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How Life Began

How Life Began

Abiogenesis (Greek a-bio-genesis, "non biological origins") is, in its most general sense, the generation of life from non-living matter. Today the term is primarily used to refer to theories about the chemical origin of life, such as from a primordial soup. Earlier notions of abiogenesis, now more commonly known as spontaneous generation, held that living organisms are generated by decaying organic substances, e.g. that mice spontaneously appear in stored grain or maggots spontaneously appear in meat. (That idea, which has long been known to be incorrect, will be called "Aristotelian abiogenesis" in this article.)

History of abiogenesis hypotheses

Aristotelian abiogenesis, also known as spontaneous generation, (and, in older texts, Generatio aequivoca, Generatio primaria, archegenesis, autogenesis, and archebiosis), was the theory according to which fully formed living organisms sometimes arise from not-living matter. Aristotle explicitly taught this form of abiogenesis, and laid it down as an observed fact that some animals spring from putrid matter, that plant lice arise from the dew which falls on plants, that fleas are developed from putrid matter, that mice come from dirty hay, and so forth. Alexander Ross, in commenting on Sir Thomas Browne's doubt as to "whether mice may be bred by putrefaction", gives a clear statement of the common opinion on abiogenesis held until about two centuries ago. Ross wrote: :So may he (Sir Thomas Browne) doubt whether in cheese and timber worms are generated; or if beetles and wasps in cows' dung; or if butterflies, locusts, grasshoppers, shellfish, snails, eels, and such like, be procreated of putrefied matter, which is apt to receive the form of that creature to which it is by formative power disposed. To question this is to question reason, sense and experience. If he doubts of this let him go to Egypt, and there he will find the fields swarming with mice, begot of the mud of Nylus, to the great calamity of the inhabitants. The first step in the scientific refutation of the theory of Aristotelian abiogenesis was taken by the Italian Francesco Redi, who, in 1668, proved that no maggots were bred in meat on which flies were prevented by wire screens from laying their eggs. From the seventeenth century onwards it was gradually shown that, at least in the case of all the higher and readily visible organisms, spontaneous generation did not occur, but that omne vivum ex ovo, every living thing came from a pre-existing living thing. The discovery of the microscope carried the refutation further. In 1683 Antoni van Leeuwenhoek discovered bacteria, and it was soon found that however carefully organic matter might be protected by screens, or by being placed in stoppered receptacles, putrefaction set in, and was invariably accompanied by the appearance of myriads of bacteria and other low organisms. As knowledge of microscopic forms of life increased, so the apparent possibilities of abiogenesis increased, and it became a tempting hypothesis that whilst the higher forms of life arose only by generation from their kind, there was a perpetual abiogenetic fount by which the first steps in the evolution of living organisms continued to arise, under suitable conditions, from inorganic matter. This was mostly disproved by Lazzaro Spallanzani, who, in 1768, proved that microbes came from the air, and could be killed by boiling. His work paved the way for Louis Pasteur. It was due chiefly to Louis Pasteur that the occurrence of abiogenesis in the microscopic world was disproved as much as its occurrence in the macroscopic world. If organic matter were first sterilized and then prevented from contamination from without, putrefaction did not occur, and the matter remained free from microbes. The nature of sterilization, and the difficulties in securing it, as well as the extreme delicacy of the manipulations necessary, made it possible for a very long time to be doubtful as to the application of the phrase omne vivum e vivo to the microscopic world, and there still remain a few belated supporters of abiogenesis. Subjection to the temperature of boiling water for, say, half an hour seemed an efficient mode of sterilization, until it was discovered that the spores of bacteria are so involved in heat-resisting membranes, that only prolonged exposure to dry, baking heat can be recognized as an efficient process of sterilization. Moreover, the presence of bacteria, or their spores, is so universal that only extreme precautions guard against a re-infection of the sterilized material. It was thus concluded definitely that all known living organisms arise only from pre-existing living organisms.

Modern concept of abiogenesis

Main article: Origin of life Even as Aristotelian abiogenesis was being disproven, many scientists, such as T. H. Huxley, continued to postulate a "primordial archebiosis", in which the living organisms observed in the present world had originally arisen in a series of stages from non-living matter. (This hypothetical scenario is not greatly different from the original Aristotelian hypothesis). Such scientists pointed out that the disproof of Aristotelian abiogenesis applied only to "known existing organisms", not to unknown forms of life or proto-life which may have existed under the vastly different conditions of the early Earth. The modern definition of abiogenesis is concerned with the formation of the earliest forms of life on earth from primordial chemicals. This is a significantly different thing from the concept of Aristotelian abiogenesis, which postulated the formation of complex organisms. Different hypotheses for modern abiogenetic processes are currently under debate with no clear frontrunner; see, for example, RNA world hypothesis, proteinoid, Miller experiment.

Critics

Though modern concept of abiogenesis has been criticised by scientists such as Sir Fred Hoyle, Erwin Schrödinger and Hubert Yockey, it should be noted that despite the success these scientists have had in their respective fields of study, they do or did not have expertise in biology. Leading biologists point to fundamental assumptions in their arguments which have little to no bearing on abiogenesis theories or research. Francis Crick is an exception on this incomplete list, in that he has training in biology, despite starting off in physics.

Schrödinger

In 1944, physicist Erwin Schrödinger, in his book What is life?, asserted that the mechanism of genetics defies the laws of thermodynamics, since a relatively small number of molecules, which form the genetic material, have such a huge influence on so many other molecules. Although not direct criticism of abiogenesis, Schrödinger's book asserts that life can't be explained by the laws of physics thus implying that it can't be created from lifeless matter. Today, scientists believe that the distinction between large numbers and small numbers is eminently important to understand biological systems, because they are small number systems rather than the convenient large number systems that physicists prefer. What thermodynamics (which Schrödinger based his book on) describes as a random fluctuation is a signaling process to cell biology. Hormonal signals depend on the behavior of small systems, where fluctuations can push a system beyond a threshold level where a chemical reaction suddenly becomes spontaneous; as opposed to, say, a balloon filled with gas, where a fluctuation (for example, a change of speed) of a few hundreds of molecules will not change the state of the gas in the balloon as a whole (for example, it will not change its temperature). [http://www.whatislife.com/about.html]

Yockey

Information theorist Hubert Yockey argued that chemical evolutionary research raises the question: :Research on the origin of life seems to be unique in that the conclusion has already been authoritatively accepted … . What remains to be done is to find the scenarios which describe the detailed mechanisms and processes by which this happened. :One must conclude that, contrary to the established and current wisdom a scenario describing the genesis of life on earth by chance and natural causes which can be accepted on the basis of fact and not faith has not yet been written. (Yockey, 1977. A calculation of the probability of spontaneous biogenesis by information theory, Journal of Theoretical Biology 67:377–398, quotes from pp. 379, 396.) In a book he wrote 15 years later, Yockey argued that the idea of abiogenesis from a primordial soup is a failed paradigm: :Although at the beginning the paradigm was worth consideration, now the entire effort in the primeval soup paradigm is self-deception on the ideology of its champions. … :The history of science shows that a paradigm, once it has achieved the status of acceptance (and is incorporated in textbooks) and regardless of its failures, is declared invalid only when a new paradigm is available to replace it. Nevertheless, in order to make progress in science, it is necessary to clear the decks, so to speak, of failed paradigms. This must be done even if this leaves the decks entirely clear and no paradigms survive. It is a characteristic of the true believer in religion, philosophy and ideology that he must have a set of beliefs, come what may (Hoffer, 1951). Belief in a primeval soup on the grounds that no other paradigm is available is an example of the logical fallacy of the false alternative. In science it is a virtue to acknowledge ignorance. This has been universally the case in the history of science as Kuhn (1970) has discussed in detail. There is no reason that this should be different in the research on the origin of life. (Yockey, 1992. Information Theory and Molecular Biology, p. 336, Cambridge University Press, UK, ISBN 0-521-80293-8). Yockey, in general, possesses a highly critical attitude toward people who give credence toward natural origins of life, often invoking words like "faith" and "ideology". Yockey's publications have become favorites to quote among creationists, though he is not a creationist himself (as noted in this 1995 email [http://www.asa3.org/archive/evolution/199602/0125.html]).

Panspermia advocates

Panspermia, the idea that life came to Earth from elsewhere in the universe, is viewed by some as a criticism of abiogenesis. However, panspermia hypotheses simply transfer the origin problem elswhere without offering a solution, so it does not necessarily address or criticize abiogenesis.

