Abiogenesis

A simpler version of this page is at Origin of life.
Non-scientific ideas how life arose on Earth can be found at creation myth, creationism, guided evolution.

Stromatolites from Bolivia, from the Proterozoic (2.3 bilion years ago. Vertical polished section.

Stromatolites growing in Yalgorup National Park in Australia

Abiogenesis is the study of how the first life forms grew out of non-living matter. The word is made of two parts. One part comes from the Ancient Greek bios, which means life. The other comes from genesis, which means birth or origin. Some people talk about 'chemical evolution'; others talk about the beginnings of biological evolution. The study of abiogenesis uses many sciences. It is related to chemistry, molecular biology, cell biology, earth science, and astronomy.

People like Jean-Baptiste de Lamarck and Charles Darwin saw that the plants and animals on earth change over time. They developed theories to explain this phenomenon: Lamarck developed Lamarckism, and Darwin developed the Darwinian theory of Evolution. This article is not about biological theories of evolution, it looks at the borderland between chemistry and life.

A scientific study from 2002 shows that geological formations of stromatolites 3.45 billion years old contain fossilized cyanobacteria. It is now widely agreed that stromatolites are oldest known life on earth which have left a record of their existence.

Therefore it follows that abiogenesis happened sometime between 4.4 billion years ago, when water vapor first liquefied, and 3.5 billion years ago.

By 2.5 million years ago the ratio of stable isotopes of carbon, iron and sulfur point to a biogenic origin of minerals and sediments, and molecular biomarkers indicate photosynthesis.

Even though it has a great potential impact on how people understand the world, abiogenesis is a limited field of study. It is not well funded, and progress is slow. This is mainly because there are few established facts, and it is not clear how research should proceed. A single well-established fact, such as the discovery of clear evidence of life on another planet, would have a great effect. This is because the question investigated is so important.

There are several main theories: The most notable are called iron-sulfur world theory (also called metabolism first) and RNA world hypothesis (also called genetics first).

History of the concept in science

Aleksandr Oparin (right) at the laboratory

Until the early 19th century many people believed in the regular spontaneous generation of life from non-living matter.

Spontaneous generation

The theory known as spontaneous generation held that complex, living organisms appear in decaying organic substances. Mice might appear in stored grain or maggots could appear in meat.

According to Aristotle, it was an easily seen truth that aphids arise from the dew which falls on plants, fleas from decaying matter, mice from dirty hay, crocodiles from logs rotting at the bottom of bodies of water, and so forth.

In the 17th century these things started to be questioned. Sir Thomas Browne wrote a book called Pseudodoxia Epidemica, in 1646. He gave it the subtitle Enquiries into very many received tenets, and commonly presumed truths. Browne wrote it as an attack on false beliefs and "vulgar errors". Others did not accept his views. Browne's contemporary, Alexander Ross, wrote: "To question this (spontaneous generation) is to question reason, sense and experience. If he doubts of this let him go to Egypt, and there he will find the fields swarming with mice, begot of the mud of Nylus, to the great calamity of the inhabitants".

In 1546 the physician Girolamo Fracastoro thought epidemic diseases were caused by tiny, invisible particles or "spores", which might not be living creatures. This was a landmark idea, but it was not widely accepted. Next, Robert Hooke published the first drawings of a microorganism in 1665. He was the first to use the term 'cell' for the things he saw in cork samples. In 1676, Anthony van Leeuwenhoek discovered microorganisms that were probably protozoa and bacteria. This sparked new interest in the microscopic world.

The first experiments were done by the Italian Francesco Redi. In 1668, Redi showed that no maggots appeared in meat when flies were prevented from laying eggs. From the 17th century onwards it was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous ideas were false. The alternative seemed to be omne vivum ex ovo: that every living thing came from a pre-existing living thing (literally, everything from an egg).

In 1768 Lazzaro Spallanzani proved that microbes came from the air, and could be killed by boiling. Yet it was not until 1861 that Louis Pasteur performed a series of careful experiments. With these experiments, Pasteur proved that organisms such as bacteria and fungi do not appear in nutrient-rich media of their own accord in non-living material. This experiment supported the cell theory. All ideas of antisepsis in surgery date from this work.

