Paleontology (/ˌpeɪliɒnˈtɒlədʒi,ˌpæli-,-ən-/PAY-lee-on-TOL-ə-jee,PAL-ee-, -ən-), also spelledpalaeontology[a] orpalæontology, is the scientific study of the history of life in the geologic past.[1][2] It includes the study offossils to classifyorganisms and study theirinteractions with each other and their environments (theirpaleoecology). Paleontological observations have been documented as far back as the 5th century BC. The science became established in the 18th century as a result ofGeorges Cuvier's work oncomparative anatomy, and developed rapidly in the 19th century. The term has been used since 1822[3][b] formed fromGreekπαλαιός ('palaios', "old, ancient"),ὄν ('on', (gen.'ontos'), "being, creature"), andλόγος ('logos', "speech, thought, study").[5]
Paleontology lies on the border betweenbiology andgeology, but it differs fromarchaeology in that it excludes the study ofanatomically modern humans. It now uses techniques drawn from a wide range of sciences, includingbiochemistry,mathematics, and engineering. Use of all these techniques has enabled paleontologists to discover much of theevolutionary history of life, almost back to when Earth became capable of supporting life, nearly 4 billion years ago.[6] As knowledge has increased, paleontology has developed specialised sub-divisions, some of which focus on different types of fossil organisms while others studyecology and environmental history, such asancient climates.
Body fossils andtrace fossils are the principal types of evidence about ancient life, andgeochemical evidence has helped to decipher the evolution of life before there were organisms large enough to leave body fossils. Estimating the dates of these remains is essential but difficult: sometimes adjacent rock layers allowradiometric dating, which providesabsolute dates that are accurate to within 0.5%, but more often paleontologists have to rely on relative dating by solving the "jigsaw puzzles" ofbiostratigraphy (arrangement of rock layers from youngest to oldest). Classifying ancient organisms is also difficult, as many do not fit well into theLinnaean taxonomy classifying living organisms, and paleontologists more often usecladistics to draw up evolutionary "family trees". The final quarter of the 20th century saw the development ofmolecular phylogenetics, which investigates how closely organisms are related by measuring the similarity of theDNA in theirgenomes. Molecular phylogenetics has also been used to estimate the dates when species diverged, but there is controversy about the reliability of themolecular clock on which such estimates depend.
The simplest definition of "paleontology" is "the study of ancient life".[7] The field seeks information about several aspects of past organisms: "their identity and origin, their environment and evolution, and what they can tell us about the Earth's organic and inorganic past".[8]
William Whewell (1794–1866) classified paleontology as one of the historical sciences, along witharchaeology, geology,astronomy,cosmology,philology and history itself:[9] paleontology aims to describe phenomena of the past and to reconstruct their causes.[10] Hence it has three main elements: description of past phenomena; developing a general theory about the causes of various types of change; and applying those theories to specific facts.[11]When trying to explain the past, paleontologists and other historical scientists often construct a set of one or morehypotheses about the causes and then look for a "smoking gun", a piece of evidence that strongly accords with one hypothesis over any others.[12]Sometimes researchers discover a "smoking gun" by a fortunate accident during other research. For example, the 1980 discovery byLuis andWalter Alvarez ofiridium, a mainly extraterrestrial metal, in theCretaceous–Paleogene boundary layer madeasteroid impact the most favored explanation for theCretaceous–Paleogene extinction event – although debate continues about the contribution of volcanism.[10]
A complementary approach to developing scientific knowledge,experimental science,[13]is often said[by whom?] to work by conducting experiments todisprove hypotheses about the workings and causes of natural phenomena. This approach cannot prove a hypothesis, since some later experiment may disprove it, but the accumulation of failures to disprove is often compelling evidence in favor. However, when confronted with totally unexpected phenomena, such as the first evidence for invisibleradiation, experimental scientists often use the same approach as historical scientists: construct a set of hypotheses about the causes and then look for a "smoking gun".[10]
Paleontology lies betweenbiology and geology since it focuses on the record of past life, but its main source of evidence isfossils in rocks.[14][15] For historical reasons, paleontology is part of the geology department at many universities: in the 19th and early 20th centuries, geology departments found fossil evidence important for dating rocks, while biology departments showed little interest.[16]
