View source for Paleontology

{{redirect|Palaeontology|the scientific journal|Palaeontology (journal)}}
[[File:Geological time spiral - sharper.png|thumb|right|400px|Paleontology investigates the whole history of life on Earth]]
[[File:Joda paleontologist.jpg|thumb|Paleontologist at work, [[John Day Fossil Beds National Monument]].]]
[[Image:Europasaurus Praeparation.JPG|thumb|right|Preparation of the fossilized bones of ''[[Europasaurus|Europasaurus holgeri]]'']]

'''Paleontology''' (British: '''palaeontology''')<ref group=derivation>from Greek: ''παλαιός (palaeos)'' "old, ancient", ''ὄν, ὀντ-'' (''on, ont''-) "being, creature", and ''λόγος (logos)'' "speech, thought"</ref> is the study of [[prehistory|prehistoric]] [[life]], including organisms' [[evolution]] and interactions with each other and their environments (their [[paleoecology]]). As a "historical science" it tries to explain causes rather than conduct experiments to observe effects.  Paleontological observations have been documented as far back as the 5th century [[Before Christ|BC]].  The science became established in the 18th century as a result of [[Georges Cuvier]]'s work on  [[comparative anatomy]], and developed rapidly in the 19th century. Fossils found in China since the 1990s have provided new information about the earliest evolution of [[animal]]s, early [[fish]], [[dinosaur]]s and the evolution of birds and [[mammal]]s. Paleontology lies on the border between [[biology]] and [[geology]], and shares with [[archeology]] a border that is difficult to define. It now uses techniques drawn from a wide range of sciences, including [[biochemistry]], [[mathematic]]s and [[engineering]]. As knowledge has increased, paleontology has developed specialized subdivisions, some of which focus on different types of [[fossil]] organisms while others study [[paleoecology|ecological]] and environmental history, such as [[paleoclimatology|ancient climates]].

Body fossils and [[trace fossil]]s are the principal types of evidence about ancient life, and [[geochemical]] evidence has helped to decipher the evolution of life before there were organisms large enough to leave fossils. Estimating the dates of these remains is essential but difficult: sometimes adjacent rock layers allow [[radiometric dating]], which provide absolute dates that are accurate to within 0.5%, but more often paleontologists have to rely on relative dating by solving the "[[jigsaw puzzle]]s" of [[biostratigraphy]]. Classifying ancient organisms is also difficult, as many do not fit well into the [[Linnean taxonomy]] that is commonly used for classifying living organisms, and paleontologists more often use [[cladistics]] to draw up evolutionary "family trees". The final quarter of the 20th century saw the development of [[molecular phylogenetics]], which investigates how closely organisms are related by measuring how similar the [[DNA]] is in their [[genome]]s. 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.

Use of all these techniques has enabled paleontologists to discover much of the [[evolutionary history of life]], almost all the way back to when [[Earth]] became capable of supporting life, about {{ma|3800}}. For about half of that time the only life was [[unicellular|single-celled]] [[micro-organism]]s, mostly in [[microbial mat]]s that formed [[ecosystem]]s only a few millimeters thick. [[Earth's atmosphere]] originally contained virtually no [[oxygen]], and its [[Oxygen#Build-up_in_the_atmosphere|oxygenation]] began about {{ma|2400}}. This may have caused an accelerating increase in the diversity and complexity of life, and early [[multicellular]] [[plant]]s and [[fungi]] have been found in rocks dated from {{ma|1700|1200}}. The earliest multicellular animal fossils are much later, from about {{ma|580}}, but animals diversified very rapidly and there is a lively debate about whether most of this happened in a relatively short [[Cambrian explosion]] or started earlier but has been hidden by lack of fossils. All of these organisms lived in water, but plants and [[invertebrate]]s started colonizing land from about {{ma|490}} and [[vertebrate]]s followed them about {{ma|370}}. The first [[dinosaur]]s appeared about {{ma|230}} and [[bird]]s evolved from one dinosaur group about {{ma|150}}. During the time of the dinosaurs, [[mammal]]s' ancestors survived only as small, mainly nocturnal insectivores, but after the non-avian dinosaurs became extinct in the [[Cretaceous–Tertiary extinction event]] {{ma|65}} mammals diversified rapidly. [[Flowering plant]]s appeared and rapidly diversified between 130 million years ago and 90 million years ago, possibly helped by [[coevolution]] with [[pollination|pollinating]] insects. [[Social insect]]s appeared around the same time and, although they have relatively few species, now form over 50% of the total mass of all insects. Humans evolved from a lineage of upright-walking apes that appeared {{ma|6|7}}, and anatomically modern humans appeared under 200,000&nbsp;years ago. The course of evolution has been changed several times by [[mass extinction]]s that wiped out previously dominant groups and allowed other to rise from obscurity to become major components of ecosystems.

==Definition==
[[Image:Paleontologist chipping.jpg|thumb|200px|right|A paleontologist carefully chips rock from a column of [[dinosaur]] [[vertebrae]].]]
The simplest definition is "the study of ancient life"<ref name="CowenHistLifeEd3Pxi">{{ cite book 
| author=Cowen, R. | title=History of Life | year=2000 | page=xi 
| publisher=Blackwell Science | edition=3rd | isbn=063204444-6 
}}</ref> Paleontology 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".<ref>{{cite journal
| author= Laporte, L.F. | title=What, after All, Is Paleontology? 
| journal=PALAIOS | volume=3 | issue=5 | month= October | year= 1988   	   
| url=http://www.jstor.org/pss/3514718 | accessdate=September 17, 2008 | pages=453
| doi= 10.2307/3514718 
}}</ref>

===A historical science===
Paleontology is one of the historical sciences, along with [[archaeology]], [[geology]], [[biology]],  [[astronomy]], [[cosmogony]], [[philology]] and [[history]] itself.<ref name="Laudan1992WhatSpecialP58">{{ Cite book 
| author=Laudan, R. | chapter=What's so Special about the Past? | page=58
| title=History and Evolution | editor=Nitecki, M.H., and Nitecki, D.V. | date=1992
| publisher=SUNY Press | isbn=0791412113
}}</ref> This means that it aims to describe phenomena of the past and reconstruct their causes.<ref name="Cleland2002MethodologicalAndEpistemicDifferences" /> 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.<ref name="Laudan1992WhatSpecialP58" />

When trying to explain past phenomena, paleontologists and other historical scientists often construct a set of hypotheses about the causes and then look for a "[[smoking gun]]", a piece of evidence which indicates that one of the hypotheses is a better explanation than the others. Sometimes the "smoking gun" is discovered by a fortunate accident during other research, for example the discovery by [[Luis Alvarez]] and [[Walter Alvarez]] of an [[iridium]]-rich layer at the [[Cretaceous]]-[[Tertiary]] boundary made [[asteroid impact]] and [[volcanism]] the most favored explanations for the [[Cretaceous–Tertiary extinction event]].<ref name="Cleland2002MethodologicalAndEpistemicDifferences">{{cite journal 
| author=Cleland, C.E. 
| title=Methodological and Epistemic Differences between Historical Science and Experimental Science
| journal=Philosophy of Science | volume=69 | month=September | year=2002 | pages=474–496 
| url=http://spot.colorado.edu/~cleland/articles/Cleland.PS.Pdf  |format=PDF| accessdate=September 17, 2008 
| doi=10.1086/342453 
}}</ref>

The other main type of science is experimental science, which is often said to work by conducting [[experiment]]s to ''disprove'' hypotheses about the workings and causes of natural phenomena – note that this approach cannot prove a hypothesis is correct, since some later experiment may disprove it. However, when confronted with totally unexpected phenomena, such as the first evidence for invisible [[radiation]], 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".<ref name="Cleland2002MethodologicalAndEpistemicDifferences" />

===Related sciences===
Paleontology lies on the boundary between [[biology]] and [[geology]] since paleontology focuses on the record of past life in [[fossil]]s, its main source of evidence, which are found in rocks.<ref>{{cite book 
| title=McGraw-Hill Encyclopedia of Science & Technology | publisher=McGraw-Hill | year=2002 | page=58 
| isbn=0079136656 
| author=
}}</ref> For historical reasons paleontology is part of the geology departments of many universities, because in the 19th and early 20th centuries geology departments found paleontological evidence important for estimating the ages of rocks while biology departments showed little interest.<ref name="Laudan1992WhatSpecialP57">{{ Cite book 
| author=Laudan, R. | chapter=What's so Special about the Past? | page=57
| title=History and Evolution | editor=Nitecki, M.H., and Nitecki, D.V. | date=1992
| publisher=SUNY Press | isbn=0791412113
}}</ref>

Paleontology also has some overlap with [[archaeology]], which primarily works with objects made by humans and with human remains, while paleontologists are interested in the characteristics and evolution of humans as organisms. When dealing with evidence about humans, archaeologists and paleontologists may work together – for example paleontologists might identify animal or plant fossils around an archaeological site, to discover what the people who lived there ate; or they might analyze the climate at the time when the site was inhabited by humans.<ref name="UCMPfaqAnthro">{{cite web 
| url=http://www.ucmp.berkeley.edu/faq.php#anthro | accessdate=September 17, 2008 
| title=How does paleontology differ from anthropology and archaeology? 
| publisher=University of California Museum of Paleontology 
}}</ref>