Darwin and Pasteur

By the middle of the 19th century, Pasteur and other scientists had shown that living organisms did not arise spontaneously from non-living matter. This raised the question of how life might have come about within a naturalistic framework.

In a letter to Joseph Dalton Hooker on 11 February 1871, Charles Darwin proposed a natural process for the origin of life.

He suggested 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. A protein compound was then 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, because there is life on Earth, a laboratory with its controlled (sterile) conditions is needed to search for the origin of life.

Haldane and Oparin

No real progress was made until 1924 when Alexander Oparin reasoned that atmospheric oxygen prevented the synthesis of the organic molecules. Organic molecules are the necessary building blocks for the evolution of life. In his The Origin of Life, Oparin argued that a "primeval soup" of organic molecules could be created in an oxygen-less atmosphere through the action of sunlight. These would combine in ever-more complex fashions until they formed droplets. These droplets would "grow" by fusion with other droplets, and "reproduce" through fission into daughter droplets, and so have a primitive metabolism in which those factors which promote "cell integrity" survive, 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 also suggested that the Earth's pre-biotic oceans, which were very different from what oceans are now, would have formed a "hot dilute soup". In this soup, organic compounds, the building blocks of life, could have formed. This idea was called biopoiesis, the process of living matter evolving from self-replicating but nonliving molecules.

Early conditions

A study done in 1998 suggests that oceans may have appeared in the Hadean eon, about 200 million years after the earth had formed. According to the study, it was very hot then (about 100 °C at the start, dropping to 70 °C during the Hadean). The environment helped chemical reductions. The pH was about 5.8 at first, but it rapidly rose towards neutral. Another study supports this theory. The date of the zircon crystals found in the metamorphosed quartzite of Mount Narryer in Western Australia, has been pushed to 4.404 billion years. Beforehand, they were thought to be 4.1–4.2 billion years old. This means that oceans and continental crust existed within 150 million years of Earth's formation.

Despite this, the environment that existed in the Hadean was hostile to life, but how much so is not known. Large objects, which could be up to 500 km across, often collided. This might have been enough to make the ocean disappear within a few months of the impact. This would lead to high-altitude vapour clouds 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.

There was a time between 3.8 and 4.1 billion years ago, that is known as Late Heavy Bombardment today. It is named that way because many lunar craters are thought to have formed then. The situation on other planets, such as Earth, Venus, Mercury and Mars must have been similar. These impacts would likely have sterilised the earth (and killed all life), if it had existed at that time.

Different environments existed between such devastating events. The time when life came to be depends on where it came into existence. A study shows that if life came to be in the deep ocean, near a hydrothermal vent, life could have originated as early as 4 to 4.2 billion years ago. If, on the other hand, life originated at the surface of the planet, a common (but not proven) opinion is it could only have done so between 3.5 and 4 billion years ago.

Life might also have started in a colder environment. A study has shown that freezing temperatures have advantages for the origin of life. This is because precursors such as HCN are more concentrated at lower temperatures. Another study suggested that adenine and guanine require freezing temperatures for synthesis, but cytosine and uracil are better synthesised at boiling temperatures. Based on this research it was suggested the beginning of life involved freezing conditions and exploding meteorites. An article in a scientific publication suggests that seven different amino acids and 11 types of nucleobases formed in ice when ammonia and cyanide were left in a freezer from 1972–1997. This article also describes research 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. 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 Islands. Carbon entering into rock formations has a concentration of elemental δ¹³C of about −5.5, where because of a preferential biotic uptake of ¹²C, biomass 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. They estimate that the development of a 100 kilobase genome of a DNA/protein primitive heterotroph into a 7000 gene filamentous cyanobacterium would have required only 7 million years.