Crick

Francis Crick, molecular biologist and neuroscientist, most noted for being one of the co-discoverers of the structure of the DNA molecule, and chemist Leslie Orgel co-proposed Directed Panspermia as the mechanism through which life started on Earth.

Hoyle

Sir Fred Hoyle, with Chandra Wickramasinghe, was a proponent of Panspermia, first proposed by the Greek philosopher Anaxagoras. Hoyle became a staunch critic of chemical evolution to explain the naturalistic origin of life. Critics have shown that Hoyle's understanding of evolution is radically out of touch with modern biology. Although the hypothesis of panspermia is not in conflict with the idea of abiogenesis, Hoyle's interpretation of panspermia is in conflict.

References

- Things Come to Life by Henry Harris (2002) ISBN 0198515383
- Buehler, Lukas K. (2000-2005) The physico-chemical basis of life, http://www.whatislife.com/about.html accessed 27 October 2005.

External links

- [http://www.talkorigins.org/faqs/abioprob/spontaneous-generation.html Spontaneous Generation and the Origin of Life] — an article part of the Talk.Origins FAQ
- [http://www.talkorigins.org/faqs/abioprob/abioprob.html Probability of Abiogenesis Calculations] — part of the Talk.Origins FAQ Category:Origin of life ko:자연발생설 ja:自然発生説

Greek language

Greek (Greek Ελληνικά, IPA – "Hellenic") is an Indo-European language with a documented history of 3,500 years. Today, it is spoken by 15 million people in Greece, Cyprus, the former Yugoslavia, particularly The Former Yugoslav Republic of Macedonia, Bulgaria, Albania and Turkey. There are also many Greek emigrant communities around the world, such as those in Melbourne, Australia which is the third-largest Greek-populated city in the world, after Athens and Thessaloniki. Greek has been written in the Greek alphabet, the first true alphabet, since the 9th century B.C. and before that, in Linear B and the Cypriot syllabaries. Greek literature has a long and rich tradition.

History

This article does not cover the reconstructed history of Greek prior to the use of writing. For more information, see main article on Proto-Greek language. Greek has been spoken in the Balkan Peninsula since the 2nd millennium BC. The earliest evidence of this is found in the Linear B tablets dating from 1500 BC. The later Greek alphabet (q.v.) is unrelated to Linear B, and was derived from the Phoenician alphabet (abjad); with minor modifications, it is still used today. Greek is conventionally divided into the following periods:
- Mycenean Greek: the language of the Mycenean civilisation. It is recorded in the Linear B script on tablets dating from the 16th century BC onwards.
- Classical Greek (also known as Ancient Greek): In its various dialects was the language of the Archaic and Classical periods of Greek civilisation. It was widely known throughout the Roman empire. Classical Greek fell into disuse in western Europe in the Middle Ages, but remained known in the Byzantine world, and was reintroduced to the rest of Europe with the Fall of Constantinople and Greek migration to Italy.
- Hellenistic Greek (also known as Koine Greek): The fusion of various ancient Greek dialects with Attic (the dialect of Athens) resulted in the creation of the first common Greek dialect, which gradually turned into one of the world's first international languages. Koine Greek can be initially traced within the armies and conquered territories of Alexander the Great, but after the Hellenistic colonisation of the known world, it was spoken from Egypt to the fringes of India. After the Roman conquest of Greece, an unofficial diglossy of Greek and Latin was established in the city of Rome and Koine Greek became a first or second language in the Roman Empire. Through Koine Greek it is also traced the origin of Christianity, as the Apostles used it to preach in Greece and the Greek-speaking world. It is also known as the Alexandrian dialect, Post-Classical Greek or even New Testament Greek (after its most famous work of literature).
- Medieval Greek: The continuation of Hellenistic Greek during medieval Greek history as the official and vernacular (if not the literary nor the ecclesiastic) language of the Byzantine Empire, and continued to be used until, and after the fall of that Empire in the 15th century. Also known as Byzantine Greek.
- Modern Greek: Stemming independently from Koine Greek, Modern Greek usages can be traced in the late Byzantine period (as early as 11th century). Two main forms of the language have been in use since the end of the medieval Greek period: Dhimotikí (Δημοτική), the Demotic (vernacular) language, and Katharévousa (Καθαρεύουσα), an imitation of classical Greek, which was used for literary, juridic, and scientific purposes during the 19th and early 20th centuries. Demotic Greek is now the official language of the modern Greek state, and the most widely spoken by Greeks today. It has been claimed that an "educated" speaker of the modern language can understand an ancient text, but this is surely as much a function of education as of the similarity of the languages. Still, Koinē , the version of Greek used to write the New Testament and the Septuagint, is relatively easy to understand for modern speakers. Greek words have been widely borrowed into the European languages: astronomy, democracy, philosophy, thespian, etc. Moreover, Greek words and word elements continue to be productive as a basis for coinages: anthropology, photography, isomer, biomechanics etc. and form, with Latin words, the foundation of international scientific and technical vocabulary. See English words of Greek origin, and List of Greek words with English derivatives.

Classification

Greek is an independent branch of the Indo-European language family. The ancient languages which were probably most closely related to it, Ancient Macedonian language (which may be regarded as a dialect of Greek) and Phrygian, are not well enough documented to permit detailed comparison. Among living languages, Armenian seems to be the most closely related to it.

Geographic distribution

Modern Greek is spoken by about 15 million people mainly in Greece and Cyprus. There are also Greek-speaking populations in Georgia, Ukraine, Egypt, Turkey, Albania, Former Yugoslav Republic of Macedonia and Southern Italy. The language is spoken also in many other countries where Greeks have settled, including Armenia, Australia, Austria, Belgium, Bulgaria, Canada, Denmark, France, Germany, Netherlands, Sweden, United Kingdom, and the United States.

Official status

Greek is the official language of Greece where it is spoken by about 99.5% of the population. It is also, alongside Turkish, the official language of Cyprus. Due to the membership of Greece and Cyprus, Greek is one of the 20 official languages of the European Union.

Phonology

This section generally describes the post-Classic phonology of the Greek language. :All phonetic transcriptions in this section use the International Phonetic Alphabet

Vowel sounds

Greek has 5 vowel sounds, all phonemic:

Origin of life

:This article focuses on modern scientific research on the origin of life. For alternate uses, see origin of life (disambiguation). origin of life (disambiguation) in the Siyeh Formation, Glacier National Park. In 2002, William Schopf of UCLA published a controversial paper in the scientific journal Nature arguing that geological formations such as this possess 3.5 billion year old fossilized algae microbes. [http://www.abc.net.au/science/news/space/SpaceRepublish_497964.htm] 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 biology and human understanding of the natural world. Progress in this field is generally slow and sporadic, though it still draws the attention of many due to the gravity of the question being investigated. A few facts give insight into the conditions in which life may have emerged, but the mechanisms by which non-life became life are still elusive. For the observed evolution of life on earth, see the timeline of life.

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 Law of Biogenesis (omne vivum ex ovo or "all life from an egg") became firmly established in modern biology as a result of the work of Louis Pasteur. Charles Darwin in a letter to J.D. Hooker of February 1st 1871, made the suggestion that life may have begun in a "warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed." Thus, it is the presence of life itself, operating in an oxygen rich atmosphere, itself a product of life, which prevents "spontaneous generation" from occurring on Earth today. This modern definition of abiogenesis is concerned with the formation of the simplest forms of life from primordial chemicals, in an environment regarded as similar to that at the time shortly after the formation of the Earth. This is significantly different 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 a number of discoveries concerning the origin of molecular and cellular components for life, which are listed in a rough order of postulated emergence: # Plausible pre-biotic conditions result in the creation of certain basic small molecules (monomers) of life, such as amino acids. This was demonstrated in the Urey-Miller experiment by Stanley L. Miller and Harold C. Urey in 1953. # Phospholipids (of an appropriate length) can spontaneously form lipid bilayers, one of the two basic components of a cell membrane. # The polymerization of nucleotides into random RNA molecules might have resulted in self-replicating ribozymes (RNA world hypothesis). # Selection pressures for catalytic efficiency and diversity result in ribozymes which catalyse peptidyl transfer (hence formation of small proteins), since oligopeptides complex with RNA to form better catalysts. Thus the first ribosome is born, and protein synthesis becomes more prevalent. # Proteins outcompete ribozymes in catalytic ability, and therefore become the dominant biopolymer. Nucleic acids are restricted to predominantly genomic use. The origin (see Origin of organic molecules) of the basic biomolecules, while not settled, is less controversial than the significance and order of steps 2 and 3. The basic inorganic chemicals from which life was formed are methane (CH₄), ammonia (NH₃), water (H₂O), hydrogen sulfide (H₂S), carbon dioxide (CO₂), and phosphate (PO₄³⁺). As of 2004, no one has yet synthesized a "protocell" using basic components which has the necessary properties of life (the so-called "bottom-up-approach"). Without such a proof-of-principle, explanations have tended to be short on specifics. However, some researchers are working in this field, notably Jack Szostak at Harvard. Others have argued that a "top-down approach" is more feasible. One such approach attempted by Craig Venter and others at The Institute for Genomic Research involved engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life were reached. The biologist John Desmond Bernal, in coining the term Biopoesis for this process suggested that there were a number of clearly defined "stages" that could be recognised in explaining the origin of life. Stage 1: The origin of biological monomers
Stage 2: The origin of biological polymers
Stage 3: The evolution from molecules to cell Bernal suggested that Darwinian evolution may have commenced early, some time between Stage 1 and 2.

