Current models of the origin of life

There is no truly "standard" model of the origin of life, however most currently accepted models build in one way or another upon the following discoveries, which are listed in a rough order of postulated emergence:

Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the Urey-Miller experiment by Stanley L. Miller and Harold C. Urey in 1953.
Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.
Procedures for producing random RNA molecules can produce "ribozymes", which are able to produce more of themselves under certain specific conditions.
The origin (see Origin of organic molecules) of basic biomolecules such as components of amino acids, while not settled, is less controversial than the significance and order of steps 2 and 3. As of 2004, no one has yet synthesized a "protocell" using basic components which has 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 Jack Szostak at Harvard. 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 involved engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life were reached.

Origin of organic molecules: Miller and Wächtershäuser's theories

The "Miller experiments" (including the original Miller–Urey experiment of 1953, by Harold Urey and his graduate student Stanley Miller) are performed under simulated conditions resembling those thought at the time to have existed shortly after Earth first accreted from the primordial solar nebula. The experiment used a reducing mixture of gases (methane, ammonia and hydrogen). However, it should be noted that the exact composition of the prebiotic atmosphere of earth is currently somewhat controversial. Other less reducing gases produce a lower yield and variety and the presence of free oxygen prevents any organomolecules from forming.

The experiment showed that many of the basic organic molecules that form the building blocks of modern life can be formed spontaneously. Simple organic molecules are of course long way from fully functional self-replicating life forms; however, in an environment with no pre-existing life these molecules may have accumulated and provided a rich environment for chemical evolution ("soup theory"). On the other hand, spontaneous formation of complex polymers from abiotically generated monomers under these conditions is not at all a straightforward process. And there is no evidence in the geological record that any soup existed. Brooks and Shaw (1973) commented:

"If there ever was a primitive soup, then we would expect to find at least somewhere on this planet either massive sediments containing enormous amounts of the various nitrogenous organic compounds, acids, purines, pyrimidines, and the like; or in much metamorphosed sediments we should find vast amounts of nitrogenous cokes. In fact no such materials have been found anywhere on earth."
Other sources of complex molecules have been postulated, including sources of extra-terrestrial 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, the most complex molecule, to that date, found in space.

It can be argued that the most crucial challenge unanswered by this theory is how the relatively simple organic building blocks polymerise and form even more complex structures, interacting in consistent ways to form a protocell. In the absence of a reliable source of energy, these processes contradict the laws of thermodynamics (especially the increase of entropy). For one thing, hydrolysis is favored over condensation polymerization; for another, the Miller experiment produces many substances that would undergo cross-reactions with the amino acids or terminate the peptide chain.

A possible answer to this polymerization conundrum was provided in 1980s by Günter Wächtershäuser, 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 (e. g. simulated lightning or UV irradiation), "Wächtershäuser systems" come with a built-in source of energy, sulfides of iron and other minerals (e. g. pyrite). The energy released from redox reactions of these metal sulfides is not only available for the synthesis of organic molecules, but also for the formation of oligomers and polymers. It is therefore hypothesized that such systems may be able to evolve into autocatalytic sets of self-replicating, metabolically active entities that would predate the life forms known today. The experiment as performed, produced a relatively small yield of dipeptides (0.4–12.4%) and a smaller yield of tripeptides (0.003%) and the authors note that: "under these same conditions dipeptides hydrolysed rapidly." Another criticism of the result is that the experiment did not include any organomolecules that would most likely cross-react or chain-terminate. (Huber and Wächtershäuser, 1998)

The latest modification of the iron-sulfur-hypothesis has been provided by William Martin and Michael Russell in 2002. According to their scenario, the first cellular life forms may have evolved inside so-called [black smokers] at seafloor spreading zones in the deep sea. These structures consist of microscale caverns that are coated by thin membraneous metal sulfide walls. Therefore, these structures would solve several critical points of the "pure" Wächtershäuser systems at once: (1) the micro-caverns provide a means of concentrating newliy synthesised molecules, thereby increasing the chance of forming oligomers; (2) the steep temperature gradients inside a black smoker allow for establishing "optimum zones" of partial reactions in different regions of the black smoker (e. g. monomer synthesis in the hotter, oligomerisation in the colder parts); (3) the flow of hydrothermal water through the structure provides a constant source of building blocks and energy (Freshly precipitated metal sulfides); (4) the model allows for a succession of different steps of cellular evolution (prebiotic chemistry, monomer and oligomer synthesis, peptide and protein synthesis, RNA world, ribonucleoprotein assembly and DNA world) in a single structure, facilitating exchange between all developmental stages; (5) synthesis of lipids as a means of "closing" the cells against the environment is not necessary, until basically all cellular functions are developed. This model locates the "last universal common ancestor" (LUCA) inside a black smoker, rather than assuming the existence of a free-living form of LUCA. The last evolutionary step would be the synthesis of a lipid membran that finally allows the organisms to leave the microcavern system of the black smokers and start their independent lives. This postulated late acquisition of lipids is consistent with the presence of completely different types of membrane lipids in archaebacteria and eubacteria (plus eukaryotes) with highly similar cellular physiology of all life forms in most other aspects.

Another unsolved issue in chemical evolution is the origin of homochirality, i.e. all monomers having the same "handedness". This is essential for both proteins and DNA, yet many prebiotic simulations produce a racemic, or 50/50 mixture of left- and right-handed forms.

Main Nighet1 Nighet2abiogen1 abiogen2 abiogen3 abiogen4 abiogen5

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