[[Image:Palais de la Decouverte Tyrannosaurus rex p1050042.jpg| thumb | right | 175px | Analyses using [[engineering]] techniques show that ''[[Tyrannosaurus]]'' had a devastating bite, but raise doubts about how fast it could move.]]
In addition paleontology often uses techniques derived from other sciences, including biology, [[ecology]], [[chemistry]], [[physics]] and [[mathematics]].<ref name="CowenHistLifeEd3Pxi" /> For example [[geochemical]] signatures from rocks may help to discover when life first arose on Earth,<ref name="BrasierMcLoughlinEtAl2006FreshLook">{{cite journal 
| author=Brasier, M., McLoughlin, N., Green, O., and Wacey, D. | month=June | year=2006 
| title=A fresh look at the fossil evidence for early Archaean cellular life
| journal=Philosophical Transactions of the Royal Society: Biology | volume=361 | issue=1470 
| pages=887–902 | doi=10.1098/rstb.2006.1835
| url=http://physwww.mcmaster.ca/~higgsp/3D03/BrasierArchaeanFossils.pdf | accessdate=August 30, 2008
|format=PDF 
| pmid=16754605
}}</ref> and analyses of [[carbon]] [[isotope analysis|isotope ratios]] may help to identify climate changes and even to explain major transitions such as the [[Permian–Triassic extinction event]].<ref name="Twitchett">{{cite journal
|title=Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis
|author=Twitchett RJ, Looy CV, Morante R, Visscher H, Wignall PB |journal=Geology |year=2001 |volume=29 
|issue=4 |pages=351–354 |doi=10.1130/0091-7613(2001)029<0351:RASCOM>2.0.CO;2
}}</ref> A relatively recent discipline, [[molecular phylogenetics]], often helps by using comparisons of different modern organisms' [[DNA]] and [[RNA]] to re-construct evolutionary "family trees"; 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]]".<ref name="PetersonEtAl2005">{{cite journal
| doi = 10.1073/pnas.0503660102 | pmid = 15983372 | author = Peterson, Kevin J., and Butterfield, N.J.
| journal = Proceedings of the National Academy of Sciences | volume = 102 | issue = 27 | pages = 9547 | year = 2005
| title = Origin of the Eumetazoa: Testing ecological predictions of molecular clocks against the Proterozoic fossil record
}}</ref> 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.<ref name="HutchinsonGarcia2002TrexSlow">{{cite journal | authors=Hutchinson, J. R. and Garcia, M. 
| date=28 February 2002 | title=''Tyrannosaurus'' was not a fast runner | journal=Nature | volume=415 
| pages=1018–1021 | doi=10.1038/4151018a | author = Hutchinson, John R. | url=http://www.nature.com/nature/journal/v415/n6875/full/4151018a.html}}
Summary in press release [http://www.sciencenews.org/articles/20020302/fob1.asp No Olympian: Analysis hints ''T. rex'' ran slowly, if at all]
</ref><ref name="MM03">{{cite journal |last=Meers |first=M.B. |month=August | year=2003 
|url= http://www.ingentaconnect.com/content/tandf/ghbi/2003/00000016/00000001/art00001 
|title=Maximum bite force and prey size of ''Tyrannosaurus rex'' and their relationships to the inference of feeding behavior 
|journal=Historical Biology: A Journal of Paleobiology |volume=16 |issue=1 |pages=1–12 |doi=10.1080/0891296021000050755
}}</ref>

Paleontology even contributes to [[astrobiology]], the investigation of possible life on other [[planet]]s, by developing models of how life may have arisen and by providing techniques for detecting evidence of life.<ref>{{cite journal 
| title=Astrobiology: A New Frontier for 21st Century Paleontologists | author=Cady, S.L.
| journal=PALAIOS | volume=13 | issue=2  | month=April | year=1998 | pages=95–97 
| url=http://www.jstor.org/stable/3515482 | accessdate=September 17, 2008 
| doi=10.2307/3515482 
}}</ref>

===Subdivisions===
As knowledge has increased, paleontology has developed specialised subdivisons.<ref>{{cite journal 
| author=Plotnick, R.E. | title=A Somewhat Fuzzy Snapshot of Employment in Paleontology in the United States 
| journal=Palaeontologia Electronica | publisher=Coquina Press | volume=11 | issue=1 | issn=1094-8074 
| url=http://palaeo-electronica.org/2008_1/commentary/comment.htm | accessdate=September 17, 2008  
}}</ref> [[Vertebrate paleontology]] concentrates on [[fossil]]s of [[vertebrates]], from the earliest [[fish]] to the immediate ancestors of modern [[mammal]]s. [[Invertebrate paleontology]] deals with fossils of [[invertebrate]]s such as [[mollusc]]s, [[arthropod]]s, [[annelid]] worms and [[echinoderm]]s. [[Paleobotany]] focuses on the study of fossil [[Embryophyte|plants]], but traditionally includes the study of fossil [[algae]] and [[fungi]]. [[Palynology]], the study of [[pollen]] and [[spores]] produced by land plants and [[protist]]s, straddles the border between paleontology and [[botany]], as it deals with both living and fossil organisms. [[Micropaleontology]] deals with all microscopic fossil organisms, regardless of the group to which they belong.<ref name="UCMPpaleoSpecialisms">{{cite web 
| url=http://www.ucmp.berkeley.edu/FAQ/whatispaleo.html | accessdate=September 17, 2008  
| title=What is Paleontology? | publisher=University of California Museum of Paleontology 
}}</ref>

[[Image:LateCarboniferousGlobal.jpg|thumb | right | 250px | In the [[Carboniferous]] period, the continents were not in the same places as they are today, and there was extensive [[glaciation]].]]
Instead of focusing on individual organisms, [[paleoecology]] examines the interactions between different organisms, such as their places in [[food chain]]s, and the two-way interaction between organisms and their environment<ref>{{cite journal
| author=Kitchell, J.A. | title=Evolutionary Paleocology: Recent Contributions to Evolutionary Theory 
| journal=Paleobiology | volume=11 | issue=1   | year=1985 | pages=91–104
| url=http://www.ucmp.berkeley.edu/FAQ/whatispaleo.html | accessdate=September 17, 2008  
}}</ref> – for example the development of [[Oxygen#Photosynthesis_and_respiration|oxygenic photosynthesis]] by [[bacteria]] hugely increased the productivity and diversity of [[ecosystem]]s,<ref>{{ cite journal | title=The role of microbial mats in the production of reduced gases on the early Earth 
| author=Hoehler, T.M., Bebout, B.M., and Des Marais, D.J. | journal=Nature | volume=412 
| pages=324–327 | date=19 July 2001 | doi=10.1038/35085554 
| url=http://www.nature.com/nature/journal/v412/n6844/full/412324a0.html | accessdate=July 14, 2008
}}</ref> and also caused the [[Oxygen#Build-up_in_the_atmosphere|oxygenation of the atmosphere]], which in turn was a prerequisite for the evolution of the most complex [[eucaryotic]] cells, from which all [[multicellular]] organisms are built.<ref name="HedgesBlairEtAl2004molecularTimescaleOfEukaryoteEvolution">{{ cite journal 
| title=A molecular timescale of eukaryote evolution and the rise of complex multicellular life 
| author=Hedges, S.B., Blair, J.E, Venturi, M.L., and Shoe, J.L. 
| journal=BMC Evolutionary Biology| volume=4 | issue=2 | doi=10.1186/1471-2148-4-2 | month=January | year=2004 
| url=http://www.biomedcentral.com/1471-2148/4/2/abstract/ | accessdate=July 14, 2008 
| pages=2 
}}</ref> [[Paleoclimatology]], although sometimes treated as part of paleoecology,<ref name="UCMPpaleoSpecialisms" /> focuses more on the history of Earth's climate and the mechanisms which have changed it<ref>{{cite web  
| url=http://matrix.geology.ohio-state.edu/ProspectiveGradStudents/school-of-earth-sciences-directory/specialties/global-and-environmental-change/paleoclimatology
| title=Paleoclimatology | publisher=Ohio State University | accessdate=September 17, 2008 
}}</ref> – which have sometimes included [[evolution]]ary developments, for example the rapid expansion of land plants in the [[Devonian]] period removed more [[carbon dioxide]] from the atmosphere, reducing the [[greenhouse effect]] and thus helping to cause an [[ice age]] in the [[Carboniferous]] period.<ref name="AlgeoScheckler1998errestrialMarineTeleconnectionsInDevonian">{{cite journal 
| author=Algeo, T.J., and Scheckler, S.E. 
| title=Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events
| journal=Philosophical Transactions of the Royal Society: Biology | year=1998 | volume=353 | pages=113–130 
| url=http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1692181&blobtype=pdf | accessdate=September 5, 2008 
| doi=10.1098/rstb.1998.0195 
}}</ref>

[[Biostratigraphy]], the use of fossils to work out the chronological order in which rocks were formed, is useful to both paleontologists and geologists.<ref>{{cite web 
| url=http://evolution.berkeley.edu/evosite/history/biostratigraphy.shtml | accessdate=September 17, 2008 
| title=Biostratigraphy: William Smith 
}}</ref> [[Biogeography]] studies the spatial distribution of organisms, and is also linked to geology, which explains how Earth's geography has changed over time.<ref>{{cite web 
| url=http://evolution.berkeley.edu/evosite/history/biogeography.shtml | accessdate=September 17, 2008  
| title=Biogeography: Wallace and Wegener (1 of 2) 
| publisher=University of California Museum of Paleontology and University of California at Berkeley
}}</ref>
<!-- *********** commented out as unreferenced
Modern paleontology sets [[Prehistoric life|ancient life]] in its context by studying how long-term physical changes of global geography [[Palaeogeography|paleogeography]] and climate [[paleoclimate]] have affected the [[evolution]] of life, how ecosystems have responded to these changes and have adapted the planetary environment in turn and how these mutual responses have affected today's patterns of [[biodiversity]]. Hence, paleontology overlaps with [[geology]] (the study of rocks and rock formations) as well as with [[botany]], [[biology]], [[zoology]] and [[ecology]] – fields concerned with [[Organism|life forms]] and how they interact.