Origin of organic molecules: Miller, Eigen and Wächtershäuser's theories

Darwinian evolution The "Miller experiments" (including the original Miller–Urey experiment of 1953, by Harold Urey and his graduate student Stanley Miller) are performed under simulated conditions resembling those thought at the time to have existed shortly after Earth first accreted from the primordial solar nebula. The experiment used a highly reduced mixture of gases (methane, ammonia and hydrogen). However, it should be noted that the composition of the prebiotic atmosphere of earth is currently controversial. Other less reducing gases produce a lower yield and variety. It was once thought that appreciable amounts of molecular oxygen were present in the prebiotic atmosphere, which would have essentially prevented the formation of organic molecules; however, the current scientific consensus is that such was not the case. The experiment showed that some of the basic organic monomers (such as amino acids) that form the polymeric building blocks of modern life can be formed spontaneously. Simple organic molecules are of course a long way from a fully functional self-replicating life form; however, in an environment with no pre-existing life these molecules may have accumulated and provided a rich environment for chemical evolution ("soup theory"). On the other hand, the spontaneous formation of complex polymers from abiotically generated monomers under these conditions is not at all a straightforward process. Besides the necessary basic organic monomers, also compounds that would have prohibited the formation of polymers were formed in high concentration during the experiments. Further, according to Brooks and Shaw (1973), there is no evidence in the geological record that any soup existed. :"If there ever was a primitive soup, then we would expect to find at least somewhere on this planet either massive sediments containing enormous amounts of the various nitrogenous organic compounds, acids, purines, pyrimidines, and the like; or in much metamorphosed sediments we should find vast amounts of nitrogenous cokes. In fact no such materials have been found anywhere on earth." Other sources of complex molecules have been postulated, including sources of extra-terrestrial stellar or interstellar origin. For example, from spectral analyses, organic molecules are known to be present in comets and meteorites. In 2004, a team detected traces of polycyclic aromatic hydrocarbons (PAH's) in a nebula, the most complex molecule, to that date, found in space. It can be argued that the most crucial challenge unanswered by this theory is how the relatively simple organic building blocks polymerise and form more complex structures, interacting in consistent ways to form a protocell. For example, in an aqueous environment hydrolysis of oligomers/polymers into their constituent monomers would be favored over the condensation of individual monomers into polymers. Also, the Miller experiment produces many substances that would undergo cross-reactions with the amino acids or terminate the peptide chain. In the early 1970s a major attack on the problem of the origin of life was organised by a team of scientists gathered around Manfred Eigen of the Max Planck Institute. They tried to examine the transient stages between the molecular chaos in a prebiotic soup and the transient stages of a self replicating hypercycle, between the molecular chaos in a prebiotic soup and simple macromolecular self-reproducing systems. In a hypercycle, the information storing system (possibly RNA) produces an enzyme, which aids catalyse the formation of another information system, in sequence until the product of the last aids in the formation of the first information system. Mathematically treated, hypercycles could create quasispecies, which through natural selection entered into a form of Darwinian evolution. A boost to hypercycle theory was the discovery that RNA, in certain circumstances forms itself into ribozymes, a form of RNA enzyme. Another possible answer to this polymerization conundrum was provided in 1980s by Günter Wächtershäuser, in his iron-sulfur world theory. In this theory, he postulated the evolution of (bio)chemical pathways as fundamentals of the evolution of life. Moreover, he presented a consistent system of tracing today's biochemistry back to ancestral reactions that provide alternative pathways to the synthesis of organic building blocks from simple gaseous compounds. In contrast to the classical Miller experiments, which depend on external sources of energy (e. g. simulated lightning or UV irradiation), "Wächtershäuser systems" come with a built-in source of energy, sulfides of iron and other minerals (e. g. pyrite). The energy released from redox reactions of these metal sulfides is not only available for the synthesis of organic molecules, but also for the formation of oligomers and polymers. It is therefore hypothesized that such systems may be able to evolve into autocatalytic sets of self-replicating, metabolically active entities that would predate the life forms known today. The experiment as performed, produced a relatively small yield of dipeptides (0.4–12.4%) and a smaller yield of tripeptides (0.003%) and the authors note that: "under these same conditions dipeptides hydrolysed rapidly." Another criticism of the result is that the experiment did not include any organomolecules that would most likely cross-react or chain-terminate. (Huber and Wächtershäuser, 1998) The latest modification of the iron-sulfur-hypothesis has been provided by William Martin and Michael Russell in 2002. According to their scenario, the first cellular life forms may have evolved inside so-called black smokers at seafloor spreading zones in the deep sea. These structures consist of microscale caverns that are coated by thin membraneous metal sulfide walls. Therefore, these structures would solve several critical points of the "pure" Wächtershäuser systems at once: # the micro-caverns provide a means of concentrating newly synthesised molecules, thereby increasing the chance of forming oligomers; # the steep temperature gradients inside a black smoker allow for establishing "optimum zones" of partial reactions in different regions of the black smoker (e.g. monomer synthesis in the hotter, oligomerisation in the colder parts); # the flow of hydrothermal water through the structure provides a constant source of building blocks and energy (freshly precipitated metal sulfides); # the model allows for a succession of different steps of cellular evolution (prebiotic chemistry, monomer and oligomer synthesis, peptide and protein synthesis, RNA world, ribonucleoprotein assembly and DNA world) in a single structure, facilitating exchange between all developmental stages; # synthesis of lipids as a means of "closing" the cells against the environment is not necessary, until basically all cellular functions are developed. This model locates the "last universal common ancestor" (LUCA) inside a black smoker, rather than assuming the existence of a free-living form of LUCA. The last evolutionary step would be the synthesis of a lipid membrane that finally allows the organisms to leave the microcavern system of the black smokers and start their independent lives. This postulated late acquisition of lipids is consistent with the presence of completely different types of membrane lipids in archaebacteria and eubacteria (plus eukaryotes) with highly similar cellular physiology of all life forms in most other aspects. Another unsolved issue in chemical evolution is the origin of homochirality, i.e. all monomers having the same "handedness" (amino acids being left handed, and nucleic acid sugars being right handed). Homochirality is essential for the formation of functional ribozymes (and probably proteins too). The origin of homochirality might simply be explained by an initial asymmetry by chance followed by common descent. Work performed in 2003 by scientists at Purdue identified the amino acid serine as being a probable root cause of organic molecules' homochirality. Serine forms particularly strong bonds with amino acids of the same chirality, resulting in a cluster of eight molecules that must be all right-handed or left-handed. This property stands in contrast with other amino acids which are able to form weak bonds with amino acids of opposite chirality. Although the mystery of why left-handed serine became dominant is still unsolved, this result suggests an answer to the question of chiral transmission: how organic molecules of one chirality maintain dominance once asymmetry is established.

From organic molecules to protocells

The question "How do simple organic molecules form a protocell?" is largely unanswered. However, there are many different hypotheses regarding the path that might have been taken. Some of these postulate the early appearance of nucleic acids ("genes-first") whereas others postulate the evolution of biochemical reactions and pathways first ("metabolism-first"). Recently, trends are emerging to create hybrid models that combine aspects of both.

