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| :''This article focuses on modern scientific research on the origin of life. For alternate uses, see ].'' | |||
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| ] ] in the Siyeh Formation, ]. In 2002, William Schopf of ] published a controversial paper in ] arguing that formations such as this possess 3.5 billion year old ] ] microbes. If true, they would be the earliest known life on earth.]] | |||
| Research into the '''origin of life''' is a limited field of research despite its profound impact on ] and human understanding of our world. Progress in this field is slow and sporadic, but it still draws the attention of many. | |||
| A few facts give insight into the conditions in which life may have emerged, but the mechanisms by which non-life became ] are elusive. | |||
| For the observed evolution of life on earth, see the ]. | |||
| ==History of the concept: abiogenesis== | |||
| ''Main article'': ] | |||
| Research into the origin of life is the modern incarnation of the ancient concept of '''abiogenesis'''. Abiogenesis, in its most general sense, is the generation of life from non-living matter. The term is primarily used in the context of biology and the origin of life. Abiogenesis was long considered to be a very common occurrence until the concept of "]" (all life from other life) became firmly established in modern biology. | |||
| The ''modern'' definition of abiogenesis is concerned with the formation of the simplest forms of life from primordial chemicals. This is a significantly different thing from the concept of Aristotelian abiogenesis, which postulated the formation of complex organisms. This article reviews different hypotheses for modern abiogenetic processes that are currently under debate. | |||
| ==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 ] by ] and ] in ]. | |||
| # ]s spontaneously form ]s, the basic structure of a ]. | |||
| # Procedures for producing random ] molecules can produce "]s", which are able to produce more of themselves under very specific conditions. | |||
| The origin (see ]) of basic ]s such as components of ]s, while not settled, is less controversial than the significance and order of steps 2 and 3. ], nobody has 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 ] at ]. Others have argued that a ''"top-down approach"'' is more feasible. One such approach attempted by ] and others at ] 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 "]s" (including the original Miller-Urey experiment of 1953) are performed under simulated conditions resembling those thought to have existed shortly after Earth first accreted. They show that many of the basic organic molecules that form the building blocks of modern life are formed spontaneously. Simple organic molecules are of course a far cry from a fully functional ] life form, but in an environment with no pre-existing life these molecules might have accumulated and provided a rich environment for ] ("soup theory"). However, spontaneous formation of complex ]s from abiotically generated ]s under these conditions is not at all a straightforward process. | |||
| 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. In ], a team ] traces of ] (polycyclic aromatic hydrocarbons) in a ], the most complex molecule, to that date, found in space. | |||
| However, the most crucial challenge unanswered by this theory is how the relatively simple organic building blocks may 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 ] (especially the increase of entropy). | |||
| A possible answer to this conudrum was provided in ] by ], in his ]. 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 metals (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. Therefore, such systems may be able to evolve to ] of self-replicating, metabolically active entities that would predate the life forms known today. | |||
| ===From organic molecules to protocells=== | |||
| As the question how organic molecules form a protocell is largely unanswered, there are many different hypotheses regarding the path that might have been taken from simple organic molecules to protocells, cells, and metabolism. Some of these postulate early appearance of nucleic acids ("]s-first"), whereas the evolution of biochemical reactions and pathways is regarded as moving force of early evolution ("]-first"). Recently, trends are emerging to create hybrid models that combine aspects of both. | |||
| ===="Genes first" models: the RNA world==== | |||
| ''Main article:'' ] | |||
| The RNA world hypothesis, for example, suggests that short ] molecules could have spontaneously formed that would then catalyze their own continuing replication. Early cell membranes could have formed spontaneously from ]s, protein-like molecules that are produced when amino acid solutions are heated. Other possibilities include systems of chemical reactions taking place within ] substrates or on the surface of ] rocks. None of these various hypotheses have strong evidence behind them at this time, however. Many of them can be simulated and tested in the lab, but a lack of undisturbed sedimentary rock from that early in Earth's history leaves few opportunities to determine what may have actually happened in practice. | |||