Paleontology also has some overlap witharchaeology, which primarily works with objects made by humans and with human remains, while paleontologists are interested in the characteristics and evolution of humans as a species. When dealing with evidence about humans, archaeologists and paleontologists may work together – for example paleontologists might identify animal or plant fossils around anarchaeological site, to discover the people who lived there, and what they ate; or they might analyze the climate at the time of habitation.[17]
In addition, paleontology often borrows techniques from other sciences, including biology,osteology, ecology,chemistry,physics and mathematics.[7] For example,geochemical signatures from rocks may help to discover when life first arose on Earth,[18] and analyses ofcarbonisotope ratios may help to identify climate changes and even to explain major transitions such as thePermian–Triassic extinction event.[19] A relatively recent discipline,molecular phylogenetics, compares theDNA andRNA of modern organisms to re-construct the "family trees" of their evolutionary ancestors. It has also been used to estimate the dates of important evolutionary developments, although this approach is controversial because of doubts about the reliability of the "molecular clock".[20] Techniques from engineering have been used to analyse how the bodies of ancient organisms might have worked, for example the running speed and bite strength ofTyrannosaurus,[21][22] or the flight mechanics ofMicroraptor.[23] It is relatively commonplace to study the internal details of fossils usingX-ray microtomography.[24][25] Paleontology, biology, archaeology, andpaleoneurobiology combine to study endocranial casts (endocasts) of species related to humans to clarify the evolution of the human brain.[26]
Paleontology even contributes toastrobiology, the investigation of possible life on otherplanets, by developing models of how life may have arisen and by providing techniques for detecting evidence of life.[27]
Analyses using engineering techniques show thatTyrannosaurus had a devastating bite, but raise doubts about its running ability.
Instead of focusing on individual organisms,paleoecology examines the interactions between different ancient organisms, such as theirfood chains, and the two-way interactions with their environments.[30] For example, the development ofoxygenic photosynthesis by bacteria caused theoxygenation of the atmosphere and hugely increased the productivity and diversity ofecosystems.[31] Together, these led to the evolution of complexeukaryotic cells, from which allmulticellular organisms are built.[32]
Paleoclimatology, although sometimes treated as part of paleoecology,[29] focuses more on the history of Earth's climate and the mechanisms that have changed it[33] – which have sometimes includedevolutionary developments, for example the rapid expansion of land plants in theDevonian period removed morecarbon dioxide from the atmosphere, reducing thegreenhouse effect and thus helping to cause anice age in theCarboniferous period.[34]
Biostratigraphy, the use of fossils to work out the chronological order in which rocks were formed, is useful to both paleontologists and geologists.[35]Biogeography studies the spatial distribution of organisms, and is also linked to geology, which explains how Earth's geography has changed over time.[36]
This illustration of anIndian elephant jaw (top) and amammoth jaw (bottom) is fromCuvier's 1796 paper on living and fossil elephants.
Although paleontology became established around 1800, earlier thinkers had noticed aspects of thefossil record. The ancient GreekphilosopherXenophanes (570–480 BCE) concluded from fossil sea shells that some areas of land were once under water.[37] During theMiddle Ages the Persian naturalistIbn Sina, known asAvicenna in Europe, discussed fossils and proposed a theory of petrifying fluids on whichAlbert of Saxony elaborated in the 14th century.[37] The Chinese naturalistShen Kuo (1031–1095) proposed a theory of climate change based on the presence ofpetrifiedbamboo in regions that in his time were too dry for bamboo.[38]
Inearly modern Europe, the systematic study of fossils emerged as an integral part of the changes innatural philosophy that occurred during theAge of Reason. In the Italian Renaissance,Leonardo da Vinci made various significant contributions to the field as well as depicted numerous fossils. Leonardo's contributions are central to the history of paleontology because he established a line of continuity between the two main branches of paleontology – ichnology and body fossil paleontology.[39][40][41] He identified the following:[39]
The biogenic nature of ichnofossils, i.e. ichnofossils were structures left by living organisms;
The utility of ichnofossils as paleoenvironmental tools – certain ichnofossils show the marine origin of rock strata;
The importance of the neoichnological approach – recent traces are a key to understanding ichnofossils;
The independence and complementary evidence of ichnofossils and body fossils – ichnofossils are distinct from body fossils, but can be integrated with body fossils to provide paleontological information
Georges Cuvier's 1812 sketch of a skeletal and muscle reconstruction ofAnoplotherium commune. This sketch was amongst the first instances of prehistoric animal reconstructions based on fossil remains.
