically from amino acids. Some theories of abiogenesis propose that proteinoids were a precursor to the first living cells....

s, protein-like molecules that are produced when amino acid solutions are heated–when present at the correct concentration in aqueous solution, these form microspheres which are observed to behave similarly to membrane-enclosed compartments. Other possibilities include systems of chemical reactions taking place within clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 substrates or on the surface of pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...

 rocks. Factors supportive of an important role for RNA in early life include its ability to act both to store information and catalyse chemical reactions (as a ribozyme
Ribozyme
A ribozyme is an RNA molecule with a well defined tertiary structure that enables it to catalyze a chemical reaction. Ribozyme means ribonucleic acid enzyme. It may also be called an RNA enzyme or catalytic RNA. Many natural ribozymes catalyze either the hydrolysis of one of their own...

); its many important roles as an intermediate in the expression and maintenance of the genetic information (in the form of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

) in modern organisms; and the ease of chemical synthesis of at least the components of the molecule under conditions approximating the early Earth. Relatively short RNA molecules which can duplicate others have been artificially produced in the lab. Such replicase RNA, which functions as both code and catalyst provides a template upon which copying can occur. Jack Szostak has shown that certain catalytic RNAs can, indeed, join smaller RNA sequences together, creating the potential, in the right conditions for self-replication. If these were present, Darwinian selection would favour the proliferation of such self-catalysing structures, to which further functionalities could be added. Lincoln and Joyce identified an RNA enzyme capable of self sustained replication.

Researchers have pointed out difficulties for the abiotic synthesis of nucleotides from cytosine
Cytosine
Cytosine is one of the four main bases found in DNA and RNA, along with adenine, guanine, and thymine . It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached . The nucleoside of cytosine is cytidine...

 and uracil
Uracil
Uracil is one of the four nucleobases in the nucleic acid of RNA that are represented by the letters A, G, C and U. The others are adenine, cytosine, and guanine. In RNA, uracil binds to adenine via two hydrogen bonds. In DNA, the uracil nucleobase is replaced by thymine.Uracil is a common and...

. Cytosine has a half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...

 of 19 days at 100 °C (212 °F) and 17,000 years in freezing water. Larralde et al., say that "the generally accepted prebiotic synthesis of ribose, the formose reaction, yields numerous sugars without any selectivity." and they conclude that their "results suggest that the backbone of the first genetic material could not have contained ribose or other sugars because of their instability." The ester linkage of ribose and phosphoric acid in RNA is known to be prone to hydrolysis.

A slightly different version of the RNA-world hypothesis is that a different type of nucleic acid
Nucleic acid
Nucleic acids are biological molecules essential for life, and include DNA and RNA . Together with proteins, nucleic acids make up the most important macromolecules; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information...

, such as PNA, TNA or GNA, was the first one to emerge as a self-reproducing molecule, to be replaced by RNA only later. Pyrimidine ribonucleosides and their respective nucleotides have been prebiotically synthesised by a sequence of reactions which by-pass the free sugars, and are assembled in a stepwise fashion by going against the dogma that nitrogenous and oxygenous chemistries should be avoided. In a series of publications, The Sutherland Group at the School of Chemistry, University of Manchester have 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. One of the steps in this sequence allows the isolation of enantiopure ribose aminooxazoline if the enantiomeric excess of glyceraldehyde is 60 % or greater. This can be viewed as a prebiotic purification step, where the said compound spontaneously crystallised out from a mixture of the other pentose aminooxazolines. Ribose aminooxazoline can then react with cyanoacetylene in a mild and highly efficient manner to give the alpha cytidine ribonucleotide. Photoanomerization with UV light allows for inversion about the 1' anomeric centre to give the correct beta stereochemistry. 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 polymerise into RNA. This paper also highlights the possibility for the photo-sanitization of the pyrimidine-2',3'-cyclic phosphates. James Ferris's studies have shown that clay minerals of montmorillonite
Montmorillonite
Montmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming a clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral sheets sandwiching a central...

 will catalyze the formation of RNA in aqueous solution, by joining activated mono RNA nucleotides to join together to form longer chains. Although these chains have random sequences, the possibility that one sequence began to non-randomly increase its frequency by increasing the speed of its catalysis is possible to "kick start" biochemical evolution.