"Genes first" models: the RNA world

Main article: RNA world hypothesis The RNA world hypothesis, for example, suggests that relatively short RNA molecules could have spontaneously formed that were capable of catalyzing 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. At this time however, these various hypotheses have incomplete evidence supporting them. 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 reality. At this time however, no prebiotically plausible experiment has confirmed this assumption. Further, recent experiments suggest that the original estimates of the size of an RNA molecule capable of self-replication were most probably vast underestimates. Worse, RNA itself does not appear to be a prebiotically plausible molecule; therefore, more-modern forms of the RNA World theory propose that a simpler molecule was capable of self-replication (that other "World" then evolved over time to produce the RNA World).

"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. However, the idea that a closed metabolic cycle, such as the reductive citric acid cycle proposed by Günter Wächtershäuser, could form spontaneously remains unsupported. Further, according to Leslie Orgel, a leader in origin-of-life studies for the past several decades, there is reason to believe the assertion will remain so. In an article entitled "Self-Organizing Biochemical Cycles" (PNAS, vol. 97, no. 23, November 7 2000, p12503-12507), Orgel summarizes his analysis of the proposal by stating, "There is at present no reason to expect that multistep cycles such as the reductive citric acid cycle will self-organize on the surface of FeS/FeS2 or some other mineral."

The Bubble Theory

Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation. While bubbles comprised of mostly water burst quickly, oily bubbles happen to be much more stable, lending more time to the particular bubble to perform these crucial experiments. The phospholipid is a good example of an oily compound believed to have been prevalent in the prebiotic seas. Because phospholipids contain a hydrophilic head on one end, and a hydrophobic tail on the other, they have the tendency to spontaneously form lipid membranes in water. A lipid monolayer bubble can only contain oil, and is therefore not conducive to harbouring water-soluble organic molecules. On the other hand, a lipid bilayer bubble can contain water, and was a likely precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first prokaryotes, eukaryotes, and multicellular organisms could be achieved. This theory is expanded upon in the book, "The Cell: Evolution of the First Organism" by Joseph Panno Ph.D.

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.

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". Cairns-Smith is a staunch critic of other models of chemical evolution (see Genetic Takeover: And the Mineral Origins of Life ISBN 0-52123-312-7). However, he admits, that like many models of the origin of life, his own also has its shortcomings (Horgan 1991).

"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. 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 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 after reactions initiated mostly by irradiation by ultraviolet light. It is supposed that a rain of cometary material on the early Earth could have brought significant quantities of complex organic molecules, and that it is possible that primitive life itself may have formed in space was 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, and neither of them actually answers the question of how life first originated, merely shifting it to another planet/comet. However, this hypothesis extends tremendously the array of conditions under which life may have have formed, from early Earth plausible conditions to literally any conditions possible in the universe.

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

References

- PMID 11882894
- (Cited on p. 108).
- (Cited on p. 108).
- Brooks, J. and Shaw, G., 1973. Origins and Development of Living Systems. Academic Press, London and New York, p. 359.
- .

External links

- [http://www.accessexcellence.org/bioforum/bf02/awramik/bf02a1.html Astrobiology and the origins of life]
- [http://genetics.mgh.harvard.edu/szostakweb/publications/Szostak_pdfs/Hanczyc_and_Szostak_2004_COChemBio.pdf Martin M Hanczyc and Jack W Szostak. Replicating vesicles as models of primitive cell growth and division. Current Opinion in Chemical Biology 2004, 8:660–664.]
- [http://www.santafe.edu/sfi/People/kauffman/sak-peptides.html "SELF-REPLICATION: Even peptides do it"] by Stuart A. Kauffman
- [http://originoflife.net/ Cairns Smith illustration of a possible solution using crystalline behaviors of clays]
- [http://www-news.uchicago.edu/releases/98/980331.origin.of.life.shtml Model of origin of life involving zeolite, press release for PNAS paper]
- [http://pokey.arc.nasa.gov/~astrochm/LifeImplications.html Possible Connections Between Interstellar Chemistry and the Origin of Life on the Earth]
- [http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=207 Scientists Find Clues That Life Began in Deep Space — NASA Astrobiology Institute]
- [http://people.cornell.edu/pages/tg21/DHB.html The Deep Hot Biosphere Theory (Thomas Gold)]
- [http://www.pnas.org/cgi/content/full/97/23/12503?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=biochemical+cycles&searchid=1119837712082_3423&stored_search=&FIRSTINDEX=0&journalcode=pnas Self-organizing biochemical cycles — by Leslie Orgel]
- [http://www.evowiki.org/index.php/Category:Creationist_claims Evolution wiki] Category:Origin of life Category:Evolution Category:Metabolism ja:生命の起源

Aristotle

by Lysippos. Louvre Museum.]] Aristotle (Greek: Αριστοτέλης Aristotelēs; 384 BC – March 7, 322 BC) was an ancient Greek philosopher, student of Plato and teacher of Alexander the Great. He wrote many books about physics, poetry, zoology, logic, rhetoric, government, and biology. Aristotle, along with Plato and Socrates, is generally considered one of the most influential ancient Greek philosophers in Western thought. Among them they transformed Presocratic Greek philosophy into the foundations of Western philosophy as we know it. The writings of Plato and Aristotle form the core of Ancient philosophy. Aristotle placed much more value on knowledge gained from the senses and would correspondingly be better classed among modern empiricists (see materialism and empiricism). He also achieved a "grounding" of dialectic in the Topics by allowing interlocutors to begin from commonly held beliefs (Endoxa); his goal being non-contradiction rather than Truth. He set the stage for what would eventually develop into the scientific method centuries later. Although he wrote dialogues early in his career, no more than fragments of these have survived. The works of Aristotle that still exist today are in treatise form and were, for the most part, unpublished texts. These were probably lecture notes or texts used by his students, and were almost certainly revised repeatedly over the course of years. As a result, these works tend to be eclectic, dense and difficult to read. Among the most important ones are Physics, Metaphysics, Nicomachean Ethics, Politics, De Anima (On the Soul) and Poetics. Their works, although connected in many fundamental ways, are very different in both style and substance. Aristotle is known for being one of the few figures in history who studied almost every subject possible at the time. In science, Aristotle studied anatomy, astronomy, embryology, geography, geology, meteorology, physics, and zoology. In philosophy, Aristotle wrote on aesthetics, economics, ethics, government, metaphysics, politics, psychology, rhetoric and theology. He also dealt with education, foreign customs, literature and poetry. His combined works practically comprise an encyclopedia of Greek knowledge.

Biography

Early life and studies at the Academy

encyclopedia.]] Aristotle was born at Stageira, a colony of Andros on the Macedonian peninsula of Chalcidice in 384 BC. His father, Nicomachus, was court physician to King Amyntas III of Macedon. It is believed that Aristotle's ancestors held this position under various kings of Macedonia. As such, Aristotle's early education would probably have consisted of instruction in medicine and biology from his father. About his mother, Phaestis, little is known. It is known that she died early in Aristotle's life. When Nicomachus also died, in Aristotle's tenth year, he was left an orphan and placed under the guardianship of his uncle, Proxenus of Atarneus. He taught Aristotle Greek, rhetoric, and poetry (O'Connor et al., 2004). Aristotle was probably influenced by his father's medical knowledge; when he went to Athens at the age of 18, he was likely already trained in the investigation of natural phenomena. From the age of 18 to 37 Aristotle remained in Athens as a pupil of Plato and distinguished himself at the Academy. The relations between Plato and Aristotle have formed the subject of various legends, many of which depict Aristotle unfavourably. No doubt there were divergences of opinion between Plato, who took his stand on sublime, idealistic principles, and Aristotle, who even at that time showed a preference for the investigation of the facts and laws of the physical world. It is also probable that Plato suggested that Aristotle needed restraining rather than encouragement, but not that there was an open breach of friendship. In fact, Aristotle's conduct after the death of Plato, his continued association with Xenocrates and other Platonists, and his allusions in his writings to Plato's doctrines prove that while there were conflicts of opinion between Plato and Aristotle, there was no lack of cordial appreciation or mutual forbearance. Besides this, the legends that reflect Aristotle unfavourably are traceable to the Epicureans, who were known as slanderers. If such legends were circulated widely by patristic writers such as Justin Martyr and Gregory Nazianzen, the reason lies in the exaggerated esteem Aristotle was held in by the early Christian heretics, not in any well-grounded historical tradition.