Current models

There is no "standard model" on how life started. Most accepted models at the moment are mostly about a few of the same discoveries. These discoveries are on how different parts of cells and molecules started. They are listed here, in the order they started:

1. Because there are the right conditions, some basic small molecules are created. These are called monomers of life. Amino acids are one type of these molecules. This was proved by the Miller–Urey experiment by Stanley L. Miller and Harold C. Urey in 1953.
2. Phospholipids can form lipid bilayers, a main component of the cell membrane.
3. The polymerization of nucleotides into random RNA molecules might have resulted in self-replicating ribozymes (RNA world hypothesis).
4. Selection pressures for catalytic efficiency and diversity result in ribozymes which catalyse peptidyl transfer (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. Thus the first ribosome is born, and protein synthesis becomes more prevalent.
5. Proteins outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer. Nucleic acids are restricted to predominantly genomic use.

The origin of the basic biomolecules, while not settled, is less controversial than the significance and order of steps 2 and 3. The basic chemicals from which life is thought to have formed are:

• Methane (CH₄),
• Ammonia (NH₃),
• Water (H₂O),
• Hydrogen sulfide (H₂S),
• Carbon dioxide (CO₂) or carbon monoxide (CO), and
• Phosphate (PO₄^3-).

Molecular oxygen (O₂) and ozone (O₃) were either rare or absent.

As of 2008, no one has yet 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 at Los Alamos National Laboratory and Jack Szostak at Harvard University. Others have argued that a "top-down approach" is more feasible. One such approach, attempted by Craig Venter and others at The Institute for Genomic Research, 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 may have commenced early, some time between Stage 1 and 2.

Origin of organic molecules

There are three sources of organic molecules on the early Earth:

1. organic synthesis by energy sources (such as ultraviolet light or electrical discharges).
2. delivery by extraterrestrial objects such as carbonaceous meteorites (chondrites);
3. organic synthesis driven by impact shocks.

Recently estimates of these sources suggest that the heavy bombardment before 3.5 Gyr ago within the early atmosphere made available quantities of organics comparable to those produced by other energy sources.

Miller's experiment and the Primordial soup theory

Main page: Miller–Urey experiment

In 1953 a graduate student, Stanley Miller, and his professor, Harold Urey, performed an experiment that showed how organic molecules could have formed on early Earth from inorganic precursors.

The now-famous Miller–Urey experiment used a highly reduced mixture of gases – methane, ammonia and hydrogen – to form basic organic monomers, such as amino acids.

Whether the mixture of gases used in the Miller-Urey experiment truly reflects the atmosphere of early Earth is not clear. Other less reducing gases produce a lower yield and variety. We do know that for more than the first half of the Earth's history its atmosphere had almost no oxygen.

The next most important step in research on prebiotic organic synthesis was the demonstration by John Oró that the nucleic acid purine base, adenine, was formed by the simple heating of solutions of ammonium cyanide.

Simple organic molecules are, of course, a long way from a fully functional self-replicating life form. But in an environment with no pre-existing life these molecules may have accumulated and provided a rich environment for chemical evolution (primordial soup theory).

The spontaneous formation of complex polymers from amino acids and purines 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 experiments.

The most crucial unanswered challenge is how the relatively simple organic building blocks form more complex structures, and form a protocell.

Fox's experiments

In the 1950s and 1960s, Sidney W. Fox studied the spontaneous formation of peptide 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 microspheres. This was a step forward in spontaneous biosynthesis.

Special conditions

Some scientists have suggested special physical conditions which could make cell synthesis easier.

Clay world

A clay model for the origin of life was suggested by A. Graham Cairns-Smith. Clay theory postulates that complex organic molecules arose gradually on a pre-existing non-organic platform, namely, silicate crystals in solution.

Deep-hot biosphere model

In the 1970s, Thomas Gold proposed the theory that life first developed not on the surface of the Earth, but several kilometers below the surface. The discovery in the late 1990s of nanobes (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 five kilometers below the surface) in the form of extremophile archaea, 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 would lend significant credence to this theory.

