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Extraterrestrial Life

Extraterrestrial life

:This article is about the scientific study of extraterrestrial life; for treatment in popular culture, see Extraterrestrial life in popular culture. Extraterrestrial life in popular culture Extraterrestrial life is life that may exist and originate outside the planet Earth. Its existence is currently hypothetical: there is as yet no evidence of extraterrestrial life that has been widely accepted by scientists. Most scientists hold that if extraterrestial life exists, its evolution would have occurred independently in different places in the universe. An alternative hypothesis, held by a minority, is panspermia, which suggests that life in the universe could have stemmed from a single initial distribution of spores that provide the basis for living beings to develop. If true, this theory would suggest that life in various forms might exist throughout the universe. Speculative forms of extraterrestrial life range from humanoid and monstrous beings seen in works of science fiction to life at the much smaller scale of bacteria and viruses. Extraterrestrial life forms, especially intelligent ones, are often referred to in popular culture as aliens or ETs. The putative study and theorisation of ET life is known as astrobiology or xenobiology.
Possible basis of extraterrestrial life

Biochemistry
All life on Earth is based on the building block element carbon with water as the solution in which bio-chemical reactions take place. Given their relative abundance and usefulness in sustaining life it has long been assumed that life forms elsewhere in the universe will also utilize these basic components. However, other elements and solvents might be capable of providing a basis for life (Main article: Alternative biochemistry). Silicon is usually considered the most likely alternative to carbon, though this remains improbable. Life forms based in ammonia rather than water are also considered, though less frequently. Nor can the possibility be rejected that a completely new substance may be found that may react in a similar way to carbon or that wholly unique, non-chemical life-forms may possibly flourish through exotic physics. Along with a building block element and a solvent life also requires an energy source. Energy from a parent star is the most obvious source for extraterrestrial life but this is not the only possibility, as the example of terrestrial extremophiles shows. Geothermal energy from a planet's interior, for instance, may drive sub-surface or oceanic life, while tidal flexing (e.g., for bodies orbiting a gas giant) provides another possible motor to sustain living things. The scientific study of the possible biochemical basis for extraterrestrial life is often called xenobiology.
Theoretical Evolution and Morphology
Along with the biochemical basis of extraterrestrial life, there remains a broader consideration of evolution and morphology. What might an alien look like? Science fiction has long shown a bias towards humanoid or (often in the case of villains) reptilian forms. The classical alien is light green or grey skinned, with an enormous head, small body, and the typical four limb and two to five digit structure—i.e., it is fundamentally humanoid with a large brain to indicate great intelligence. Other subjects from our animal mythos (felines, insects) have also featured strongly in fictional representations of aliens. While such bias is predictable, it is also curiously unimaginative and almost certain to be proven wrong should human beings encounter extraterrestrials. In considering the subject more seriously, a useful division has been suggested between universal and parochial characteristics. Universals are features which have evolved independently more than once on Earth (and thus presumably are not difficult to develop) and are so intrinsically useful that species will inevitably tend towards them. These include flight, sight, photosynthesis and limbs, all of which have evolved several times here on Earth with differing materialization. There are a huge variety of eyes, for example, many of which have radically different working schematics as well as different visual foci: the visual spectrum, infrared, polarity and echolocation. Parochials, by contrast, are essentially arbitrary evolutionary forms which often serve little utility (or at least have a function which can be equally served by dissimilar morphology) and probably will not be replicated. Parochials include the five digits of mammals, the genitalia and sexual mechanics of animals, as well as the curious and often fatal conjunction of the feeding and breathing passages found within many animals. A consideration of which features are ultimately parochial challenges many taken for granted notions about morphological necessity. Skeletons, in some form, are likely to be replicated elsewhere, yet the vertebrate spine—while a profound development on Earth—is just as likely to be unique. Similarly, it is reasonable to expect some type of egg laying amongst off-Earth creatures but the mammary glands which set apart mammals may be a singular case. The assumption of radical diversity amongst putative extraterrestrials is by no means settled. While many exobiologists do stress that the enormously heterogeneous nature of Earth life foregrounds even greater variety in space, others point out that convergent evolution dictates substantial similarities between Earth and off-Earth life. These two schools of thought are called "divergionism" and "convergionism", respectively [http://www.daviddarling.info/encyclopedia/E/etlifevar.html].
Beliefs in extraterrestrial life

