1.
Chronology
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Chronology is the science of arranging events in their order of occurrence in time. Consider, for example, the use of a timeline or sequence of events and it is also the determination of the actual temporal sequence of past events. It is also part of the discipline of history, including history, the earth sciences. Chronology is the science of locating historical events in time and it relies upon chronometry, which is also known as timekeeping, and historiography, which examines the writing of history and the use of historical methods. Radiocarbon dating estimates the age of living things by measuring the proportion of carbon-14 isotope in their carbon content. Dendrochronology is used in turn as a reference for radiocarbon dating curves. The familiar terms calendar and era concern two complementary concepts of chronology. Dionysius Exiguus was the founder of that era, which is nowadays the most widespread dating system on earth, an epoch is the date when an era begins. Ab Urbe condita is Latin for from the founding of the City and it was used to identify the Roman year by a few Roman historians. Modern historians use it more frequently than the Romans themselves did. Before the advent of the critical edition of historical Roman works, AUC was indiscriminately added to them by earlier editors. It was used systematically for the first time only about the year 400, pope Boniface IV, in about the year 600, seems to have been the first who made a connection between these this era and Anno Domini. Dionysius Exiguus’ Anno Domini era was extended by Bede to the complete Christian era, while of critical importance to the historian, methods of determining chronology are used in most disciplines of science, especially astronomy, geology, paleontology and archaeology. In the absence of written history, with its chronicles and king lists and this method of dating is known as seriation. Known wares discovered at strata in sometimes quite distant sites, the product of trade, laboratory techniques developed particularly after mid-20th century helped constantly revise and refine the chronologies developed for specific cultural areas. Unrelated dating methods help reinforce a chronology, an axiom of corroborative evidence, ideally, archaeological materials used for dating a site should complement each other and provide a means of cross-checking. Conclusions drawn from just one unsupported technique are usually regarded as unreliable, the fundamental problem of chronology is to synchronize events. By synchronizing an event it becomes possible to relate it to the current time, among historians, a typical need to is to synchronize the reigns of kings and leaders in order to relate the history of one country or region to that of another
2.
Archaeology
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Archaeology, or archeology, is the study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, Archaeology can be considered both a social science and a branch of the humanities. In North America, archaeology is considered a sub-field of anthropology, archaeologists study human prehistory and history, from the development of the first stone tools at Lomekwi in East Africa 3.3 million years ago up until recent decades. Archaeology as a field is distinct from the discipline of palaeontology, Archaeology is particularly important for learning about prehistoric societies, for whom there may be no written records to study. Prehistory includes over 99% of the human past, from the Paleolithic until the advent of literacy in societies across the world, Archaeology has various goals, which range from understanding culture history to reconstructing past lifeways to documenting and explaining changes in human societies through time. The discipline involves surveying, excavation and eventually analysis of data collected to learn more about the past, in broad scope, archaeology relies on cross-disciplinary research. Archaeology developed out of antiquarianism in Europe during the 19th century, Archaeology has been used by nation-states to create particular visions of the past. Nonetheless, today, archaeologists face many problems, such as dealing with pseudoarchaeology, the looting of artifacts, a lack of public interest, the science of archaeology grew out of the older multi-disciplinary study known as antiquarianism. Antiquarians studied history with attention to ancient artifacts and manuscripts. Tentative steps towards the systematization of archaeology as a science took place during the Enlightenment era in Europe in the 17th and 18th centuries, in Europe, philosophical interest in the remains of Greco-Roman civilization and the rediscovery of classical culture began in the late Middle Age. Antiquarians, including John Leland and William Camden, conducted surveys of the English countryside, one of the first sites to undergo archaeological excavation was Stonehenge and other megalithic monuments in England. John Aubrey was a pioneer archaeologist who recorded numerous megalithic and other monuments in southern England. He was also ahead of his time in the analysis of his findings and he attempted to chart the chronological stylistic evolution of handwriting, medieval architecture, costume, and shield-shapes. Excavations were also carried out in the ancient towns of Pompeii and Herculaneum and these excavations began in 1748 in Pompeii, while in Herculaneum they began in 1738. The discovery of entire towns, complete with utensils and even human shapes, however, prior to the development of modern techniques, excavations tended to be haphazard, the importance of concepts such as stratification and context were overlooked. The father of archaeological excavation was William Cunnington and he undertook excavations in Wiltshire from around 1798, funded by Sir Richard Colt Hoare. Cunnington made meticulous recordings of neolithic and Bronze Age barrows, one of the major achievements of 19th century archaeology was the development of stratigraphy. The idea of overlapping strata tracing back to successive periods was borrowed from the new geological and paleontological work of scholars like William Smith, James Hutton, the application of stratigraphy to archaeology first took place with the excavations of prehistorical and Bronze Age sites
3.
Geology
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Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, and the processes by which they change over time. Geology can also refer generally to the study of the features of any terrestrial planet. Geology gives insight into the history of the Earth by providing the evidence for plate tectonics, the evolutionary history of life. Geology also plays a role in engineering and is a major academic discipline. The majority of data comes from research on solid Earth materials. These typically fall into one of two categories, rock and unconsolidated material, the majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three types of rock, igneous, sedimentary, and metamorphic. The rock cycle is an important concept in geology which illustrates the relationships between three types of rock, and magma. When a rock crystallizes from melt, it is an igneous rock, the sedimentary rock can then be subsequently turned into a metamorphic rock due to heat and pressure and is then weathered, eroded, deposited, and lithified, ultimately becoming a sedimentary rock. Sedimentary rock may also be re-eroded and redeposited, and metamorphic rock may also undergo additional metamorphism, all three types of rocks may be re-melted, when this happens, a new magma is formed, from which an igneous rock may once again crystallize. Geologists also study unlithified material which typically comes from more recent deposits and these materials are superficial deposits which lie above the bedrock. Because of this, the study of material is often known as Quaternary geology. This includes the study of sediment and soils, including studies in geomorphology, sedimentology and this theory is supported by several types of observations, including seafloor spreading, and the global distribution of mountain terrain and seismicity. This coupling between rigid plates moving on the surface of the Earth and the mantle is called plate tectonics. The development of plate tectonics provided a basis for many observations of the solid Earth. Long linear regions of geologic features could be explained as plate boundaries, mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, were explained as divergent boundaries, where two plates move apart. Arcs of volcanoes and earthquakes were explained as convergent boundaries, where one plate subducts under another, transform boundaries, such as the San Andreas Fault system, resulted in widespread powerful earthquakes. Plate tectonics also provided a mechanism for Alfred Wegeners theory of continental drift and they also provided a driving force for crustal deformation, and a new setting for the observations of structural geology
4.
Relative dating
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Relative dating is the science of determining the relative order of past events, without necessarily determining their absolute age. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another. Prior to the discovery of radiometric dating which provided a means of dating in the early 20th century. Though relative dating can determine the sequential order in which a series of events occurred, not when they occur. Relative dating by biostratigraphy is the method in paleontology, and is in some respects more accurate. The regular order of occurrence of fossils in rock layers was discovered around 1800 by William Smith, while digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers. As he continued his job as a surveyor, he found the same patterns across England and he also found that certain animals were in only certain layers and that they were in the same layers all across England. Due to that discovery, Smith was able to recognize the order that the rocks were formed, sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras. Methods for relative dating were developed when geology first emerged as a formal science, geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events. The principle of Uniformitarianism states that the processes observed in operation that modify the Earths crust at present have worked in much the same way over geologic time. A fundamental principle of geology advanced by the 18th century Scottish physician, in Huttons words, the past history of our globe must be explained by what can be seen to be happening now. The principle of intrusive relationships concerns crosscutting intrusions, in geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills, the principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut. Finding the key bed in these situations may help determine whether the fault is a fault or a thrust fault. The principle of inclusions and components states that, with rocks, if inclusions are found in a formation. For example, in rocks, it is common for gravel from an older formation to be ripped up. A similar situation with igneous rocks occurs when xenoliths are found and these foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them, the principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds
5.