Aristotle as philosopher and tutor

After the death of Plato (347 BC), Aristotle was considered as the next head of the Academy, a post that was eventually awarded to Plato's nephew. Aristotle then went with Xenocrates to the court of Hermias, ruler of Atarneus in Asia Minor, and married his niece and adopted daughter, Pythia. In 344 BC, Hermias was murdered in a rebellion, and Aristotle went with his family to Mytilene. It is also reported that he stopped on Lesbos and briefly conducted biological research. Then, one or two years later, he was summoned to Pella, the Macedonian capital, by King Philip II of Macedon to become the tutor of Alexander the Great, who was then 13. Plutarch wrote that Aristotle not only imparted to Alexander a knowledge of ethics and politics, but also of the most profound secrets of philosophy. We have much proof that Alexander profited by contact with the philosopher, and that Aristotle made prudent and beneficial use of his influence over the young prince (although Bertrand Russell disputes this). Due to this influence, Alexander provided Aristotle with ample means for the acquisition of books and the pursuit of his scientific investigation. It is possible that Aristotle also participated in the education of Alexander's boyhood friends, which may have included for example Hephaestion and Harpalus. Aristotle maintained a long correspondence with Hephaestion, eventually collected into a book, unfortunately now lost. According to sources such as Plutarch and Diogenes, Philip had Aristotle's hometown of Stageira burned during the 340s BC, and Aristotle successfully requested that Alexander rebuild it. During his tutorship of Alexander, Aristotle was reportedly considered a second time for leadership of the Academy; his companion Xenocrates was selected instead.

Founder and master of the Lyceum

In about 335 BC, Alexander departed for his Asiatic campaign, and Aristotle, who had served as an informal adviser (more or less) since Alexander ascended the Macedonian throne, returned to Athens and opened his own school of philosophy. He may, as Aulus Gellius says, have conducted a school of rhetoric during his former residence in Athens; but now, following Plato's example, he gave regular instruction in philosophy in a gymnasium dedicated to Apollo Lyceios, from which his school has come to be known as the Lyceum. (It was also called the Peripatetic School because Aristotle preferred to discuss problems of philosophy with his pupils while walking up and down -- peripateo -- the shaded walks -- peripatoi -- around the gymnasium). During the thirteen years (335 BC–322 BC) which he spent as teacher of the Lyceum, Aristotle composed most of his writings. Imitating Plato, he wrote Dialogues in which his doctrines were expounded in somewhat popular language. He also composed the several treatises (which will be mentioned below) on physics, metaphysics, and so forth, in which the exposition is more didactic and the language more technical than in the Dialogues. These writings show to what good use he put the resources Alexander had provided for him. They show particularly how he succeeded in bringing together the works of his predecessors in Greek philosophy, and how he pursued, either personally or through others, his investigations in the realm of natural phenomena. Pliny claimed that Alexander placed under Aristotle's orders all the hunters, fishermen, and fowlers of the royal kingdom and all the overseers of the royal forests, lakes, ponds and cattle-ranges, and Aristotle's works on zoology make this statement more believable. Aristotle was fully informed about the doctrines of his predecessors, and Strabo asserted that he was the first to accumulate a great library. During the last years of Aristotle's life the relations between him and Alexander became very strained, owing to the disgrace and punishment of Callisthenes, whom Aristotle had recommended to Alexander. Nevertheless, Aristotle continued to be regarded at Athens as a friend of Alexander and a representative of Macedonia. Consequently, when Alexander's death became known in Athens, and the outbreak occurred which led to the Lamian war, Aristotle shared in the general unpopularity of the Macedonians. The charge of impiety, which had been brought against Anaxagoras and Socrates, was now, with even less reason, brought against Aristotle. He left the city, saying (according to many ancient authorities) that he would not give the Athenians a chance to sin a third time against philosophy. He took up residence at his country house at Chalcis, in Euboea, and there he died the following year, 322 BC. His death was due to a disease, reportedly 'of the stomach', from which he had long suffered. The story that his death was due to hemlock poisoning, as well as the legend that he threw himself into the sea "because he could not explain the tides," is without historical foundation. Very little is known about Aristotle's personal appearance except from hostile sources. The statues and busts of Aristotle, possibly from the first years of the Peripatetic School, represent him as sharp and keen of countenance, and somewhat below the average height. His character—as revealed by his writings, his will (which is undoubtedly genuine), fragments of his letters and the allusions of his unprejudiced contemporaries—was that of a high-minded, kind-hearted man, devoted to his family and his friends, kind to his slaves, fair to his enemies and rivals, grateful towards his benefactors. When Platonism ceased to dominate the world of Christian speculation, and the works of Aristotle began to be studied without fear and prejudice, the personality of Aristotle appeared to the Christian writers of the 13th century, as it had to the unprejudiced pagan writers of his own day, as calm, majestic, untroubled by passion, and undimmed by any great moral defects, "the master of those who know". Aristotle's legacy also had a profound influence on Islamic thought and philosophy during the middle ages. The likes of Avicenna, Farabi, and Yaqub ibn Ishaq al-Kindi[http://www.ummah.net/history/scholars/KINDI.html 1] were a few of the major proponents of the Aristotelian school of thought during the Golden Age of Islam.

Methodology

Aristotle defines philosophy in terms of essence, saying that philosophy is "the science of the universal essence of that which is actual". Plato had defined it as the "science of the idea", meaning by idea what we should call the unconditional basis of phenomena. Both pupil and master regard philosophy as concerned with the universal; Aristotle, however, finds the universal in particular things, and called it the essence of things, while Plato finds that the universal exists apart from particular things, and is related to them as their prototype or exemplar. For Aristotle, therefore, philosophic method implies the ascent from the study of particular phenomena to the knowledge of essences, while for Plato philosophic method means the descent from a knowledge of universal ideas to a contemplation of particular imitations of those ideas. In a certain sense, Aristotle's method is both inductive and deductive, while Plato's is essentially deductive. In Aristotle's terminology, the term natural philosophy corresponds to the phenomena of the natural world, which include: motion, light, and the laws of physics. Many centuries later these subjects would later become the basis of modern science, as studied through the scientific method. The term philosophy is distinct from metaphysics, which is what moderns term philosophy. In the larger sense of the word, he makes philosophy coextensive with reasoning, which he also called "science". Note, however, that his use of the term science carries a different meaning than that which is covered by the scientific method. "All science (dianoia) is either practical, poetical or theoretical." By practical science he understands ethics and politics; by poetical, he means the study of poetry and the other fine arts; while by theoretical philosophy he means physics, mathematics, and metaphysics. The last, philosophy in the stricter sense, he defines as "the knowledge of immaterial being," and calls it "first philosophy", "the theologic science" or of "being in the highest degree of abstraction." If logic, or, as Aristotle calls it, Analytic, be regarded as a study preliminary to philosophy, we have as divisions of Aristotelian philosophy (1) Logic; (2) Theoretical Philosophy, including Metaphysics, Physics, Mathematics, (3) Practical Philosophy; and (4) Poetical Philosophy.

Aristotle's epistemology

Logic

History

Aristotle "says that 'on the subject of reasoning' he 'had nothing else on an earlier date to speak about'" (Bocheński, 1951). However, Plato reports that syntax was thought of before him, by Prodikos of Keos, who was concerned by the right use of words. Logic seems to have emerged from dialectics, the earlier philosophers used concepts like reductio ad absurdum as a rule when discussing, but never understood its logical implications. Even Plato had difficulties with logic. Although he had the idea of constructing a system for deduction, he was never able to construct one. Instead, he relied on his dialectic, which was a confusion between different sciences and methods (Bocheński, 1951). Plato thought that deduction would simply follow from premises, so he focused on having good premises so that the conclusion would follow. Later on, Plato realised that a method for obtaining the conclusion would be beneficial. Plato never obtained such a method, but his best attempt was published in his book Sophist, where he introduced his division method (Rose, 1968).

Analytics and the Organon

What we call today Aristotelian logic, Aristotle himself would have labelled analytics. The term logic he reserved to mean dialectics. Most of Aristotle's work is probably not authentic, since it was most likely edited by students and later lecturers. The logical works of Aristotle were compiled into six books at about the time of Christ: #Categories #On Interpretation #Prior Analytics #Posterior Analytics #Topics #On Sophistical Refutations The order of the books (or the teachings from which they are composed) is not certain, but this list was derived from analysis of Aristotle's writings. There is one volume of Aristotle's concerning logic not found in the Organon, namely the fourth book of Metaphysics. (Bocheński, 1951).