Ancient and Early Modern ideas
Belief in extraterrestrial life may have been present in ancient Egypt, Babylon, and Sumer, although in these societies, cosmology was fundamentally supernatural and the notion of aliens is difficult to distinguish from that of gods, demons, and such. The first important Western thinkers to argue systematically for a universe full of other planets and, therefore, possible extraterrestrial life were the ancient Greek writers Thales and his student Anaximander in the 7th and 6th centuries B.C.E. The atomists of Greece took up the idea, arguing that an infinite universe ought to have an infinity of populated worlds. Ancient Greek cosmology worked against the idea of extraterrestrial life in one critical respect, however: the geocentric universe, championed by Aristotle and codified by Ptolemy, privileged the Earth and Earth-life (Aristotle denied there could be a plurality of worlds) and seemingly rendered extraterrestrial life impossible. Ptolemy Ancient Jewish sources also considered extraterrestrial life. The Talmud suggests that there are at least 18,000 other worlds, but provides little eloboration. The book Sefer Habrit (Book of the Covenant) writes that extraterrestrial creatures exist but that they have no free will (and are thus equivalent to animal life). It adds that human beings should not expect creatures from another world to resemble earthly life, any more than sea creatures resemble land animals. [http://ohr.edu/ask_db/ask_main.php/318/Q1/] [http://www.torah.org/features/secondlook/extraterrestrial.html] When Christianity spread through the West the Ptolemaic system became dogma and although the Church never issued any formal pronouncement on the question of alien life [http://www.crisismagazine.com/november2002/feature7.htm], at least tacitly the idea was heretical. In 1277 the Bishop of Paris, Etienne Tempier did overturn Aristotle on one point: God could have created more than one world (given His omnipotence) yet we know by revelation he only made one. To take a further step and argue that aliens actually existed remained dangerous. The best known early-modern proponent of extra-solar planets and widespread life off Earth was Giordano Bruno, who was burned at the stake for this and other unorthodox ideas in 1600. The Church, however, could not contain the storm that accompanied the invention of the telescope and the Copernican assault on geocentric cosmology. Once it became clear that the Earth was merely one planet amongst countless bodies in the universe the extraterrestrial idea moved towards the scientific mainstream. In the early 17th century the Czech astronomer Anton Maria Schyrleus of Rheita mused that "if Jupiter has…inhabitants…they must be larger and more beautiful than the inhabitants of the Earth, in proportion to the [size] of the two spheres;" he did not dare to confirm the existence of Jovian beings due to potential theological difficulties. Later, this bold step would be taken. William Herschel, the discoverer of Uranus, was one of many 18th-19th century astronomers convinced that our Solar System, and perhaps others, would be well populated by alien life. Other luminaries of the period who championed "cosmic pluralism" included Immanuel Kant and Benjamin Franklin. At the height of the Enlightenment even the Sun and Moon were considered candidates for hosting aliens. The Christian attitude towards extraterrestrials turned from denial to ambivalence. Theological criticisms had been partially stalemated by a critical counter-argument that had remained in the background since the pronouncements of 1277: God's omnipotence not only allowed for other worlds and other life, on some level it necessitated them.
Extraterrestrials and the Modern era
This enthusiasm towards the possibility of alien life continued well into the 20th century. Indeed, the roughly three centuries from the Scientific Revolution through the beginning of the modern era of solar system probes were essentially the highpoint for belief in extraterrestrials in the West: many astronomers and other secular thinkers, at least some religious thinkers, and much of the general public were largely satisfied that aliens were a reality. This trend was finally tempered as actual probes visited potential alien abodes in the solar system. The moon was decisively ruled out as a possibility, while Venus and Mars—long the two main candidates for extraterrestrials—showed no obvious evidence of current life. The other large moons of our system which have been visited appear similarly lifeless, though interesting geothermic forces observed (Io's volcanism, Europa's ocean, Titan's thick atmosphere) has underscored how broad the range of potentially habitable environments may be. Finally, the failure of NASA's SETI program to detect anything resembling an intelligent radio signal after four decades of effort has partially dimmed the optimism that prevailed at the beginning of the space age and emboldened critics who view the search for extraterrestrials as unscientific. [http://www.crichton-official.com/speeches/speeches_quote04.html] Thus, the three decades preceding the turn of the second millenium saw a crossroads reached in beliefs in alien life. The prospect of ubiquitous, intelligent, space-faring civilizations in our solar system appears increasingly dubious to many scientists ("All we know for sure is that the sky is not littered with powerful microwave transmitters" in the words of SETI's Frank Drake). At the same time, the data returned by space probes and giant strides in detection methods have allowed science to begin delineating habitability criteria on other worlds and to confirm that, at least, other planets are plentiful though aliens remain a question mark. Amongst the general public belief and interest in extraterrestrials remains high and skepticism towards galaxy-exploring alien civilizations is not shared by many individuals. At present, some enthusiasts in the topic believe that extraterrestrial beings regularly visit or have visited the Earth. Some think that unidentified flying objects observed in the skies are in fact sightings of the spacecraft of intelligent extraterrestrials, and even claim to have met such beings. Crop circle patterns have also been attributed to the actions of extraterrestrials, although many were later found to be hoaxes. While at least one recent scientific paper published in a respected, peer-reviewed journal has urged a re-evaluation of the UFO phenomenon (Deardorff et al., 2005) [http://www.ufoskeptic.org/JBIS.pdf], as of this time mainstream scientific opinion holds that such claims are unsupportable by the evidence currently available and unlikely to be true. The possible existence of primitive (microbial) life outside of Earth is much less controversial to mainstream scientists although at present no direct evidence of such life has been found. Indirect evidence has been offered for the current existence of primitive life on the planet Mars; however, the conclusions that should be drawn from such evidence remain in debate.
Scientific search for extraterrestrial life
The scientific search for extraterrestrial life is being carried out in two different ways, directly and indirectly.
Direct search
Scientists are directly searching for evidence of unicellular life within the solar system, carrying out studies on the surface of Mars and examining meteors that have fallen to Earth. A mission is also proposed to Europa, one of Jupiter's moons with a liquid water layer under its surface, which might contain life. There is some limited evidence that microbial life might possibly exist or have existed on Mars. An experiment on the Viking Mars lander reported gas emissions from heated Martian soil that some argue are consistent with the presence of microbes. However, the lack of corroborating evidence from other experiments on the Viking indicates that a non-biological reaction is a more likely hypothesis. Independently in 1996 structures resembling bacteria were reportedly discovered in a meteorite, ALH84001, known to be formed of rock ejected from Mars. Again, this report is vigorously disputed. In February 2005, NASA scientists reported that they had found strong evidence of present life on Mars (Berger, 2005). The two scientists, Carol Stoker and Larry Lemke of NASA’s Ames Research Center, based their claims on methane signatures found in Mars’ atmosphere that resemble the methane production of some forms of primitive life on Earth, as well as their own study of primitive life near the Rio Tinto river in Spain. NASA officials soon denied the scientists’ claims, and Stoker herself backed off from her initial assertions (spacetoday.net, 2005). However, only a few days after Stoker and Lemke made their claims, scientists from the European Space Agency reported that their own measurements of methane on Mars suggested an organic origin (Michelson, 2005). Though such findings are still very much in debate, support among scientists for the belief in the existence of life on Mars seems to be growing. In an informal survey of scientists attending the conference at which the European Space Agency presented its findings, 75 percent of the scientists at the conference reported to believe that life once existed on Mars; 25 percent reported a belief that life currently exists there (Michelson, 2005).
Indirect search
It is theorised that any technological society in space will be transmitting information. Projects such as SETI are conducting an astronomical search for radioactivity that would confirm the presence of intelligent life. A related suggestion is that aliens might broadcast pulsed and continuous laser signals in the optical as well as infrared spectrum [http://www.coseti.org/]; laser signals have the advantage of not "smearing" in the interstellar medium and may prove more conducive to communication between the stars. Astronomers also search for extrasolar planets that would be conducive to life. Current radiodetection methods have been inadequate for such a search, as the resolution afforded by recent technology is inadequate for detailed study of extrasolar planetary objects. Future telescopes should be able to image planets around nearby stars, which may reveal the presence of life (either directly or through spectrography which would reveal key information such as the presence of free oxygen in a planet's atmosphere). The Terrestrial Planet Finder is one NASA programme on the horizon that has generated optimism over the potential discovery of habitable planets. It has been argued that one of the best candidates for the discovery of life-supporting planets may be Alpha Centauri, the closest star system to Earth, given that two of the three stars in the system are broadly sun-like.
Extraterrestrial life in the Solar System
Many bodies in the Solar System have been suggested as being likely to contain life. The most commonly suggested ones are listed below; of these, four of the five are moons thought to have large bodies of underground liquid, and life may have evolved there in a similar fashion to deep sea vents.
- Mars - The best known of the other planets and moons in the Solar system. There was liquid water on Mars in the past and there may be liquid water beneath the surface. Recently, methane was found in the atmosphere of Mars.
- Titan - Saturn's largest moon, and the only known moon with a significant atmosphere. Recently visited by the Huygens probe. Latest discoveries indicate that there is no global or widespread ocean, but small and/or seasonal liquid hydrocarbon [http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=1577 lakes] are still possible.
- Europa - probably has a salt ocean under a thick ice crust.
- Ganymede - Jupiter's largest moon, and indeed the largest moon in the entire solar system
- Enceladus - Another one of Saturn's moons, may have liquid water beneath its surface. [http://www.newscientist.com/article.ns?id=dn7159] Numerous other bodies have been suggested as potentially life-bearing. For example, atmospheric life has been hypothesised on Venus and the gas giants. Fred Hoyle also proposed that microbial life might exist on comets. Some Earth microbes also managed to survive on a lunar probe for some years. It is considered highly unlikely that complex multicellular organisms exist in any of these places.
Dealing with extraterrestrial life
If intelligent extraterrestrial life is found and it is possible to communicate with it, the people of the world and their governments will need to determine how to manage those interactions. The development of policy guidelines for dealing with extraterrestrial beings and territory has been considered by authors such as Michael Salla and Alfred Webre and termed exopolitics.
See also

- Alien invasion
- Anomalous phenomenon
- Are We Alone?
- Astrobiology
- Astrosociobiology
- Back-contamination
- Drake equation
- Extraterrestrial life in popular culture
- Fermi paradox
- First contact
- Frank Drake
- The Galactic Federation
- Habitable zone
- Kardashev scale
- List of space aliens in fiction
- Panspermia
- Planetary habitability
- Rare Earth hypothesis
- Scientific skepticism
- Sentience Quotient
- Seth Shostak
- SETI
- Greys
- The Galactic Empire
References

- Jack Cohen and Ian Stewart (2002): Evolving the Alien: The Science of Extraterrestrial Life, Ebury Press, ISBN 0-091-87927-2
- [http://www.ufoskeptic.org/JBIS.pdf (pdf file)]
- Berger, Brian (2005). [http://www.space.com/scienceastronomy/mars_life_050216.html Exclusive: NASA Researchers Claim Evidence of Present Life on Mars]. Posted Feb. 16, 2005.
- spacetoday.net (2005). [http://www.spacetoday.net/Summary/2804 NASA denies Mars life reports]. Posted Feb 19, 2005.
- Michelson, Marcel (2005). [http://story.news.yahoo.com/news?tmpl=story&u=/nm/space_mars_dc European Scientists Believe in Life on Mars]. Posted Feb 25, 2005.
- John C. Baird. 1987. The Inner Limits of Outer Space: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings. Hanover: University Press of New England. ISBN 0-87451-406-1
- Donald Goldsmith. 1997. The Hunt for Life on Mars. New York: A Dutton Book. ISBN 0525943366
- Michael T. Lemnick. 1998. Other Worlds: The Search for Life in the Universe. New York: A Touchstone Book.
- Cliff Pickover. 2003 The Science of Aliens New York: Basic Books. ISBN 0-465-07315-8
Related books and media

- Sagan, Carl and I.S. Shklovskii, Intelligent Life in the Universe. Random House, 1966
- Sagan, Carl, Communication with Extraterrestrial Intelligence. MIT Press, 1973
External links