| ===="Metabolism first" models: iron-sulfur world and others==== | |||
| 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. One of the earliest incarnations of this idea was put forward in ] with ]'s notion of primitive self-replicating ]s which predated the discovery of the structure of DNA. More recent variants in the ] and ] include ]'s ] and models introduced by ] based on the chemistry of ]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 ] in the early ], and ]'s notion of collectively ]s discussed later in that decade. | |||
| ====Hybrid models==== | |||
| A growing realization of the inadequacy of either pure "genes-first" or "metabolism-first" models is leading the trend towards models that incorporate aspects of each. <!-- more needed here, some of Wachtershauser's ideas might fall in here --> | |||
| == The oxygen holocaust == | |||
| About 2 billion years ago, during the ] eon of history, there was a significant increase in atmospheric oxygen. Before this time life was anaerobic--that is, the metabolism of life depended on a form of ] that did not require oxygen. The presence of large amounts of free oxygen is poisonous to most anaerobic bacteria, and at this time most life on Earth died out. The only life that survived was either life that was resistant to the oxidizing and poisonous effects of oxygen, or life that spent most or all of its life-cycle in an oxygen-free environment. | |||
| However, presence of uranium in sediments dated to 3.7 Ga indicates transported in solution by oxygenated water, otherwise it would have precipitated. This in turn suggests an oxygenic atmosphere at that time. | |||
| *Rosing, M.T. and Frei, R., U-rich Archaean sea-floor sediments from Greenland—indications of >3700 Ma oxygenic photosynthesis, ''Earth and Planetary Science Letters'' '''217:'''237–244, 2004. | |||
| ==Other models== | |||
| ===Clay theory of the origin of life=== | |||
| A hypothesis for the origin of life based on ] was forwarded by Dr A. ] of ] in ] and adopted as a plausible illustration by just a handful of other scientists (including ]). ] postulates complex organic molecules arising 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 ] to serve the replication of organic molecules independently of their silicate "launch stage". | |||
| ==="Deep-hot biosphere" model of Gold=== | |||
| A controversial theory put forward by ] in the ] has life first developing not on the surface of the earth, but several kilometers below the surface. It is now known that ] life is plentiful up to five kilometers below the earth's surface in the form of ], which are generally considered to have originated around the same time or earlier than ], most of which live on the surface including the oceans. It is claimed that discovery of microbial life below the surface of another body in our ] would lend significant credence to this theory. He also noted that a trickle of food from a deep, unreachable, source promotes survival because life arising in a puddle of organic material is likely to consume all of its food and become extinct. | |||
| ==="Primitive" extraterrestrial life=== | |||
| An alternative to Earthly abiogenesis is the hypothesis that primitive life may have originally formed extraterrestrially (note that ''exogenesis'' is related to, but is not the same as the notion of ]). Organic compounds are relatively common in space, especially in the outer solar system where volatiles are not evaporated by solar heating. ]s are encrusted by outer layers of dark material, thought to be a ]-like substance composed of complex organic material formed from simple carbon compounds and ] light. The rain of cometary material on the early Earth could have brought significant quantities of complex organic molecules, and it is possible that primitive life itself may have formed in space and been brought to the surface along with it. A related hypothesis holds that life may have formed first on early ], and been transported to Earth when crustal material was blasted off of Mars by asteroid and comet impacts to later fall to Earth's surface. Both of these hypotheses are even more difficult to find evidence for, and may have to wait for samples to be taken from comets and Mars for study. | |||
| ==Creation and intelligent design== | |||
| ''Main articles: ], ]'' | |||
| ] and those ascribing to ] assert that life was created by an intelligent being, and, due to its ], could only have been created by an intelligent being; in the case of creationists, this designer is believed to be ]. | |||
| For a discussion of the creationist abiogenesis hypothesis in relationship to ] and the ], see ]. | |||
| ==Relevant fields== | |||
| * ] is a field that may shed light on the nature of life in general, instead of just life as we know it (on Earth), and may give clues as to how life originates. | |||
| * ] | |||
| ==See also== | |||
| * ] | |||
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| ==External links== | |||
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| * by ] | |||
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| * (articles critical of chemical evolution from an allegedly scientific angle, ]) | |||
| {{Biology-footer}} | |||
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