Recorded history
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Recorded history or written history is a historical narrative based on a written record or other documented communication. Recorded history can be contrasted with other narratives of the past, for broader world history, recorded history begins with the accounts of the ancient world around the 4th millennium BC, and coincides with the invention of writing. Examples of written texts, however, can be found dating as far back as 1750 BCE in Ancient Mesopotamia, for some geographic regions or cultures, written history is limited to a relatively recent period in human history because of the limited use of written records. Because of these limits, recorded history in different contexts may refer to different periods of time depending on the historical topic. The question of the nature, and even the possibility, of a method for interpreting recorded history is raised in the philosophy of history as a question of epistemology. Prehistory traditionally refers to the span of time before recorded history, prehistory refers to the past in an area where no written records exist, or where the writing of a culture is not understood. Protohistory refers to the period between prehistory and history, after the advent of literacy in a society but before the writings of the first historians. Protohistory may also refer to the period during which a culture or civilization has not yet developed writing, more complete writing systems were preceded by proto-writing. Early examples are the Jiahu symbols, Vinča signs, early Indus script, earliest recorded history, which varies greatly in quality and reliability, deals with Pharaohs and their reigns, made by ancient Egyptians. Much of the earliest recorded history was re-discovered relatively recently due to archaeological dig sites findings, since these initial accounts, a number of different traditions have developed in different parts of the world in how to handle the writing and production of historical accounts. In China, the earliest history was recorded in oracle bone script which was deciphered, the Zuo Zhuan, attributed to Zuo Qiuming in the 5th century BCE, is the earliest written of narrative history in the world and covers the period from 722 to 468 BCE. The Classic of History is one of the Five Classics of Chinese classic texts and it is traditionally attributed to Confucius. Zhan Guo Ce was a renowned ancient Chinese historical compilation of materials on the Warring States period compiled between the 3rd and 1st centuries BCE. Sima Qian was the first in China to lay the groundwork for professional historical writing and his written work was the Shiji, a monumental lifelong achievement in literature. His work influenced every subsequent author of history in China, including the prestigious Ban family of the Eastern Han Dynasty era, Herodotus of Halicarnassus has generally been acclaimed as the father of history composing his The Histories written from 450s to the 420s BCE. However, his contemporary Thucydides is credited with having first approached history with a historical method in his work the History of the Peloponnesian War. Thucydides, unlike Herodotus, regarded history as being the product of the choices and actions of human beings, saint Augustine was influential in Christian and Western thought at the beginning of the medieval period. Through the Medieval and Renaissance periods, history was studied through a sacred or religious perspective
6.
Dendrochronology
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Dendrochronology is the scientific method of dating tree rings to the exact year they were formed in order to analyze atmospheric conditions during different periods in history. Dendrochronology is useful for determining the timing of events and rates of change in the environment and also in works of art and architecture, such as old panel paintings on wood, buildings and it is also used in radiocarbon dating to calibrate radiocarbon ages. New growth in trees occurs in a layer of cells near the bark, a trees growth rate changes in a predictable pattern throughout the year in response to seasonal climate changes, resulting in visible growth rings. Each ring marks a complete cycle of seasons, or one year, as of 2013, the oldest tree-ring measurements in the Northern Hemisphere extend back 13,900 years. Dendrochronology derives from Ancient Greek, δένδρον, meaning tree limb, χρόνος, meaning time, and -λογία, the Greek botanist Theophrastus first mentioned that the wood of trees has rings. In his Trattato della Pittura, Leonardo da Vinci was the first person to mention that trees form rings annually, in 1737, French investigators Henri-Louis Duhamel du Monceau and Georges-Louis Leclerc de Buffon examined the effect of growing conditions on the shape of tree rings. They found that in 1709, a severe winter produced a distinctly dark tree ring, the English polymath Charles Babbage proposed using dendrochronology to date the remains of trees in peat bogs or even in geological strata. During the latter half of the century, the scientific study of tree rings. In 1859, the German-American Jacob Kuechler used crossdating to examine oaks in order to study the record of climate in western Texas, in 1866, the German botanist, entomologist, and forester Julius Ratzeburg observed the effects on tree rings of defoliation caused by insect infestations. By 1882, this observation was already appearing in forestry textbooks, in the 1870s, the Dutch astronomer Jacobus C. Kapteyn was using crossdating to reconstruct the climates of the Netherlands, in 1881, the Swiss-Austrian forester Arthur von Seckendorff-Gudent was using crossdating. From 1869 to 1901, Robert Hartig, a German professor of forest pathology, wrote a series of papers on the anatomy, in 1892, the Russian physicist Fedor Nikiforovich Shvedov wrote that he had used patterns found in tree rings to predict droughts in 1882 and 1891. During the first half of the 20th century, the astronomer A. E. Douglass founded the Laboratory of Tree-Ring Research at the University of Arizona. Growth rings, also referred to as tree rings or annual rings, growth rings are the result of new growth in the vascular cambium, a layer of cells near the bark that is classified as a lateral meristem, this growth in diameter is known as secondary growth. Visible rings result from the change in speed through the seasons of the year, thus, critical for the title method. If the bark of the tree has been removed in a particular area, the rings are more visible in temperate zones, where the seasons differ more markedly. The inner portion of a ring is formed early in the growing season, when growth is comparatively rapid and is known as early wood. Many trees in temperate zones make one growth ring each year, hence, for the entire period of a trees life, a year-by-year record or ring pattern is formed that reflects the age of the tree and the climatic conditions in which the tree grew
7.
Radiocarbon dating
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Radiocarbon dating is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon. The method was developed by Willard Libby in the late 1940s, Libby received the Nobel Prize for his work in 1960. The radiocarbon dating method is based on the fact that radiocarbon is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen. The resulting radiocarbon combines with oxygen to form radioactive carbon dioxide. When the animal or plant dies, it stops exchanging carbon with its environment, and from that point onwards the amount of 14C it contains begins to decrease as the 14C undergoes radioactive decay. Measuring the amount of 14C in a sample from a plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died. The idea behind radiocarbon dating is straightforward, but years of work were required to develop the technique to the point where accurate dates could be obtained. Research has been ongoing since the 1960s to determine what the proportion of 14C in the atmosphere has been over the past fifty thousand years. The resulting data, in the form of a curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the samples calendar age. Other corrections must be made to account for the proportion of 14C in different types of organisms, additional complications come from the burning of fossil fuels such as coal and oil, and from the above-ground nuclear tests done in the 1950s and 1960s. Conversely, nuclear testing increased the amount of 14C in the atmosphere, measurement of radiocarbon was originally done by beta-counting devices, which counted the amount of beta radiation emitted by decaying 14C atoms in a sample. The development of dating has had a profound impact on archaeology. In addition to permitting more accurate dating within archaeological sites than previous methods, histories of archaeology often refer to its impact as the radiocarbon revolution. Radiocarbon dating has allowed key transitions in prehistory to be dated, such as the end of the last ice age, and they synthesized 14C using the laboratorys cyclotron accelerator and soon discovered that the atoms half-life was far longer than had been previously thought. This was followed by a prediction by Serge A. Korff, then employed at the Franklin Institute in Philadelphia and it had previously been thought that 14C would be more likely to be created by deuterons interacting with 13C. At some time during World War II, Willard Libby, who was then at Berkeley, learned of Korffs research, in 1945, Libby moved to the University of Chicago where he began his work on radiocarbon dating. He published a paper in 1946 in which he proposed that the carbon in living matter might include 14C as well as non-radioactive carbon, by contrast, methane created from petroleum showed no radiocarbon activity because of its age. The results were summarized in a paper in Science in 1947, Libby and James Arnold proceeded to test the radiocarbon dating theory by analyzing samples with known ages
8.
Thermoluminescence dating
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As a crystalline material is heated during measurements the process of thermoluminescence starts. Thermoluminescence emits a light signal that is proportional to the radiation dose absorbed by the material. It is a type of luminescence dating, the technique has wide application, and is relatively cheap at some US$300–700 per object, ideally a number of samples are tested. Sediments are more expensive to date, the destruction of a relatively significant amount of sample material is necessary, which can be a limitation in the case of artworks. The heating must have taken the object above 500° C, which covers most ceramics and it will often work well with stones that have been heated by fire. The clay core of bronze sculptures made by lost wax casting can also be tested, different materials vary considerably in their suitability for the technique, depending on several factors. Subsequent irradiation, for if an x-ray is taken, can affect accuracy. Ideally this is assessed by measurements made at the precise findspot over a long period, for artworks, it may be sufficient to confirm whether a piece is broadly ancient or modern, and this may be possible even if a precise date cannot be estimated. These imperfections lead to local humps and dips in the crystalline materials electric potential, where there is a dip, a free electron may be attracted and trapped. The flux of ionizing radiation—both from cosmic radiation and from natural radioactivity—excites electrons from atoms in the lattice into the conduction band where they can move freely. Most excited electrons will recombine with lattice ions, but some will be trapped. Depending on the depth of the traps the storage time of trapped electrons will vary as some traps are sufficiently deep to store charge for hundreds of thousands of years. In the process of recombining with an ion, they lose energy and emit photons. The amount of light produced is proportional to the number of trapped electrons that have been freed which is in turn proportional to the dose accumulated. In order to relate the signal to the dose that caused it, it is necessary to calibrate the material with known doses of radiation since the density of traps is highly variable. Thermoluminescence dating presupposes a zeroing event in the history of the material, either heating or exposure to sunlight, therefore, at that point the thermoluminescence signal is zero. As time goes on, the radiation field around the material causes the trapped electrons to accumulate. In the laboratory, the radiation dose can be measured
9.