- [http://www.paranormalnetwork.net/wiki/index.php/Star_cruiser Possible Extraterrestrial Ships Spotted by SOHO]
- [http://www.mysterymap.com MysteryMap.com Extra Terrestrial Sightings]
- [http://www.exopolitics.com Exopolitics.com by Alfred Webre]
- [http://www.daviddarling.info/encyclopedia/S/siliconlife.html Silicon-based life by David Darling]
- [http://www.daviddarling.info/encyclopedia/A/ammonialife.html Ammonia-based life by David Darling]
- [http://www.pbs.org/lifebeyondearth/index.html PBS: Life Beyond Earth a film by Timothy Ferris]
- [http://www.ufoskeptic.org ufoskeptic.org by Bernard Haisch]
- [http://www.rfreitas.com/Astro/Xenopsychology.htm Xenopsychology by Robert A. Freitas Jr.]
- [http://www.projectrho.com/rocket/rocket3aa.html Let's Build an Extraterrestrial]
- [http://www.ufocasebook.com UFO Casebook]
- [http://www.maar.us Malevolent Alien Abduction Research]
- [http://www.answersdepot.com/doaliensexist.html A Christian view on the possibility of aliens existing] Category:Astrobiology
- ja:地球外生命

Extraterrestrial life in popular culture
In popular fiction and conspiracy theories, life forms, especially intelligent life forms, that are of extraterrestrial origin, i.e. not coming from the Earth are referred to as alien and collectively as aliens. Prime examples of how aliens are viewed are found in the movies Alien, E.T. the Extra-Terrestrial, Signs, Independence Day, and The War of the Worlds. This usage is clearly anthropocentric: when humans in fictional accounts accomplish interstellar travel and land on a planet elsewhere in the universe, the local inhabitants of these other planets are usually still referred to as "alien," even though they are the native life form and the humans are the intruders. In general they are seen as unfriendly life forms. This may be seen as a reversion to the classic meaning of "alien" as referring to "other," in contrast to "us" in the context of the writer's frame of reference.
Typical characteristics
In popular culture, such as movies and comics, "aliens" are often depicted as somewhat humanoid in their appearance (See Greys, Little green men). There are several reasons for this humanoid depiction in popular culture. It makes it easier for an alien in a movie scene to simply be a disguised human actor. Aliens in movies, in order to catch our attention, must trigger instantaneous emotional reaction; this requires a design based on recognizable human facial features and expressions. It is easier to relate to an alien with features we recognize such as arms and legs, two eyes, a nose and a mouth, as well as behavior we recognize such as baring its teeth in anger or widening its eyes in shock or surprise. However, if real extraterrestrial life exists, few people expect to find humanoid characteristics, believing that this would be too great a coincidence given an entirely different evolutionary scale.
Extraterrestrial life in fiction
The existence of alien beings (intelligent extraterrestrial life) has been one of the enduring themes of science fiction. One of the first fiction writers to use alien beings from another planet was H. G. Wells. The War of the Worlds is the best known of his works; it also introduced the modern reader to the recurring concept of interplanetary invasion by malign aliens. The alien invasion was one of two themes which was to crop up again and again in the 'pulp science fiction' years, the 1930s to 1960s. The Cold War made people particularly receptive to the idea of evil and incomprehensible beings coming to destroy or enslave earthly (usually American) life. Examples of these include the short story "The Liberation of Earth" by William Tenn. The contrasting picture of aliens during this time was that of the wise and civlised race coming to Earth to impart their wisdom and solve our problems. These stories were almost as popular as the invasion theme, at a time when mankind looked as though it was in the brink of destroying itself. A good example of this story is the movie The Day the Earth Stood Still. Aliens are also used in fiction to examine the human race from a different perspective; the author wishes us to examine ourselves in a new way by contrasting us with an imagined alternative. This was one of the functions of Mr. Spock in Star Trek. One of the most frequently portrayed alien races from our own solar system are the Martians, Mars being the most romanticized of the other planets whose surface conditions are closest to being amenable to life. See Mars in fiction for more details on the red planet's numerous roles. Popular fictional tales of the first half of the 20th century also include many fanciful tales of alien races and civilizations on our nearest neighbor, the Moon. Many of the aliens have been hostile and alien invasion has been a very popular idea in English-language science fiction. Writers have created a long list of extraterrestrial creatures and intelligent beings; see the list of aliens in fiction.
Silicon based life
There are examples of silicon-based life in science fiction, such as an episode of the original Star Trek series, which included a silicon life form called the Horta or the Chenjesu and Taalo in the science-fiction adventure game Star Control II. The Doctor Who story The Hand of Fear featured a silicon-based being, and indeed made great play of the exceptional rarity of silicon-based lifeforms.
See also

- Alien abduction
- Alien invasion
- Alien races
- Ancient astronaut theory
- Black triangles
- Conspiracy theory
- Crop circle
- first contact
- Grey alien
- Invader Zim
- Destroy All Humans!
- Hollow earth
- List of magazines of anomalous phenomena
- Military flying saucers
- Roswell rods
- Starchild skull
- UFO conspiracy theory
- Ufology
- Ummo
- Unidentified flying object
- Xenobiology
Further reading

- Sagan, Carl. 1996. The Demon-Haunted World: Science as a Candle in the Dark: chapter 4: "Aliens" Category:Extraterrestrials
- Category:Science fiction themes Category:UFOs