At the end of the 18th centuryGeorges Cuvier's work establishedcomparative anatomy as a scientific discipline and, by proving that some fossil animals resembled no living ones, demonstrated that animals could becomeextinct, leading to the emergence of paleontology.[42] The expanding knowledge of the fossil record also played an increasing role in the development of geology, particularlystratigraphy.[43] Cuvier proved that the different levels of deposits represented different time periods in the early 19th century. The surface-level deposits in the Americas contained later mammals like the megatheriid ground slothMegatherium and themammutidproboscideanMammut (later known informally as a "mastodon"), which were some of the earliest-named fossil mammal genera with official taxonomic authorities. They today are known to date to theNeogene-Quaternary. In deeper-level deposits in western Europe are early-aged mammals such as thepalaeothereperissodactylPalaeotherium and theanoplotheriidartiodactylAnoplotherium, both of which were described earliest after the former two genera, which today are known to date to thePaleogene period. Cuvier figured out that even older than the two levels of deposits with extinct large mammals is one that contained an extinct "crocodile-like" marine reptile, which eventually came to be known as themosasauridMosasaurus of theCretaceous period.[44]
The first half of the 19th century saw geological and paleontological activity become increasingly well organised with the growth of geologic societies and museums[45][46] and an increasing number of professional geologists and fossil specialists. Interest increased for reasons that were not purely scientific, as geology and paleontology helped industrialists to find and exploit natural resources such as coal.[37]This contributed to a rapid increase in knowledge about the history of life on Earth and to progress in the definition of thegeologic time scale, largely based on fossil evidence. Although she was rarely recognised by the scientific community,[47]Mary Anning was a significant contributor to the field of palaeontology during this period; she uncovered multiple novelMesozoic reptile fossils and deducted that what were then known asbezoar stones are in factfossilised faeces.[48] In 1822Henri Marie Ducrotay de Blainville, editor ofJournal de Physique, coined the word "palaeontology" to refer to the study of ancient living organisms through fossils.[49] As knowledge of life's history continued to improve, it became increasingly obvious that there had been some kind of successive order to the development of life. This encouraged early evolutionary theories on thetransmutation of species.[50]AfterCharles Darwin publishedOrigin of Species in 1859, much of the focus of paleontology shifted to understandingevolutionary paths, includinghuman evolution, and evolutionary theory.[50]
Haikouichthys, from about518 million years ago in China, may be the earliest known fish[51]
The last half of the 19th century saw a tremendous expansion in paleontological activity, especially in North America.[52] The trend continued in the 20th century with additional regions of the Earth being opened to systematic fossil collection. Fossils found in China near the end of the 20th century have been particularly important as they have provided new information about the earliest evolution of animals, early fish, dinosaurs and the evolution of birds.[53] The last few decades of the 20th century saw a renewed interest inmass extinctions and their role in the evolution of life on Earth.[54] There was also a renewed interest in theCambrian explosion that apparently saw the development of the body plans of most animalphyla. The discovery of fossils of theEdiacaran biota and developments inpaleobiology extended knowledge about the history of life back far before the Cambrian.[55]
Fossils of organisms' bodies are usually the most informative type of evidence. The most common types are wood, bones, and shells.[59] Fossilisation is a rare event, and most fossils are destroyed byerosion ormetamorphism before they can be observed. Hence the fossil record is very incomplete, increasingly so further back in time. Despite this, it is often adequate to illustrate the broader patterns of life's history.[60] There are also biases in the fossil record: different environments are more favorable to the preservation of different types of organism or parts of organisms.[61] Further, only the parts of organisms that were alreadymineralised are usually preserved, such as the shells of molluscs. Since most animal species are soft-bodied, they decay before they can become fossilised. As a result, although there are 30-plusphyla of living animals, two-thirds have never been found as fossils.[7]
Occasionally, unusual environments may preserve soft tissues.[62] Theselagerstätten allow paleontologists to examine the internal anatomy of animals that in other sediments are represented only by shells, spines, claws, etc. – if they are preserved at all. However, even lagerstätten present an incomplete picture of life at the time. The majority of organisms living at the time are probably not represented because lagerstätten are restricted to a narrow range of environments, e.g. where soft-bodied organisms can be preserved very quickly by events such as mudslides; and the exceptional events that cause quick burial make it difficult to study the normal environments of the animals.[63] The sparseness of the fossil record means that organisms are expected to exist long before and after they are found in the fossil record – this is known as theSignor–Lipps effect.[64]