Radiometric dating
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Radiometric dating or radioactive dating is a technique used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geological time scale. Among the best-known techniques are radiocarbon dating, potassium-argon dating and uranium-lead dating, by allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is used to date archaeological materials, including ancient artifacts. Different methods of radiometric dating vary in the timescale over which they are accurate, all ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus, a particular isotope of a particular element is called a nuclide. That is, at point in time, an atom of such a nuclide will undergo radioactive decay. This transformation may be accomplished in a number of different ways, including alpha decay, another possibility is spontaneous fission into two or more nuclides. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a daughter nuclide or decay product, Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years to over 100 billion years. For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially a constant and it is not affected by external factors such as temperature, pressure, chemical environment, or presence of a magnetic or electric field. The only exceptions are nuclides that decay by the process of capture, such as beryllium-7, strontium-85. For all other nuclides, the proportion of the nuclide to its decay products changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present. The basic equation of radiometric dating requires that neither the parent nuclide nor the product can enter or leave the material after its formation. The possible confounding effects of contamination of parent and daughter isotopes have to be considered and it is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration. Precision is enhanced if measurements are taken on multiple samples from different locations of the rock body and this can reduce the problem of contamination. In uranium-lead dating, the diagram is used which also decreases the problem of nuclide loss. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample, the procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate
10.
Radionuclide
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A radionuclide is an atom that has excess nuclear energy, making it unstable. During those processes, the radionuclide is said to undergo radioactive decay, the unstable nucleus is more stable following the emission, but will sometimes undergo further decay. Radioactive decay is a process at the level of single atoms. However, for a collection of atoms of an element the decay rate. The range of the half-lives of radioactive atoms have no known limits, Radionuclides occur naturally and are artificially produced in nuclear reactors, cyclotrons, particle accelerators or radionuclide generators. There are about 730 radionuclides with half-lives longer than 60 minutes, with the longest half lives are the 32 primordial radionuclides that have survived from the creation of the Solar System. Over 60 further radionuclides are detectable in nature, either as daughters of these, more than 2400 radionuclides have half-lives less than 60 minutes. Most of those are produced artificially, and have very short half-lives. For comparison, there are about 254 stable nuclides, even the lightest element, hydrogen, has a well-known radionuclide, tritium. Elements heavier than lead, and the elements technetium and promethium, unplanned exposure to radionuclides generally has a harmful effect on living organisms including humans, although low levels of exposure occur naturally without harm. However, radionuclides with suitable properties are used in medicine for both diagnosis and treatment. An imaging tracer made with radionuclides is called a radioactive tracer, a pharmaceutical drug made with radionuclides is called a radiopharmaceutical. On Earth, naturally occurring radionuclides fall into three categories, primordial radionuclides, secondary radionuclides, and cosmogenic radionuclides, Radionuclides are produced in stellar nucleosynthesis and supernova explosions along with stable nuclides. Most decay quickly but can still be observed astronomically and can play a part in understanding astronomic processes, primordial radionuclides, such as uranium and thorium, exist in the present time because their half-lives are so long they have not yet completely decayed. It is possible decay may be observed in other nuclides adding to this list of primordial radionuclides, secondary radionuclides are radiogenic isotopes derived from the decay of primordial radionuclides. They have shorter half-lives than primordial radionuclides and they arise in the decay chain of the primordial isotopes thorium-232, uranium-238 and uranium-235. Examples include the natural isotopes of polonium and radium, cosmogenic isotopes, such as carbon-14, are present because they are continually being formed in the atmosphere due to cosmic rays. Many of these radionuclides exist only in trace amounts in nature, secondary radionuclides will occur in proportion to their half-lives, so short-lived ones will be very rare
11.
Accelerator mass spectrometry
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Accelerator mass spectrometry differs from other forms of mass spectrometry in that it accelerates ions to extraordinarily high kinetic energies before mass analysis. The special strength of AMS among the mass spectrometric methods is its power to separate a rare isotope from an abundant neighboring mass, the method suppresses molecular isobars completely and in many cases can separate atomic isobars also. This makes possible the detection of naturally occurring, long-lived radio-isotopes such as 10Be, 36Cl, 26Al and their typical isotopic abundance ranges from 10−12 to 10−18. AMS can outperform the competing technique of decay counting for all isotopes where the half-life is long enough, generally, negative ions are created in an ion source. In fortunate cases this allows the suppression of an unwanted isobar. The pre-accelerated ions are separated by a first mass spectrometer of sector-field type. At the connecting point between the two stages, the ions change charge from negative to positive by passing through a layer of matter. Molecules will break apart in this stripping stage, the complete suppression of molecular isobars is one reason for the exceptional abundance sensitivity of AMS. Additionally, the impact strips off several of the ions electrons, in the second half of the accelerator the now positively charged ion is accelerated away from the highly positive center of the electrostatic accelerator which previously attracted the negative ion. When the ions leave the accelerator they are charged and are moving at several percent of the speed of light. In a second stage of mass spectrometer, the fragments from the molecules are separated from the ions of interest and this spectrometer may consist of magnetic or electric sectors, and so called velocity selectors, which utilizes both electric fields and magnetic fields. After this stage, no background is left, unless a stable isobar forming negative ions exists, thanks to the high energy of the ions, these can be separated by methods borrowed from nuclear physics, like degrader foils and gas-filled magnets. Individual ions are detected by single-ion counting. Thanks to the energy of the ions, these detectors can provide additional identification of background isobars by nuclear-charge determination. The above is not an example, in 1977, inspired by this early work, Richard A. His paper was the inspiration for other groups using cyclotrons. K. Purser and colleagues published the successful detection of radiocarbon using their tandem at Rochester. Soon afterwards the Berkeley and French teams reported the detection of 10Be
12.
Potassium-40
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Potassium-40 is a radioactive isotope of potassium which has a very long half-life of 1. 251×109 years. It makes up 0. 012% of the amount of potassium found in nature. Potassium-40 is an example of an isotope that undergoes all three types of beta decay. About 89. 28% of the time, it decays to calcium-40 with emission of a particle with a maximum energy of 1.33 MeV. About 10. 72% of the time it decays to argon-40 by electron capture, with the emission of a 1.460 MeV gamma ray, the radioactive decay of this particular isotope explains the large abundance of argon in the earths atmosphere. Very rarely it will decay to 40Ar by emitting a positron, potassium-40 is especially important in potassium–argon dating. Argon is a gas that does not ordinarily combine with other elements, so, when a mineral forms – whether from molten rock, or from substances dissolved in water – it will be initially argon-free, even if there is some argon in the liquid. However, if the mineral contains any potassium, then decay of the 40K isotope present will create fresh argon-40 that will remain locked up in the mineral. Since the rate at which this occurs is known, it is possible to determine the elapsed time since the mineral formed by measuring the ratio of 40K. It follows that most of the terrestrial argon derives from potassium-40 that decayed into argon-40, the radioactive decay of 40K in the Earths mantle ranks third, after 232Th and 238U, as the source of radiogenic heat. The core also likely contains radiogenic sources, although how much is uncertain and it has been proposed that significant core radioactivity may be caused by high levels of U, Th, and K. 40K is the largest source of natural radioactivity in animals including humans
13.
Geochronology
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Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is provided by such as palaeomagnetism. By combining multiple geochronological indicators the precision of the age can be improved. Biostratigraphy does not directly provide an absolute age determination of a rock, both disciplines work together hand in hand however, to the point where they share the same system of naming rock layers and the time spans utilized to classify layers within a stratum. By measuring the amount of decay of a radioactive isotope with a known half-life. A number of isotopes are used for this purpose. More slowly decaying isotopes are useful for longer periods of time, two or more radiometric methods can be used in concert to achieve more robust results. Some of the commonly used techniques are, Radiocarbon dating and this technique measures the decay of carbon-14 in organic material and can be best applied to samples younger than about 60,000 years. This technique measures the ratio of two isotopes to the amount of uranium in a mineral or rock. Often applied to the mineral zircon in igneous rocks, this method is one of the two most commonly used for geologic dating. Monazite geochronology is another example of U-Pb dating, employed for dating metamorphism in particular, uranium-lead dating is applied to samples older than about 1 million years. This technique is used to date speleothems, corals, carbonates and its range is from a few years to about 700,000 years. These techniques date metamorphic, igneous and volcanic rocks and they are also used to date volcanic ash layers within or overlying paleoanthropologic sites. The younger limit of the method is a few thousand years. Electron spin resonance dating A series of related techniques for determining the age at which a surface was created. Exposure dating uses the concentration of exotic nuclides produced by cosmic rays interacting with Earth materials as a proxy for the age at which a surface, such as an alluvial fan, was created. Burial dating uses the radioactive decay of 2 cosmogenic elements as a proxy for the age at which a sediment was screened by burial from further cosmic rays exposure. Luminescence dating techniques observe light emitted from such as quartz, diamond, feldspar
14.
