LIFE : Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased (death), or else because they lack such functions and are classified as inanimate. Biology is the science concerned with the study of life. Any contiguous living system is called an organism. These animate entities undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. More complex living organisms can communicate through various means. A diverse array of living organisms can be found in the biosphere of Earth, and the properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information. Scientific evidence suggests that life began on Earth some 3.7 billion years ago. The mechanism by which life emerged is still being investigated. Since then, life has evolved into a wide variety of forms, which biologists have classified into a hierarchy of Taxa. Life can survive and thrive in a wide range of conditions. The meaning of life—its significance, purpose, and ultimate fate—is a central concept and question in philosophy and religion. Both philosophy and religion have offered interpretations as to how life relates to existence and consciousness, and both touch on many related issues, including life stance, purpose, conception of a god or gods, a soul or an afterlife. Different cultures throughout history have had widely varying approaches to these issues. Though the existence of life is only confirmed on Earth, many scientists believe extraterrestrial life is not only plausible but probable. Other planets and moons in the Solar System have been examined for evidence of having once supported simple life, and projects such as SETI have attempted to detect transmissions from possible alien civilizations. According to the panspermia hypothesis, life on Earth may have originated from meteorites that spread organic molecules or simple life that first evolved elsewhere.Origin:Evidence suggests that life on Earth has existed for about 3.7 billion years,  with the oldest traces of life found in fossils dating back 3.4 billion years.  All known life forms share fundamental molecular mechanisms, and based on these observations, theories on the origin of life attempt to find a mechanism explaining the formation of a primordial single cell organism from which all life originates. There are many different hypotheses regarding the path that might have been taken from simple organic molecules via pre-cellular life 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 combine both categories.  There is no scientific consensus as to how life originated and all proposed theories are highly speculative. However, most accepted scientific models build in one way or another on the following hypotheses:The Miller-Urey experiment, and the work of Sidney Fox, suggest that conditions on the primitive Earth may have favored chemical reactions that synthesized amino acids and other organic compounds from inorganic precursors.Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.Life synthesizes proteins, which are polymers of amino acids using instructions encoded by cellular genes; the polymers of deoxyribonucleic acid (DNA). Protein synthesis entails intermediary ribonucleic acid (RNA) polymers. One possibility for how life began is that genes originated first, followed by proteins; the alternative being that proteins came first and then genes. However, because genes are required to make proteins, and proteins are needed to make genes, the problem of considering which came first is like that of the chicken or the egg. Most scientists have adopted the hypothesis that because DNA and proteins function together so intimately, it's unlikely that they arose independently. Therefore, a possibility, apparently first suggested by Francis Crick,  is that the first life was based on the DNA-protein intermediary: RNA. RNA has the DNA-like properties of information storage, replication and the catalytic properties of some proteins. Crick and others actually favored the RNA-first hypothesis even before the catalytic properties of RNA had been demonstrated by Thomas Cech.  A significant issue with the RNA-first hypothesis is that experiments designed to synthesize RNA from simple precursors have not been nearly as successful as the Miller-Urey experiments that synthesized other organic molecules from inorganic precursors. One reason for the failure to create RNA in the laboratory is that RNA precursors are very stable and do not react with each other under ambient conditions. However, the successful synthesis of certain RNA molecules under conditions hypothesized to exist prior to life on Earth has been achieved by adding alternative precursors in a specified order with the precursor phosphate present throughout the reaction.[60] This study makes the RNA-first hypothesis more plausible.  Recent experiments have demonstrated true Darwinian evolution of unique RNA enzymes (ribozymes) made up of two separate catalytic components that replicate each other in vitro.[62] In describing this work from his laboratory, Gerald Joyce stated: "This is the first example, outside of biology, of evolutionary adaptation in a molecular genetic system."