Trace fossils consist mainly of tracks and burrows, but also includecoprolites (fossilfeces) and marks left by feeding.[59][65] Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily fossilised hard parts, and they reflect organisms' behaviours. Also many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them.[66] Whilst exact assignment of trace fossils to their makers is generally impossible, traces may for example provide the earliest physical evidence of the appearance of moderately complex animals (comparable toearthworms).[65]
Geochemical observations may help to deduce the global level of biological activity at a certain period, or the affinity of certain fossils. For example, geochemical features of rocks may reveal when life first arose on Earth,[18] and may provide evidence of the presence ofeukaryotic cells, the type from which allmulticellular organisms are built.[67] Analyses ofcarbonisotope ratios may help to explain major transitions such as thePermian–Triassic extinction event.[19]
Simple example cladogram Warm-bloodedness evolved somewhere in the synapsid–mammal transition. ? Warm-bloodedness must also have evolved at one of these points – an example ofconvergent evolution.[7]
Naming groups of organisms in a way that is clear and widely agreed is important, as some disputes in paleontology have been based just on misunderstandings over names.[68]Linnaean taxonomy is commonly used for classifying living organisms, but runs into difficulties when dealing with newly discovered organisms that are significantly different from known ones. For example: it is hard to decide at what level to place a new higher-level grouping, e.g.genus orfamily ororder; this is important since the Linnaean rules for naming groups are tied to their levels, and hence if a group is moved to a different level it must be renamed.[69]
Paleontologists generally use approaches based oncladistics, a technique for working out the evolutionary "family tree" of a set of organisms.[68] It works by the logic that, if groups B and C have more similarities to each other than either has to group A, then B and C are more closely related to each other than either is to A. Characters that are compared may beanatomical, such as the presence of anotochord, ormolecular, by comparing sequences ofDNA orproteins. The result of a successful analysis is a hierarchy of clades – groups that share a common ancestor. Ideally the "family tree" has only two branches leading from each node ("junction"), but sometimes there is too little information to achieve this, and paleontologists have to make do with junctions that have several branches. The cladistic technique is sometimes fallible, as some features, such as wings orcamera eyes, evolved more than once,convergently – this must be taken into account in analyses.[7]
Commonindex fossils used to date rocks in the northeast United States
Paleontology seeks to map out how living things have changed through time. A substantial hurdle to this aim is the difficulty of working out how old fossils are. Beds that preserve fossils typically lack the radioactive elements needed forradiometric dating. This technique is our only means of giving rocks greater than about 50 million years old an absolute age, and can be accurate to within 0.5% or better.[72] Although radiometric dating requires very careful laboratory work, its basic principle is simple: the rates at which various radioactive elementsdecay are known, and so the ratio of the radioactive element to the element into which it decays shows how long ago the radioactive element was incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are a few volcanic ash layers.[72]
Consequently, paleontologists must usually rely onstratigraphy to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is thesedimentary record, and has been compared to ajigsaw puzzle.[73] Rocks normally form relatively horizontal layers, with each layer younger than the one underneath it. If a fossil is found between two layers whose ages are known, the fossil's age must lie between the two known ages.[74] Because rock sequences are not continuous, but may be broken up byfaults or periods oferosion, it is very difficult to match up rock beds that are not directly next to one another. However, fossils of species that survived for a relatively short time can be used to link up isolated rocks: this technique is calledbiostratigraphy. For instance, the conodontEoplacognathus pseudoplanus has a short range in the Middle Ordovician period.[75] If rocks of unknown age are found to have traces ofE. pseudoplanus, they must have a mid-Ordovician age. Suchindex fossils must be distinctive, be globally distributed and have a short time range to be useful. However, misleading results are produced if the index fossils turn out to have longer fossil ranges than first thought.[76] Stratigraphy and biostratigraphy can in general provide only relative dating (A was beforeB), which is often sufficient for studying evolution. However, this is difficult for some time periods, because of the problems involved in matching up rocks of the same age across differentcontinents.[76]
Family-tree relationships may also help to narrow down the date when lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved more than X million years ago.