"Metabolism first" models


Several models reject the idea of the self-replication of a "naked-gene" and postulate the emergence of a primitive metabolism which could provide an environment for the later emergence of RNA replication. The centrality of the Krebs cycle to energy production in aerobic organisms, and in drawing in carbon dioxide and hydrogen ions in biosynthesis of complex organic chemicals, including amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s and nucleotides, suggests that it was one of the first parts of the metabolism to evolve. Harold Morowitz concludes that given sufficient concentrations of ingredients the cycle will "spin" of its own, as the concentration of each intermediate rises, it tends to convert into the next intermediate spontaneously. It thus appears to be in origin, not a creation of the genes, but the product of thermodynamics and chemistry alone. Somewhat in agreement with these notions, physicist Sean Carroll
Sean M. Carroll
Sean Michael Carroll is a senior research associate in the Department of Physics at the California Institute of Technology. He is a theoretical cosmologist specializing in dark energy and general relativity...

 has proposed that "the purpose of life is to hydrogenate carbon dioxide" (as part of a "metabolism-first", rather than a "genetics-first", scenario).
Iron-sulfur world

One of the earliest incarnations of this idea was put forward in 1924 with Alexander Oparin's notion of primitive self-replicating vesicles
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...

 which predated the discovery of the structure of DNA. More recent variants in the 1980s and 1990s include Günter Wächtershäuser
Günter Wächtershäuser
Günter Wächtershäuser , a German chemist turned patent lawyer, is mainly known for his work on the origin of life, and in particular his iron-sulfur world theory, a theory that life on Earth had hydrothermal origins...

's iron-sulfur world theory
Iron-sulfur world theory
The iron-sulfur world theory is a set of proposals for the origin of life and the early evolution of life advanced by Günter Wächtershäuser, a Munich patent lawyer with a degree in chemistry who had been encouraged and supported by philosopher Karl R. Popper to publish his ideas. The theory...

 and models introduced by Christian de Duve
Christian de Duve
Christian René, viscount de Duve is a Nobel Prize-winning cytologist and biochemist. De Duve was born in Thames Ditton, Surrey, Great Britain, as a son of Belgian refugees. They returned to Belgium in 1920...

 based on the chemistry of thioester
Thioester
Thioesters are compounds with the functional group C-S-CO-C. They are the product of esterification between a carboxylic acid and a thiol. Thioesters are widespread in biochemistry, the best-known derivative being acetyl-CoA.-Synthesis:...

s. 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
Freeman Dyson
Freeman John Dyson FRS is a British-born American theoretical physicist and mathematician, famous for his work in quantum field theory, solid-state physics, astronomy and nuclear engineering. Dyson is a member of the Board of Sponsors of the Bulletin of the Atomic Scientists...

 in the early 1980s and Stuart Kauffman
Stuart Kauffman
Stuart Alan Kauffman is an American theoretical biologist and complex systems researcher concerning the origin of life on Earth...

's notion of collectively autocatalytic set
Autocatalytic set
An autocatalytic set is a collection of entities, each of which can be created catalytically by other entities within the set, such that as a whole, the set is able to catalyze its own production. In this way the set as a whole is said to be autocatalytic...

s, discussed later in that decade.

However, the idea that a closed metabolic cycle, such as the reductive citric acid cycle
Citric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...

, could form spontaneously (proposed by Günter Wächtershäuser) remains debated. In an article entitled "Self-Organizing Biochemical Cycles", the late Leslie Orgel
Leslie Orgel
Leslie Eleazer Orgel FRS was a British chemist.Born in London, England, Orgel received his B.A. in chemistry with first class honours from Oxford University in 1949...

 summarized his analysis of the proposal by stating, "There is at present no reason to expect that multistep cycles such as the reductive citric acid cycle will self-organize on the surface of FeS/FeS2 or some other mineral." It is possible that another type of metabolic pathway was used at the beginning of life. For example, instead of the reductive citric acid cycle, the "open" acetyl-CoA
Acetyl-CoA
Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...