Argon
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Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 of the table and is a noble gas. Argon is the third-most abundant gas in the Earths atmosphere, at 0. 934% and it is more than twice as abundant as water vapor,23 times as abundant as carbon dioxide, and more than 500 times as abundant as neon. Argon is the most abundant noble gas in Earths crust, comprising 0. 00015% of the crust, nearly all of the argon in Earths atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in the Earths crust. In the universe, argon-36 is by far the most common argon isotope, the name argon is derived from the Greek word ἀργόν, neuter singular form of ἀργός meaning lazy or inactive, as a reference to the fact that the element undergoes almost no chemical reactions. The complete octet in the atomic shell makes argon stable. Its triple point temperature of 83.8058 K is a fixed point in the International Temperature Scale of 1990. Argon is produced industrially by the distillation of liquid air. Argon is also used in incandescent, fluorescent lighting, and other gas-discharge tubes, Argon makes a distinctive blue-green gas laser. Argon is also used in fluorescent glow starters, Argon has approximately the same solubility in water as oxygen and is 2.5 times more soluble in water than nitrogen. Argon is colorless, odorless, nonflammable and nontoxic as a solid, liquid or gas, Argon is chemically inert under most conditions and forms no confirmed stable compounds at room temperature. Although argon is a gas, it can form some compounds under extreme conditions. Argon fluorohydride, a compound of argon with fluorine and hydrogen that is stable below 17 K, has been demonstrated. Although the neutral ground-state chemical compounds of argon are presently limited to HArF, ions, such as ArH+, and excited-state complexes, such as ArF, have been demonstrated. Theoretical calculation predicts several more argon compounds that should be stable but have not yet been synthesized, Argon was suspected to be a component of air by Henry Cavendish in 1785. Argon was first isolated from air in 1894 by Lord Rayleigh and Sir William Ramsay at University College London by removing oxygen, carbon dioxide, water and they had determined that nitrogen produced from chemical compounds was 0. 5% lighter than nitrogen from the atmosphere. The difference was slight, but it was important enough to attract their attention for many months and they concluded that there was another gas in the air mixed in with the nitrogen. Argon was also encountered in 1882 through independent research of H. F. Newall, each observed new lines in the color spectrum of air that did not match known elements
15.
Geomagnetic reversal
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The time spans of chrons are randomly distributed with most being between 0.1 and 1 million years with an average of 450,000 years. Most reversals are estimated to take between 1,000 and 10,000 years, the latest one, the Brunhes–Matuyama reversal, occurred 780,000 years ago, and may have happened very quickly, within a human lifetime. A brief complete reversal, known as the Laschamp event, occurred only 41,000 years ago during the last glacial period and that reversal lasted only about 440 years with the actual change of polarity lasting around 250 years. During this change the strength of the magnetic field weakened to 5% of its present strength, brief disruptions that do not result in reversal are called geomagnetic excursions. In the early 20th century, geologists first noticed that some rocks were magnetized opposite to the direction of the local Earths field. The first estimate of the timing of magnetic reversals was made by Motonori Matuyama in the 1920s, at the time, the Earths polarity was poorly understood, and the possibility of reversal aroused little interest. Three decades later, when Earths magnetic field was better understood, most paleomagnetic research in the late 1950s included an examination of the wandering of the poles and continental drift. In the absence of reliable methods for obtaining absolute ages for rocks, the next major advance in understanding reversals came when techniques for radiometric dating were developed in the 1950s. They produced the first magnetic-polarity time scale in 1959, as they accumulated data, they continued to refine this scale in competition with Don Tarling and Ian McDougall at the Australian National University. A group led by Neil Opdyke at the Lamont-Doherty Geological Observatory showed that the pattern of reversals was recorded in sediments from deep-sea cores. During the 1950s and 1960s information about variations in the Earths magnetic field was gathered largely by means of research vessels, but the complex routes of ocean cruises rendered the association of navigational data with magnetometer readings difficult. Only when data were plotted on a map did it become apparent that remarkably regular, thus, sea floor spreading from a central ridge will produce pairs of magnetic stripes parallel to the ridge. The Morley–Vine–Matthews hypothesis was the first key scientific test of the spreading theory of continental drift. The same magnetic anomalies were found over most of the worlds oceans, past field reversals can be and have been recorded in the frozen ferromagnetic minerals of consolidated sedimentary deposits or cooled volcanic flows on land. The past record of geomagnetic reversals was first noticed by observing the magnetic anomalies on the ocean floor. Because no existing unsubducted sea floor is more than about 180 million years old, most sedimentary rocks incorporate tiny amounts of iron rich minerals, whose orientation is influenced by the ambient magnetic field at the time at which they formed. These rocks can preserve a record of the if it is not later erased by chemical, physical or biological change. Because the magnetic field is global, similar patterns of magnetic variations at different sites may be used to age in different locations
16.
Luminescence dating
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Luminescence dating refers to a group of methods of determining how long ago mineral grains were last exposed to sunlight or sufficient heating. It is useful to geologists and archaeologists who want to know such an event occurred. It uses various methods to stimulate and measure luminescence It includes techniques such as optically stimulated luminescence, infrared stimulated luminescence, optical dating typically refers to OSL and IRSL, but not TL. All sediments and soils contain trace amounts of isotopes of elements such as potassium, uranium, thorium. These slowly decay over time and the radiation they produce is absorbed by mineral grains in the sediments such as quartz. The radiation causes charge to remain within the grains in structurally unstable electron traps, the trapped charge accumulates over time at a rate determined by the amount of background radiation at the location where the sample was buried. Most luminescence dating methods rely on the assumption that the grains were sufficiently bleached at the time of the event being dated. For example, in quartz a short exposure in the range of 1–100 seconds before burial is sufficient to effectively “reset” the OSL dating clock. This is usually, but not always, the case with aeolian deposits, such as dunes and loess. Quartz OSL ages can be determined typically from 100 to 350,000 years BP, ages can be obtained outside this these ranges, but they should be regarded with caution. The uncertainty of an OSL date is typically 5-10% of the age of the sample, experimental tests on archaeological ceramics followed a few years later in 1960 by Grögler et al. Over the next few decades, thermoluminescence research was focused on heated pottery and ceramics, burnt flints, baked hearth sediments, oven stones from burnt mounds and other heated objects. In 1963, Aitken et al. noted that TL traps in calcite could be bleached by sunlight as well as heat, throughout the 70s and early 80s TL dating of light-sensitive traps in geological sediments of both terrestrial and marine origin became more widespread. Optically stimulated luminescence was developed in 1984 by David Huntley and colleagues, hütt et al. laid the groundwork for the infrared stimulated luminescence dating of potassium feldspars in 1988. In 1994, the principles behind optical and thermoluminescence dating were extended to include surfaces made of granite, basalt and sandstone, ioannis Liritzis, the initiator of ancient buildings luminescence dating, has shown this in several cases of various monuments. The dose rate is usually in the range 0.5 -5 grays/1000 years, the total absorbed radiation dose is determined by exciting specific minerals extracted from the sample with light and measuring the amount of light emitted as a result. The photons of the light must have higher energies than the excitation photons in order to avoid measurement of ordinary photoluminescence. A sample in which the grains have all been exposed to sufficient daylight can be said to be of zero age
17.
Electron
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The electron is a subatomic particle, symbol e− or β−, with a negative elementary electric charge. Electrons belong to the first generation of the lepton particle family, the electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the include a intrinsic angular momentum of a half-integer value, expressed in units of the reduced Planck constant. As it is a fermion, no two electrons can occupy the same state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of particles and waves, they can collide with other particles and can be diffracted like light. Since an electron has charge, it has an electric field. Electromagnetic fields produced from other sources will affect the motion of an electron according to the Lorentz force law, electrons radiate or absorb energy in the form of photons when they are accelerated. Laboratory instruments are capable of trapping individual electrons as well as electron plasma by the use of electromagnetic fields, special telescopes can detect electron plasma in outer space. Electrons are involved in applications such as electronics, welding, cathode ray tubes, electron microscopes, radiation therapy, lasers, gaseous ionization detectors. Interactions involving electrons with other particles are of interest in fields such as chemistry. The Coulomb force interaction between the positive protons within atomic nuclei and the negative electrons without, allows the composition of the two known as atoms, ionization or differences in the proportions of negative electrons versus positive nuclei changes the binding energy of an atomic system. The exchange or sharing of the electrons between two or more atoms is the cause of chemical bonding. In 1838, British natural philosopher Richard Laming first hypothesized the concept of a quantity of electric charge to explain the chemical properties of atoms. Irish physicist George Johnstone Stoney named this charge electron in 1891, electrons can also participate in nuclear reactions, such as nucleosynthesis in stars, where they are known as beta particles. Electrons can be created through beta decay of isotopes and in high-energy collisions. The antiparticle of the electron is called the positron, it is identical to the electron except that it carries electrical, when an electron collides with a positron, both particles can be totally annihilated, producing gamma ray photons. The ancient Greeks noticed that amber attracted small objects when rubbed with fur, along with lightning, this phenomenon is one of humanitys earliest recorded experiences with electricity. In his 1600 treatise De Magnete, the English scientist William Gilbert coined the New Latin term electricus, both electric and electricity are derived from the Latin ēlectrum, which came from the Greek word for amber, ἤλεκτρον
18.