Life
:For other uses, see Life and Living Life is a multi-faceted concept. Life may refer to the ongoing process of which living things are a part, the period between the conception (or a point at which the entity can be considered to be an individualized being) and death of an organism, the condition of an entity that has been born (or reached the point in its existence at which it can be established to be alive) and has yet to die, and that which makes a living thing alive.
Defining the concept of life
How can one tell when an entity is a lifeform? It would be relatively straightforward to offer a practical set of guidelines if one's only concern were life on Earth as we know it (see biosphere), but as soon as one considers questions about life's origins on Earth, or the possibility of extraterrestrial life, or the concept of artificial life, it becomes clear that the question is fundamentally difficult and comparable in many respects to the problem of defining intelligence. Also, loosely speaking, some theories are grounded in the basic assumption that "ideas have a life of their own".
A conventional definition
In biology, a lifeform has traditionally been considered to be a member of a population whose members can exhibit all the following phenomena at least once during their existence: #Growth, full development, maturity #Metabolism, consuming, transforming and storing energy/mass; growing by absorbing and reorganizing mass; excreting waste #Motion, either moving itself, or having internal motion #Reproduction, the ability to create entities that are similar to, yet separate from, itself or consisting solely of entities that exhibit the quality of reproduction. #Response to stimuli - the ability to measure properties of its surrounding environment, and act upon certain conditions. This property is also called homeostasis.
Exceptions to the conventional definition
These criteria are not without their uses, but their disparate nature makes them unsatisfactory from a number of perspectives; in fact, it is not difficult to find counterexamples and examples that require further elaboration. For example, according to the above definition, one could say:
- (most) mules and people who are infertile cannot reproduce and thus would not qualify as lifeforms. Also worker bees and other organisms living in colonies would not qualify; only the queen and the drones (or the whole colony) can be considered 'alive'.
- Fire and stars could be considered lifeforms.
- A virus does not grow and cannot reproduce outside of a host cell and thus would not qualify as a lifeform. Many individual organisms are incapable of reproduction and yet are still considered to be lifeforms; see mules and ants for examples. This is because the term "lifeform" applies on the level of entire species or of individual genes. (For example, see kin selection for information about one way by which non-reproducing individuals can still enhance the spread of their genes and the survival of their species.) It is important to keep in mind the difference between a "lifeform" and "a being that is alive." One example of sterility does not render the rest of the species a non-lifeform, any more than one dead animal renders the rest of the species dead. Note also that the two cases of fire and stars fitting the definition of life can be simply remedied by defining metabolism in a more biochemically exact way. Fundamentals of Biochemistry by Donald Voet and Judith Voet (ISBN 0471586501) defines metabolism as follows: "Metabolism is the overall process through which living systems acquire and utilize the free energy they need to carry out their various functions. They do so by coupling the exergonic reactions of nutrient oxidation to the endergonic processes required to maintain the living state, such as the performance of mechanical work, the active transport of molecules against concentration gradients, and the biosynthesis of complex molecules." This definition, in use by most biochemists, makes it clear that fire is not alive, because fire releases all the oxidative energy of its fuel as heat. (Note: Actually, the definition does not help much at all, for it is circular. What we are looking for, after all, is a definition of "living entity." We agreed that part of the definition is "capable of metabolism." We then tried to define "metabolism" in order to get clear on which entities are capable of it and which not. But the definition of "metabolism" just offered is in terms of living systems, and those are exactly what we are trying to define!) This could also be remedied by adding the requirement of locality, where there is an obvious structure that delineates the spatial extension of the living being, such as a cell membrane. A conceptual problem with saying that fire is life is that it collapses the distinction between "growth" and "reproduction." It is possible to think of a spreading flame as either growing or reproducing, but what would it mean to say that the same act is both growth and reproduction? Viruses reproduce, flames grow, some software programs mutate and evolve, future software programs will probably evince (even high-order) behavior, machines move, and some form of proto-life consisting of metabolizing cells without the ability to reproduce presumably existed. Still, some would not call these entities alive. Generally, all five characteristics are required for a population to be considered a lifeform.
Other definitions
Biologists who are content to focus on terrestrial organisms often note some additional signs of life, including these: # Living organisms contain molecular components such as: carbohydrates, lipids, nucleic acids, and proteins. # Living organisms require both energy and matter in order to continue living. # Living organisms are composed of at least one cell. # Living organisms maintain homeostasis for some period of time. # Species of living organisms will evolve. All life on Earth is based on the chemistry of carbon compounds. Some assert that this must be the case for all possible forms of life throughout the universe; others describe this position as 'carbon chauvinism'. The systemic definition is that living things are self-organizing and autopoietic (self-producing). These objects are not to be confused with dissipative structures (e.g. fire). Variations of this definition include:
- Francisco Varela and Humberto Maturana's definition of life (also widely used by Lynn Margulis) as an autopoietic (self-producing), water based, lipid-protein bound, carbon metabolic, nucleic acid replicated, protein readout system
- "a system of inferior negative feedbacks subordinated to a superior positive feedback" ([http://www.mol.uj.edu.pl/~benio/cyber_def_life.pdf J. theor Biol. 2001])
- Tom Kinch's definition of life as a highly organized auto-cannibalizing system naturally emerging from conditions common on planetary bodies, and consisting of a population of replicators capable of mutation, around each set of which a homeostatic metabolizing organism, which actively helps reproduce and/or protect the replicator(s), has evolved
- Stuart Kauffman's definition of life as an autonomous agent or a multi-agent system capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle
- Robert Pirsig's definition of life, found in his book Lila: An Inquiry into Morals, as that which maximizes its range of possible futures, in other words, that which makes decisions that result in the most future choices, or that which strives to keep its options open.
- A system converting entropy to negentropy, using flow of energy. Other definitions:
- That which seeks to continue its own existence (attributed to Clifford A. Schaffer).
- A self-replicating system that evolves through mutation.
Descent with modification: a "useful" characteristic
A useful characteristic upon which to base a definition of life is that of descent with modification: the ability of a life form to produce offspring that are like its parent or parents, but with the possibility of some variation due to chance. Descent with modification is sufficient by itself to allow evolution, assuming that the variations in the offspring allow for differential survival. The study of this form of heritability is called genetics. In all known life forms (assuming prions are not counted as such), the genetic material is primarily DNA or the related molecule, RNA. Another exception might be the software code of certain forms of viruses and programs created through genetic programming, but whether computer programs can be alive even by this definition is still a matter of some contention.
Origin of life
Main article: Origin of life There is no truly "standard" model of the origin of life, but most currently accepted scientific models build in one way or another on the following discoveries, which are listed roughly in 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. #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. There are many different hypotheses regarding the path that might have been taken from simple organic molecules to protocells and metabolism. Many models fall into the "genes-first" category or the "metabolism-first" category, but a recent trend is the emergence of hybrid models that do not fit into either of these categories.
The possibility of extraterrestrial life
Main articles: Extraterrestrial life, Astrobiology As of 2005, Earth is the only planet in the universe known by humans to support life. The question of whether life exists elsewhere in the universe remains open, but analyses such as the Drake equation have been used to estimate the probability of such life existing. There have been a number of claims of the discovery of life elsewhere in the universe, but none of these have yet survived scientific scrutiny. Today, the closest that scientists have come to finding extraterrestrial life is fossil evidence of possible bacterial life on Mars (via the ALH84001 meteorite). Searches for extraterrestrial life are currently focusing on planets and moons believed to possess liquid water, at present or in the past. Recent evidence from the NASA rovers Spirit and Opportunity supports the theory that Mars once had surface water. See Life on Mars for further discussion. Jupiter's moons are also considered good candidates for extraterrestrial life, especially Europa, which seems to possess oceans of liquid water. Other highly speculative and somewhat doubtful places for present or past life include the atmosphere of Venus, Titan cryovolcanoes, or even Enceladus.
See also

- Animal
- Artificial life
- Bacteria
- Biology
- Death
- Fungi
- Biological kingdom
- Biological life cycle
- Monera
- Odic force
- Origin of life (disambiguation)
- Plant
- Prehistoric life
- Protista
References

- Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from [http://www.edge.org/3rd_culture/kauffman03/kauffman_index.html]
External links

- [http://www.lifetheory.com Express your theory and meaning of life]
- [http://www.edge.org/3rd_culture/kauffman03/kauffman_index.html "The Adjacent Possible: A Talk with Stuart Kauffman"]
- [http://www.quotesandpoem.com/poems/SelectedPoetryTopic/Life Poems and Quotes about life and living]
- [http://www.angelfire.com/linux/vjtorley/ Animals and other living things: their interests, mental capacities and moral entitlements]
- [http://tolweb.org/tree?group=life Tree of Life Web Project - Life on Earth]
- [http://plato.stanford.edu/entries/life/ Stanford Encyclopedia of Philosophy entry]
- [http://web.archive.org/web/20041030074958/http://people.cornell.edu/pages/tg21/DHB.html The Deep Hot Biosphere Theory (Thomas Gold)] Category:Biology ja:生命 ko:생명 ms:Benda hidup simple:Life

Panspermia

Panspermia is the hypothesis that the seeds of life are prevalent throughout the universe, and furthermore that life on Earth began by such seeds landing on Earth and propagating themselves. The idea has its origins in the writings of Anaxagoras, but was first proposed in its modern form by Hermann von Helmholtz in 1879. Panspermia can be said to be either interstellar (between star systems) or interplanetary (between planets in the same solar system). There is as yet no compelling evidence to support or contradict it, although the majority view holds that panspermia — especially in its interstellar form — is unlikely given the challenges of survival and transport in space. Sir Fred Hoyle (1915–2001) and Chandra Wickramasinghe were important proponents of the hypothesis who further contended that lifeforms continue to enter the Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution. Panspermia does not remove the need for life to originate somewhere, but does extend the time frame and environments available. Similarly it does not necessarily suggest that life originated only once and subsequently spread through the entire universe, but instead that once started it may be able to spread to other environments suitable for replication. The mechanisms proposed for interstellar panspermia are hypothetical and currently unproven. Interplanetary transfer of material is well documented, as evidenced by meteorites of Martian origin found on Earth. However, claims that these carry evidence of extraterrestrial lifeforms — let alone viable dormant lifeforms — have either been proven unfounded as a result of terrestrial contamination, misinterpretation, or hoaxing; or are currently hotly disputed. Interestingly, space probes may also be a viable transport mechanism for interplanetary panspermia in our solar system (or even beyond) especially as terrestrial bacteria were shown to have survived in a dormant state on the Moon. Since then, however, NASA has implemented strict abiotic procedures to avoid planetary contamination. Exogenesis is a related, but less radical, hypothesis that simply proposes life originated elsewhere in the universe and was transferred to Earth, with no prediction about how widespread life is. The term "panspermia" is more well-known, however, and tends to be used in reference to what would properly be called exogenesis, too.