 pathway (another one of the five recognised ways of carbon dioxide fixation in nature today) would be compatible with the idea of self-organisation on a metal sulfide surface. The key enzyme of this pathway, carbon monoxide dehydrogenase/acetyl-CoA synthase harbours mixed nickel-iron-sulfur clusters in its reaction centers and catalyses the formation of acetyl-CoA (which may be regarded as a modern form of acetyl-thiol) in a single step.
Thermosynthesis world

Today’s bioenergetic process of fermentation
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...

 is related to the just mentioned citric acid cycle or the Acetyl-CoA pathway that have been connected to the primordial iron-sulfur world. In a different approach, today’s bioenergetic process of chemiosmosis
Chemiosmosis
Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration....

, which plays an essential role in cellular respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...

 and photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, is considered as more fundamental than fermentation: in Anthonie Muller’s “thermosynthesis
Thermosynthesis
Thermosynthesis is a theoretical mechanism proposed by Anthonie Muller for biological use of the free energy in a temperature gradient to drive energetically uphill anabolic reactions. It makes use of this thermal gradient, or the dissipative structure of convection in this gradient, to drive a...

 world” the ATP Synthase
ATP synthase
right|thumb|300px|Molecular model of ATP synthase by X-ray diffraction methodATP synthase is an important enzyme that provides energy for the cell to use through the synthesis of adenosine triphosphate . ATP is the most commonly used "energy currency" of cells from most organisms...

 enzyme that sustains chemiosmosis is proposed as today’s enzyme that is the closest connected to the first metabolic process.

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
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

. 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 after a temperature change was released upon a thermal unfolding.

The energy source of the thermosynthesis world was thermal cycling, the result of suspension of the protocell in a convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 current, as is plausible in a volcanic hot spring; the convection accounts for the self-organization
Self-organization
Self-organization is the process where a structure or pattern appears in a system without a central authority or external element imposing it through planning...

 and dissipative structure required in any origin of life model. The still ubiquitous role of thermal cycling in germination and cell division is considered a relic of primordial thermosynthesis.

By phosphorylating cell membrane lipids, this ‘First Protein’ gave a selective advantage to the lipid protocell that contained the protein. In the beginning this First Protein also synthesized a library with many proteins, of which only a minute fraction had thermosynthesis capabilities. Just as proposed by Dyson
Freeman Dyson
Freeman John Dyson FRS is a British-born American theoretical physicist and mathematician, famous for his work in quantum field theory, solid-state physics, astronomy and nuclear engineering. Dyson is a member of the Board of Sponsors of the Bulletin of the Atomic Scientists...

  for the first proteins, the First Protein propagated functionally: it made daughters with similar capabilities, but it did not copy itself. Functioning daughters consisted of different amino acid sequences.

Over a long time, RNA sequences were selected among the at first randomly synthesized RNAs by the criterion of speed and efficiency increase of First Protein synthesis, for instance by the creation of RNA that functioned as messenger RNA, Transfer RNA and ribosomal RNA, or, even more generally, all the components of the RNA World were also generated and selected. The thermosynthesis world therefore in theory accounts for the origin of the genetic machinery.

Whereas the iron-sulfur world identifies a circular pathway as the most simple—and therefore assumes the existence of enzymes—the thermosynthesis world does not even invoke a pathway, and does not assume the existence of regular enzymes: 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. The RNA World also implies the existence of several enzymes. But even the emergence of a single enzyme by chance is implausible. The thermosynthesis world is therefore more simple, and thus more plausible, than the iron-sulfur and RNA worlds.
Possible role of bubbles

Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

. While bubbles composed mostly of water burst quickly, water containing amphiphiles
Amphiphiles
Amphiphile is a term describing a chemical compound possessing both hydrophilic and lipophilic properties...

 forms much more stable bubbles, lending more time to the particular bubble to perform these crucial reactions.

Amphiphiles are oily compounds containing a hydrophilic head on one or both ends of a hydrophobic molecule. Some amphiphiles have the tendency to spontaneously form membranes in water. A spherically closed membrane contains water and is a hypothetical precursor to the modern cell membrane. If a protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 would increase the integrity of its parent bubble, that bubble had an advantage, and was placed at the top of the natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

 waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the 'experiment' into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first prokaryotes, eukaryotes, and multicellular organisms could be achieved.