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 from the Earth's mantle; more conventional explanations of the food supply of deep microbes (away from sedimentary carbon compounds) is that the organisms subsist on hydrogen released by an interaction between water and (reduced) iron compounds in rocks

Homochirality

Chirality is a special issue. Some process in chemical evolution must account for the origin of homochirality, i.e. all building blocks in living organisms having the same "handedness". Amino acids are left-handed, nucleic acid sugars (ribose and deoxyribose) right-handed.

Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst are formed in a 50/50 mixture of both kinds. This is called a racemic mixture. Clark has suggested that homochirality may have started in space, as the studies of the amino acids on the Murchison meteorite 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 kind within the proto-planetary disk. Noyes showed that beta decay caused the breakdown of D-leucine, in a racemic mixture, and that the presence of ¹⁴C, 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

Features of self-organization and self-replication are the hallmark of living systems. There are instances of abiotic molecules exhibiting such characteristics under proper conditions. For example, Martin and Russel show that cell membranes separating contents from the environment and self-organization of self-contained redox reactions are the most conserved attributes of living things.

They argue that inorganic matter with such attributes would be life's most likely last common ancestor.

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 ("genes-first") whereas others postulate the evolution of biochemical reactions and pathways first ("metabolism-first"). Recently, trends are emerging to create hybrid models that combine aspects of both.

"Genes first" models: the RNA world

The RNA world hypothesis suggests that relatively short RNA molecules could have spontaneously formed that were capable of catalyzing their own continuing replication. It is difficult to gauge the probability of this formation. A number of theories of modes of formation have been put forward. Early cell membranes could have formed spontaneously from proteinoids, 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 substrates or on the surface of pyrite 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); its many important roles as an intermediate in the expression and maintenance of the genetic information (in the form of DNA) in modern organisms; and the ease of chemical synthesis of at least the components of the molecule under conditions approximating the early Earth. Relatively short RNA molecules which can duplicate others have been artificially produced in the lab.

Researchers have pointed out difficulties for the abiogenic synthesis of nucleotides from cytosine and uracil. Cytosine has a half-life of 19 days at 100 °C 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 this hypothesis is that a different type of nucleic acid, such as PNA, TNA or GNA, was the first one to emerge as a self-reproducing molecule, to be replaced by RNA only later.

'Metabolism first' models

Many think the idea that a "naked gene" replicates all by itself is false. Rather, they say that a primitive metabolism emerged. This metabolism would provide an environment, so that RNA replication could emerge later.

One of the first to think this idea could be true was Oparin. In 1924, Oparin talked about primitive vesicles that replicated themselves. This was before the structure of the DNA was found. In the 1980s and 1990s, Günter Wächtershäuser came up with the iron-sulfur world theory and Christian de Duve introduced models based on the chemistry of thioesters. Some people also had more abstract and theoretical models as an explanation why a metabolism could emerge without genes. Freeman Dyson introduced a mathematical model in the 1980s. Stuart Kauffman developed the notion of autocatalytic sets, which was discussed later in that decade.

However, the idea that a closed metabolic cycle, such as the reductive citric acid cycle, could form spontaneously (proposed by Günter Wächtershäuser) remains unsupported. According to Leslie Orgel, a leader in origin-of-life studies for the past several decades, there is reason to believe the assertion will remain so. In an article entitled "Self-Organizing Biochemical Cycles", Orgel summarizes 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 pathway (another one of the four recognised ways of carbon dioxide fixation in nature today) would be even more 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.

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. While bubbles composed mostly of water burst quickly, water containing amphiphiles forms much more stable bubbles, lending more time to the particular bubble to perform these crucial experiments.

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 came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection 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 microspheres, 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, coacervate).

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.

Other models

Primitive extraterrestrial life

An alternative to Earthly abiogenesis is the hypothesis that primitive life may have originally formed extraterrestrially, either in space or on a nearby planet (Mars). (Note that exogenesis is related to, but not the same as, the notion of panspermia). A supporter of this theory was Francis Crick.