Evidence

Until a large portion of the galaxy is surveyed for signs of life or contact is made with other civilizations, the panspermia hypothesis in its fullest meaning will remain difficult to test. There is, however, circumstantial evidence for exogenesis:

Narrow time window for geogenesis

circumstantial evidence in the Siyeh Formation, Glacier National Park. It is in formations such as this that 3.5 billion year old fossilized algae microbes, the earliest known life on earth, were discovered.]] The Precambrian fossil record indicates that life appeared soon after the Earth was formed. Unless the Earth just happened to be the site of a large number of fortuitous coincidences, this would imply that life appears in several hundred million years when conditions are favourable.
- Generally accepted scientific estimates of the age of the Earth place its formation (along with the rest of the Solar system) at about 4.55 Ga.
- The oldest known sedimentary rocks are somewhat altered Hadean formations from the southern tip of Akilia island, West Greenland. These rocks have been dated as no younger than 3.85 Ga (they are likely older). The Greenland sediments include banded iron beds, thought to be the result of oxygen released by photosynthetic organisms combining with dissolved iron to form insoluble iron oxides. Carbon deposits in the rock show low levels of carbon-13. Kerogen deposits (derived from organic matter) are isotopically light (i.e. more negative δ¹³C values) which is indicative of photosynthesis (see Schidlowski, 1988). However, this interpretation is under doubt as the Akilia rocks have undergone high-temperature metamorphosis which is known to be fractionating itself (Gilmour & Wright, 1997). There is also a lack of corroborating sulphur isotope fractionation (Nisbet, 2000). Both the sedimentary origin and the carbon content of the rocks have been questioned (Lepland et al, 2005).
- Fossilized stromatolites or bacterial aggregates, the oldest of which are dated at 3.5 billion years old. The bacteria that form stromatolites, cyanobacteria, are photosynthetic. Most models of the origin of life have the earliest organisms obtaining energy from reduced chemicals, with the more complex mechanisms of photosynthesis evolving later.
- During the Late Heavy Bombardment of the Earth's Moon about 3.9 Ga (as evidenced by Apollo lunar samples) impact intensities may have been up to 100x those immediately before or after (Cohen et al., 2000). From analysis of lunar melts and observations of similar cratering on Mars' highlands, Kring and Cohen (2002) suggest that the LHB was caused by asteroid impacts that affected the entire inner solar system. This is likely to have effectively sterilised Earth's entire planetary surface, including submarine hydrothermal systems that would be otherwise protected (Cohen et al., 2000).
- The best estimate of the origin of the universe, from the Wilkinson Microwave Anisotropy Probe, is 13700 million years ago (13.7 Ga). However, at least one subsequent cycle of star birth/death is required for nucleosynthesis of the C, N and O essential to life, and this process may have taken up to several Ga to produce sufficient quantities (Gilmour et al., 1997). This puts the earliest possible emergence of life in the universe at ~12.7 Ga, although there is large uncertainty in the length of the necessary time period. If life originated on Earth it did so in a window of at most 1 Ga (4.55 Ga to 3.5 Ga), most plausibly 400 Ma (3.9 Ga to 3.5 Ga), and possibly <100 Ma (3.9 Ga to 3.85 Ga) if the Greenland (3.85 Ga) isotope signal is correct. If life originated elsewhere, the window expands to ~9 Ga. That full length of time might not be available on a single planet, but the Earth has provided a life-friendly environment for at least 3.5 Ga.

Extremophiles

Evidence has accumulated that some bacteria are more resistant to extreme conditions than previously recognised, and may be able to survive for very long periods of time even in deep space. These extremophiles could possibly travel in a dormant state between environments suitable for ongoing life such as planetary surfaces.
- Streptococcus mitis bacteria that had accidentally been taken to the moon on the Surveyor 3 spacecraft in 1967, could easily be revived after being taken back to Earth by the Apollo 12 astronauts 31 months later. (This report has be called into question by an observer who claims to have seen a lapse in sterile procedure in handling the sample after it was returned to Earth. [http://www.astrobio.net/news/article1311.html])
- Bacteria and more complex organisms have been found in more extreme environments than thought possible, such as black smokers or oceanic volcanic vents. Some extremophile bacteria have been found living at temperatures above 100 °C, others in strongly caustic environments, and others in extreme pressures 11 km under the ocean. [http://news.bbc.co.uk/2/hi/science/nature/4235979.stm]
- Semi-dormant bacteria found in ice cores over a mile beneath the Antarctic — this lends credibility to the concept of sustaining the components of life on the surface of icy comets.
- Bacteria which don't rely on photosynthesis for energy. In particular, endolithic bacteria using chemosynthesis found inside rocks and in subterranean lakes.
- Deinococcus radiodurans is a radioresistant bacterium that can survive high radiation levels.
- Dormant bacteria isolated from insects in amber 10s Ma old (Gilmour et al., 1997)

Wider range of potential habitats for life

Another line of evidence comes from research that shows there are many more potential habitats for life than Earth-like planets.
- The presence of past liquid water on Mars, suggested by river-like formations on the red planet, was confirmed by the Mars Exploration Rover missions.
- Possible water oceans on Europa and perhaps other moons in the Solar system. Even moons that are now frozen ice balls might earlier have been heated internally by radioactive rocky cores. Bodies like this may be extremely common throughout the universe.

Evidence of extraterrestrial life

No undisputed evidence has ever been published in a mainstream scientific journal to suggest that intelligent alien species have visited the Earth. The majority view in the scientific community seems to be an acceptance that the existence of intelligent life elsewhere in the universe is at least highly probable, due to the sheer number of potential sites where life could take hold. However, the special theory of relativity holds that travel over the vast distances between stars would be limited to the speed of light, and so take such a long time that many scientists think it unlikely that that such travel would be practical for life forms as we know them. Nevertheless, a small core of researchers continue to monitor the skies for signs of transmissions from other stars. The Search for Extra-Terrestrial Intelligence (SETI) project is the most popular example. Over the past century, thousands of people have reported UFO sightings in countries all over the world. Some remain unexplained. While such sightings were mostly ignored by the scientific community in the last half of the twentieth century, a few peer-reviewed scientfic journals have published reports assessing physical evidence associated with a few of these sightings, for example, the Journal of Scientific Exploration ([http://www.jse.com/]).

Disputed

UFO
- A meteorite originating from Mars known as ALH84001 was shown in 1996 to contain microscopic structures resembling small terrestrial microfossils. When the discovery was announced, many immediately conjectured that the fossils were the first true evidence of extraterrestrial life—making headlines around the world, and even prompting U.S. President Bill Clinton to make a formal televised announcement to mark the event. As of 2003 however, most experts agree that these are not indicative of life, but may instead be formed abiotically from organic molecules. It has not yet conclusively been shown how they formed.
- Narlikar et al. (2003) took air samples at 41 km over Hyderabad, India — above the tropopause where mixing from the lower atmosphere is unexpected — from which rod and coccoid bacteria were isolated. Two bacterial and one fungal species were later independently isolated from these filters which were identified as Bacillus simplex, Staphylococcus pasteuri and Engyodontium album respectively (Wainright, 2003). The experimental procedure suggested that these were not the result of laboratory contamination, although similar isolation experiments at separate laboratories were unsuccessful. That these are common terrestrial organisms is not necessarily contraindicative of panspermia, since a prediction of the hypothesis is that life throughout the universe is derived from the same ancestral stock. Assuming they are not contaminants, did the micro-organisms come from the Earth or space? That there were no volcanic eruptions — the only known way for terrestrial particles to mix up beyond the tropopause — prior to sampling suggests against a terrestrial source. In either case, Wainright (2003) points out that some part of the panspermia hypothesis is validated: either terrestrial micro-organisms are indeed derived from space, or they are capable of contaminating our local space in a viable form.
- Of three biological experiments on the Mars lander Viking, two gave results that were initially indicative of life. However, the similar results from heated controls; how the release of indicative gas tapered off; and the lack of organic molecules in soil samples all suggest the results were the result of an abiotic chemical reaction rather than biological metabolism. Later experiments showed that terrestrial clays could reproduce the results of the two positive Viking experiments. Despite this, some of the Viking experiments' designers remain convinced that they are diagnostic for life.