The major subdivisions of paleontology include [[paleozoology]] (animals), [[paleobotany]] (plants) and [[micropaleontology]] (microfossils). Paleozoologists may specialise in [[invertebrate paleontology]], which deals with animals without backbones or in [[vertebrate paleontology]], dealing with fossils of animals with backbones, including fossil hominids ([[paleoanthropology]]). Micropaleontologists study microscopic fossils, including organic-walled microfossils whose study is called [[palynology]].

There are many developing specialties such as [[paleobiology]], [[paleoecology]], [[ichnology]] (the study of tracks and burrows) and [[taphonomy]] (the study of what happens to organisms after they expire). Major areas of study include the correlation of [[rock strata]] with their [[geologic ages]] and the study of evolution of lifeforms.

Paleontology utilises the same classic [[binomial nomenclature]] scheme, devised for the biology of living things by the mid-18th century [[Swedish people|Swedish]] biologist [[Carolus Linnaeus]] and increasingly sets these species in a genealogical framework, showing their degrees of interrelatedness using the still somewhat controversial technique of '[[cladistics]]'.

The primary economic importance of paleontology lies in the use of fossils to determine the age and nature of the rocks that contain them or the layers above or below. This information is vital to the [[mining]] industry and especially the [[petroleum]] industry. Simply looking at the fossils contained in a rock remains one of the fastest and most accurate means of telling how old that rock is.

Fossils were known by primitive humans and were sometimes identified correctly as the remains of ancient lifeforms. The organized study of paleontology dates from the late 18th century. For a more complete historical overview see the article [[History of paleontology]].
***********  -->

==Sources of evidence==
===Body fossils===
[[Image:Marella200x155.png|thumb| This ''[[Marrella]]'' specimen illustrates how clear and detailed the fossils from the [[Burgess Shale]] [[lagerstätte]] are.]]
Fossils of organisms' bodies are usually the most informative type of evidence. The most common types are wood, bones, and shells.<ref name="UCMPWhatIsPaleo">{{cite web 
| url=http://www.ucmp.berkeley.edu/faq.php#paleo | accessdate = September 17, 2008 
| title=What is paleontology? | publisher=University of California Museum of Paleontology 
}}</ref> Fossilisation is a rare event, and most fossils are destroyed by [[erosion]] or [[metamorphism]] 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.<ref name="BentonQualityFossilRecord">{{cite journal |author=Benton MJ, Wills MA, Hitchin R 
|title=Quality of the fossil record through time |journal=Nature |volume=403 |issue=6769 |pages=534–7 
|year=2000 |pmid=10676959 |doi=10.1038/35000558}}
: Non-technical [http://palaeo.gly.bris.ac.uk/cladestrat/news.html summary]
</ref> There are also biases in the fossil record: different environments are more favourable to the preservation of different types of organism or parts of organisms.<ref name="Butterfield2003ExceptionalFossilPreservation">{{cite journal | author = Butterfield, N.J. | year =2003 
| title = Exceptional Fossil Preservation and the Cambrian Explosion | journal = Integrative and Comparative Biology 
| volume = 43 | issue = 1 | pages = 166–177 | doi = 10.1093/icb/43.1.166 
| url=http://icb.oxfordjournals.org/cgi/content/abstract/43/1/166 | accessdate=June 28, 2008
}}</ref> Further, only the parts of organisms that were already [[Mineralization|mineralised]] 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-plus [[phylum|phyla]] of living animals, two-thirds have never been found as fossils.<ref name="CowenHistLifeEd3P61">{{ cite book 
| author=Cowen, R. | title=History of Life | year=2000 | page=61 
| publisher=Blackwell Science | edition=3rd | isbn=063204444-6 
}}</ref> 

Occasionally, unusual environments may preserve soft tissues. These [[lagerstätte]]n allow paleontologists to examine the internal anatomy of animals that in other sediments are only represented 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.<ref>{{cite journal | author = Butterfield, N.J. | year = 2001 | title = Ecology and evolution of Cambrian plankton 
| journal = The Ecology of the Cambrian Radiation |publisher = Columbia University Press |location = New York 
| pages = 200–216 | url = http://66.102.1.104/scholar?hl=en&lr=&ie=UTF-8&q=cache:9xeRu1SdF0QJ:www.earthscape.org/r3/ES14785/ch09.pdf+ | accessdate = September 27, 2007
}}</ref> 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 the [[Signor-Lipps effect]].<ref name="Signor1982">{{cite journal
| author = Signor, P.W. |year= 1982
| title = Sampling bias, gradual extinction patterns and catastrophes in the fossil record
| journal = Geological implications of impacts of large asteroids and comets on the earth
| id = A 84–25651 10–42 | location= Boulder, CO  |publisher = Geological Society of America | pages = 291–296
| url = http://www.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8425675AH | accessdate = January 1, 2008
}}</ref>

===Trace fossils===
[[Image:Cruziana2.jpg|thumb|[[Trace fossil]] of the type called ''Cruziana'', possibly made by a [[trilobite]]. ]]
{{Main|Trace fossil}}

[[Trace fossils]] consist mainly of tracks and burrows, but also include [[coprolite]]s (fossil [[feces]]) and marks left by feeding.<ref name="UCMPWhatIsPaleo" /><ref name="FedonkinGehlingEtAl2007RiseOfAnimals" /> Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily-fossilized hard parts, and which reflects organisms' behaviour. Also many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them.<ref name="Seilacher1994">e.g. {{cite journal
| author = Seilacher, A. | year = 1994 | title = How valid is Cruziana Stratigraphy?
| journal = International Journal of Earth Sciences | volume = 83 | issue = 4 | pages = 752–758
| url = http://www.springerlink.com/index/WP279834395100KH.pdf | accessdate = September 9, 2007
|format=PDF}}</ref> 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 to [[earthworm]]s).<ref name="FedonkinGehlingEtAl2007RiseOfAnimals">{{cite book 
| author=Fedonkin, M.A., Gehling, J.G., Grey, K., Narbonne, G.M., Vickers-Rich, P.
| title=The Rise of Animals: Evolution and Diversification of the Kingdom Animalia
| publisher=JHU Press | date=2007 | isbn=0801886791 | pages=213–216 
| url=http://books.google.co.uk/books?id=OFKG6SmPNuUC&pg=PA213&lpg=PA213&dq=trace+fossil+complex+animal&source=web&ots=gxRn0yEcyh&sig=0GXGIpXruR6rHt27iOCJf08QcTw&hl=en&sa=X&oi=book_result&resnum=2&ct=result#PPA216,M1 
| accessdate=November 14, 2008
}}</ref>

===Geochemical observations===
{{main|Geochemistry}}
Geochemical observations may help to deduce the global level of biological activity, or the affinity of a certain fossil. For example geochemical features of rocks may reveal when life first arose on Earth,<ref name="BrasierMcLoughlinEtAl2006FreshLook" /> and may provide evidence of the presence of [[eucaryotic]] cells, the type from which all [[multicellular]] organisms are built.<ref name="BrocksLoganEtAl1999RiseOfEukaryotes">{{cite journal 
| author=Brocks, J.J., Logan, G.A., Buick, R., and Summons, R.E. | year=1999 
| title=Archaean molecular fossils and the rise of eukaryotes | journal=Science | volume=285 | pages=1033–1036 
| url=http://www.sciencemag.org/cgi/content/abstract/285/5430/1033 | accessdate=September 2, 2008 
| doi=10.1126/science.285.5430.1033 | pmid=10446042 
}}</ref> Analyses of [[carbon]] [[isotope analysis|isotope ratios]] may help to explain major transitions such as the [[Permian–Triassic extinction event]].<ref name="Twitchett" />