Modal logic

Aristotle is also the creator of syllogisms with modalities (modal logic). The word modal refers to the word 'modes', explaining the fact that modal logic deals with the modes of truth. Aristotle introduced the qualification of 'necessary' and 'possible' premises. He constructed a logic which helped in the evaluation of truth but which was very difficult to interpret. (Rose, 1968).

Science

Aristotelian discussions about science had only been qualitative, not quantitative. By the modern definition of the term, Aristotelian philosophy was not science, as this worldview did not attempt to probe how the world actually worked through experiment. For example, in his book The history of animals he claimed that human males have more teeth than females. Had he only made some observations, he would have discovered that this claim is false. Rather, based on what one's senses told one, Aristotelian philosophy then depended upon the assumption that man's mind could elucidate all the laws of the universe, based on simple observation (without experimentation) through reason alone. One of the reasons for this was that Aristotle held that physics was about changing objects with a reality of their own, whereas mathematics was about unchanging objects without a reality of their own. In this philosophy, he could not imagine that there was a relationship between them. In contrast, today's "science" assumes that thinking alone often leads people astray, and therefore one must compare one's ideas to the actual world through experimentation; only then can one see if one's ideas are based in reality. This position is known as empiricism or the scientific method.

Aristotle's metaphysics

Aristotle's four causes

Aristotle names four "causes" of things, but the word cause (Greek: , aitia) is not used in the modern sense of "cause and effect", under which causes are events or states of affairs. Rather, the four causes are like different ways of explaining something: ; The Material Cause (That from which it comes): This is the material that makes up an object, for example, "the bronze and silver ... are causes of the statue and the bowl." ; The Formal Cause (That which it is): This is the blueprint or the idea commonly held of what an object should be. Aristotle says, "The form is the account (and the genera of the account) of the essence (for instance, the cause of an octave is the ratio two to one, and in general number), and the parts that are in the account." ; The Efficient Cause (That which moves it): This is the person who makes an object, or "unmoved movers" (gods) who move nature. For example, "a father is a cause of his child; and in general the producer is a cause of the product and the initiator of the change is a cause." This is closest to the modern definition of "cause". ; The Final Cause (That of which its purpose is): The final cause or telos is the purpose or end that something is supposed to serve. This includes "all the intermediate steps that are for the end ... for example, slimming, purging, drugs, or instruments are for health; all of these are for the end, though they differ in that some are activities while others are instruments." An example of an artifact that has all four causes would be a table, which has material causes (wood and nails), a formal cause (the blueprint, or a generally agreed idea of what tables are), an efficient cause (the carpenter), and a final cause (using it to dine on). Aristotle argues that natural objects such as an "individual man" have all four causes. The material cause of an individual man would be the flesh and bone that make up an individual man. The formal cause would be the blueprint of man, that which is used as a guide to create an individual man and to keep him in a certain state called man. The efficient cause of an individual man would be the father of that man, or in the case of all men an �unmoved mover� who breathed (anima: breath) into the soul (anima: soul) of man. The final cause of man would be as Aristotle stated, �Now we take the human�s function to be a certain kind of life, and take this life to be the soul�s activity and actions that express reason. Hence the excellent man�s function is to do this finely and well. Each function is completed well when its completion expresses the proper virtue. Therefore the human good turns out to be the souls� activity that expresses virtue.�

The difference between natural objects and artifacts

The difference between natural objects and an artifact is that natural objects have self movement. Aristotle defined the difference between a natural object and an artifact when he stated, �In contrast to these, a bed, a cloak, or any other artifact-insofar as it is described as such i.e., as a bed, a cloak, or whatever, and to the extent that it is a product of a craft-has no innate impulse to change; but insofar as it is coincidentally made of stone or earth or a mixture of these, it has an innate impulse to change and just to that extent. This is because a nature is a type of principle and cause of motion and stability within those things to which it primarily belongs in their own right and not coincidentally.� The natural objects are changed to artifacts through crafts but they have an innate impulse of self movement to convert through time to their natural state, and they will all turn into that state when all animals with reason are extinct from the earth.

Modes of causation

Aristotle states two modes of causation:
- Proper Causation: Things take place for the sake of something, and the result is that which is intended.
- Accidental Causation: Things that take place not out of necessity. E.g. things that take place by chance/coincidence. This cause is indeterminable.

Chance

Chance lies in the realm of accidental causes. It is "from what is spontaneous" (but note that what is spontaneous does not come from chance). For a better understanding of Aristotle's conception of "chance" it might be better to think of "coincidence": Something takes place by chance if a person sets out with the intent of having one thing take place, but with the result of another thing (not intended) taking place. For example: A person seeks donations. That person may find another person willing to donate a substantial sum. However, if the person seeking the donations met the person donating, not for the purpose of collecting donations, but for some other purpose, Aristotle would call the collecting of the donation by that particular donator a result of chance. It must be unusual that something happens by chance. In other words, if something happens all or most of the time, we cannot say that it is by chance. However, chance can only apply to human beings. According to Aristotle, chance must involve choice (and thus deliberation), and only humans are capable of deliberation and choice. "What is not capable of action cannot do anything by chance" (Physics, 2.6).

The Five Elements

- Fire which is hot and dry.
- Earth which is cold and dry.
- Air which is hot and wet.
- Water which is cold and wet.
- Aether which is the divine substance that makes up the heavens These four elements interchange (i.e. Fire ↔ Air ↔ Water ↔ Earth etc.), while aether is on its own. The Sun keeps this cycle going. God keeps the Sun going (and thus the Sun is eternal).

Aristotle's ethics

Although Aristotle wrote several works on ethics, the major one was the Nicomachean Ethics, which is considered one of Aristotle's greatest works; it discusses virtues. The ten books which comprise it are based on notes from his lectures at the Lyceum and were either edited by or dedicated to Aristotle's son, Nicomachus. Aristotle believed that ethical knowledge is not certain knowledge (like metaphysics and epistemology) but is general knowledge. Also, as it is not a theoretical discipline, he thought a person had to study in order to become "good." Thus, if a person was to become virtuous, they could not simply study what virtue is, they had to actually do virtuous activity. In order to do this, Aristotle had to first establish what was virtuous. He began by determining that everything was done with some goal in mind and that goal is 'good.' The ultimate goal he called the Highest Good. Aristotle contested that happiness could not be found only in pleasure or only in fame and honor. He finally finds happiness "by ascertaining the specific function of man. But what is this function that will bring happiness? To determine this, Aristotle analyzed the soul and found it to have three parts: the Nutritive Soul (plants, animals and humans), the Perceptive Soul (animals and humans) and the Rational Soul (humans only). Thus, a human's function is to do what makes it human, to be good at what sets it apart from everything else: the ability to reason or Nous. A person that does this is the happiest because they are fulfulling their purpose or nature as found in the rational soul. Depending on how well they did this, Aristotle said people belonged to one of four categories: the Virtuous, the Continent, the Incontinent and the Vicious. Aristotle believes that every ethical virtue is an intermediate condition between excess and deficiency. This does not mean Aristotle believed in moral relativism, however. He set certain emotions (e.g., hate, envy, jealousy, spite, etc.) and certain actions (e.g., adultery, theft, murder, etc.) as being always wrong, regardless of the situation or the circumstances.

Nicomachean ethics

In Nicomachean Ethics, Aristotle focuses on the importance of continually behaving virtuously and developing virtue rather than committing specific good actions. This can be opposed to Kantian ethics, in which the primary focus is on individual action. Nicomachean Ethics emphasizes the importance of context to ethical behaviour — what might be right in one situation might be wrong in another. Aristotle believed that happiness is the end of life and that as long as a person is striving for goodness, good deeds will result from that struggle, making the person virtuous and therefore happy.