Similarly, bubbles formed entirely out of protein-like molecules, called microsphere
Microsphere
Microsphere are small spherical particles, with diameters in the micrometer range . Microspheres are sometimes referred to as microparticles....

s, will form spontaneously under the right conditions. But they are not a likely precursor to the modern cell membrane, as cell membranes are composed primarily of lipid compounds rather than amino-acid compounds (for types of membrane spheres associated with abiogenesis, see protobionts, micelle
Micelle
A micelle is an aggregate of surfactant molecules dispersed in a liquid colloid. A typical micelle in aqueous solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single tail regions in the micelle centre. This phase is...

, coacervate
Coacervate
A coacervate is a tiny spherical droplet of assorted organic molecules which is held together by hydrophobic forces from a surrounding liquid....

).

A recent model by Fernando and Rowe suggests that the enclosure of an autocatalytic non-enzymatic metabolism within protocells may have been one way of avoiding the side-reaction problem that is typical of metabolism first models.
Possible role of pumice rafts

An alternative (or perhaps adjunct) theory, to the formation of bubbles via waves breaking on the shore creating delicate foam, is the hypothetical creation of bubbles formed within pores of a pumice raft
Pumice raft
A pumice raft is a floating raft of pumice occasionally created by ocean-based volcanic activity.Volcanic activity in the South Pacific near Tonga on August 12, 2006 caused the emergence of a new island...

. Like the windblown foam, the pumice rafts would also have made landfall, and this is observed in modern times. Paleontological evidence of pumice rafts associated with Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...

 life have been discovered in Australia.

Although the windblown concentration of organic molecules may have been a key part of the abiogenesis puzzle, even with amphiphilic stabilization, exposure to the elements may have rendered the fragile foam too unstable to be an abiogenesis precursor and/or its ongoing natural selection actor.

A possibly more probable bubble formation environment for the 'cradle of life' to occur (due to its greater stability-longer 'lifetime') and optimum size (micron) range would have been the protected environment within the pores of the pumice. The crucial reaction time necessary could have been greatly extended in this protected environment. Relatively rapid selection pressure could have been applied if the pumice raft landed on active geothermal outgassing percolation (acting something like an airstone in an aquarium) pumping out massive quantities of various bubble quasispecies and then species probabilistically interacting and evolving.

Autocatalysis


In 1993 Stuart Kauffman proposed that life initially arose as autocatalytic chemical networks.

British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 ethologist Richard Dawkins
Richard Dawkins
Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...

 wrote about autocatalysis
Autocatalysis
A single chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product itself is the catalyst for that reaction....

 as a potential explanation for the origin of life in his 2004 book The Ancestor's Tale
The Ancestor's Tale
The Ancestor's Tale is a 2004 popular science book by Richard Dawkins, with contributions from Dawkins' research assistant Yan Wong. It follows the path of humans backwards through evolutionary history, meeting humanity's cousins as they converge on common ancestors...

. Autocatalysts are substances which catalyze the production of themselves, and therefore have the property of being a simple molecular replicator. In his book, Dawkins cites experiments performed by Julius Rebek
Julius Rebek
Julius Rebek, Jr. is a Hungarian-born American chemist and expert on molecular self-assembly.Rebek was born in Beregszasz , Hungary in 1944 and lived in Austria from 1945 to 1949. In 1949 he and his family immigrated to the United States and settled in Kansas. Rebek graduated from the University...

 and his colleagues at the Scripps Research Institute in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

 in which they combined amino adenosine and pentafluorophenyl ester with the autocatalyst amino adenosine triacid ester (AATE). One system from the experiment contained variants of AATE which catalysed the synthesis of themselves. 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
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

.