==Classifying ancient organisms==
{{main | Biological classification | Cladistics | Phylogenetic nomenclature | Evolutionary taxonomy}}
<div style="width:auto; border:solid 1px silver; padding:2px; margin:2px; float:right; font-size:90%">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
   |label1=[[Tetrapod]]s 
   |1={{clade
         |1=[[Amphibian]]s
         |label2=[[Amniote]]s 
         |2={{clade
               |label1=[[Synapsid]]s
               |1={{clade
                      |1=Extinct Synapsids
                      |label2=<span style="background-color:red">&nbsp;&nbsp;&nbsp;</span>
                      |2=[[Mammal]]s
                   }}
               |label2=[[Reptile]]s
               |2={{clade
                     |1=Extinct reptiles
                     |2=[[Lizard]]s and [[snake]]s
                     |label3=[[Archosaur]]s<br /><span style="background-color:red; color:yellow">&nbsp;'''?'''&nbsp;</span>
                     |3={{clade
                          |1=Extinct<br />Archosaurs
                          |2=[[Crocodilian]]s
                          |label3=[[Dinosaur]]s<br /><span style="background-color:red; color:yellow">&nbsp;'''?'''&nbsp;</span>
                          |3={{clade
                                 |1=Extinct<br />Dinosaurs
                                 |label2=<br /><span style="background-color:red; color:yellow">&nbsp;'''?'''&nbsp;</span>
                                 |2=[[Bird]]s
                              }}
                        }}
                  }}
            }}
       }}
}}
</div>Simple example cladogram.<br /><span style="background-color:red">&nbsp;&nbsp;&nbsp;</span> Warm-bloodedness evolved somewhere in the<br />synapsid-mammal transition.<br /><span style="background-color:red; color:yellow">&nbsp;'''?'''&nbsp;</span> Warm-bloodedness must also have evolved at one of<br /> these points - an example of [[convergent evolution]].<ref name="CowenHistLifeEd3P47To50" /></div>
[[Image:Biological classification L Pengo.svg| thumb | left | 100px | Levels in the [[Linnean taxonomy]].]]
Naming groups of organisms in a way that is clear and widely agreed is important, as some disputes in palaeontology have just been based on misunderstandings over names.<ref name="BrochuSumrall2001">{{cite journal 
| author=Brochu, C.A, and Sumrall, C.D.
| title=Phylogenetic Nomenclature and Paleontology
| journal=Journal of Paleontology | volume=75 | issue=4 | month=July | year=2001 | pages=754–757
| url=http://www.jstor.org/stable/1306999 | accessdate=September 20, 2008 
| doi=10.1666/0022-3360(2001)075<0754:PNAP>2.0.CO;2 
}}</ref> [[Linnean 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]] or [[family (biology)|family]] or [[Order (biology)|order]]; this is important since the Linnean rules for naming groups are tied to their levels, and hence if a group is moved to a different level it has to be renamed.<ref>{{cite book 
| author=Ereshefsky, M.  | page=5
| title=The Poverty of the Linnaean Hierarchy: A Philosophical Study of Biological Taxonomy 
| publisher= Cambridge University Press | year=2001 | isbn=0521781701 
| url=http://books.google.co.uk/books?id=tM6E8-_vSD0C&dq=%22The+Poverty+of+the+Linnaean+Hierarchy:+A+Philosophical+Study+of+Biological+Taxonomy%22&pg=PP1&ots=uNGrsGENBA&sig=vgHTcJgf1qhSEh8Zc5bzGSEoSmA&hl=en&sa=X&oi=book_result&resnum=5&ct=result#PPA5,M1
}}</ref>

Paleontologists generally use approaches based on [[cladistics]], a technique for working out the evolutionary "family tree" of a set of organisms.<ref name="BrochuSumrall2001" /> 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 be [[Anatomy|anatomical]], such as the presence of a [[notochord]], or [[Molecular phylogeny|molecular]], by comparing sequences of [[DNA]] or [[protein]]s. 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 or [[Evolution of the eye|camera eyes]], evolved more than once, [[convergent evolution|convergent]]ly – this must be taken into account in analyses.<ref name="CowenHistLifeEd3P47To50">{{ cite book 
| author=Cowen, R. | title=History of Life | year=2000 | pages=47–50
| publisher=Blackwell Science | edition=3rd | isbn=063204444-6 
}}</ref>

[[Evolutionary developmental biology]], commonly abbreviated to "Evo Devo", also helps paleontologists to produce "family trees". For example the [[embryological]] development of some modern [[brachiopod]]s suggests that brachiopods may be descendants of the [[halkieriid]]s, which became extinct in the [[Cambrian]] period.<ref>{{ cite journal 
| author=Cohen, B. L. and Holmer, L. E. and Luter, C. | year=2003 | title=The brachiopod fold: a neglected body plan hypothesis
| url=http://eprints.gla.ac.uk/2920/01/Cohen_2920.pdf |format=PDF| accessdate=August 7, 2008 | journal=Palaeontology 
| volume=46 | issue=1 | pages=59–65 | doi=10.1111/1475-4983.00287c 
| doi_brokendate=2009-01-01 
}}</ref>
{{-}}

==Estimating the dates of organisms==
{{ Annotated image | caption=Common [[index fossil]]s used to date rocks in North East USA.| image=Index fossils blank 01.png | width=332
| height=261 | image-width=338 | image-left=0 | image-top=0 | float=right | annot-font-size=10 
| annotations = 
<!-- eras -->
{{Annotation|1|15|[[Cenozoic]]}}
{{Annotation|1|70|[[Mesozoic]]}}
{{Annotation|1|165|[[Paleozoic]]}}
{{Annotation|1|252|[[Proterozoic]]}}
<!-- periods -->
{{Annotation|57|0|[[Quaternary|Quater-<br />nary]]}}
{{Annotation|57|25|[[Tertiary]]}}
{{Annotation|57|42|[[Cretaceous|Creta-<br />ceous]]}}
{{Annotation|57|68|[[Jurassic]]}}
{{Annotation|57|89|[[Triassic]]}}
{{Annotation|57|109|[[Permian]]}}
{{Annotation|57|122|[[Mississippian|Missis-<br />sippian]]}}
{{Annotation|57|146|[[Pennsylvanian|Pennsyl-<br / >vanian]]}}
{{Annotation|57|166|[[Devonian|Devo-<br />nian]]}}
{{Annotation|57|191|[[Silurian]]}}
{{Annotation|57|207|[[Ordovician|Ordo-<br />vician]]}}
{{Annotation|57|227|[[Cambrian|Camb-<br />rian]]}}
<!-- fossils: left col -->
{{Annotation|112|2|''[[Pecten gibbus]]''}}
{{Annotation|120|20|''[[Calyptraphorus velatus|Calyptraphorus<br />velatus]]''}}
{{Annotation|125|40|''[[Scaphites hippocrepis|Scaphites<br />hippocrepis]]''}}
{{Annotation|129|63|''[[Perisphinctes tiziani|Perisphinctes<br />tiziani]]''}}
{{Annotation|125|83|''[[Trophites subbullatus|Trophites<br />subbullatus]]''}}
{{Annotation|123|105|''[[Leptodus americanus|Leptodus<br />americanus]]''}}
{{Annotation|110|125|''[[Cactocrinus multibrachiatus|Cactocrinus<br />multibrachiatus]]''}}
{{Annotation|128|143|''[[Dictyoclostus americanus|Dictyoclostus<br />americanus]]''}}
{{Annotation|99|166|''[[Mucrospinifer mucronatus|Mucrospinifer<br />mucronatus]]''}}
{{Annotation|125|186|''[[Cystiphyllum niagarense|Cystiphyllum<br />niagarense]]''}}
{{Annotation|105|211|''[[Bathyurus extans]]''}}
{{Annotation|127|231|''[[Paradoxides pinus]]''}}
<!-- fossils: rightcol -->
{{Annotation|225|6|''[[Neptunea tabulata]]''}}
{{Annotation|257|20|''[[Venericardia planicosta|Venericardia<br />planicosta]]''}}
{{Annotation|249|40|''[[Inoceramus labiatus|Inoceramus<br />labiatus]]''}}
{{Annotation|253|63|''[[Nerinea trinodosa]]''}}
{{Annotation|252|82|''[[Monotis subcircularis|Monotis<br />subcircularis]]''}}
{{Annotation|275|103|''[[Parafusilina bosei|Parafusilina<br />bosei]]''}}
{{Annotation|230|123|''[[Lophophyllidium proliferum|Lophophyllidium<br />proliferum]]''}}
{{Annotation|255|143|''[[Prolecanites gurleyi]]''}}
{{Annotation|240|165|''[[Palmatolepus unicornis|Palmatolepus<br />unicornis]]''}}
{{Annotation|251|185|''[[Hexamocaras hertzeri]]''}}
{{Annotation|250|206|''[[Tetragraptus fructicosus]]''}}
{{Annotation|260|226|''[[Billingsella corrugata]]''}}
}}
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 which preserve fossils typically lack the radioactive elements needed for [[radiometric dating]]. This technique is our only means of giving rocks greater than about 50 million years old an absolute age,<!-- <ref group=note>Astrochronology can date sediments back to the Eocene{{unclear}}</ref> the link for this note appears not to work --> and can be accurate to within 0.5% or better.<ref name="Martin2000">{{cite journal
| author = Martin, M.W. 
| coauthors = Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L.
| year = 2000 | date = May 5, 2000
| title = Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution
| journal = Science | volume = 288
| issue = 5467 | pages = 841 | doi = 10.1126/science.288.5467.841
| accessdate = May 10, 2007 | format = abstract | pmid = 10797002
}}</ref> Although radiometric dating requires very careful laboratory work, its basic principle is simple: the rates at which various radioactive elements [[radioactive decay|decay]] are known, 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 only common in rocks with a volcanic origin, so the only fossil-bearing rocks that can be dated radiometrically are a few volcanic ash layers.<ref name="Martin2000" /> 