Bristol Zoo
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Bristol Zoo is a zoo in the city of Bristol in South West England. Among species now on view at Bristol which are rare or absent in other UK zoos are Livingstones fruit bats and successful breeding groups of western lowland gorillas, the lakes islands are home to gorillas, tamarins, marmosets, gibbons and pelicans. Seal and Penguin Coasts is an attraction at the zoo, opened in 1999, it allows South American fur seals. The two pools contain 145,000 imperial gallons of water, with waves, waterfalls, rocks. The exhibit has a net over the top to allow Inca terns. Explorers Creek opened in May 2009 and features three areas – a water area, a tropical bird house and a walk-through lorikeet feeding area. Gorilla Island is home to a family of western lowland gorillas, as well as an indoor house which is also home to okapis, they have a large island which they share with the De Brazzas monkeys from Monkey Jungle. The gorillas are herbivores, and are not aggressive, however the keepers do not enter their island home because it is the zoos policy to keep the animals captive environment as similar as possible to that of their natural African forest habitat. The Terrace is one of the oldest parts of the zoo, by exchanging night and day, the animals can be observed during daylight hours. The lights allow a transition from night to day and vice versa. Twilight world is split into four zones, the Desert, the Rainforest, the Cave, the Reptile House houses a collection of reptiles and amphibians. The house itself is heated and gives a sense of the heat of the rainforest, there are three sections to the house, Desert, Rainforest and the Rearing Room where visitors can view the raising of reptiles and amphibian and also learn about the zoos conservation work. Outside, but still considered part of the house, is a giant tortoise and rhinoceros iguana enclosure where the animals have access to a heated indoor house. The Aquarium has around 70 species of fish, the aquarium has three sections, The Amazon River, Africa and the coral reef. On the outside of the building there is a water sculpture, there are several exhibits of conservation significance on view. Notably, there is a display of endangered cichlids from Lake Barombi Mbo in Cameroon, bug World, the zoos collection of invertebrates, includes species such as Partula snails, stick and leaf insects, corals and peacock mantis shrimp. Other displays include tarantulas, black widow spiders, giant millipedes, honey bees, leaf-cutting ants, zona Brazil includes a tropical house that has Amazon tree boas and tarantulas. The two monkey enclosures house Geoffreys marmosets, black lion tamarins and titi monkeys, outside, there are aviaries for red-tailed amazon parrots, an enclosure for golden lion tamarins and three linked paddocks for tapirs and capybaras
19.
Paleoecology
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Paleoecology uses data from fossils and subfossils to reconstruct the ecosystems of the past. Such interpretations aid the reconstruction of past environments, paleoecologists have studied the fossil record to try to clarify the relationship animals have to their environment, in part to help understand the current state of biodiversity. They have identified links between vertebrate taxonomic and ecological diversity, that is, between the diversity of animals and the niches they occupy. The aim of paleoecology is therefore to build the most detailed model possible of the environment of previously living organisms found today as fossils. Such reconstruction takes into consideration complex interactions among environmental factors such as temperatures, food supplies, often much of this information is lost or distorted by the fossilization process or diagenesis of the enclosing sediments, making interpretation difficult. The environmental complexity factor is normally tackled through statistical analysis of the numerical data. Much of the original research has focused on the last two million years, because older environments are less well represented in the fossil timeline of evolution. Indeed, many studies concentrate on the Holocene epoch, or the last glacial stage of the Pleistocene epoch, such studies are useful for understanding the dynamics of ecosystem change and for reconstructing pre-industrialization ecosystems. Many public policy decision-makers have pointed to the importance of using palaeoecological studies as a basis for choices made in conservation ecology, historical ecology Palaeogeography Palaeogeography, Palaeoclimatology, Palaeoecology Fox D. Taylor, P. D. and Wilson, M. A.2003, Palaeoecology and evolution of marine hard substrate communities
20.
Ecology
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Ecology is the scientific analysis and study of interactions among organisms and their environment. It is a field that includes biology, geography. Ecology includes the study of interactions that organisms have with other, other organisms. Ecosystems are composed of dynamically interacting parts including organisms, the communities make up. Ecosystem processes, such as production, pedogenesis, nutrient cycling. These processes are sustained by organisms with specific life history traits, biodiversity, which refers to the varieties of species, genes, and ecosystems, enhances certain ecosystem services. Ecology is not synonymous with environment, environmentalism, natural history and it is closely related to evolutionary biology, genetics, and ethology. An important focus for ecologists is to improve the understanding of how biodiversity affects ecological function, Ecology is a human science as well. For example, the Circles of Sustainability approach treats ecology as more than the environment out there and it is not treated as separate from humans. Organisms and resources compose ecosystems which, in turn, maintain biophysical feedback mechanisms that moderate processes acting on living and non-living components of the planet, the word ecology was coined in 1866 by the German scientist Ernst Haeckel. Ecological thought is derivative of established currents in philosophy, particularly from ethics and politics, ancient Greek philosophers such as Hippocrates and Aristotle laid the foundations of ecology in their studies on natural history. Modern ecology became a more rigorous science in the late 19th century. Evolutionary concepts relating to adaptation and natural selection became the cornerstones of modern ecological theory, the scope of ecology contains a wide array of interacting levels of organization spanning micro-level to a planetary scale phenomena. Ecosystems, for example, contain abiotic resources and interacting life forms, an ecosystems area can vary greatly, from tiny to vast. A single tree is of consequence to the classification of a forest ecosystem. Several generations of a population can exist over the lifespan of a single leaf. Each of those aphids, in turn, support diverse bacterial communities, biodiversity describes the diversity of life from genes to ecosystems and spans every level of biological organization. The term has several interpretations, and there are ways to index, measure, characterize
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Dating methodology (archaeology)
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Chronological dating, or simply dating, is the process of attributing to an object or event a date in the past, allowing such object or event to be located in a previously established chronology. This usually requires what is known as a dating method. Dating methods are most commonly classified following two criteria, relative dating and absolute dating, but this method is also useful in many other disciplines. Thus,1587 is the post quem dating of Shakespeares play Henry V and that means that the play was without fail written after 1587. The same inductive mechanism is applied in archaeology, geology and paleontology, for example, in a stratum presenting difficulties or ambiguities to absolute dating, paleopalynology can be used as a relative referent by means of the study of the pollens found in the stratum. This is admitted because of the reason that some botanical species. Thus, to be considered as archaeological, the remains, objects or artifacts to be dated must be related to human activity. It is commonly assumed that if the remains or elements to be dated are older than the human species, nevertheless, the range of time within archaeological dating can be enormous compared to the average lifespan of a singular human being. As an example Pinnacle Points caves, in the southern coast of South Africa, on the other hand, remains as recent as a hundred years old can also be the target of archaeological dating methods. It was the case of an 18th-century sloop whose excavation was led in South Carolina in 1992, thus, from the oldest to the youngest, all archaeological sites are likely to be dated by an appropriate method. Dating is carried out mainly post excavation, but to good practice. Dating is very important in archaeology for constructing models of the past, as it relies on the integrity of dateable objects, numismatics - many coins have the date of their production written on them or their use is specified in the historical record. Palaeography - the study of ancient writing, including the practice of deciphering, reading, seriation is a relative dating method. An example of an application of seriation, is the comparison of the known style of artefacts such as stone tools or pottery. For example, if a context is sealed between two other contexts of known date, it can be inferred that the context must date to between those dates. Geochronology Geologic time scale Geological history of Earth Age of the universe Age of the Earth
22.