Clay hypothesis


A model for the origin of life based on clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 was forwarded by A. Graham Cairns-Smith
Graham Cairns-Smith
Alexander Graham Cairns-Smith is an organic chemist and molecular biologist at the University of Glasgow, most famous for his controversial 1985 book, Seven Clues to the Origin of Life...

 of the University of Glasgow
University of Glasgow
The University of Glasgow is the fourth-oldest university in the English-speaking world and one of Scotland's four ancient universities. Located in Glasgow, the university was founded in 1451 and is presently one of seventeen British higher education institutions ranked amongst the top 100 of the...

 in 1985 and explored as a plausible illustration by several other scientists, including Richard Dawkins
Richard Dawkins
Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...

. Clay hypothesis postulates that complex organic molecules arose gradually on a pre-existing, non-organic replication platform—silicate crystals in solution. Complexity in companion molecules developed as a function of selection pressures on types of clay crystal is then exapted
Exaptation
Exaptation, cooption, and preadaptation are related terms referring to shifts in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common in both anatomy and behaviour...

 to serve the replication of organic molecules independently of their silicate "launch stage".

Cairns-Smith is a staunch critic of other models of chemical evolution. However, he admits that like many models of the origin of life, his own also has its shortcomings (Horgan 1991).

In 2007, Kahr and colleagues reported their experiments to examine the idea that crystals can act as a source of transferable information, using crystals of potassium hydrogen phthalate
Potassium hydrogen phthalate
Potassium hydrogen phthalate, often called simply KHP, is an acidic salt compound. It forms white powder, colorless crystals, a colorless solution, and an ionic solid that is the monopotassium salt of phthalic acid...

. "Mother" crystals with imperfections were cleaved and used as seeds to grow "daughter" crystals from solution. They then examined the distribution of imperfections in the crystal system and found that the imperfections in the mother crystals were indeed reproduced in the daughters, but the daughter crystals had many additional imperfections. 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, and it did not. Thus Kahr concludes that the crystals "were not faithful enough to store and transfer information from one generation to the next".

Gold's "deep-hot biosphere" model


In the 1970s, Thomas Gold
Thomas Gold
Thomas Gold was an Austrian-born astrophysicist, a professor of astronomy at Cornell University, a member of the U.S. National Academy of Sciences, and a Fellow of the Royal Society . Gold was one of three young Cambridge scientists who in the 1950s proposed the now mostly abandoned 'steady...

 proposed the theory that life first developed not on the surface of the Earth, but several kilometers below the surface. The discovery in the late 1990s of nanobe
Nanobe
A nanobe is a tiny filamental structure first found in some rocks and sediments. Some scientists hypothesize that nanobes are the smallest form of life, 1/10 the size of the smallest known bacteria...

s (filamental structures that are smaller than bacteria, but that may contain DNA) in deep rocks might be seen as lending support to Gold's theory.

It is now reasonably well established that microbial life is plentiful at shallow depths in the Earth, up to 5 kilometres (3.1 mi) below the surface, in the form of extremophile archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

, rather than the better-known eubacteria (which live in more accessible conditions). It is claimed that discovery of microbial life below the surface of another body in our solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 would lend significant credence to this theory. Thomas 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. Gold's theory is that flow of food is due to out-gassing of primordial methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 from the Earth's mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....

; 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.

"Primitive" extraterrestrial life


An alternative to Earthly abiogenesis is the hypothesis that primitive life may have originally formed extraterrestrially, either in space or on Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

, a nearby planet. (Note that exogenesis is related to, but not the same as, the notion of panspermia). A supporter of this theory was Francis Crick
Francis Crick
Francis Harry Compton Crick OM FRS was an English molecular biologist, biophysicist, and neuroscientist, and most noted for being one of two co-discoverers of the structure of the DNA molecule in 1953, together with James D. Watson...

.

Organic compounds are relatively common in space, especially in the outer solar system where volatiles are not evaporated by solar heating. Comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s are encrusted by outer layers of dark material, thought to be a tar
Tar
Tar is modified pitch produced primarily from the wood and roots of pine by destructive distillation under pyrolysis. Production and trade in tar was a major contributor in the economies of Northern Europe and Colonial America. Its main use was in preserving wooden vessels against rot. The largest...