Consequently, palaeontologists must usually rely on [[stratigraphy]] to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is the [[sediment]]ary record, and has been compared to a [[jigsaw puzzle]].<ref>{{cite journal 
| author=Pufahl, P.K., Grimm, K.A., Abed, A.M., and Sadaqah, R.M.Y.
| title=Upper Cretaceous (Campanian) phosphorites in Jordan: implications for the formation of a south Tethyan phosphorite giant 
| journal=Sedimentary Geology | volume=161 | issues=3-4 | month=October | year=2003 | pages=175–205 
| doi=10.1016/S0037-0738(03)00070-8 
| url=http://www.geo.edu.ro/~sedim/Cursuri/Petr-sed-Tehn/2003-161-175.pdf |format=PDF| accessdate=October 10, 2008
}}</ref> 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.<ref>{{cite web 
| url=http://pubs.usgs.gov/gip/geotime/radiometric.html | accessdate=September 20, 2008 
| title=Geologic Time: Radiometric Time Scale | publisher=U.S. Geological Survey
}}</ref> Because rock sequences are not continuous, but may be broken up by [[Fault (geology)|faults]] or periods of [[erosion]], 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 called ''biostratigraphy''. For instance, the conodont  ''Eoplacognathus pseudoplanus'' has a short range in the Middle Ordovician period.<ref>{{Cite journal
| author = L&ouml;fgren, A. | year = 2004
| title = The conodont fauna in the Middle Ordovician ''Eoplacognathus pseudoplanus'' Zone of Baltoscandia
| journal = Geological Magazine | volume = 141 | issue = 04 | pages = 505–524
| doi = 10.1017/S0016756804009227 | accessdate = November 17, 2008
| url = http://journals.cambridge.org/production/action/cjoGetFulltext?fulltextid=235488
}}</ref> If rocks of unknown age are found to have traces of  ''E. pseudoplanus'', they must have a mid-Ordovician age. Such [[index fossil]]s must be distinctive, 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.<ref>{{Cite journal
  | last = Gehling | first = James | last2 = Jensen | first2 = Sören | last3 = Droser
  | first3 = Mary | last4 = Myrow | first4 = Paul | last5 = Narbonne | first5 = Guy
  | title = Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland
  | journal = Geological Magazine | volume = 138 | issue = 2 | pages = 213–218
  | month = March | year = 2001 | accessdate = November 17, 2008
  | url = http://www.journals.cambridge.org/action/displayAbstract?fromPage=online&aid=74669
  | doi = 10.1017/S001675680100509X
}}</ref> Stratigraphy and biostratigraphy can in general provide only relative dating (''A'' was before ''B''), 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 different [[continent]]s.<ref name="Gehling2001">e.g. {{Cite journal
| last = Gehling | first = James | last2 = Jensen | first2 = Sören | last3 = Droser | first3 = Mary
| last4 = Myrow | first4 = Paul | last5 = Narbonne | first5 = Guy
| title = Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland
| journal = Geological Magazine | volume = 138 | issue = 2 | pages = 213–218 | month = March | year = 2001
| url = http://www.journals.cambridge.org/action/displayAbstract?fromPage=online&aid=74669
| doi = 10.1017/S001675680100509X | accessdate = November 17, 2008
}}</ref>

Family tree relationships may also help to narrow down the date on which lineages first appeared. For instance, if fossils of B or C date to X&nbsp;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&nbsp;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 DNA [[mutation]]s accumulate at a constant rate. These  "[[molecular clock]]s", 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 the [[Cambrian explosion]] first evolved,<ref>{{ cite journal 
| author=Hug, L.A., and Roger, A.J. 
| title=The Impact of Fossils and Taxon Sampling on Ancient Molecular Dating Analyses 
| journal=Molecular Biology and Evolution | year=2007 | volume=24 | issue=8 | pages=889–1897 
| doi=10.1093/molbev/msm115 | pmid=17556757 
}}</ref> and estimates produced by different techniques may vary by a factor of two.<ref name="PetersonEtAl2005">{{cite journal
| doi = 10.1073/pnas.0503660102 | pmid = 15983372 | author = Peterson, Kevin J., and Butterfield, N.J.
| journal = Proceedings of the National Academy of Sciences | volume = 102 | issue = 27
| pages = 9547 | year = 2005
| title = Origin of the Eumetazoa: Testing ecological predictions of molecular clocks against the Proterozoic fossil record
}}</ref>

==Overview of the history of life==
{{Main|Evolutionary history of life}}
The evolutionary history of life stretches back to over {{ma|3000|}}, possibly as far as {{ma|3800}}. [[Earth]] formed about {{ma|4540}} and, after a collision that formed the [[Moon]] about 40 million years later, may have cooled quickly enough to have oceans and an atmosphere about {{ma|4440}}.<ref>* {{cite web | title = Early Earth Likely Had Continents And Was Habitable | date = [[2005-11-17]] | url = http://www.colorado.edu/news/releases/2005/438.html }}<br>* {{cite journal
| last = Cavosie | first = A. J. | coauthors = J. W. Valley, S. A., Wilde, and E.I.M.F. | date = July 15, 2005
| title = Magmatic δ<sup>18</sup>O in 4400-3900 Ma detrital zircons: A record of the alteration and recycling of crust in the Early Archean
| journal = Earth and Planetary Science Letters | volume = 235 | issue = 3-4 | pages = 663–681
| doi = 10.1016/j.epsl.2005.04.028
| url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-4GDKB05-3&_coverDate=07%2F15%2F2005&_alid=382434001&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=5801&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=be47e49c535d059be188b66c6e596dd5
}}</ref> However there is evidence on the Moon of a [[Late Heavy Bombardment]] from {{ma|4000|3800}}. If, as seem likely, such a bombardment struck Earth at the same time, the first atmosphere and oceans may have been stripped away.<ref>{{cite journal 
| author=Dauphas, N., Robert, F., and Marty, B. | month=December | year=2000 
| title=The Late Asteroidal and Cometary Bombardment of Earth as Recorded in Water Deuterium to Protium Ratio 
| journal=Icarus | issue=2 | pages=508–512 | doi=10.1006/icar.2000.6489 | volume=148 
}}</ref> The oldest undisputed evidence of life on Earth dates to {{ma|3000}}, although there have been reports, often disputed, of [[fossil]] [[bacteria]] from {{ma|3400}} and of geochemical evidence for the presence of life {{ma|3800}}.<ref name="BrasierMcLoughlinEtAl2006FreshLook"/><ref>{{cite journal |author=Schopf, J. |title=Fossil evidence of Archaean life |journal=Philos Trans R Soc Lond B Biol Sci |volume=361 |issue=1470 |pages=869&ndash;85 |year=2006 |pmid=16754604 |doi=10.1098/rstb.2006.1834
}}</ref> Even the simplest modern organisms are too complex to have emerged directly from non-living materials.<ref name="CowenHistLifeEd3P6">{{cite book 
| author=Cowen, R. | title=History of Life | edition=3rd | publisher=Blackwell Science 
| isbn=0632044446 | page=6 
| year=2000}}</ref> Some scientists have proposed that life on Earth was [[panspermia|"seeded" from elsewhere]],<ref>* {{cite journal 
| author=Arrhenius, S. | title=The Propagation of Life in Space | year=1903 
| journal=Die Umschau volume=7 }} Reprinted in {{ cite book 
| editor=Goldsmith, D., | title=The Quest for Extraterrestrial Life 
| publisher=University Science Books | isbn=0198557043 
| year= 
}}<br>* {{cite journal 
| author=Hoyle, F., and Wickramasinghe, C. | year=1979 
| title=On the Nature of Interstellar Grains
| journal=Astrophysics and Space Science | volume=66 | pages=77–90 
| doi=10.1007/BF00648361 
}}<br>* {{Cite journal 
| last = Crick | first =  F. H. | last2 = Orgel | first2 = L. E.| title = Directed Panspermia 
| journal = Icarus | volume = 19 | pages =  341–348 | year = 1973| doi = 10.1016/0019-1035(73)90110-3}}</ref> but most research concentrates on various explanations of how life could have [[Abiogenesis|arisen independently]] on Earth.<ref>{{cite journal |author=Peretó, J. |title=Controversies on the origin of life |url=http://www.im.microbios.org/0801/0801023.pdf |format=PDF |journal=Int. Microbiol. |volume=8 |issue=1 |pages=23&ndash;31 |year=2005 |pmid=15906258 |accessdate=October 7, 2007}}</ref>

[[Image:Runzelmarken.jpg|thumb|250px|This wrinkled "elephant skin" texture is a [[trace fossil]] of a non-[[stromatolite]] [[microbial mat]].<br />The image shows the location, in the [[Burgsvik beds]] of [[Sweden]], where the texture was first identified as evidence of a microbial mat.<ref name="Manten1966">
{{cite journal
| author = Manten, A.A. | year = 1966 | title = Some problematic shallow-marine structures
| journal = Marine Geol | volume = 4 | pages = 227–232 | doi = 10.1016/0025-3227(66)90023-5
| url = http://igitur-archive.library.uu.nl/geo/2006-1213-210452/UUindex.html | accessdate = June 18, 2007
}}
</ref>]]For about 2,000 million years [[microbial mat]]s, multi-layered colonies of different types of bacteria, were the dominant life on Earth.<ref name="KrumbeinBrehmEtAl2003BiofilmBiodictyonBiomatEtc">{{ Cite book 
| contribution=Biofilm, Biodictyon, Biomat Microbialites, Oolites, Stromatolites, Geophysiology, Global Mechanism, Parahistology | author=Krumbein, W.E., Brehm, U., Gerdes, G., Gorbushina, A.A., Levit, G. and Palinska, K.A. | pages=1–28 | title=Fossil and Recent Biofilms: A Natural History of Life on Earth | editor=Krumbein, W.E., Paterson, D.M., and Zavarzin, G.A. | date=2003 | publisher=Kluwer Academic| isbn=1402015976 | url=http://134.106.242.33/krumbein/htdocs/Archive/397/Krumbein_397.pdf | accessdate=July 9, 2008
}}</ref> The evolution of [[Oxygen#Photosynthesis_and_respiration|oxygenic photosynthesis]] enabled them to play the major role in the [[Oxygen#Build-up_in_the_atmosphere|oxygenation of the atmosphere]].<ref name="HoehlerBeboutMarais2001MicrobialMatsInProductionOfReducedGasesOnEarlyEarth">{{cite journal
| title=The role of microbial mats in the production of reduced gases on the early Earth | author=Hoehler, T.M., Bebout, B.M., and Des Marais, D.J. | journal=Nature | volume=412 | pages=324–327 | date=July 19, 2001 | doi=10.1038/35085554 | url=http://www.nature.com/nature/journal/v412/n6844/full/412324a0.html | accessdate=July 14, 2008
}}</ref> from about {{ma|2400}}. This change in the atmosphere increased their effectiveness as nurseries of evolution.<ref name="NisbetFowler1999ArchaeanMetabolicEvolution">{{ cite journal
 | title=Archaean metabolic evolution of microbial mats | author=Nisbet, E.G., and Fowler, C.M.R.
 | journal=Proceedings of the Royal Society: Biology | date=December 7, 1999
 | volume=266 | issue=1436 | pages=2375 | doi=10.1098/rspb.1999.0934 |format=PDF
 | url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1690475 | accessdate=July 16, 2008}}
</ref> While [[eukaryote]]s, cells with complex internal structures, may have been present earlier, their evolution speeded up when they acquired the ability to transform oxygen from a [[poison]] to a powerful source of energy in their [[metabolism]]. This innovation may have come from primitive eukaryotes capturing oxygen-powered bacteria as [[endosymbiont]]s and transforming them into [[organelle]]s called [[mitochondrion|mitochondria]].<ref>{{cite journal|author=Gray MW, Burger G, Lang BF |title=Mitochondrial evolution |journal=Science (journal) |volume=283 |issue=5407 |pages=1476–81 |year=1999 |month=March |pmid=10066161}}</ref> The earliest evidence of complex eukaryotes with organelles such as mitochondria, dates from {{ma|1850}}.<ref name="HedgesBlairEtAl2004molecularTimescaleOfEukaryoteEvolution">{{ cite journal 
| title=A molecular timescale of eukaryote evolution and the rise of complex multicellular life 
| author=Hedges, S.B., Blair, J.E, Venturi, M.L., and Shoe, J.L. 
| journal=BMC Evolutionary Biology| volume=4 | issue=2 | doi=10.1186/1471-2148-4-2 | month=January | year=2004 
| url=http://www.biomedcentral.com/1471-2148/4/2/abstract/ | accessdate=July 14, 2008 | pages=2 
}}</ref>