Paleobiology
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Paleobiology is a growing and comparatively new discipline which combines the methods and findings of the natural science biology with the methods and findings of the earth science paleontology. It is occasionally referred to as geobiology, Paleobiological research uses biological field research of current biota and of fossils millions of years old to answer questions about the molecular evolution and the evolutionary history of life. In this scientific quest, macrofossils, microfossils and trace fossils are typically analyzed, however, the 21st-century biochemical analysis of DNA and RNA samples offers much promise, as does the biometric construction of phylogenetic trees. An investigator in this field is known as a paleobiologist, paleobotany applies the principles and methods of paleobiology to flora, especially green land plants, but also including the fungi and seaweeds. See also mycology, phycology and dendrochronology, paleozoology uses the methods and principles of paleobiology to understand fauna, both vertebrates and invertebrates. See also vertebrate and invertebrate paleontology, as well as paleoanthropology, micropaleontology applies paleobiologic principles and methods to archaea, bacteria, protists and microscopic pollen/spores. Paleovirology examines the history of viruses on paleobiological timescales. Paleobiochemistry uses the methods and principles of chemistry to detect and analyze molecular-level evidence of ancient life. Paleoecology examines past ecosystems, climates, and geographies so as to better comprehend prehistoric life, taphonomy analyzes the post-mortem history of an individual organism in order to gain insight on the behavior, death and environment of the fossilized organism. Paleoichnology analyzes the tracks, borings, trails, burrows, impressions, stratigraphic paleobiology studies long-term secular changes, as well as the bed-by-bed sequence of changes, in organismal characteristics and behaviors. See also stratification, sedimentary rocks and the time scale. Evolutionary developmental paleobiology examines the evolutionary aspects of the modes and trajectories of growth, see also adaptive radiation, cladistics, evolutionary biology, developmental biology and phylogenetic tree. The founder or father of modern paleobiology was Baron Franz Nopcsa and he initially termed the discipline paleophysiology. However, credit for coining the word itself should go to Professor Charles Schuchert. He proposed the term in 1904 so as to initiate a new science joining traditional paleontology with the evidence and insights of geology. On the other hand, Charles Doolittle Walcott, a Smithsonian adventurer, has cited as the founder of Precambrian paleobiology. In 1899 he discovered the first acritarch fossil cells, a Precambrian algal phytoplankton he named Chuaria, lastly, in 1914, Walcott reported minute cells and chains of cell-like bodies belonging to Precambrian purple bacteria. Eleven years later, Barghoorn and J. William Schopf reported finely-preserved Precambrian microflora at their Bitter Springs site of the Amadeus Basin, Central Australia
23.
Forensic science
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Forensic scientists collect, preserve, and analyze scientific evidence during the course of an investigation. In addition to their role, forensic scientists testify as expert witnesses in both criminal and civil cases and can work for either the prosecution or the defense. While any field could technically be forensic, certain sections have developed over time to encompass the majority of forensically related cases, the word forensic comes from the Latin term forensis, meaning of or before the forum. The history of the term originates from Roman times, during which a charge meant presenting the case before a group of public individuals in the forum. Both the person accused of the crime and the accuser would give speeches based on their sides of the story, the case would be decided in favor of the individual with the best argument and delivery. This origin is the source of the two usages of the word forensic – as a form of legal evidence and as a category of public presentation. In modern use, the term forensics in the place of science can be considered correct. However, the term is now so closely associated with the field that many dictionaries include the meaning that equates the word forensics with forensic science. The ancient world lacked standardized forensic practices, which aided criminals in escaping punishment, Criminal investigations and trials heavily relied on forced confessions and witness testimony. However, ancient sources do contain several accounts of techniques that foreshadow concepts in science that were developed centuries later. The first written account of using medicine and entomology to solve criminal cases is attributed to the book of Xi Yuan Lu, written in China by Song Ci in 1248, during the Song Dynasty. In one of the accounts, the case of a person murdered with a sickle was solved by an investigator who instructed everyone to bring his sickle to one location, flies, attracted by the smell of blood, eventually gathered on a single sickle. In light of this, the murderer confessed, methods from around the world involved saliva and examination of the mouth and tongue to determine innocence or guilt, as a precursor to the Polygraph test. In ancient India, some suspects were made to fill their mouths with dried rice, similarly, in Ancient China, those accused of a crime would have rice powder placed in their mouths. In ancient middle-eastern cultures, the accused were made to lick hot metal rods briefly, in 16th-century Europe, medical practitioners in army and university settings began to gather information on the cause and manner of death. Ambroise Paré, a French army surgeon, systematically studied the effects of violent death on internal organs, two Italian surgeons, Fortunato Fidelis and Paolo Zacchia, laid the foundation of modern pathology by studying changes that occurred in the structure of the body as the result of disease. In the late 18th century, writings on these began to appear. Two examples of English forensic science in individual legal proceedings demonstrate the use of logic
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Taphonomy
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Taphonomy is the study of decaying organisms over time and how they may become fossilized. e. Taphonomists study such phenomena as biostratinomy, decomposition, diagenesis, one motivation behind taphonomy is to better understand biases present in the fossil record. During the late century, taphonomic data began to be applied to other paleontological subfields such as paleobiology, paleoceanography, ichnology. Archaeologists study taphonomic processes in order to determine how plant and animal remains accumulate and this is critical to determining whether these remains are associated with human activity. In addition, taphonomic processes may alter biological remains after they are deposited at a site, some remains survive better than others over time, and can therefore bias an excavated collection. Forensic taphonomy is concerned with the study of the decomposition of human remains, Experimental taphonomy testing usually consists of exposing the remains of organisms to various altering processes, and then examining the effects of the exposure. Taphonomy has undergone an explosion of interest since the 1980s, with focusing on certain areas. Microbial, biogeochemical, and larger-scale controls on the preservation of different tissue types, in particular, covered within this field is the dominance of biological versus physical agents in the destruction of remains from all major taxonomic groups. Processes that concentrate biological remains, especially the degree to which different types of assemblages reflect the species composition, the spatio-temporal resolution and ecological fidelity of species assemblages, particularly the relatively minor role of out-of-habitat transport contrasted with the major effects of time-averaging. The Mars Science Laboratory mission objectives evolved from assessment on ancient Mars habitability to developing predictive models on taphonomy. It has been suggested that biominerals could be important indicators of extraterrestrial life, furthermore, organic components that are often associated with biominerals are believed to play crucial roles in both pre-biotic and biotic reactions. The search for evidence of habitability, taphonomy, and organic carbon on the planet Mars is now a primary NASA objective, because of the very select processes that cause preservation, not all organisms have the same chance of being preserved. Any factor that affects the likelihood that an organism is preserved as a fossil is a source of bias. Some of the most common sources of bias are listed below and this perhaps represents the biggest source of bias in the fossil record. First and foremost, organisms that contain hard parts have a far greater chance of being represented in the record than organisms consisting of soft tissue only. As a result, animals with bones or shells are overrepresented in the fossil record, many animals that moult, on the other hand, are overrepresented, as one animal may leave multiple fossils due to its discarded body parts. Among plants, wind-pollinated species produce so much more pollen than animal-pollinated species, most fossils form in conditions where material is deposited to the bottom of water bodies. In continental environments, fossilization is especially likely in small lakes that fill in with organic and inorganic material
25.
Amino acid
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Amino acids are organic compounds containing amine and carboxyl functional groups, along with a side chain specific to each amino acid. The key elements of an acid are carbon, hydrogen, oxygen. About 500 amino acids are known and can be classified in many ways, in the form of proteins, amino acids comprise the second-largest component of human muscles, cells and other tissues. Outside proteins, amino acids perform critical roles in such as neurotransmitter transport. In biochemistry, amino acids having both the amine and the acid groups attached to the first carbon atom have particular importance. They are known as 2-, alpha-, or α-amino acids and they include the 22 proteinogenic amino acids, which combine into peptide chains to form the building-blocks of a vast array of proteins. These are all L-stereoisomers, although a few D-amino acids occur in bacterial envelopes, as a neuromodulator, twenty of the proteinogenic amino acids are encoded directly by triplet codons in the genetic code and are known as standard amino acids. The other two are selenocysteine, and pyrrolysine, pyrrolysine and selenocysteine are encoded via variant codons, for example, selenocysteine is encoded by stop codon and SECIS element. N-formylmethionine is generally considered as a form of methionine rather than as a separate proteinogenic amino acid, codon–tRNA combinations not found in nature can also be used to expand the genetic code and create novel proteins known as alloproteins incorporating non-proteinogenic amino acids. Many important proteinogenic and non-proteinogenic amino acids also play critical roles within the body. Nine proteinogenic amino acids are called essential for humans because they cannot be created from other compounds by the human body, others may be conditionally essential for certain ages or medical conditions. Essential amino acids may also differ between species, because of their biological significance, amino acids are important in nutrition and are commonly used in nutritional supplements, fertilizers, and food technology. Industrial uses include the production of drugs, biodegradable plastics, the first few amino acids were discovered in the early 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound in asparagus that was subsequently named asparagine, cystine was discovered in 1810, although its monomer, cysteine, remained undiscovered until 1884. Glycine and leucine were discovered in 1820, usage of the term amino acid in the English language is from 1898. Proteins were found to yield amino acids after enzymatic digestion or acid hydrolysis, in the structure shown at the top of the page, R represents a side chain specific to each amino acid. The carbon atom next to the group is called the α–carbon. Amino acids containing an amino group bonded directly to the alpha carbon are referred to as amino acids
26.