It is also possible to estimate how long ago two living clades diverged – i.e. approximately how long ago their last common ancestor must have lived – by assuming that DNAmutations accumulate at a constant rate. These "molecular clocks", however, are fallible, and provide only a very approximate timing: for example, they are not sufficiently precise and reliable for estimating when the groups that feature in theCambrian explosion first evolved,[77] and estimates produced by different techniques may vary by a factor of two.[20]
Earth formed about4,570 million years ago and, after a collision that formed theMoon about 40 million years later, may have cooled quickly enough to have oceans and an atmosphere about4,440 million years ago.[79][80] There is evidence on the Moon of aLate Heavy Bombardment by asteroids from4,000 to 3,800 million years ago. If, as seems likely, such a bombardment struck Earth at the same time, the first atmosphere and oceans may have been stripped away.[81]
Paleontology traces the evolutionary history of life back to over3,000 million years ago, possibly as far as3,800 million years ago.[82] The oldest clear evidence of life on Earth dates to3,000 million years ago, although there have been reports, often disputed, offossil bacteria from3,400 million years ago and of geochemical evidence for the presence of life3,800 million years ago.[18][83] Some scientists have proposed that life on Earth was"seeded" from elsewhere,[84][85][86] but most research concentrates on various explanations of how life could havearisen independently on Earth.[87]
For about 2,000 million yearsmicrobial mats, multi-layered colonies of different bacteria, were the dominant life on Earth.[88] The evolution ofoxygenic photosynthesis enabled them to play the major role in theoxygenation of the atmosphere[31] from about2,400 million years ago. This change in the atmosphere increased their effectiveness as nurseries of evolution.[89] Whileeukaryotes, cells with complex internal structures, may have been present earlier, their evolution speeded up when they acquired the ability to transform oxygen from apoison to a powerful source ofmetabolic energy. This innovation may have come from primitive eukaryotes capturing oxygen-powered bacteria asendosymbionts and transforming them intoorganelles calledmitochondria.[82][90] The earliest evidence of complex eukaryotes with organelles (such as mitochondria) dates from1,850 million years ago.[32]Multicellular life is composed only of eukaryotic cells, and the earliest evidence for it is theFrancevillian Group Fossils from2,100 million years ago,[91] although specialisation of cells for different functions first appears between1,430 million years ago (a possible fungus) and1,200 million years ago (a probablered alga).Sexual reproduction may be a prerequisite for specialisation of cells, as an asexual multicellular organism might be at risk of being taken over by rogue cells that retain the ability to reproduce.[92][93]
The earliest known animals arecnidarians from about580 million years ago, but these are so modern-looking that they must be descendants of earlier animals.[94] Early fossils of animals are rare because they had not developedmineralised, easily fossilized hard parts until about548 million years ago.[95] The earliest modern-lookingbilaterian animals appear in the EarlyCambrian, along with several "weird wonders" that bear little obvious resemblance to any modern animals. There is a long-running debate about whether thisCambrian explosion was truly a very rapid period of evolutionary experimentation; alternative views are that modern-looking animals began evolving earlier but fossils of their precursors have not yet been found, or that the "weird wonders" areevolutionary "aunts" and "cousins" of modern groups.[55]Vertebrates remained a minor group until the first jawed fish appeared in the LateOrdovician.[96][97]
At about 13 centimetres (5.1 in) the Early CretaceousYanoconodon was longer than the average mammal of the time.[98]
The spread of animals and plants from water to land required organisms to solve several problems, including protection against drying out and supporting themselves againstgravity.[99][100][101][102] The earliest evidence of land plants and land invertebrates date back to about476 million years ago and490 million years ago respectively.[101][103] Those invertebrates, as indicated by their trace and body fossils, were shown to be arthropods known aseuthycarcinoids.[104] The lineage that produced land vertebrates evolved later but very rapidly between370 million years ago and360 million years ago;[105] recent discoveries have overturned earlier ideas about the history and driving forces behind their evolution.[106] Land plants were so successful that their detritus caused anecological crisis in the LateDevonian, until the evolution of fungi that could digest dead wood.[34]