[[Multicellular]] life is composed only of eukaryotic cells, and the earliest evidence for it is from {{ma|1700}}, although specialization of cells for different functions first appears between {{ma|1430}} (a possible [[fungus]]) and {{ma|1200}} (a probable [[red alga]]). [[Sexual reproduction]] may be a prerequisite for specialization of cells, as an asexual multicellular organism might be at risk of being taken over by rogue cells that retain the ability to reproduce.<ref name="Butterfield2000Bangiomorpha">{{cite journal
| author=Butterfield, N.J. | month=September | year=2000 
| title=''Bangiomorpha pubescens'' n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes
| journal=Paleobiology | volume=26 | issue=3 | pages=386–404 
| doi=10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2
| url=http://paleobiol.geoscienceworld.org/cgi/content/abstract/26/3/386 | accessdate=2008-09-02 
}}</ref><ref name="Butterfield2005ProterozoicFungi">{{cite journal
| author = Butterfield, N.J. | year = 2005 | title = Probable Proterozoic fungi
| journal = Paleobiology | volume = 31 | issue = 1 | pages = 165–182
| doi =10.1666/0094-8373(2005)031<0165:PPF>2.0.CO;2 
| url=http://paleobiol.geoscienceworld.org/cgi/content/abstract/31/1/165 | accessdate=2008-09-02 
}}</ref>

[[Image:Opabinia BW2.jpg| thumb | left| 150px | ''[[Opabinia]]'' made the largest single contribution to modern interest in the [[Cambrian explosion]] ]]
The earliest known [[animal]]s are [[cnidaria]]ns from about {{ma|580}}, but these are so modern-looking that the earliest animals must have appeared before then.<ref>{{cite journal 
| author=Chen, J.-Y., Oliveri, P., Gao, F., Dornbos, S.Q., Li, C-W., Bottjer, D.J. and Davidson, E.H. 
| title=Precambrian Animal Life: Probable Developmental and Adult Cnidarian Forms from Southwest China 
| journal=Developmental Biology | volume=248 | issue=1 | month=August | year=2002 
| doi=10.1006/dbio.2002.0714 |format=PDF | pages=182–196
| url=http://www.uwm.edu/~sdornbos/PDF's/Chen%20et%20al.%202002.pdf | accessdate=September 3, 2008 
}}</ref> Early fossils of animals are rare because they did not develop [[biomineralization|mineralized]] hard parts that fossilize easily until about {{ma|548}}.<ref name="Bengtson2004">{{Cite journal | contribution=Early skeletal fossils
| author = Bengtson, S. | editor=Lipps, J.H., and Waggoner, B.M. 
| title=Neoproterozoic - Cambrian Biological Revolutions
| year = 2004 | journal = Palentological Society Papers | volume = 10 | pages = 67–78
| url = http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf 
| accessdate=July 18, 2008
}}</ref> The earliest modern-looking [[bilateria]]n animals appear in the Early [[Cambrian]], along with several "weird wonders" that bear little obvious resemblance to any modern animals. There is a long-running debate about whether this [[Cambrian 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" are [[stem group|evolutionary "aunts" and "cousins"]] of modern groups.<ref name="Marshall2006Explaining">{{cite journal
| author=Marshall, C.R. | title=Explaining the Cambrian “Explosion” of Animals
| journal=Annu. Rev. Earth Planet. Sci. | year=2006 | volume=34 | pages=355–384
| doi=10.1146/annurev.earth.33.031504.103001
| url=http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.earth.33.031504.103001?journalCode=earth
| accessdate=November 6, 2007
}}</ref> [[Vertebrates]] remained an obscure group until the first fish with jaws appeared in the Late [[Ordovician]].<ref>{{ cite journal 
| author=Conway Morris, S. | title=Once we were worms | date=August 2, 2003 
| journal=New Scientist | volume=179 | issue=2406 
| url=http://cas.bellarmine.edu/tietjen/Evolution/once_we_were_worms.htm | accessdate=September 5, 2008  | pages=34 
}}</ref><ref>{{Cite book 
| author=Sansom I.J., Smith, M.M., and Smith, M.P. | date=2001 
| contribution=The Ordovician radiation of vertebrates | pages=156–171
| editor=Ahlberg, P.E.  |title=Major Events in Early Vertebrate Evolution 
| publisher=Taylor and Francis | isbn=0-415-23370-4 
}}</ref>

The spread of life from water to land required organisms to solve several problems, including protection against drying out and supporting themselves against [[gravity]].<ref name="Selden2001TerrestrializationOfAnimals">{{Cite book 
| contribution="Terrestrialization of Animals" | author=Selden, P.A. | pages=71–74
| title=Palaeobiology II: A Synthesis | editor=Briggs, D.E.G., and Crowther, P.R. | date=2001 
| publisher=Blackwell | isbn=0632051493  | accessdate=September 5, 2008 
| url=http://books.google.co.uk/books?id=AHsrhGOTRM4C&pg=PA71&lpg=PA71&dq=%22Terrestrialization+of+Animals%22+selden&source=web&ots=ImrDW71qDp&sig=JptdVx34SMIjKamHXDnEpKSE78s&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA74,M1
}}</ref><ref name="KenrickCrane1997EvolutionOfLandPlants">{{cite journal 
| author=Kenrick, P., and Crane, P.R. | title=The origin and early evolution of plants on land 
| journal=Nature | volume=389 | month=September | year=1997 
| url=http://botit.botany.wisc.edu/courses/botany_940/06EvidEvol/papers/KendrickCrane1997.pdf 
| accessdate=September 5, 2008  |format=PDF | doi=10.1038/37918 | pages=33
}}</ref> The earliest evidence of land plants and land invertebrates date back to about {{ma|476}} and {{ma|490}} respectively.<ref name="KenrickCrane1997EvolutionOfLandPlants" /><ref>{{cite journal 
| author=MacNaughton, R.B., Cole, J.M., Dalrymple, R.W., Braddy, S.J., Briggs, D.E.G., and Lukie, T.D.
| title=First steps on land: Arthropod trackways in Cambrian-Ordovician eolian sandstone, southeastern Ontario, Canada
| journal=Geology | month=May | year=2002 | volume=30| issue=5 | pages=391–394 
| doi=10.1130/0091-7613(2002)030<0391:FSOLAT>2.0.CO;2 
| url=http://geology.geoscienceworld.org/cgi/content/abstract/30/5/391 | accessdate=2008-09-05 
}}</ref> The lineage that produced land vertebrates evolved later but very rapidly between {{ma|370}} and {{ma|360}};<ref>{{cite journal 
| author=Gordon, M.S, Graham, J.B., and Wang, T. | title=Revisiting the Vertebrate Invasion of the Land 
| journal=Physiological and Biochemical Zoology | volume=77 | issue=5 
| date= September/October 2004 | doi=10.1086/425182 | pages=697–699 
}}</ref> recent discoveries have overturned earlier ideas about the history and driving forces behind their evolution.<ref name="Clack2005GettingLegUp">{{cite newspaper
| author=[[Jennifer A. Clack|Clack, J.A.]] | title=Getting a Leg Up on Land 
| newspaper=Scientific American | date=November 2005 | url=http://www.sciam.com/article.cfm?id=getting-a-leg-up-on-land | accessdate=September 6, 2008 
}}</ref>  Land plants were so successful that they caused an [[ecological crisis]] in the Late [[Devonian]], until the evolution and spread of fungi that could digest dead wood.<ref name="AlgeoScheckler1998errestrialMarineTeleconnectionsInDevonian"/>