Aristotle's critics

goodness (right), a detail of The School of Athens, a fresco by Raphael.]] Aristotle has been criticised on several grounds.
- His analysis of procreation is frequently criticised on the grounds that it presupposes an active, ensouling masculine element bringing life to an inert, passive, lumpen female element; it is on these grounds that some feminist critics refer to Aristotle as a misogynist.
- At times, the objections that Aristotle raises against the arguments of his own teacher, Plato, appear to rely on faulty interpretations of those arguments.
- Although Aristotle advised, against Plato, that knowledge of the world could only be obtained through experience, he frequently failed to take his own advice. Aristotle conducted projects of careful empirical investigation, but often drifted into abstract logical reasoning, with the result that his work was littered with conclusions that were not supported by empirical evidence; for example, his assertion that objects of different mass fall at different speeds under gravity, which was later refuted by John Philoponus. Credit is often given to Galileo, even though Philopinus lived centuries before him.
- In the Middle Ages, roughly from the 12th century to the 15th century, the philosophy of Aristotle became firmly established dogma. Although Aristotle himself was far from dogmatic in his approach to philosophical inquiry, two aspects of his philosophy might have assisted its transformation into dogma. His works were wide-ranging and systematic so that they could give the impression that no significant matter had been left unsettled. He was also much less inclined to employ the sceptical methods of his predecessors, Socrates and Plato.
- Some academics have suggested that Aristotle was unaware of much of the current science of his own time, and that he was a far lesser mathematician than many of his learned contemporaries. Aristotle was called not a great philosopher, but "The Philosopher" by Scholastic thinkers. These thinkers blended Aristotelian philosophy with Christianity, bringing the thought of Ancient Greece into the Middle Ages. It required a repudiation of some Aristotelian principles for the sciences and the arts to free themselves for the discovery of modern scientific laws and empirical methods. The Western mind is "Aristotelian". By this we mean that it formats the external world into factual and "scien"-tific categories. (By "Scien"-tific we mean that something is knowable or known. Latin scientia = knowledge). Under the premise of external categorization, the Aristotelian mind has come to equate "experience" with the unified chronical and spatial ontological structure that is the "external" universe -- visible, audible and sensible by the handful of our common, well-identified senses. By so equating the two, the Aristotelian mind is fully confident, or fully "positive" of the meanings of its utterances and the purposes of all actions. That is to say, it dismisses the possibility of dubious meanings as interpreted by subjects that are at variance in perspectives or phenomenology, and it dismisses the importance of anything other than an objectively defined "purpose" to an action. Therefore, the Aristotelian mind assumes that when subject A utters "I am X," he or she is referring to the same experience and is expressing the same purpose as subject B who also utters "I am X."

Bibliography

Note: Bekker numbers are often used to uniquely identify passages of Aristotle. They are identified below where available.

Major works

The extant works of Aristotle are broken down according to the five categories in the Corpus Aristotelicum. Not all of these works are considered genuine, but differ with respect to their connection to Aristotle, his associates and his views. Some, such as the Athenaion Politeia or the fragments of other politeia are regarded by most scholars as products of Aristotle's "school" and compiled under his direction or supervision. Other works, such On Colours may have been products of Aristotle's successors at the Lyceum, e.g., Theophrastus and Straton. Still others acquired Aristotle's name through similarities in doctrine or content, such as the De Plantis, possibly by Nicolaus of Damascus. A final category, omitted here, includes medieval palmistries, astrological and magical texts whose connection to Aristotle is purely fanciful and self-promotional. Those that are seriously disputed are marked with an asterisk.

Logical writings

- Organon (collected works on logic):
- (1a) Categories (or Categoriae)
- (16a) On Interpretation (or De Interpretatione)
- (24a) Prior Analytics (or Analytica Priora)
- (71a) Posterior Analytics (or Analytica Posteriora)
- (100b) Topics (or Topica)
- (164a) On Sophistical Refutations (or De Sophisticis Elenchis)

Physical and scientific writings

- (184a) Physics (or Physica)
- (268a) On the Heavens (or De Caelo)
- (314a) On Generation and Corruption (or De Generatione et Corruptione)
- (338a) Meteorology (or Meteorologica)
- (391a) On the Cosmos (or De Mundo, or On the Universe)
-
- (402a) On the Soul (or De Anima)
- (436a) Little Physical Treatises (or Parva Naturalia):
- On Sense and the Sensible (or De Sensu et Sensibilibus)
- On Memory and Reminiscence (or De Memoria et Reminiscentia)
- On Sleep and Sleeplessness (or De Somno et Vigilia)
- On Dreams (or De Insomniis)
-
- On Prophesying by Dreams (or De Divinatione per Somnum)
- On Longevity and Shortness of Life (or De Longitudine et Brevitate Vitae)
- On Youth and Old Age (On Life and Death) (or De Juventute et Senectute, De Vita et Morte)
- On Breathing (or De Respiratione)
- (481a) On Breath (or De Spiritu)
-
- (486a) History of Animals (or Historia Animalium, or On the History of Animals, or Description of Animals)
- (639a) On the Parts of Animals (or De Partibus Animalium)
- (698a) On the Gait of Animals (or De Motu Animalium, or On the Movement of Animals)
- (704a) On the Progression of Animals (or De Incessu Animalium)
- (715a) On the Generation of Animals (or De Generatione Animalium)
- (791a) On Colours (or De Coloribus)
-
- (800a) De audibilibus
- (805a) Physiognomics (or Physiognomonica)
-
- On Plants (or De Plantis)
-
- (830a) On Marvellous Things Heard (or Mirabilibus Auscultationibus, or On Things Heard)
-
- (847a) Mechanical Problems (or Mechanica)
-
- (859a) Problems (or Problemata)
-
- (968a) On Indivisible Lines (or De Lineis Insecabilibus)
-
- (973a) Situations and Names of Winds (or Ventorum Situs)
-

Metaphysical writings

- (980a) Metaphysics (or Metaphysica)

Ethical writings

- (1094a) Nicomachean Ethics (or Ethica Nicomachea, or The Ethics)
- (1181a) Great Ethics (or Magna Moralia)
-
- (1214a) Eudemian Ethics (or Ethica Eudemia)
- (1249a) Virtues and Vices (or De Virtutibus et Vitiis Libellus, Libellus de virtutibus)
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- (1252a) Politics (or Politica)
- (1343a) Economics (or Oeconomica)

Aesthetic writings

- (1354a) Rhetoric (or Ars Rhetorica, or The Art of Rhetoric or Treatise on Rhetoric)
- Rhetoric to Alexander (or Rhetorica ad Alexandrum)
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- (1447a) Poetics (or Ars Poetica)

Writings absent from Corpus Aristotelicum

- The Constitution of the Athenians (or Athenaion Politeia, or The Athenian Consitution)
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- On Melissus, On Xenophanes, and On Gorgias. These are sometimes grouped together and called the "MXG" writings. They clearly are not written by Aristotle, and are believed to date from the fifth century AD. However, because they have frequently been attributed to him in the past, they are often included in compilations of his writings (for example, in the Loeb Classical Library).

Specific editions

- Princeton University Press: The Complete Works of Aristotle: The Revised Oxford Translation (2 Volume Set; Bollingen Series, Vol. LXXI, No. 2), edited by Jonathan Barnes ISBN 0-691-09950-2 (The most complete recent translation of Aristotle's extant works)
- Oxford University Press: Clarendon Aristotle Series. [http://www.oup.com/us/catalog/general/series/ClarendonAristotleSeries/?view=usa Scholarly edition]
- Harvard University Press: Loeb Classical Library (hardbound; publishes in Greek, with English translations on facing pages)

Named after Aristotle

- Aristoteles crater on the Moon.
- The Aristotle University of Thessaloniki
- Aristotle's Cockney legacy - The name of Aristotle, like that of J. Arthur Rank, became a common expression in Cockney rhyming slang.

References

Needless to say, the secondary literature on Aristotle is vast. The following references are only a small selection.
- A popular exposition for the general reader.
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- A detailed and scholarly work, but very readable.
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- For the general reader.