-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by irradiation by ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

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

An alternative but related hypothesis, proposed to explain the presence of life on Earth so soon after the planet had cooled down, with apparently very little time for prebiotic evolution, is that life formed first on early Mars. Due to its smaller size Mars cooled before Earth (a difference of hundreds of millions of years), allowing prebiotic processes there while Earth was still too hot. Life was then transported to the cooled Earth when crustal material was blasted off Mars by asteroid and comet impacts. Mars continued to cool faster and eventually became hostile to the continued evolution or even existence of life (it lost its atmosphere due to low volcanism); Earth is following the same fate as Mars, but at a slower rate.

Neither hypothesis actually answers the question of how life first originated, but merely shifts it to another planet or a comet. However, the advantage of an extraterrestrial origin of primitive life is that life is not required to have evolved 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. Evidence to support the hypothesis is scant, but it finds support in recent study of Martian meteorites found in Antarctica and in studies of extremophile
Extremophile
An extremophile is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles...

 microbes. Additional support comes from a recent discovery of a bacterial ecosystem whose energy source is radioactivity.

A 2001 experiment led by Jason Dworkin
subjected a frozen mixture of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

, ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 to UV radiation, mimicking conditions found in an extraterrestrial environment. This combination yielded large amounts of organic material that self-organised to form bubbles or micelles when immersed in water. Dworkin considered these bubbles to resemble cell membranes that enclose and concentrate the chemistry of life, separating their interior from the outside world.

The bubbles produced in these experiments were between 10 to 40 µm (0.000393700787401575 to 0.0015748031496063 in), or about the size of red blood cells. Remarkably, the bubbles fluoresced, or glowed, when exposed to UV light. Absorbing UV and converting it into visible light in this way was considered one possible way of providing energy to a primitive cell. If such bubbles played a role in the origin of life, the fluorescence could have been a precursor to primitive photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. Such fluorescence also provides the benefit of acting as a sunscreen, diffusing any damage that otherwise would be inflicted by UV radiation. Such a protective function would have been vital for life on the early Earth, since the ozone layer
Ozone layer
The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...

, which blocks out the sun's most destructive UV rays, did not form until after photosynthetic life began to produce oxygen.

Extraterrestrial organic molecules


Another idea is that amino acids which were formed extraterrestrially arrived on Earth via comets. In 2009 it was announced by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 that scientists had identified one of the fundamental chemical building blocks of life in a comet for the first time: glycine
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...

, an amino acid, was detected in the material ejected from Comet Wild-2 in 2004 and grabbed by NASA's Stardust
Stardust (spacecraft)
Stardust is a 300-kilogram robotic space probe launched by NASA on February 7, 1999 to study the asteroid 5535 Annefrank and collect samples from the coma of comet Wild 2. The primary mission was completed January 15, 2006, when the sample return capsule returned to Earth...

 probe. Tiny grains, just a few thousandths of a millimetre in size, were collected from the comet and returned to Earth in 2006 in a sealed capsule, and distributed among the world's leading astro-biology labs. NASA said in a statement that it took some time for the investigating team, led by Dr Jamie Elsila, to convince itself that the glycine signature found in Stardust's sample bay was genuine and not just Earthly contamination. Glycine has been detected in meteorites before and there are also observations in interstellar gas clouds claimed for telescopes, but the Stardust find is described as a first in cometary material. Isotope analysis indicates that the Late Heavy Bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...

 included cometary impacts after the Earth coalesced but before life evolved. Dr. Carl Pilcher, who leads NASA's Astrobiology Institute commented that "The discovery of glycine in a comet supports the idea that the fundamental building blocks of life are prevalent in space, and strengthens the argument that life in the Universe may be common rather than rare."

Recent observations suggests 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
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

 activities. Studies reported in 2008, based on 12C/13C isotopic ratios of organic compounds found in the Murchison meteorite
Murchison meteorite
The Murchison meteorite is named after Murchison, Victoria, in Australia. It is one of the most studied meteorites due to its large mass , the fact that it was an observed fall, and it belongs to a group of meteorites rich in organic compounds....