Debunked

- In 1962, Claus et al. announced the discovery of 'organised elements' embedded in the Orgueil meteorite. These elements were subsequently shown to be either pollens (including that of ragwort) and fungal spores (Fitch & Anders, 1963) that had contaminated the sample, or crystals of the mineral olivine.
- Unpublished claims ([http://arxiv.org/abs/astro-ph/0310120], [http://arxiv.org/abs/astro-ph/0312639]) that spores detected in the red rain that fell near Kerala, India in 2001 were of cometary extraterrestrial origin, were subsequently debunked. As of 2003, it is now generally accepted that the rain was coloured red by a dust cloud from the Gulf region that contained terrestrial fungal spores (see [http://web.archive.org/web/20041028040513/www.indiaexpress.com/news/regional/kerala/20030619-0.html]).
- In 2002, the discovery of glycine (the simplest amino acid) in interstellar clouds was reported. [http://physicsweb.org/article/news/7/8/7]. Subsequent investigation has refuted these claims.

Hoaxes

- A separate fragment of the Orgueil meteorite (kept in a sealed glass jar since its discovery) was found in 1965 to have a seed capsule embedded in it, whilst the original glassy layer on the outside remained undisturbed. Despite great initial excitement, it was found to be that of a European rush that had been glued into the fragment and camouflaged using coal dust. The outer 'fusion layer' was in fact glue. Whilst the perpetrator of this hoax is unknown, it is thought he sought to influence the 19th century debate on spontaneous generation — rather than panspermia — by demonstrating the transformation of inorganic to biological matter.

Objections to panspermia and exogenesis

- Life requires heavy elements carbon, nitrogen and oxygen (C, N and O, respectively) to exist at sufficient densities and temperatures for the chemical reactions between them to occur. These conditions are not widespread in the universe, so this limits the distribution of life as an ongoing process. First, the elements C, N and O are only created after at least one cycle of star birth/death: this is a limit to the earliest time life could have arisen. Second, densities of elements sufficient for the formation of more complex molecules necessary to life (such as amino acids) only occur in molecular dust clouds (10⁹–10¹² particles/m³), and (following their collapse) in solar systems. Third, temperatures must be lower than those in stars (elements are stripped of electrons: a plasma state) but higher than in interstellar space (reaction rates are too low). This restricts ongoing life to planetary environments where heavy elements are present at high densities, so long as temperatures are sufficient for plausible reaction rates. Note this does not restrict dormant forms of life to these environments, so this argument only contradicts the widest interpretation of panspermia — that life is ongoing and spread in many different environments throughout the universe — and presupposes that any life needs those elements, which the proponents of alternative biochemistries do not consider certain.
- Space is a damaging environment for life, as it would be exposed to radiation, cosmic rays and stellar winds. However, some bacteria may be able to survive these conditions. Also, environments may exist within meteorites or comets that are somewhat shielded from these hazards.
- Bacteria would not survive the immense heat and forces of an impact on earth — no conclusions (whether positive or negative) have yet been reached on this point. However most of the heat generated when a meteor enters the Earth's atmosphere is carried away by ablation and the interiors of freshly landed meteorites are rarely heated much and are often cold. For example, a sample of hundreds of nematode worms on the Columbia space shuttle survived its crash landing from 63 km inside of a 4 kg locker, and samples of already dead moss were not damaged. Though this is not a very good example, being protected by the man-made locker and possibly pieces of the shuttle, it lends some support to the idea that life could survive a trip through the atmosphere. [http://news.bbc.co.uk/1/hi/sci/tech/2992123.stm] The existence of Martian meteorites and Lunar meteorites in captivity suggests that transfer of material from other planets to Earth happens regularly.
- Occam's Razor states that when multiple explanations are available for a phenomenon, the simplest version is preferred. From this perspective, geogenesis appears to be the default assumption when compared with panspermia or exogenesis. The former assumes a single step — that life originated on Earth — ahead of the more elaborate idea that life formed elsewhere and was subsequently transplanted to the Earth biosphere. Given that an understanding of life's emergence remains partly speculative, however, the perception of which possibility is the "simplest" explanation is not always clear. Geogenesis eliminates the step of transferring life across space, but requires a lot to happen in a relatively short time frame. Also, on a larger scale, for life to emerge in one place in the universe and subsequently spread to other planets would be simpler than life emerging separately on different planets. Occam's razor is usually a double-edged knife.

Directed panspermia

A second prominent proponent of panspermia is Nobel prize winner Francis Crick, who along with Leslie Orgel proposed the theory of directed panspermia in 1973. This suggests that the seeds of life may have been purposely spread by an advanced extraterrestrial civilization. Crick argues that small grains containing DNA, or the building blocks of life, fired randomly in all directions is the best, most cost effective strategy for seeding life on a compatible planet at some time in the future. The strategy might have been pursued by a civilization facing catastrophic annihilation, or hoping to terraform planets for later colonization. Other proponents of panspermia believe that life never evolved from inorganic molecules, but that it has existed as long as all other forms of matter. This is an extension of panspermia called cosmic ancestry.

Science fiction

The theory of panspermia has been explored in a number of works of science fiction, notably Jack Finney's Invasion of the Body Snatchers (twice made into a film) and the Dragonriders of Pern books of Anne McCaffrey. In John Wyndham's book, The Day of the Triffids (also made into a film), the first person narrator, writing in historical mode, takes care to reject the theory of panspermia in favour of the conclusion that the eponymous carnivorous plants are a product of Soviet biotechnology. The book and film of The Andromeda Strain examines the consequences of a pathogenic extraterrestrial organism arriving on Earth. Some works of science fiction advance a derivative of the theory as a rationalization for the improbable tendency of fictional extra-terrestrials to be strongly humanoid in form as well as living on earth-compatible worlds (see Class M planet). In Star Trek, the humanoid aliens, as well as humans themselves, are results of the cells spread through the universe by the Progenitors. Fiction writer Dan Brown also includes panspermia in the novel Deception Point. The novel The Gripping Hand by Larry Niven and Jerry Pournelle mentions that panspermia is a commonly accepted theory in that universe.

References

- Rhawn Joseph, "Astrobiology, the Origin of Life and the Death of Darwinism", University Press, 2001, ISBN 0970073380
- Cohen, B., Swindle, T. and Kring, D. (2000) Support for the lunar cataclysm hypothesis from lunar meteorite impact melt ages. Science, 290(5497):1754 -- 1756.
- Crick F, 'Life, Its Origin and Nature', Simon and Schuster, 1981, ISBN 0708822355
- Gilmour I, Wright I, Wright J 'Origins of Earth and Life', The Open University, 1997, ISBN 0749281820
- FITCH FW, ANDERS E (1963) ORGANIZED ELEMENT — POSSIBLE IDENTIFICATION IN ORGUEIL METEORITE. SCIENCE 140 (357): 1097
- Hoyle F, 'The Intelligent Universe', Michael Joseph Limited, London 1983, ISBN 0718122984
- Kring DA, Cohen BA (2002) Cataclysmic bombardment throughout the inner solar system 3.9-4.0 Ga. J GEOPHYS RES-PLANET 107 (E2): art. no. 5009
- Lepland A, van Zuilen M, Arrhenius G, Whitehouse M and Fedo C, Questioning the evidence for Earth's earliest life—Akilia revisited, Geology; January 2005; v. 33; no. 1; p. 77-79; DOI: 10.1130/G20890.1
- NAGY B, CLAUS G, HENNESSY DJ (1962) ORGANIC PARTICLES EMBEDDED IN MINERALS IN ORGUEIL AND IVUNA CARBONACEOUS CHONDRITES. NATURE 193 (4821): 1129
- Narlikar JV, Lloyd D, Wickramasinghe NC, et al. (2003) Balloon experiment to detect micro-organisms in the outer space. ASTROPHYS SPACE SCI 285 (2): 555-562
- Nisbet, E. (2000) The realms of Archaean life. Nature, 405(6787):625 -- 626.
- SCHIDLOWSKI, M. (1988) A 3,800-MILLION-YEAR ISOTOPIC RECORD OF LIFE FROM CARBON IN SEDIMENTARY-ROCKS. Nature, 333(6171):313 -- 318.
- Wainwright, M. (2003) A microbiologist looks at panspermia. Astrophysics and Space Science, 285(2):563 -- 570