External links

Aristotle Aristotle
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- [http://Aristotle.thefreelibrary.com/ A brief biography and e-texts presented one chapter at a time]
- [http://www.utm.edu/research/iep/a/aristotl.htm The Internet Encyclopedia of Philosophy: Aristotle.], 2004.
- [http://www.non-contradiction.com/ An extensive collection of Aristotle's philosophy and works, including lesser known texts]
- [http://www.virtuescience.com/nicomachean-ethics.html Nicomachean Ethics by Aristotle.]
- [http://uk.arxiv.org/abs/physics/0505172 Aristotle and Indian logic]
- O'Connor, J. John & Robertson, Edmund F., [http://www-history.mcs.st-andrews.ac.uk/Mathematicians/Aristotle.html Aristotle], 2004.
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- [http://www.greektexts.com/library/Aristotle/index.html Large collection of Aristotle's texts, presented page by page]
- [http://www.greek-literature-online.com/aristotle/ Read Aristotle's works online]
- [http://www.newadvent.org/cathen/01713a.htm Source of most of the Biography and Methodology sections, as well as more overview]
- Category:Aristotle Category:Ancient Greek philosophers Category:Aristotelian philosophers Category:Ancient Greek mathematicians Category:Empiricists Category:Rhetoric Category:Greek logicians Category:Meteorologists ko:아리스토텔레스 ms:Aristotle ja:アリストテレス simple:Aristotle th:อริสโตเติล

Flea

:For other meanings of flea see flea (disambiguation) Tungidae - Sticktight and Chigoe fleas (Chiggers)
Pulicidae - Common fleas
Coptopsyllidae
Vermipsyllidae - Carnivore fleas
Rhopalopsyllidae - Marsupial fleas
Hypsophthalmidae
Stephanocircidae
Pygiopsyllidae
Hystrichopsyllidae - Rat and mouse fleas
Leptopsyllidae - Bird and rabbit fleas
Ischnopsyllidae - Bat fleas
Ceratophyllidae
Amphipsyllidae
Malacopsyllidae
Dolichopsyllidae - Rodent fleas
Ctenopsyllidae Flea is the common name for any of the small wingless insects of the order Siphonaptera. Fleas are external parasites, living by hematophagy off the blood of mammals and birds. Note: There is also a genus of Protozoa named Siphonaptera Some well known flea species include:
- Cat Flea (Ctenocephalides felis),
- Dog Flea (Ctenocephalides canis),
- Northern Rat Flea (Nosopsyllus fasciatus),
- Oriental Rat Flea (Xenopsylla cheopis). Protozoa In most cases fleas are just a nuisance to their hosts, but some people and some animals suffer allergic reactions to flea saliva resulting in rashes. Flea bites generally result in the formation of a slightly-raised swollen itching spot with a single puncture point at the center. However, fleas can transmit disease. One devastating example of this was the bubonic plague, transmitted between rodents and humans. Murine typhus (endemic typhus) fever, and in some cases tapeworms can also be transmitted by fleas.

Life Cycle

tapeworm]] Fleas pass through a complete life cycle consisting of egg, larva, pupa and adult. Completion of the life cycle from egg to adult varies from two weeks to eight months depending on the temperature, humidity, food, and species. Normally after a blood meal, the female flea lays about 15 to eggs per day – up to 600 in its lifetime – usually on the host (dogs, cats, rats, rabbits, mice, squirrels, chipmunks, raccoons, opossums, foxes, chickens, humans, etc.). Eggs loosely laid in the hair coat drop out almost anywhere, especially where the host rests, sleeps or nests (rugs, carpets, upholstered furniture, cat or dog boxes, kennels, sand boxes, etc.). Eggs hatch between two days to two weeks into larvae found indoors in and along floor cracks, crevices, along baseboards, under rug edges and in furniture or beds. Outdoor development occurs in sandy gravel soils (moist sand boxes, dirt crawlspace under the house, under shrubs, etc.) where the host may rest or sleep. Sand and gravel are very suitable for larval development which is the reason fleas are erroneously called "sand fleas." Larvae are blind, avoid light, pass through three larval instars and take a week to several months to develop. Their food consists of digested blood from adult flea feces, dead skin, hair, feathers, and other organic debris; larvae do not suck blood. Pupae mature to adulthood within a silken cocoon woven by the larva to which pet hair, carpet fiber, dust, grass cuttings, and other debris adheres. In about five to fourteen days, adult fleas emerge or may remain resting in the cocoon until the detection of vibration (pet and people movement), pressure (host animal lying down on them), heat, noise, or carbon dioxide (meaning a potential blood source is near). Most fleas overwinter in the larval or pupal stage with survival and growth best during warm, moist winters and spring. [http://www.flea-i.com Flea Bites] can be treated with Calamine Lotion or 0.5-1% conc. hydrocortisone cream. Lufenuron is a veterinary medicine that attacks the larval flea's ability to produce chitin.

External links

- [http://parents.berkeley.edu/advice/health/fleas.html Advice about Fleas] from the Berkeley Parents Network
- [http://www.thepetcenter.com/gen/fleB.html Fleas on dogs and cats] Category:insects
- Category:Dog health Category:Cat health ja:ノミ simple:Flea

Egypt

The Arab Republic of Egypt, commonly known as Egypt, (in Arabic: مصر, romanized Misr), is a republic in North Africa. While it is geographically located in Africa, it is sometimes associated with the Middle East for political reasons. Covering an area of about 1,020,000 km², Egypt shares land borders with Libya to the west, Sudan to the south, and Israel and the Gaza Strip to the northeast and has coasts on the north and east by the Mediterranean Sea and the Red Sea, respectively. Egypt is the second most populous country in Africa, second only to Nigeria, and the vast majority of its 77 million population (2005) live near the banks of the Nile River (about 40,000 km²), where the only arable agricultural land is found. Large areas of land are part of the Sahara Desert and are sparsely inhabited. The majority of Egyptians today are urban, living in the great Arab population centers of greater Cairo, the largest city in Africa, and Alexandria. Egypt is famous for its ancient civilization and some of the world's most stunning ancient monuments, including the Giza Pyramids, the Karnak Temple and the Valley of the Kings; the southern city of Luxor contains a particularly large number of ancient artifacts. Today, Egypt is widely regarded as the main political and cultural centre of the Arab and Middle Eastern regions.

Origin and history of the name

Misr, the Arabic and official name for modern Egypt, is of Semitic origin directly cognate with the Hebrew מִצְרַיִם Misráyim meaning "the two straits", and possibly means "a country" or "a state." The ancient name for the country, kemet, or "black land," is derived from the fertile black soils deposited by the Nile floods, distinct from the 'red land' (deshret) of the desert. This name became keme in a later stage of Coptic. The English name "Egypt" came via the Latin word Aegyptus derived from the ancient Greek word Αίγυπτος Aiguptos (see also List of traditional Greek place names), which in turn is derived from the ancient Egyptian phrase ḥwt-k3-ptḥ ("Hwt ka Ptah") meaning "home of the Ka (part of the soul) of Ptah," the name of a temple of the god Ptah at Memphis. For details see the article Copt.

History

Main article: History of Egypt The regularity and richness of the annual Nile River flood, coupled with semi-isolation provided by deserts to the east and west, allowed for the development of one of the world's great civilizations. A unified kingdom was founded circa 3200 BC by King Menes, and a series of dynasties ruled in Egypt for the next three millennia. The last native dynasty, known as the Thirtieth Dynasty, fell to the Persians in 341 BC who dug the predecessor of the Suez canal and connected the Red Sea to the Mediterranean. Later, Egypt fell to the Greeks, Romans, Byzantines and Persians again. It was the Muslim Arabs who introduced Islam and the Arabic language in the seventh cen

How Life Began

History of abiogenesis hypotheses

Modern concept of abiogenesis

Critics

Schrödinger

Yockey

Panspermia advocates

Crick

Hoyle

References

External links

Greek language

History

Classification

Geographic distribution

Official status

Phonology

Vowel sounds

Origin of life

History of the concept: abiogenesis

Current models of the origin of life

Origin of organic molecules: Miller, Eigen and Wächtershäuser's theories

From organic molecules to protocells

"Genes first" models: the RNA world

"Metabolism first" models: iron-sulfur world and others

The Bubble Theory

Hybrid models

Other models

Clay theory of the origin of life

"Deep-hot biosphere" model of Gold

"Primitive" extraterrestrial life

Relevant fields

See also

References

External links

Aristotle

Biography

Early life and studies at the Academy

Aristotle as philosopher and tutor

Founder and master of the Lyceum

Methodology

Aristotle's epistemology

Logic

History

Analytics and the Organon

Modal logic

Science

Aristotle's metaphysics

Aristotle's four causes

The difference between natural objects and artifacts

Modes of causation

Chance

The Five Elements

Aristotle's ethics

Nicomachean ethics

Aristotle's critics

Bibliography

Major works

Logical writings

Physical and scientific writings

Metaphysical writings

Ethical writings

Aesthetic writings

Writings absent from Corpus Aristotelicum

Specific editions

Named after Aristotle

See also

References

External links

Flea

Life Cycle

External links

Egypt

Origin and history of the name

History