, suggested that the RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 component uracil
Uracil
Uracil is one of the four nucleobases in the nucleic acid of RNA that are represented by the letters A, G, C and U. The others are adenine, cytosine, and guanine. In RNA, uracil binds to adenine via two hydrogen bonds. In DNA, the uracil nucleobase is replaced by thymine.Uracil is a common and...

 and related molecules, including xanthine
Xanthine
Xanthine , is a purine base found in most human body tissues and fluids and in other organisms. A number of stimulants are derived from xanthine, including caffeine and theobromine....

, were formed extraterrestrially. On August 8, 2011, a report, based on NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 studies with meteorites found on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, was published suggesting DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 components (adenine
Adenine
Adenine is a nucleobase with a variety of roles in biochemistry including cellular respiration, in the form of both the energy-rich adenosine triphosphate and the cofactors nicotinamide adenine dinucleotide and flavin adenine dinucleotide , and protein synthesis, as a chemical component of DNA...

, guanine
Guanine
Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...

 and related organic molecules) were made in outer space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

. More recently, scientists found that the cosmic dust
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...

 permeating the universe contains complex organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars. As one of the scientists noted, "Coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

 and kerogen
Kerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...

 are products of life and it took a long time for them to form ... How do stars make such complicated organics under seemingly unfavorable conditions and [do] it so rapidly?" Further, the scientist suggested that these compounds may have been related to the development of life on earth and 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."

Lipid world


The lipid world
Gard model
The GARD model In the context of abiogenesis, the Lipid-world suggests assemblies of simple molecules, such as lipids, can store and propegate information, thus undergo evolution....

 theory postulates that the first self-replicating object was lipid-like. It is known that phospholipids form bilayers in water while under agitation – the same structure as in cell membranes. These molecules were not present on early Earth, however other amphiphilic long chain molecules also form membranes. 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. 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 favorably. Still, no biochemical mechanism has been offered to support the lipid world theory.

Polyphosphates


The problem with most scenarios of abiogenesis is that the thermodynamic equilibrium of amino acid versus peptides is in the direction of separate amino acids. What has been missing is some force that drives polymerization. The resolution of this problem may well be in the properties of polyphosphate
Polyphosphate
Triphosphates are salts or esters of polymeric oxyanions formed from tetrahedral PO4 structural units linked together by sharing oxygen atoms. When two corners are shared the polyphosphate may have a linear chain structure or a cyclic ring structure. In biology the polyphosphate esters AMP, ADP...

s. Polyphosphates are formed by polymerization of ordinary monophosphate ions PO4−3. Several mechanisms for such polymerization have been suggested. Polyphosphates cause polymerization of amino acids into peptides . They are also logical precursors in the synthesis of such key biochemical compounds as ATP. A key issue seems to be that calcium reacts with soluble phosphate to form insoluble calcium phosphate
Calcium phosphate
Calcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....

 (apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...

), so some plausible mechanism must be found to keep calcium ions from causing precipitation of phosphate.
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.

PAH world hypothesis



Other sources of complex molecules have been postulated, including extraterrestrial stellar or interstellar origin. For example, from spectral analyses, organic molecules are known to be present in comets and meteorites. In 2004, a team detected traces of polycyclic aromatic hydrocarbons (PAHs) in a nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

. More recently, in 2010, another team also detected PAHs, along with fullerenes (or "buckyballs"), in nebulae. PAHs are the most complex molecules so far found in space. The use of PAHs has also been proposed as a precursor to the RNA world in the PAH world hypothesis
PAH world hypothesis
The PAH world hypothesis is a speculative hypothesis that proposes that polycyclic aromatic hydrocarbons , assumed to be abundant in the primordial soup of the early Earth, played a major role in the origin of life by mediating the synthesis of RNA molecules, leading into the RNA world...

. The Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

 has recently detected a star, HH 46-IR, which is forming by a process similar to that by which the sun formed. In the disk of material surrounding the star, there is a very large range of molecules, including cyanide compounds, hydrocarbons, and carbon monoxide. PAHs have also been found all over the surface of galaxy M81, which is 12 million light years away from the Earth, confirming their widespread distribution in space.