{| align="right" border="0" cellspacing="0" cellpadding="3"
|- valign="top"
|[[Image:Yanoconodon BW.jpg| thumb| 150px | The Early Cretaceous [[Yanoconodon]] was only about {{convert|13|cm|in}} long, but was longer than the average mammal of its time.<ref name="">{{cite journal 
| author=Luo, Z., Chen, P., Li, G., & Chen, M. | date=March 2007 
| title=A new eutriconodont mammal and evolutionary development in early mammals 
| journal=[[Nature (journal)|Nature]] | volume=446 | pages=288–293 | doi=10.1038/nature05627
}}</ref> ]]
|[[Image:House sparrow04.jpg|thumb|right|[[Bird]]s are the last surviving [[dinosaur]]s.<ref name="padian2004" />]]
|}During the [[Permian]] period [[synapsid]]s, including the ancestors of [[mammal]]s, may have dominated land environments,<ref>{{cite journal 
| author=Sidor, C.A., O'Keefe, F.R., Damiani, R., Steyer, J.S., Smith, R.M.H., Larsson, H.C.E., Sereno, P.C., Ide, O, and Maga, A. 
| title=Permian tetrapods from the Sahara show climate-controlled endemism in Pangaea | month=April | year=2005 
| journal=Nature | volume=434 | pages=886–889 | doi=10.1038/nature03393 
| url=http://www.nature.com/nature/journal/v434/n7035/full/nature03393.html | accessdate=September 8, 2008 
}}</ref> but the [[Permian–Triassic extinction event]] {{ma|251}} came very close to wiping out complex life.<ref>{{cite book 
| author=Benton M.J. | year=2005 | title=When Life Nearly Died: The Greatest Mass Extinction of All Time 
|publisher=Thames & Hudson | isbn=978-0500285732 
}}</ref> During the slow recovery from this catastrophe a previously obscure group, [[archosaur]]s, became the most abundant and diverse terrestrial vertebrates. One archosaur group, the [[dinosaur]]s, were the dominant land vertebrates for the rest of the [[Mesozoic]],<ref>{{Cite journal
| author=Benton, M.J. | date=March 1983
| title=Dinosaur Success in the Triassic: a Noncompetitive Ecological Model 
| journal=Quarterly Review of Biology | volume=58 | issue=1 
| url=http://palaeo.gly.bris.ac.uk/Benton/reprints/1983success.pdf | accessdate=September 8, 2008  
}}</ref> and [[bird]]s evolved from one group of dinosaurs.<ref name="padian2004">{{cite book |last=Padian |first=Kevin. |year=2004 |chapter=Basal Avialae |editor=[[David B. Weishampel|Weishampel, David B.]]; [[Peter Dodson|Dodson, Peter]]; & Osmólska, Halszka (eds.) |title=The Dinosauria |edition=Second |location=Berkeley |publisher=University of California Press |pages=210–231 |isbn=0-520-24209-2}}</ref> During this time mammals' ancestors survived only as small, mainly nocturnal [[insectivore]]s, but this apparent set-back may have accelerated the development of mammalian traits such as [[endothermy]] and [[hair]].<ref name="RubenJones2000FurAndFeathers">{{ cite journal | author=Ruben, J.A., and Jones, T.D. | title=Selective Factors Associated with the Origin of Fur and Feathers | journal=American Zoologist | year=2000 | volume=40 | issue=4 | pages=585–596 |doi=10.1093/icb/40.4.585 | url=http://icb.oxfordjournals.org/cgi/content/full/40/4/585}}</ref> After the [[Cretaceous–Tertiary extinction event]] {{ma|65}} killed off the non-avian dinosaurs – birds are the only surviving dinosaurs – mammals increased rapidly in size and diversity, and some took to the air and the sea.<ref>{{cite journal 
| author=Alroy J. | month=March | year=1999 
| title=The fossil record of North American mammals: evidence for a Paleocene evolutionary radiation
| journal=Systematic biology | volume=48 | issue=1 | pages=107–18 | pmid=12078635 
| doi=10.1080/106351599260472 
}}</ref><ref>{{cite journal 
| author=Simmons, N.B., Seymour,K.L., Habersetzer, J.,and Gunnell, G.F. | month=February | year=2008 
| title=Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation 
| journal=Nature | volume=451 | pages=818–821 | doi=10.1038/nature06549
}}</ref><ref>{{cite journal|author = J. G. M. Thewissen, S. I. Madar, and S. T. Hussain|title = ''Ambulocetus natans'', an Eocene cetacean (Mammalia) from Pakistan|journal = Courier Forschungsinstitut Senckenberg|volume = 191|pages = 1–86|year = 1996 
}}</ref>

[[Image:bee pollenating a rose.jpg|thumb|200px|left|A modern [[social insect]] collects pollen from a modern [[flowering plant]].<!--An ''[[Andrena]]'' bee collects pollen among the [[stamens]] of a [[rose]]. The female [[carpel]] structure of the rose appears rough and globular to the left. The bee's stash of pollen is on its hind leg.-->]]
Fossil evidence indicates that [[flowering plant]]s appeared and rapidly diversified in the Early [[Cretaceous]], between {{ma|130}} and {{ma|90}}.<ref name="CraneFriisPedersen2000OriginAndDiversificationOfAngiosperms">{{Cite book 
| author=Crane, P.R., Friis, E.M., and Pedersen, K.R. 
| chapter=The Origin and Early Diversification of Angiosperms | pages=233–250
| title=Shaking the Tree: Readings from Nature in the History of Life | editor=Gee, H. 
| publisher=University of Chicago Press | date=2000 | isbn=0226284964
| url=http://books.google.co.uk/books?hl=en&lr=&id=ZJe_Dmdbm-QC&oi=fnd&pg=PA233&dq=evolution+flowering+plant+angiosperm&ots=abVpqx_cP8&sig=z1HvmrRLdJP9oPkN0bffbAyriEI#PPA233,M1 
| accessdate=September 9, 2008 
}}</ref> Their rapid rise to dominance of terrestrial ecosystems is thought to have been propelled by [[coevolution]] with [[pollenation|pollinating]] insects.<ref name="Crepet2000">{{cite journal 
| author=Crepet, W.L. | month=November | year=2000                                                             
| title=Progress in understanding angiosperm history, success, and relationships: Darwin’s abominably "perplexing phenomenon"     
| journal=Proceedings of the National Academy of Sciences | volume=97  | issue=24 | pages= 12939–12941 
| url=http://www.pnas.org/content/97/24/12939.full.pdf+html | accessdate=September 9 , 2008  
| doi=10.1073/pnas.97.24.12939 
| pmid=11087846
}}</ref> [[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.<ref name="WilsonHölldobler2005EusocialityOriginConsequences">{{cite journal 
| author=Wilson, E.O., and Hölldobler, B. | title=Eusociality: Origin and consequences
| journal=Proceedigs of the National Acedemy of Sciences | month=September | year=2005 | volume=102 | issue=38 
| pages=13367–13371 | doi=10.1073/pnas.0505858102 
| url=http://www.pnas.org/content/102/38/13367.full.pdf+html | accessdate=September 7, 2008  
| pmid=16157878 
}}</ref>

Humans evolved from a lineage of upright-walking [[ape]]s whose earliest fossils date from over {{ma|6}}.<ref>{{cite journal
| author=Brunet M., Guy, F., Pilbeam, D., Mackaye, H.T., ''et al.''
| title=A new hominid from the Upper Miocene of Chad, Central Africa
| journal=Nature  | volume=418 | pages=145–151 | month=July | year=2002 | doi=10.1038/nature00879
| url=http://www.nature.com/nature/journal/v418/n6894/abs/nature00879.html | accessdate=September 9, 2008  
}}</ref> Although early members of this lineage had [[chimp]]-sized [[brain]]s, about 25% as big as modern humans', there are signs of a steady increase in brain size after about {{ma|3}}.<ref>{{cite journal 
| author=De Miguel, C., and M. Henneberg, M. 
| title=Variation in hominid brain size: How much is due to method? 
| journal=HOMO - Journal of Comparative Human Biology | issue=1 | year=2001    
| doi=10.1078/0018-442X-00019 | accessdate=September 9, 2008 | volume=52 | pages=3–58 
| url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B7GW4-4DPCHXC-2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_version=1&_urlVersion=0&_userid=10&md5=aef79dbca1f189c885cfe9f36636b131 
}}</ref> There is a long-running debate about whether ''modern'' humans are descendants of a [[Recent African origin of modern humans|single small population in Africa]], which then migrated all over the world less than 200,000 years ago and replaced previous [[hominine]] species, or [[Multiregional origin of modern humans|arose worldwide at the same time]] as a result of [[interbreeding]].<ref>{{cite book|last=Leakey|first=Richard|title=The Origin of Humankind|publisher=Basic Books|location=New York, NY|year=1994|series=Science Masters Series|pages=87–89|isbn=0465053130
}}</ref>
{{clear}}

==Mass extinctions==
{{annotated image/Extinction|caption=Apparent extinction intensity, i.e. the fraction of [[genus|genera]] going extinct at any given time, as reconstructed from the [[fossil record]].  (Graph not meant to include recent epoch of [[Holocene extinction event]])}}
{{main|Mass extinction}}
Life on earth has suffered occasional mass extinctions at least since {{ma|542}}. Although they are disasters at the time, mass extinctions have sometimes accelerated the evolution of [[life on earth]]. When dominance of particular [[ecological niche]]s passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one.<ref>{{cite book | last=Benton | first =M.J. | title=Vertebrate Palaeontology | publisher=Blackwell | year=2004 | chapter=6. Reptiles Of The Triassic | url=http://www.blackwellpublishing.com/book.asp?ref=0632056371 | accessdate= November 17, 2008
}}</ref><ref>{{cite journal| last=Van Valkenburgh | first= B.| year=1999| url=http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.earth.27.1.463 | title=Major patterns in the history of xarnivorous mammals | journal=Annual Review of Earth and Planetary Sciences| volume= 26| pages= 463–493 | doi= 10.1146/annurev.earth.27.1.463
}}</ref>

