Online Encyclopedia
Origin of life
- This article focuses on modern scientific research on the origin of life. For alternate uses, see origin of life (disambiguation).
Research into the origin of life is a limited field of research despite its profound impact on biology 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 life are elusive.
For the observed evolution of life on earth, see the timeline of life.
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History of the concept: abiogenesis
Main article: Abiogenesis
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 "omne vivum ex ovo" (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 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 very 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, nobody has synthesized a protocell using basic components which has the necessary properties of life (the so-called "bottom-up-approach"), thus without 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 extant prokaryotic cells with progressively few genes, attempting to discern at which point the most minimal requirements for life were reached.
Origin of organic molecules: Miller experiments
"Miller experiments" (including the original Miller-Urey experiment of 1953) have shown that under simulated conditions resembling those thought to have existed shortly after Earth first accreted, many of the basic organic molecules that form the building blocks of modern life are able to spontaneously form. Simple organic molecules are of course a far cry from a fully functional self-replicating life form, but in an environment with no pre-existing life these molecules could accumulate and provide a rich environment for chemical evolution. The spontaneous formation of complex polymers from abiotically generated monomers under these conditions is straightforward.
Other sources of complex molecules have been postulated including sources of extra-terrestrial, stellar or interstellar origin. In 2004, a team detected traces of PAH (polycyclic aromatic hydrocarbons) in a nebula, the most complex molecule, to that date, found in space.
However the not yet answered challenge is how these relatively simple building blocks began to form complex structures interacting in consistent ways to form a protocell.
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. Many of these tend to fall into either "genes-first" or "metabolism-first", a recent trend is the emergence of hybrid models that combine aspects of both.
"Genes first" models: the RNA world
Main article: RNA world hypothesis
The RNA world hypothesis, for example, suggests that short RNA molecules could have spontaneously formed that would then catalyze their own continuing replication. Early cell membranes could have formed spontaneously from proteinoids, protein-like molecules that are produced when amino acid solutions are heated. Other possibilities include systems of chemical reactions taking place within clay substrates or on the surface of pyrite 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 1924 with Alexander Oparin's notion of primitive self-replicating vesicles which predated the discovery of the structure of DNA. More recent variants in the 1980s and 1990s include Günter Wächtershäuser's iron-sulfur world theory and models introduced by Christian de Duve based on the chemistry of thioesters. 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 in the early 1980s, and Stuart Kauffman's notion of collectively autocatalytic sets 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.
The oxygen holocaust
About 2 billion years ago, during the paleoproterozoic 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 cellular respiration 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.
Other models
Clay theory of the origin of life
A hypothesis for the origin of life based on clay was forwarded by Dr A. Graham Cairns-Smith of Glasgow University in 1985 and adopted as a plausible illustration by just a handful of other scientists (including Richard Dawkins). Clay theory 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 exapted 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 Thomas Gold in the 1990s has life first developing not on the surface of the earth, but several kilometers below the surface. It is now known that microbial life is plentiful up to five kilometers below the earth's surface in the form of archaea, which are generally considered to have originated around the same time or earlier than bacteria, 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 solar system would lend significant credence to this theory.
"Primitive" extraterrestrial life
An alternative to Earthly abiogenesis is the hypothesis that primitive life may have originally formed extraterrestrially (note this is related to, but is not the same as the notion of panspermia). 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 and ultraviolet 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 Mars, 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.
Relevant fields
- Astrobiology 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.
- Complex systems
See also
- Category:Origin of life
- Anthropic principle
- Biogenesis
- Drake equation
- Fine-tuned universe
- Important publications in origin of life
- Panspermia
- Universal common ancestor
- Zeolites
External links
- Astrobiology and the origins of life http://www.accessexcellence.org/bioforum/bf02/awramik/bf02a1.html
- Cairns Smith illustration of a possible solution using crystalline behaviors of clays http://originoflife.net/
- Model of origin of life involving zeolite, press release for PNAS paper http://www-news.uchicago.edu/releases/98/980331.origin.of.life.shtml
- The Birth of Complex Cells, and the oxygen holocaust - by Christian de Duve http://www.biosci.uga.edu/almanac/archive/fall_96/bio_340/docs/deduve.html
- Possible Connections Between Interstellar Chemistry and the Origin of Life on the Earth http://pokey.arc.nasa.gov/~astrochm/LifeImplications.html
- Scientists Find Clues That Life Began in Deep Space - NASA Astrobiology Institute http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=207
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