Glycine
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Glycine is the amino acid that has a single hydrogen atom as its side chain. It is the simplest possible amino acid, the chemical formula of glycine is NH2‐CH2‐COOH. Glycine is one of the amino acids. Its codons are GGU, GGC, GGA, GGG of the genetic code, Glycine is a colorless, sweet-tasting crystalline solid. It is unique among the amino acids in that it is achiral. It can fit into hydrophilic or hydrophobic environments since it exists as zwitterion at natural pH, Glycine was first isolated from gelatin in 1820. The name comes from the ancient Greek word γλυκύς sweet tasting, Glycine was discovered in 1820, by Henri Braconnot who boiled a gelatinous object with sulfuric acid. Glycine is manufactured industrially by treating chloroacetic acid with ammonia, ClCH2COOH +2 NH3 → H2NCH2COOH + NH4Cl About 15 million kg are produced annually in this way, in the USA and in Japan, glycine is produced via the Strecker amino acid synthesis. Glycine is also cogenerated as an impurity in the synthesis of EDTA, arising from reactions of the ammonia coproduct. In aqueous solution, glycine itself is amphoteric, at low pH the molecule can be protonated with a pKa of about 2.4, the nature of glycine in aqueous solution has been investigated by theoretical methods. In solution the ratio of concentrations of the two isomers is independent of both the concentration and of pH. This ratio is simply the equilibrium constant for isomerization, K = / Both isomers of glycine have been observed by microwave spectroscopy in the gas phase. The solid-state structure has been analyzed in detail and this conversion is readily reversible, CO2 + NH+4 + N5, N10-Methylene tetrahydrofolate + NADH + H+ → Glycine + tetrahydrofolate + NAD+ Glycine is coded by codons GGU, GGC, GGA and GGG. Most proteins incorporate only small quantities of glycine, a notable exception is collagen, which contains about 35% glycine. Glycine is degraded via three pathways, the first step is the reverse of glycine biosynthesis from serine with serine hydroxymethyl transferase. Serine is then converted to pyruvate by serine dehydratase, in the third pathway of glycine degradation, glycine is converted to glyoxylate by D-amino acid oxidase. Glyoxylate is then oxidized by lactate dehydrogenase to oxalate in an NAD+-dependent reaction. The half-life of glycine and its elimination from the body varies significantly based on dose, in one study, the half-life was between 0.5 and 4.0 hours
27.
Optical rotation
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Optical rotation or optical activity is the rotation of the plane of polarization of linearly polarized light as it travels through certain materials. Optical activity occurs only in chiral materials, those lacking microscopic mirror symmetry, unlike other sources of birefringence which alter a beams state of polarization, optical activity can be observed in fluids. This can include gases or solutions of chiral molecules such as sugars, molecules with helical secondary structure such as some proteins and it can also be observed in chiral solids such as certain crystals with a rotation between adjacent crystal planes or metamaterials. The rotation of the plane of polarization may be either clockwise, to the right, for instance, sucrose and camphor are d-rotary whereas cholesterol is l-rotary. For a given substance, the angle by which the polarization of light of a wavelength is rotated is proportional to the path length through the material and proportional to its concentration. The rotation is not dependent on the direction of propagation, unlike the Faraday effect where the rotation is dependent on the direction of the applied magnetic field. Optical activity is measured using a source and polarimeter. Modulation of a liquid crystals optical activity, viewed between two polarizers, is the principle of operation of liquid-crystal displays. The rotation of the orientation of polarized light was first observed in 1811 in quartz by French physicist François Jean Dominique Arago. Jean Baptiste Biot also observed the rotation of the axis of polarization in certain liquids, simple polarimeters have been used since this time to measure the concentrations of simple sugars, such as glucose, in solution. In fact one name for D-glucose, is dextrose, referring to the fact that it causes linearly polarized light to rotate to the right or dexter side. In a similar manner, levulose, more known as fructose. Fructose is even more strongly levorotatory than glucose is dextrorotatory, in 1849, Louis Pasteur resolved a problem concerning the nature of tartaric acid. Pasteur noticed that the crystals come in two forms that are mirror images of one another. Sorting the crystals by hand gave two forms of the compound, Solutions of one form rotate polarized light clockwise, while the other form rotate light counterclockwise, an equal mix of the two has no polarizing effect on light. If the 4 neighbors are all different, then there are two possible orderings of the neighbors around the tetrahedron, which will be mirror images of each other and this led to a better understanding of the three-dimensional nature of molecules. Optical activity occurs due to molecules dissolved in a fluid or due to the fluid itself only if the molecules are one of two stereoisomers, this is known as an enantiomer, the structure of such a molecule is such that it is not identical to its mirror image. In mathematics, this property is known as chirality
28.
Carbon
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Carbon is a chemical element with symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds, three isotopes occur naturally, 12C and 13C being stable, while 14C is a radioactive isotope, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity, Carbon is the 15th most abundant element in the Earths crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is the second most abundant element in the body by mass after oxygen. The atoms of carbon can bond together in different ways, termed allotropes of carbon, the best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form, for example, graphite is opaque and black while diamond is highly transparent. Graphite is soft enough to form a streak on paper, while diamond is the hardest naturally occurring material known, graphite is a good electrical conductor while diamond has a low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials, all carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen, the most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and transition metal carbonyl complexes. The largest sources of carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil. For this reason, carbon has often referred to as the king of the elements. The allotropes of carbon graphite, one of the softest known substances, and diamond. It bonds readily with other small atoms including other carbon atoms, Carbon is known to form almost ten million different compounds, a large majority of all chemical compounds. Carbon also has the highest sublimation point of all elements, although thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements such as iron and copper that are weaker reducing agents at room temperature. Carbon is the element, with a ground-state electron configuration of 1s22s22p2. Its first four ionisation energies,1086.5,2352.6,4620.5 and 6222.7 kJ/mol, are higher than those of the heavier group 14 elements. Carbons covalent radii are normally taken as 77.2 pm,66.7 pm and 60.3 pm, although these may vary depending on coordination number, in general, covalent radius decreases with lower coordination number and higher bond order. Carbon compounds form the basis of all life on Earth
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International Standard Book Number
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The International Standard Book Number is a unique numeric commercial book identifier. An ISBN is assigned to each edition and variation of a book, for example, an e-book, a paperback and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, the method of assigning an ISBN is nation-based and varies from country to country, often depending on how large the publishing industry is within a country. The initial ISBN configuration of recognition was generated in 1967 based upon the 9-digit Standard Book Numbering created in 1966, the 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108. Occasionally, a book may appear without a printed ISBN if it is printed privately or the author does not follow the usual ISBN procedure, however, this can be rectified later. Another identifier, the International Standard Serial Number, identifies periodical publications such as magazines, the ISBN configuration of recognition was generated in 1967 in the United Kingdom by David Whitaker and in 1968 in the US by Emery Koltay. The 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108, the United Kingdom continued to use the 9-digit SBN code until 1974. The ISO on-line facility only refers back to 1978, an SBN may be converted to an ISBN by prefixing the digit 0. For example, the edition of Mr. J. G. Reeder Returns, published by Hodder in 1965, has SBN340013818 -340 indicating the publisher,01381 their serial number. This can be converted to ISBN 0-340-01381-8, the check digit does not need to be re-calculated, since 1 January 2007, ISBNs have contained 13 digits, a format that is compatible with Bookland European Article Number EAN-13s. An ISBN is assigned to each edition and variation of a book, for example, an ebook, a paperback, and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, a 13-digit ISBN can be separated into its parts, and when this is done it is customary to separate the parts with hyphens or spaces. Separating the parts of a 10-digit ISBN is also done with either hyphens or spaces, figuring out how to correctly separate a given ISBN number is complicated, because most of the parts do not use a fixed number of digits. ISBN issuance is country-specific, in that ISBNs are issued by the ISBN registration agency that is responsible for country or territory regardless of the publication language. Some ISBN registration agencies are based in national libraries or within ministries of culture, in other cases, the ISBN registration service is provided by organisations such as bibliographic data providers that are not government funded. In Canada, ISBNs are issued at no cost with the purpose of encouraging Canadian culture. In the United Kingdom, United States, and some countries, where the service is provided by non-government-funded organisations. Australia, ISBNs are issued by the library services agency Thorpe-Bowker
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PubMed Identifier
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PubMed is a free search engine accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. The United States National Library of Medicine at the National Institutes of Health maintains the database as part of the Entrez system of information retrieval, from 1971 to 1997, MEDLINE online access to the MEDLARS Online computerized database primarily had been through institutional facilities, such as university libraries. PubMed, first released in January 1996, ushered in the era of private, free, home-, the PubMed system was offered free to the public in June 1997, when MEDLINE searches via the Web were demonstrated, in a ceremony, by Vice President Al Gore. Information about the journals indexed in MEDLINE, and available through PubMed, is found in the NLM Catalog. As of 5 January 2017, PubMed has more than 26.8 million records going back to 1966, selectively to the year 1865, and very selectively to 1809, about 500,000 new records are added each year. As of the date,13.1 million of PubMeds records are listed with their abstracts. In 2016, NLM changed the system so that publishers will be able to directly correct typos. Simple searches on PubMed can be carried out by entering key aspects of a subject into PubMeds search window, when a journal article is indexed, numerous article parameters are extracted and stored as structured information. Such parameters are, Article Type, Secondary identifiers, Language, publication type parameter enables many special features. As these clinical girish can generate small sets of robust studies with considerable precision, since July 2005, the MEDLINE article indexing process extracts important identifiers from the article abstract and puts those in a field called Secondary Identifier. The secondary identifier field is to store numbers to various databases of molecular sequence data, gene expression or chemical compounds. For clinical trials, PubMed extracts trial IDs for the two largest trial registries, ClinicalTrials. gov and the International Standard Randomized Controlled Trial Number Register, a reference which is judged particularly relevant can be marked and related articles can be identified. If relevant, several studies can be selected and related articles to all of them can be generated using the Find related data option, the related articles are then listed in order of relatedness. To create these lists of related articles, PubMed compares words from the title and abstract of each citation, as well as the MeSH headings assigned, using a powerful word-weighted algorithm. The related articles function has been judged to be so precise that some researchers suggest it can be used instead of a full search, a strong feature of PubMed is its ability to automatically link to MeSH terms and subheadings. Examples would be, bad breath links to halitosis, heart attack to myocardial infarction, where appropriate, these MeSH terms are automatically expanded, that is, include more specific terms. Terms like nursing are automatically linked to Nursing or Nursing and this important feature makes PubMed searches automatically more sensitive and avoids false-negative hits by compensating for the diversity of medical terminology. The My NCBI area can be accessed from any computer with web-access, an earlier version of My NCBI was called PubMed Cubby