External links

- [http://www.actionbioscience.org/newfrontiers/wickramasinghe/wick_hoyle.html Evolution of Life: A Cosmic Perspective], by N. Chandra Wickramasinghe & Fred Hoyle
- [http://www.actionbioscience.org/newfrontiers/wickramasinghe/article.html Life from Space: An Emerging Paradigm], by N. Chandra Wickramasinghe
- [http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=00073A97-5745-1359-94FF83414B7F0000 Did Life Come from Another World?] Scientific American, November 2005.
- [http://www.panspermia.org/thirdalt.htm Creationism versus Darwinism: A third alternative ], by N. Chandra Wickramasinghe & Brig Klyce
- [http://www.museumofhoaxes.com/orgueil.html The Orgueil meteorite hoax]
- [http://www.fossilmall.com/Science/About_Stromatolite.htm About stromatolites]
- http://www.panspermia.org/ — Modern panspermia advocates. This site claims cosmic ancestry; not only that life on Earth originated in space, but that life has existed since the beginning of time, as well as an alternative explanation of evolution.
- http://www.iscid.org/brig-klyce-chat.php — cosmic ancestry
- http://www.nature.com/nsu/040216/040216-20.html
- http://www.ideacenter.org/contentmgr/showdetails.php/id/849 — Panspermia criticism from the intelligent design community
- Francis Crick's [http://profiles.nlm.nih.gov/SC/B/B/Y/P/_/scbbyp.pdf handwritten notes (PDF)] for a lecture on Directed Panspermia, dated 5 November 1976.
- http://www.tonderai.co.uk/earth/origins.php#limits Category:Astrobiology Category:Origin of life ja:パンスペルミア説

Humanoid

: Humanoids links here. For the French comic book publisher, see Humanoids Publishing. For real humanoid robots, see humanoid robot. The term "humanoid" refers to any being whose body structure resembles that of a human. In this sense, the term describes non-human hominids and indeed most primates, as well as mythological creatures and artificial organisms (robots), especially in the context of science fiction and fantasy fiction. An android or gynoid is a humanoid robot, although the words are, in principle, synonymous. Usually, a fictional humanoid species has the same basic body outline as a human, being bipedal, but differs in details such as coloring, ear form, presence of hair, average height and weight, size of nose, form of skin, "extras" such as horns, plates, claws, tails or multiple appendages, limb structure (such as having digitigrade legs) and taxonomic lineage (being descended from reptiles, fish, rodents, marsupials, or a phylum not evolved on Earth, perhaps, instead of primates). Reptilian humanoids are a common concept. Most of the aliens in television and movies are humanoid, since it is easier for a fictional character to be a disguised human actor. However, there are various methods for presenting non-humanoid characters, for example computer graphics, creative costuming, and puppetry. Some people find it unlikely to have a universe populated by unrelated creatures that all look human, while others (including some biologists) believe that a species would naturally drift towards bipedalism when achieving sapience as we know it (e.g. Russell's hypothetical troödon-descended sapient). Occasionally, shows present a reason for this to be the case. For example, the episode "The Chase" of Star Trek: The Next Generation explained the humanoid denizens of the Star Trek universe by advancing the story of a primordial humanoid civilization, the Progenitors, that seeded the galaxy with genetically-engineered cells that guide evolution toward humanoid life (see panspermia). In most cases, however, the reason for the similarity is not explained, and it is regarded simply as a dramatic convention.

Science Fiction

Science fiction

Bacterium

Actinobacteria
Aquificae
Bacteroidetes/Chlorobi
Chlamydiae/Verrucomicrobia
Chloroflexi
Chrysiogenetes
Cyanobacteria
Deferribacteres
Deinococcus-Thermus
Dictyoglomi
Fibrobacteres/Acidobacteria
Firmicutes
Fusobacteria
Gemmatimonadetes
Nitrospirae
Planctomycetes
Proteobacteria
Spirochaetes
Thermodesulfobacteria
Thermomicrobia
Thermotogae Bacteria (singular: bacterium) are a major group of living organisms. Most are microscopic and unicellular, with a relatively simple cell structure lacking a cell nucleus, and organelles such as mitochondria and chloroplasts. Their cell structure is further described in the article about prokaryotes, because bacteria are prokaryotes, in contrast to organisms with more complex cells, called eukaryotes. The term "bacteria" has variously applied to all prokaryotes or to a major group of them, otherwise called the eubacteria, depending on ideas about their relationships. In Wikipedia, bacteria is used specifically to refer to the eubacteria. Bacteria are the most abundant of all organisms. They are ubiquitous in soil, water, and as symbionts of other organisms. Many pathogens are bacteria. Most are minute, usually only 0.5-5.0 μm in their longest dimension, although giant bacteria like Thiomargarita namibiensis and Epulopiscium fishelsoni may grow past 0.5 mm in size. They generally have cell walls, like plant and fungal cells, but with a very different composition (peptidoglycans). Many move around using flagella, which are different in structure from the flagella of other groups.

History and taxonomy

The first bacteria were observed by Antony van Leeuwenhoek in 1683 using a single-lens microscope of his own design. The name bacterium was introduced much later, by Ehrenberg in 1828, derived from the Greek word βακτηριον meaning "small stick". Louis Pasteur (1822-1895) and Robert Koch (1843-1910) described the role of bacteria as conveyors and causes of disease or pathogens.

Metabolism

Bacteria show a wide variety of different metabolisms and can accordingly be classified into primary nutritional groups. The most common division is between heterotrophs, which depend on an organic source of carbon, and autotrophs, which are able to synthesize organic compounds from carbon dioxide and water. Autotrophs that obtain energy by oxidizing chemical compounds are called chemotrophs, and those that obtain their energy from light, via photosynthesis, are called phototrophs. There are many variations on this terminology such as chemoautotrophs and photosynthetic autotrophs and so on. In addition, bacteria are distinguished based on the source of reducing equivalents they are using. Those using inorganic compounds (e. g. water, hydrogen, sulfide or ammonia) for this purpose are called lithotrophs and others needing organic compounds (e. g. sugars or organic acids) and are called organotrophs. The metabolic modes of energy metabolism (phototrophy or chemotrophy), reducing equivalent sources (lithotrophy or organotrophy) and carbon sources (autotrophy or heterotrophy) can be combined differently in any single microorganism, and even shifting between different modes frequently occurs in many species. Other nutritional requirements include nitrogen, sulfur, phosphorus, vitamins and metallic elements such as sodium, potassium, calcium, magnesium, manganese, iron, zinc, cobalt, copper and nickel for normal growth. For some species, additional trace elements such as selenium, tungsten, vanadium or boron are needed. Based on their response to oxygen, most bacteria can be placed into one of three groups: Some bacteria can grow only in the presence of oxygen and are called aerobes; others can grow only in the absence of oxygen and are called anaerobes; and some can grow in the presence or absence of oxygen and are called facultative anaerobes.

Movement

Motile bacteria can move about, either using flagella, bacterial gliding, or changes of buoyancy. A unique group of bacteria, the spirochaetes, have structures similar to flagella, called axial filaments, between two membranes in the periplasmic space. They have a distinctive helical body that twists about as it moves. Bacterial flagella are arranged in many different ways. Bacteria can have a single polar flagellum at one end of a cell, clusters of many flagella at one end or flagella scattered all over the cell, as with Peritrichous. Many bacteria (such as E.coli) have two distinct modes of movement: forward movement (swimming) and tumbling. The tumbling allows them to reorient and introduces an important element of randomness in their forward movement. (see external links below for link to videos). Motile bacteria are attracted or repelled by certain stimuli, behaviors called taxes - for instance, chemotaxis, phototaxis, mechanotaxis and magnetotaxis. In one peculiar group, the myxobacteria, individual bacteria attract to form swarms and may differentiate to form fruiting bodies. The myxobacteria move only when on solid surfaces, unlike E. coli which is motile in liquid or solid media.

Groups and identification

myxobacteria Bacteria come in a variety of different shapes. Most are rod-shaped, sphere-shaped, or helix-shaped; these are respectively referred to as bacilli, cocci, and spirilla. An additional group, vibrios, are comma-shaped. Shape is no longer considered a defining factor in the classification of bacteria, but many genera are named for their shape (e.g. Bacillus, Streptococcus, Staphylococcus) and it is an important part in their identification. Another important tool is Gram staining, named after Hans Christian Gram who developed the technique. This separates bacteria into two groups, based on the composition of their cell wall. The first formal grouping of bacteria into phyla was based largely on this test:
- Gracilicutes - bacteria with a second cell membrane containing lipids, giving them Gram-negative stains
- Firmicutes - bacteria with a single membrane and thick peptidoglycan wall, giving them Gram-positive stains
- Mollicutes - bacteria with no second membrane or wall, giving them Gram-negative stains The archeabacteria were originally included as the Mendosicutes. As given, these phyla are no longer believed to represent monophyletic groups. The Gracilicutes have been divided into many different phyla. Most gram-positive bacteria are placed in the phyla Firmicutes and Actinobacteria, which are closely related. However, the Firmicutes have been redefined to include the mycoplasmas (Mollicutes) and certain Gram-negative bacteria.