[63] Such experiments make the possibility of a primordial RNA world even more attractive to many scientists.Miller/Urey Experiment By the 1950s, scientists were in hot pursuit of the origin of life. Around the world, the scientific community was examining what kind of environment would be needed to allow life to begin. In 1953, Stanley L. Miller and Harold C. Urey, working at the University of Chicago, conducted an experiment which would change the approach of scientific investigation into the origin of life. Miller took molecules which were believed to represent the major components of the early Earth's atmosphere and put them into a closed system The gases they used were methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O). Next, he ran a continuous electric current through the system, to simulate lightning storms believed to be common on the early earth. Analysis of the experiment was done by chromotography. At the end of one week, Miller observed that as much as 10-15% of the carbon was now in the form of organic compounds. Two percent of the carbon had formed some of the amino acids which are used to make proteins. Perhaps most importantly, Miller's experiment showed that organic compounds such as amino acids, which are essential to cellular life, could be made easily under the conditions that scientists believed to be present on the early earth. This enormous finding inspired a multitude of further experiments. In 1961, Juan Oro found that amino acids could be made from hydrogen cyanide (HCN) and ammonia in an aqueous solution. He also found that his experiment produced an amazing amount of the nucleotide base, adenine. Adenine is of tremendous biological significance as an organic compound because it is one of the four bases in RNA and DNA. It is also a component of adenosine triphosphate, or ATP, which is a major energy releasing molecule in cells. Experiments conducted later showed that the other RNA and DNA bases could be obtained through simulated prebiotic chemistry with a reducing atmosphere. These discoveries created a stir within the science community. Scientists became very optimistic that the questions about the origin of life would be solved within a few decades. This has not been the case, however. Instead, the investigation into life's origins seems only to have just begun. There has been a recent wave of skepticism concerning Miller's experiment because it is now believed that the early earth's atmosphere did not contain predominantly reductant molecules. Another objection is that this experiment required a tremendous amount of energy. While it is believed lightning storms were extremely common on the primitive Earth, they were not continuous as the Miller/Urey experiment portrayed. Thus it has been argued that while amino acids and other organic compounds may have been formed, they would not have been formed in the amounts which this experiment produced. Many of the compounds made in the Miller/Urey experiment are known to exist in outer spaceTheories of origin of life1. Scientific Evolution: This theory relies strongly on the Big Bang theory of the Creation of the Universe, which was the beginning of the formation of matter. This eventually led to the creation of planets, Pangaea and life on earth as it evolved over millions of years in a natural environment of chemicals and enabling elements. Evolution of life can mean many things. Some use the word to refer to any change at all. Obviously the creation/evolution debate is not about that kind of a definition. Creationists agree that many changes take place, but disagree with the theory of evolution when it is used to mean that a gradual progression from molecules to man produced all living things by natural means, that is, without the involvement of an intelligent Creator. 2. Special Creation: According to this theory, all the different forms of life that occur today on planet earth have been created by God, the almighty. This idea is found in the ancient scriptures of almost every religion. According to Hindu mythology, Lord Brahma, the God of Creation, created the living world in accordance to his wish. According to the Christian belief, God created this universe, plants, animals and human beings in about six natural days. The Sikh mythology says that all forms of life including human beings came into being with a single word of God. Special creation theory believes that the things have not undergone any significant change since their creation. Creationists generally believe the Bible's explanation that God created a number of basic groups of animals and plants as described in the first part of Genesis. They believe that while God created each group with the possibility of a good deal of variation, they brought forth according to their own kind. By definition, the faith-based Theory of Special Creation is purely a religious concept, acceptable only on the basis of faith. It has no scientific basis. 