Organic compounds are relatively common in space, especially in the outer solar system where volatiles are not evaporated by solar heating. Comets are encrusted by outer layers of dark material, thought to be a tar-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by irradiation by ultraviolet 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 plausibility of the concept is scant, but it finds support in recent study of Martian meteorites found in Antarctica and in studies of extremophile microbes. Additional support comes from a recent discovery of a bacterial ecosytem whose energy source is radioactivity.

A recent experiment led by Jason Dworkin, subjected a frozen mixture of water, methanol, ammonia and carbon monoxide to UV radiation, mimicking conditions found in an extraterrestrial environment. This combination yielded large numbers of organic material that self-organised to form bubbles 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 micrometers, 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. 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, which blocks out the sun's most destructive UV rays, did not form until after photosynthetic life began to produce oxygen.

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 (PAH's) in a nebula. Those are the most complex molecules so far found in space. The use of PAH's has also been proposed as a precursor to the RNA world in the PAH world hypothesis. The Spitzer Space Telescope has recently detected a star, HH 46-IR, which is forming by a process similar to the sun. 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.

Lipid World

There is a theory that ascribes the first self-replicating object to be 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 polyphosphates. 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 (apatite), 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.

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 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 acids. 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.

Books and Journal articles

• Knoll, Andrew H. (2003). Life on a young planet: the first three billion years of evolution on Earth. Princeton University Press. 
• Schopf, J. William (2003). Life's origin: the beginnings of biological evolution. University of California Press. ISBN 0520233913. 
• Hazen, Robert M. (2005). Genesis: the scientific quest for life's origins. Joseph Henry Press. 
• Harris, Henry (2002). Things come to life: spontaneous generation revisited. Oxford: Oxford University Press. ISBN 0198515383. 
• NASA Astrobiology Institute: Earth's early environment and life
• NASA Specialized Center of Research and Training in Exobiology: Gustaf O. Arrhenius
• Fernando CT, Rowe, J (2007). "Natural selection in chemical evolution". Journal of Theoretical Biology 247: 152–67. doi:10.1016/j.jtbi.2007.01.028. 
• Martin, W. and Russell M.J. (2002). "On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells". Philosophical Transactions of the Royal Society: Biological sciences 358: 59–85. doi:10.1098/rstb.2002.1183. 
• Maynard Smith, John; Szathmary, Eors (2000). The Origins of life: from the birth of life to the origin of language. Oxford Paperbacks. ISBN 0-19-286209-X. 
• Hazen, Robert M. (Dec 2005). Genesis: the scientific quest for life's origins. Joseph Henry Press. ISBN 0-309-09432-1. http://newton.nap.edu/books/0309094321/html. 
• Morowitz, Harold J. 1992. "Beginnings of cellular life: metabolism recapitulates biogenesis". Yale University Press. ISBN 0-300-05483-1
• Dedicated issue of Philosophical Transactions B on Major steps in cell evolution freely available.
• Dedicated issue of Philosophical Transactions B on the Emergence of life on the early Earth freely available.
• Luisi, Pier L. (2006). Emergence of life: from chemical origins to synthetic biology. Cambridge University Press. ISBN 0-521-82117-7. http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521821179. 

Other pages

• Evolution - biological evolution

Other websites

• "Harvard Team Creates the World's 1st Synthesized Cells"
• Martin M Hanczyc and Jack W Szostak. Replicating vesicles as models of primitive cell growth and division. Current Opinion in Chemical Biology 2004, 8:660–664.PDF (192 KiB)
• "Exploring Life's Origins: a Virtual Exhibit"
• "SELF-REPLICATION: Even peptides do it" by Stuart A. Kauffman (web archive version as original page no longer accessible)
• Origins of Life website including papers, resources, by Dr. Michael Russell at the U. of Glasgow
• Possible Connections Between Interstellar Chemistry and the Origin of Life on the Earth
• Scientists Find Clues That Life Began in Deep Space—NASA Astrobiology Institute
• Self-organizing biochemical cycles—by Leslie Orgel
• How Life Began: New Research Suggests Simple Approach
• Primordial Soup's On: Scientists Repeat Evolution's Most Famous Experiment – an article in Scientific American. March 28, 2007
• Illustrations from Evolution (textbook)