During thePermian period,synapsids, including the ancestors ofmammals, may have dominated land environments,[108] but this ended with thePermian–Triassic extinction event251 million years ago, which came very close to wiping out all complex life.[109] The extinctions were apparently fairly sudden, at least among vertebrates.[110] During the slow recovery from this catastrophe a previously obscure group,archosaurs, became the most abundant and diverse terrestrial vertebrates. One archosaur group, the dinosaurs, were the dominant land vertebrates for the rest of theMesozoic,[111] and birds evolved from one group of dinosaurs.[107] During this time mammals' ancestors survived only as small, mainly nocturnalinsectivores, which may have accelerated the development of mammalian traits such asendothermy and hair.[112] After theCretaceous–Paleogene extinction event66 million years ago[113] killed off all the dinosaurs except the birds, mammals increased rapidly in size and diversity, and some took to the air and the sea.[114][115][116]
Fossil evidence indicates thatflowering plants appeared and rapidly diversified in the EarlyCretaceous between130 million years ago and90 million years ago.[117] Their rapid rise to dominance of terrestrial ecosystems is thought to have been propelled bycoevolution withpollinating insects.[118]Social insects appeared around the same time and, although they account for only small parts of the insect "family tree", now form over 50% of the total mass of all insects.[119]
Humans evolved from a lineage of upright-walkingapes whose earliest fossils date from over6 million years ago.[120] Although early members of this lineage hadchimp-sized brains, about 25% as big as modern humans', there are signs of a steady increase in brain size after about3 million years ago.[121] There is a long-running debate about whethermodern humans are descendants of asingle small population in Africa, which then migrated all over the world less than 200,000 years ago and replaced previoushominine species, orarose worldwide at the same time as a result ofinterbreeding.[122]
Apparent extinction intensity, i.e. the fraction ofgenera going extinct at any given time, as reconstructed from thefossil record (graph not meant to include recent epoch ofHolocene extinction event)
Life on earth has suffered occasional mass extinctions at least since542 million years ago. Despite their disastrous effects, mass extinctions have sometimes accelerated the evolution of life on earth. When dominance of anecological niche passes from one group of organisms to another, this is rarely because the new dominant group outcompetes the old, but usually because an extinction event allows a new group, which may possess an advantageous trait, to outlive the old and move into its niche.[123][124][125]
The fossil record appears to show that the rate of extinction is slowing down, with both the gaps between mass extinctions becoming longer and the average and background rates of extinction decreasing. However, it is not certain whether the actual rate of extinction has altered, since both of these observations could be explained in several ways:[126]
The oceans may have become more hospitable to life over the last 500 million years and less vulnerable to mass extinctions:dissolved oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk ofeutrophication andanoxic events; marine ecosystems became more diversified so thatfood chains were less likely to be disrupted.[127][128]
Reasonably completefossils are very rare: most extinct organisms are represented only by partial fossils, and complete fossils are rarest in the oldest rocks. So paleontologists have mistakenly assigned parts of the same organism to differentgenera, which were often defined solely to accommodate these finds – the story ofAnomalocaris is an example of this.[129] The risk of this mistake is higher for older fossils because these are often unlike parts of any living organism. Many "superfluous" genera are represented by fragments that are not found again, and these "superfluous" genera are interpreted as becoming extinct very quickly.[126]
"the number of distinct genera alive at any given time; that is, those whose first occurrence predates and whose last occurrence postdates that time"[130]
shows a different trend: a fairly swift rise from542 to 400 million years ago, a slight decline from400 to 200 million years ago, in which the devastatingPermian–Triassic extinction event is an important factor, and a swift rise from200 million years ago to the present.[130]
Paleobotany – Study of organic evolution of plants based on fossils
Paleogenetics – study of the past through the examination of preserved genetic material from the remains of ancient organismsPages displaying wikidata descriptions as a fallback
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^Laudan, R. (1992)."What's so Special about the Past?". In Nitecki, M.H.; Nitecki, D.V. (eds.).History and Evolution. SUNY Press. p. 58.ISBN0-7914-1211-3.[Whewell] distinguished three tasks for such a historical science (1837 [...]): ' the Description of the facts and phenomena; – the general Theory of the causes of change appropriate to the case; – and the Application of the theory to the facts.'
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