3. Biogenesis: The belief that living things come only from other living things (e.g. a spider lays eggs, which develop into spiders). It may also refer to biochemical processes of production in living organisms. The Law of Biogenesis, attributed to Louis Pasteur, states that life arises from pre-existing life, not from nonliving material. Pasteur's (and others') empirical results were summarized in the phrase Omne vivum ex vivo, Latin for "all life [is] from life", also known as the "law of biogenesis". Pasteur stated: "La génération spontanée est une chimère" ("Spontaneous generation is a dream"). 4. Abiogenesis: In the natural sciences, abiogenesis - also known as spontaneous generation - is the study of how life on Earth could have arisen from inanimate matter. This is also referred to as the "primordial soup" theory of evolution (life began in water as a result of the combination of chemicals from the atmosphere and some form of energy to make amino acids, the building blocks of proteins, which would then evolve into all the species). It should not be confused with evolution, which is the study of how groups of already living things change over time. Most amino acids, often called "the building blocks of life", can form via natural chemical reactions unrelated to life, as demonstrated in the Miller-Urey experiment and similar experiments, which involved simulating the conditions of the early Earth. In all living things, these amino acids are organized into proteins, and the construction of these proteins is mediated by nucleic acids. This of these organic molecules first arose and how they formed the first life is the focus of abiogenesis. Egyptians believed that mud of the Nile River could spontaneously give rise to many forms of life. The idea of spontaneous generation was popular almost till seventeenth century. Many scientists like Descartes, Galileo and Helmont supported this idea. 5. Theory of Chemical Evolution : This theory is also known as Materialistic Theory or Physico-chemical Theory. According this theory, the origin of life on earth is the result of a slow and gradual process of chemical evolution that probably occurred about 3.8 billion years ago. This theory was proposed independently by two scientists - A.I.Oparin, a Russian scientist in 1923 and J.B.S Haldane, an English scientist, in 1928. 6. Theory of Catastrophism: This theory on the origin of life is simply a modification of the theory of Special Creation. It states that there have been several creations of life by God, each preceded by a catastrophe resulting from some kind of geological disturbance. According to this theory, since each catastrophe completely destroyed the existing life, each new creation consisted of life form different from that of previous ones. French scientists Georges Cuvier (1769-1832) and Orbigney (1802 to 1837) were the main supporters of this theory. 7. Inorganic Incubation: Proposed by Professor William Martin, of Düsseldorf University, and Professor Michael Russell, of the Scottish Environmental Research Centre in Glasgow, this theory states that Instead of the building blocks of life forming first, and then forming a cell-like structure, the researchers say the cell came first and was later filled with living molecules. They say that the first cells were not living cells but inorganic ones made of iron sulfide and were formed not at the Earth's surface but in total darkness at the bottom of the oceans. The theory postulates that life is a chemical consequence of convection currents through the Earth's crust and, in principle, could happen on any wet, rocky planet. 8. Endosymbiotic Theory: This theory, espoused by Lynn Margulis, suggests that multiple forms of bacteria entered into symbiotic relationship to form the eukaryotic cell. The horizontal transfer of genetic material between bacteria promotes such symbiotic relationships, and thus many separate organisms may have contributed to building what has been recognized as the Last Universal Common Ancestor (LUCA) of modern organisms. James Lovelock's Gaia theory, proposes that such bacterial symbiosis establishes the environment as a system produced by and supportive of life. His arguments strongly weaken the case for life having evolved elsewhere in the solar system. 9. Panspermia - Cells From Outer Space: Some scientists believe that the simplest life-forms, whole cells (especially microbial cells), have been transported to the Earth from extraterrestrial sources. In this way, a process called panspermia (means seeds everywhere) might have initiated life on Earth. Most mainstream scientists have not supported panspermia, but early challenges have been thwarted in recent years due to discoveries such as terrestrial microbes that survive in extreme environments and incredibly aged yet viable microorganisms found in ancient rocks. In addition, water (essential for life) has been discovered on other planets and moons, and organic chemicals have been found on meteorites and in interstellar debris. 