Benefits and dangers

Bacteria are both harmful and useful to the environment, and animals, including humans. The role of bacteria in disease and infection is important. Some bacteria act as pathogens and cause tetanus, typhoid fever, pneumonia, syphilis, cholera, foodborne illness and tuberculosis. Sepsis, a systemic infectious syndrome characterized by shock and massive vasodilation, or localized infection, can be caused by bacteria such as streptococcus, staphylococcus, or many gram-negative bacteria. Some bacterial infections can spread throughout the host's body and become systemic. In plants, bacteria cause leaf spot, fireblight, and wilts. The mode of infection includes contact, air, food, water, and insect-borne microorganisms. The hosts infected with the pathogens may be treated with antibiotics, which can be classified as bacteriocidal and bacteriostatic, which at concentrations that can be reached in bodily fluids either kill bacteria or hamper their growth, respectively. Antiseptic measures may be taken to prevent infection by bacteria, for example, prior to cutting the skin during surgery or swabbing skin with alcohol when piercing the skin with the needle of a syringe. Sterilization of surgical and dental instruments is done to make them sterile or pathogen-free to prevent contamination and infection by bacteria. Sanitizers and disinfectants are used to kill bacteria or other pathogens to prevent contamination and risk of infection. In soil, microorganisms help in the transformation of nitrogen to ammonia with enzymes secreted by these microbes, which reside in the rhizosphere (a zone that includes the root surface and the soil that adheres to the root after gentle shaking). Some bacteria are able to use molecular nitrogen as their source of nitrogen, converting it to nitrogenous compounds, a process known as nitrogen fixation. Many other bacteria are found as symbionts in humans and other organisms. For example, the presence of the gut flora in the large intestine can help prevent the growth of potentially harmful microbes. The ability of bacteria to degrade a variety of organic compounds is remarkable. Highly specialized groups of microorganisms play important roles in the mineralization of specific classes of organic compounds. For example, the decomposition of cellulose, which is one of the most abundant constituents of plant tissues, is mainly brought about by aerobic bacteria that belong to the genus Cytophaga. This ability has also been utilized by humans in industry, waste processing, and bioremediation. Bacteria capable of digesting the hydrocarbons in petroleum are often used to clean up oil spills. Some beaches in Prince William Sound were fertilized in an attempt to facilitate the growth of such bacteria after the infamous 1989 Exxon Valdez oil spill. These efforts were effective on beaches that were not too thickly covered in oil. Bacteria, often in combination with yeasts and molds, are used in the preparation of fermented foods such as cheese, pickles, soy sauce, sauerkraut, vinegar, wine, and yogurt. Using biotechnology techniques, bacteria can be bioengineered for the production of therapeutic drugs, such as insulin, or for the bioremediation of toxic wastes.

Miscellaneous

Two organelles, mitochondria and chloroplasts, are generally believed to have been derived from endosymbiotic bacteria. Microorganisms are widely distributed and are most abundant where they have food, moisture, and the right temperature for their multiplication and growth. They can be carried by air currents from one place to another. The human body is home to billions of microorganisms; they can be found on skin surfaces, in the intestinal tract, in the mouth, nose, and other body openings. They are in the air one breathes, the water one drinks, and the food one eats. The great antiquity of the bacteria has enabled them to evolve a great deal of genetic diversity. They are far more diverse than, say, the mammals or insects. For instance, the genetic distance between E. coli and Thermus aquaticus is greater than the distance between humans and oak trees.

References

- Some text in this entry was merged with the Nupedia article entitled Bacteria, written by Nagina Parmar; reviewed and approved by the Biology group (editor: Gaytha Langlois, lead reviewer: Gaytha Langlois, lead copyeditors: Ruth Ifcher and Jan Hogle)
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External links

- [http://www.dsmz.de/bactnom/bactname.htm Bacterial Nomenclature Up-To-Date from DSMZ]
- [http://www.sciencenews.org/pages/sn_arc99/4_17_99/fob5.htm The largest bacteria]
- [http://tolweb.org/tree?group=Eubacteria&contgroup=Life_on_Earth Tree of Life]
- [http://www.rowland.harvard.edu/labs/bacteria/index_movies.html Videos] of bacteria swimming and tumbling, use of optical tweezers and other fine videos.
- Category:Bacteriology ko:세균 ja:真正細菌 th:แบคทีเรีย

Extraterrestrial life in popular culture

In popular fiction and conspiracy theories, life forms, especially intelligent life forms, that are of extraterrestrial origin, i.e. not coming from the Earth are referred to as alien and collectively as aliens. Prime examples of how aliens are viewed are found in the movies Alien, E.T. the Extra-Terrestrial, Signs, Independence Day, and The War of the Worlds. This usage is clearly anthropocentric: when humans in fictional accounts accomplish interstellar travel and land on a planet elsewhere in the universe, the local inhabitants of these other planets are usually still referred to as "alien," even though they are the native life form and the humans are the intruders. In general they are seen as unfriendly life forms. This may be seen as a reversion to the classic meaning of "alien" as referring to "other," in contrast to "us" in the context of the writer's frame of reference.

Typical characteristics

In popular culture, such as movies and comics, "aliens" are often depicted as somewhat humanoid in their appearance (See Greys, Little green men). There are several reasons for this humanoid depiction in popular culture. It makes it easier for an alien in a movie scene to simply be a disguised human actor. Aliens in movies, in order to catch our attention, must trigger instantaneous emotional reaction; this requires a design based on recognizable human facial features and expressions. It is easier to relate to an alien with features we recognize such as arms and legs, two eyes, a nose and a mouth, as well as behavior we recognize such as baring its teeth in anger or widening its eyes in shock or surprise. However, if real extraterrestrial life exists, few people expect to find humanoid characteristics, believing that this would be too great a coincidence given an entirely different evolutionary scale.

Extraterrestrial life in fiction

The existence of alien beings (intelligent extraterrestrial life) has been one of the enduring themes of science fiction. One of the first fiction writers to use alien beings from another planet was H. G. Wells. The War of the Worlds is the best known of his works; it also introduced the modern reader to the recurring concept of interplanetary invasion by malign aliens. The alien invasion was one of two themes which was to crop up again and again in the 'pulp science fiction' years, the 1930s to 1960s. The Cold War made people particularly receptive to the idea of evil and incomprehensible beings coming to destroy or enslave earthly (usually American) life. Examples of these include the short story "The Liberation of Earth" by William Tenn. The contrasting picture of aliens during this time was that of the wise and civlised race coming to Earth to impart their wisdom and solve our problems. These stories were almost as popular as the invasion theme, at a time when mankind looked as though it was in the brink of destroying itself. A good example of this story is the movie The Day the Earth Stood Still. Aliens are also used in fiction to examine the human race from a different perspective; the author wishes us to examine ourselves in a new way by contrasting us with an imagined alternative. This was one of the functions of Mr. Spock in Star Trek. One of the most frequently portrayed alien races from our own solar system are the Martians, Mars being the most romanticized of the other planets whose surface conditions are closest to being amenable to life. See Mars in fiction for more details on the red planet's numerous roles. Popular fictional tales of the first half of the 20th century also include many fanciful tales of alien races and civilizations on our nearest neighbor, the Moon. Many of the aliens have been hostile and alien invasion has been a very popular idea in English-language science fiction. Writers have created a long list of extraterrestrial creatures and intelligent beings; see the list of aliens in fiction.

Silicon based life

There are examples of silicon-based life in science fiction, such as an episode of the original Star Trek series, which included a silicon life form called the Horta or the Chenjesu and Taalo in the science-fiction adventure game Star Control II. The Doctor Who story The Hand of Fear featured a silicon-based being, and indeed made great play of the exceptional rarity of silicon-based lifeforms.