Multiple genesis


Different forms of life may have appeared quasi-simultaneously in the early history of Earth. The other forms may be extinct, leaving distinctive fossils through their different biochemistry (e.g., using arsenic instead of phosphorus), survive as extremophiles, or simply be unnoticed through their being analogous
Analogy (biology)
An analogy is a trait or an organ that appears similar in two unrelated organisms. The cladistic term for the same phenomenon is homoplasy, from Greek for same form. Biological anologies are often the result of convergent evolution....

 to organisms of the current life tree. Hartman for example combines a number of theories together, by proposing that:
The first organisms were self-replicating iron-rich clays which fixed carbon dioxide into oxalic and other dicarboxylic acid
Dicarboxylic acid
Dicarboxylic acids are organic compounds that contain two carboxylic acid functional groups. In molecular formulae for dicarboxylic acids, these groups are often written as HOOC-R-COOH, where R may be an alkyl, alkenyl, alkynyl, or aryl group...

s. This system of replicating clays and their metabolic phenotype then evolved into the sulfide rich region of the hotspring acquiring the ability to fix nitrogen. Finally phosphate was incorporated into the evolving system which allowed the synthesis of nucleotides and phospholipids. If biosynthesis recapitulates biopoesis, then the synthesis of amino acids preceded the synthesis of the purine and pyrimidine bases. Furthermore the polymerization of the amino acid thioesters into polypeptides preceded the directed polymerization of amino acid esters by polynucleotides.

Lynn Margulis
Lynn Margulis
Lynn Margulis was an American biologist and University Professor in the Department of Geosciences at the University of Massachusetts Amherst. She is best known for her theory on the origin of eukaryotic organelles, and her contributions to the endosymbiotic theory, which is now generally accepted...

's endosymbiotic theory
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...

 suggests that multiple forms of archea entered into symbiotic relationship to form the eukaryotic cell. The horizontal transfer of genetic material between archea promotes such symbiotic relationships, and thus many separate organisms may have contributed to building what has been recognised as the Last Universal Common Ancestor (LUCA) of modern organisms. James Lovelock
James Lovelock
James Lovelock, CH, CBE, FRS is an independent scientist, environmentalist and futurologist who lives in Devon, England. He is best known for proposing the Gaia hypothesis, which postulates that the biosphere is a self-regulating entity with the capacity to keep our planet healthy by controlling...

's Gaia theory proposes that such symbiosis establishes the environment as a system produced by and supportive of life. His arguments strongly weaken the case for life having evolved elsewhere in the solar system.

See also

  • Astrochemistry
    Astrochemistry
    Astrochemistry is the study of the abundance and reactions of chemical elements and molecules in the universe, and their interaction with radiation. The discipline is an overlap of astronomy and chemistry. The word "astrochemistry" may be applied to both the Solar System and the interstellar medium...

  • Biological immortality
    Biological immortality
    Biological immortality refers to a stable rate of mortality as a function of chronological age. Some individual cells and entire organisms in some species achieve this state either throughout their existence or after living long enough. This requires that death occur from injury or disease rather...

  • Common descent
    Common descent
    In evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor....

  • Entropy and life
    Entropy and life
    Research concerning the relationship between the thermodynamic quantity entropy and the evolution of life began in around the turn of the 20th century...

  • List of independent discoveries
  • Mediocrity principle
    Mediocrity principle
    The mediocrity principle is the notion in philosophy of science that there is nothing very unusual about the evolution of our solar system, the Earth, any one nation, or humans. It is a heuristic in the vein of the Copernican principle, and is sometimes used as a philosophical statement about the...

  • Mimivirus
    Mimivirus
    Mimivirus is a viral genus containing a single identified species named Acanthamoeba polyphaga mimivirus , or is a group of phylogenetically related large viruses . In colloquial speech, APMV is more commonly referred to as just “mimivirus”...

  • Planetary habitability
    Planetary habitability
    Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

  • Rare Earth hypothesis
    Rare Earth hypothesis
    In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances...

  • Shadow biosphere
    Shadow biosphere
    The term "shadow biosphere" was coined by Carol Cleland and Shelley Copley. A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life...


Further reading


  • Buehler, Lukas K. (2000–2005) The physico-chemical basis of life, http://www.whatislife.com/about.html accessed 27 October 2005. (Cited on p. 108). (Cited on p. 108).



External links