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Astronomy
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Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry, in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, galaxies, and comets, while the phenomena include supernovae explosions, gamma ray bursts, more generally, all astronomical phenomena that originate outside Earths atmosphere are within the purview of astronomy. A related but distinct subject, physical cosmology, is concerned with the study of the Universe as a whole, Astronomy is the oldest of the natural sciences. The early civilizations in recorded history, such as the Babylonians, Greeks, Indians, Egyptians, Nubians, Iranians, Chinese, during the 20th century, the field of professional astronomy split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the results and observations being used to confirm theoretical results. Astronomy is one of the few sciences where amateurs can play an active role, especially in the discovery. Amateur astronomers have made and contributed to many important astronomical discoveries, Astronomy means law of the stars. Astronomy should not be confused with astrology, the system which claims that human affairs are correlated with the positions of celestial objects. Although the two share a common origin, they are now entirely distinct. Generally, either the term astronomy or astrophysics may be used to refer to this subject, however, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics. Few fields, such as astrometry, are purely astronomy rather than also astrophysics, some titles of the leading scientific journals in this field includeThe Astronomical Journal, The Astrophysical Journal and Astronomy and Astrophysics. In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye, in some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose. Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye, most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon, the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the model of the Universe, or the Ptolemaic system. The Babylonians discovered that lunar eclipses recurred in a cycle known as a saros
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History
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History is the study of the past as it is described in written documents. Events occurring before written record are considered prehistory and it is an umbrella term that relates to past events as well as the memory, discovery, collection, organization, presentation, and interpretation of information about these events. Scholars who write about history are called historians and their works continue to be read today, and the gap between the culture-focused Herodotus and the military-focused Thucydides remains a point of contention or approach in modern historical writing. In Asia, a chronicle, the Spring and Autumn Annals was known to be compiled from as early as 722 BC although only 2nd-century BC texts survived. Ancient influences have helped spawn variant interpretations of the nature of history which have evolved over the centuries, the modern study of history is wide-ranging, and includes the study of specific regions and the study of certain topical or thematical elements of historical investigation. Often history is taught as part of primary and secondary education, the word history comes ultimately from Ancient Greek ἱστορία, meaning inquiry, knowledge from inquiry, or judge. It was in that sense that Aristotle used the word in his Περὶ Τὰ Ζῷα Ἱστορίαι, the ancestor word ἵστωρ is attested early on in Homeric Hymns, Heraclitus, the Athenian ephebes oath, and in Boiotic inscriptions. History was borrowed from Latin into Old English as stær, and it was from Anglo-Norman that history was borrowed into Middle English, and this time the loan stuck. In Middle English, the meaning of history was story in general, the restriction to the meaning the branch of knowledge that deals with past events, the formal record or study of past events, esp. human affairs arose in the mid-fifteenth century. With the Renaissance, older senses of the word were revived, and it was in the Greek sense that Francis Bacon used the term in the sixteenth century. For him, historia was the knowledge of objects determined by space and time, in an expression of the linguistic synthetic vs. analytic/isolating dichotomy, English like Chinese now designates separate words for human history and storytelling in general. In modern German, French, and most Germanic and Romance languages, which are synthetic and highly inflected. The adjective historical is attested from 1661, and historic from 1669, Historian in the sense of a researcher of history is attested from 1531. Historians write in the context of their own time, and with due regard to the current dominant ideas of how to interpret the past, in the words of Benedetto Croce, All history is contemporary history. History is facilitated by the formation of a discourse of past through the production of narrative. The modern discipline of history is dedicated to the production of this discourse. All events that are remembered and preserved in some authentic form constitute the historical record, the task of historical discourse is to identify the sources which can most usefully contribute to the production of accurate accounts of past. Therefore, the constitution of the archive is a result of circumscribing a more general archive by invalidating the usage of certain texts and documents
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Paleontology
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Paleontology or palaeontology is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene Epoch. It includes the study of fossils to determine organisms evolution and interactions with each other, paleontological observations have been documented as far back as the 5th century BC. The science became established in the 18th century as a result of Georges Cuviers work on comparative anatomy, and developed rapidly in the 19th century. The term itself originates from Greek παλαιός, palaios, i. e. old, ancient, ὄν, on, i. e. being, creature and λόγος, logos, i. e. speech, thought, study. Paleontology lies on the border between biology and geology, but differs from archaeology in that it excludes the study of modern humans. It now uses techniques drawn from a range of sciences, including biochemistry, mathematics. The final quarter of the 20th century saw the development of molecular phylogenetics, molecular phylogenetics has also been used to estimate the dates when species diverged, but there is controversy about the reliability of the molecular clock on which such estimates depend. The simplest definition is the study of ancient life, paleontology is one of the historical sciences, along with archaeology, geology, astronomy, cosmology, philology and history itself. This means that it aims to describe phenomena of the past, hence it has three main elements, description of the phenomena, developing a general theory about the causes of various types of change, and applying those theories to specific facts. Sometimes the smoking gun is discovered by an accident during other research. Paleontology lies on the boundary between biology and geology since paleontology focuses on the record of past life but its source of evidence is fossils. In addition paleontology often uses techniques derived from other sciences, including biology, osteology, ecology, chemistry, techniques developed in engineering have been used to analyse how ancient organisms might have worked, for example how fast Tyrannosaurus could move and how powerful its bite was. As knowledge has increased, paleontology has developed specialised subdivisions, vertebrate paleontology concentrates on fossils of vertebrates, from the earliest fish to the immediate ancestors of modern mammals. Invertebrate paleontology deals with fossils of such as molluscs, arthropods. Paleobotany focuses on the study of plants, but traditionally includes the study of fossil algae. Palynology, the study of pollen and spores produced by plants and protists. Micropaleontology deals with all microscopic fossil organisms, regardless of the group to which they belong, one example is the development of oxygenic photosynthesis by bacteria, which hugely increased the productivity and diversity of ecosystems. This also caused the oxygenation of the atmosphere, together, these were a prerequisite for the evolution of the most complex eukaryotic cells, from which all multicellular organisms are built
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Time
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Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future. Time is often referred to as the dimension, along with the three spatial dimensions. Time has long been an important subject of study in religion, philosophy, and science, nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems. Two contrasting viewpoints on time divide prominent philosophers, one view is that time is part of the fundamental structure of the universe—a dimension independent of events, in which events occur in sequence. Isaac Newton subscribed to this realist view, and hence it is referred to as Newtonian time. This second view, in the tradition of Gottfried Leibniz and Immanuel Kant, holds that time is neither an event nor a thing, Time in physics is unambiguously operationally defined as what a clock reads. Time is one of the seven fundamental physical quantities in both the International System of Units and International System of Quantities, Time is used to define other quantities—such as velocity—so defining time in terms of such quantities would result in circularity of definition. The operational definition leaves aside the question there is something called time, apart from the counting activity just mentioned, that flows. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy. Furthermore, it may be there is a subjective component to time. Temporal measurement has occupied scientists and technologists, and was a motivation in navigation. Periodic events and periodic motion have long served as standards for units of time, examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms. Time is also of significant social importance, having economic value as well as value, due to an awareness of the limited time in each day. In day-to-day life, the clock is consulted for periods less than a day whereas the calendar is consulted for periods longer than a day, increasingly, personal electronic devices display both calendars and clocks simultaneously. The number that marks the occurrence of an event as to hour or date is obtained by counting from a fiducial epoch—a central reference point. Artifacts from the Paleolithic suggest that the moon was used to time as early as 6,000 years ago. Lunar calendars were among the first to appear, either 12 or 13 lunar months, without intercalation to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months