10. Cosmogony: Cosmogony is any theory concerning the coming into existence or origin of the universe, or about how reality came to be. In the specialized context of space science and astronomy, the term refers to theories of creation of the Solar System. For example, Greek mythology and some religions of the Ancient Near East refer to chaos, the formless or void state of primordial matter preceding the creation of the universe or cosmos in creation myths. Cosmogony can be distinguished from cosmology, which studies the universe at large and throughout its existence, yet does not inquire directly into the source of life or its origins. Biological classificationThe hierarchy of biological classification's eight major taxonomic ranks, which is an example of definition by genus and differentia. Life is divided into domains, which are subdivided into further groups. Intermediate minor rankings are not shown. Traditionally, people have divided organisms into the classes of plants and animals, based mainly on their ability of movement. The first known attempt to classify organisms was conducted by the Greek philosopher Aristotle (384–322 BC). He classified all living organisms known at that time as either a plant or an animal. Aristotle distinguished animals with blood from animals without blood (or at least without red blood), which can be compared with the concepts of vertebrates and invertebrates respectively. He divided the blooded animals into five groups: viviparous quadrupeds (mammals), birds, oviparous quadrupeds (reptiles and amphibians), fishes and whales. The bloodless animals were divided into five groups: cephalopods, crustaceans, insects (which included the spiders, scorpions, centipedes, and what we define as insects in the present day), shelled animals (such as most molluscs and echinoderms) and "zoophytes." Though Aristotle's work in zoology was not without errors, it was the grandest biological synthesis of the time and remained the ultimate authority for many centuries after his death.  The exploration of the American continent revealed large numbers of new plants and animals that needed descriptions and classification. In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced and was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. In the late 1740s, Carolus Linnaeus introduced his method, still used, to formulate the scientific name of every species. Linnaeus took every effort to improve the composition and reduce the length of the many-worded names by abolishing unnecessary rhetoric, introducing new descriptive terms and defining their meaning with an unprecedented precision. By consistently using his system, Linnaeus separated nomenclature from taxonomy. This convention for naming species is referred to as binomial nomenclature. The fungi were originally treated as plants. For a short period Linnaeus had placed them in the taxon Vermes in Animalia. He later placed them back in Plantae. Copeland classified the Fungi in his Protoctista, thus partially avoiding the problem but acknowledged their special status.  The problem was eventually solved by Whittaker, when he gave them their own kingdom in his five-kingdom system. As it turned out, the fungi are more closely related to animals than to plants.  As new discoveries enabled us to study cells and microorganisms, new groups of life were revealed, and the fields of cell biology and microbiology were created. These new organisms were originally described separately in protozoa as animals and protophyta/thallophyta as plants, but were united by Haeckel in his kingdom protista, later the group of prokaryotes were split off in the kingdom Monera, eventually this kingdom would be divided in two separate groups, the Bacteria and the Archaea, leading to the six-kingdom system and eventually to the current three-domain system.  The classification of eukaryotes is still controversial, with protist taxonomy especially problematic.  As microbiology, molecular biology and virology developed, non-cellular reproducing agents were discovered, such as viruses and viroids. Sometimes these entities are considered to be alive but others argue that viruses are not living organisms since they lack characteristics such as cell membrane, metabolism and do not grow or respond to their environments. Viruses can however be classed into "species" based on their biology and genetics but many aspects of such a classification remain controversial.  Since the 1960s a trend called cladistics has emerged, arranging taxa in an evolutionary or phylogenetic tree. It is unclear, should this be implemented, how the different codes will coexist. Linnaeus 1735 Haeckel 1866 Chatton 1925 Copeland 1938 Whittaker 1969 Woese et al. 1977 Woese et al. 1990 Cavalier-Smith 2004 2 kingdoms 3 kingdoms 2 empires 4 kingdoms 5 kingdoms 6 kingdoms 3 domains 6 kingdoms (not treated) Protista Prokaryota Monera Monera Eubacteria Bacteria Bacteria Archaebacteria Archaea Eukaryota Protoctista Protista Protista Eukarya Protozoa Chromista Vegetabilia Plantae Plantae Plantae Plantae Plantae Fungi Fungi Fungi Animalia Animalia Animalia Animalia Animalia Animalia

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