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 if the actual rate of extinction has altered, since both of these observations could be explained in several ways:<ref name="MacLeod2001Extinction">{{cite web 
| date=2001-01-06| first=Norman| last= MacLeod | title=Extinction! 
| url=http://www.firstscience.com/home/articles/earth/extinction-page-2-1_1258.html  | accessdate=September 11, 2008
}}</ref>
*The oceans may have become more hospitable to life over the last 500&nbsp;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 of [[eutrophication]] and [[anoxic event]]s; and marine ecosystems became more diversified so that [[food chain]]s were less likely to be disrupted.<ref>{{cite journal| last=Martin| first= R.E.| year=1995| title=Cyclic and secular variation in microfossil biomineralization: clues to the biogeochemical evolution of Phanerozoic oceans| journal=Global and Planetary Change| volume =11| issue= 1| doi=10.1016/0921-8181(94)00011-2| pages=1
}}</ref><ref>{{cite journal| last=Martin| first= R.E.| year=1996| title=Secular increase in nutrient levels through the Phanerozoic: Implications for productivity, biomass, and diversity of the marine biosphere| journal=PALAIOS| volume= 11| pages=209–219| doi=10.2307/3515230
}}</ref>

*Reasonably complete [[fossil]]s 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 different [[genus|genera]] which were often defined solely to accommodate these finds – the story of ''[[Anomalocaris]]'' is an example of this.<ref name="WonderfulLifeP194To206">{{Cite book
| title=Wonderful Life: The Burgess Shale and the Nature of History | pages=194–206
| author=Gould, S.J. | authorlink=Stephen Jay Gould
| year=1990 | publisher=Hutchinson Radius | isbn=0091742714 
}}</ref> The risk of this mistake is higher for older fossils because these are often unlike parts of any living organism. Many of the "superfluous" genera are represented by fragments which are not found again and the "superfluous" genera appear to become extinct very quickly.<ref name="MacLeod2001Extinction" />
{{clear}}
{{Phanerozoic biodiversity}}
[[Biodiversity]] in the fossil record, which is
::"the number of distinct genera alive at any given time; that is, those whose first occurrence predates and whose last occurrence postdates that time"<ref name="RohdeMuller2005" />
shows a different trend: a fairly swift rise from {{ma|542|400}}; a slight decline from {{ma|400|200}}, in which the devastating [[Permian–Triassic extinction event]] is an important factor; and a swift rise from {{ma|200}} to the present.<ref name="RohdeMuller2005">{{cite journal 
| author=Rohde, R.A., and Muller, R.A. | title=Cycles in fossil diversity | month=March | year=2005 
| journal=Nature | volume=434 | pages=208–210 
| url=http://muller.lbl.gov/papers/Rohde-Muller-Nature.pdf |format=PDF| accessdate=September 22, 2008 
| doi=10.1038/nature03339 
}}</ref>

[[Image:Cuvier elephant jaw.jpg|thumb|left|This illustration of an [[Indian elephant]] jaw and a [[mammoth]] jaw is from [[Georges Cuvier|Cuvier]]'s 1796 paper on living and fossil elephants.]]

==History of paleontology==
{{main|History of paleontology}}

Although palaeontology became established around 1800, earlier thinkers had noticed aspects of the [[fossil]] record. The ancient Greek [[Philosophy#Ancient_philosophy|philosopher]] [[Xenophanes]] (570-480 BC) concluded from fossil sea shells that some areas of land were once under water.<ref>{{cite book 
|last=Rudwick|first=Martin J.S.|authorlink=Martin J. S. Rudwick 
|title=The Meaning of Fossils | page=39 
|publisher=The University of Chicago Press|year=1985|edition=2nd|isbn=0-226-73103-0 
}}</ref> During the [[Middle Ages]] the Persian naturalist [[Avicenna|Ibn Sina]], known as ''Avicenna'' in Europe, discussed fossils and proposed a theory of petrifying fluids that [[Albert of Saxony]] elaborated on in the 14th century.<ref>{{cite book 
|last=Rudwick|first=Martin J.S.|authorlink=Martin J. S. Rudwick 
|title=The Meaning of Fossils | page=24 
|publisher=The University of Chicago Press|year=1985|edition=2nd|isbn=0-226-73103-0 
}}</ref> The Chinese naturalist [[Shen Kuo]] (1031-1095) proposed a theory of climate change based on the presence of [[petrified]] [[bamboo]] in regions that in his time were too dry for bamboo.<ref>{{cite book
|last=Needham|first=Joseph|authorlink=Joseph Needham
|title=''Science and Civilization in China: Volume 3, Mathematics and the Sciences of the Heavens and the Earth''
|page =614 | publisher=Caves Books Ltd|year=1986|isbn=0-253-34547-2
}}</ref>

In [[early modern Europe]], the systematic study of fossils emerged as an integral part of the changes in [[natural philosophy]] that occurred during the [[Age of Reason]]. At the end of the 18th century [[Georges Cuvier]]'s work established [[comparative anatomy]] as a scientific discipline and, by proving that some fossil animals resembled no living ones, demonstrated that animals could become [[extinction|extinct]], leading to the emergence of paleontology.<ref>{{cite book
|last=McGowan|first=Christopher|title=''The Dragon Seekers'' | pages=3–4
|publisher=Persus Publishing|year=2001|isbn=0-7382-0282-7 
}}</ref> The expanding knowledge of the fossil record also played an increasing role in the development of [[geology]], particularly [[stratigraphy]].<ref name="Palmer2005EarthTime">{{cite book 
| author=Palmer, D. | year=2005 
| title=Earth Time: Exploring the Deep Past from Victorian England to the Grand Canyon 
| publisher=Wiley | isbn=780470022214 
}}</ref>

The first half of the 19th century saw geological and paleontological activity become increasingly well organized with the growth of geologic societies and museums,<ref>{{cite book 
|last=Greene|first=Marjorie|coauthors=David Depew
|title=The Philosophy of Biology: An Episodic History | pages=128–130
|publisher=Cambridge University Press|year=2004|isbn=0-521-64371-6
}}</ref><ref>{{cite book 
|last=Bowler|first=Peter J.|coauthors=Iwan Rhys Morus|title=Making Modern Science | pages=168–169 
|publisher=The University of Chicago Press|year=2005|isbn=0-226-06861-7 
}}</ref> 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]].<ref>{{cite book 
|last=Rudwick|first=Martin J.S.|authorlink=Martin J. S. Rudwick 
|title=The Meaning of Fossils | pages=200–201 
|publisher=The University of Chicago Press|year=1985|edition=2nd|isbn=0-226-73103-0 
}}</ref> 

This contributed to a rapid increase in knowledge about the history of life on Earth and to progress in the definition of the [[geologic time scale]], largely based on fossil evidence. In 1822 Henri Marie Ducrotay de Blanville, editor of ''Journal de Phisique'', coined the word "paleontology" to refer to the study of ancient living organisms through fossils.<ref>{{cite book 
|last=Rudwick|first=Martin J.S.|authorlink=Martin J. S. Rudwick
|title=Worlds Before Adam: The Reconstruction of Geohistory in the Age of Reform page=48 
|publisher=The University of Chicago Press|year=2008|isbn=0-226-73128-6 
}} </ref> 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 the [[transmutation of species]].<ref name="BucklandGould">{{cite book|author=Buckland W & Gould SJ|title=Geology and Mineralogy Considered With Reference to Natural Theology (History of Paleontology)|year=1980|publisher=Ayer Company Publishing|isbn=978-0405127069}}</ref>
After [[Charles Darwin]] published ''[[Origin of Species]]'' in 1859, much of the focus of paleontology shifted to understanding [[evolution]]ary paths, including [[human evolution]], and evolutionary theory.<ref name="BucklandGould"/>

[[Image:Haikouichthys4.png| thumb | right | 200px | ''[[Haikouichthys]]'', from about {{ma|518}} in China, may be the earliest known fish.<ref>{{cite journal 
| author=Shu, D-G., Conway Morris, S., Han, J., ''et al.'' 
| title=Head and backbone of the Early Cambrian vertebrate Haikouichthys
| journal=Nature | volume=421 | page=526-529 | date=January 2003 | doi=10.1038/nature01264; 
| url=http://www.nature.com/nature/journal/v421/n6922/abs/nature01264.html | accessdate=September 21, 2008 
| doi_brokendate=2009-01-01 
}}</ref>]]
The last half of the 19th century saw a tremendous expansion in paleontological activity, especially in [[North America]].<ref>{{cite book 
|last=Everhart|first=Michael J. 
|title=''Oceans of Kansas: A Natural History of the Western Interior Sea'' | page=17
|publisher=Indiana University Press|year=2005|isbn=0-253-34547-2
}}</ref>  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 [[animal]]s, early [[fish]], [[dinosaur]]s and the evolution of birds.<ref>{{cite book 
| editor=Gee, H. | title=Rise of the Dragon: Readings from Nature on the Chinese Fossil Record | year=2001 | isbn=0226284913 | pages=276 
| url=http://books.google.co.uk/books?hl=en&id=6WYZQwmyWy0C&dq=Rise+of+the+Dragon++By+Henry+Gee&printsec=frontcover&source=web&ots=X_e3CwmtoV&sig=HZBiAmZLqqkJBJSdmJCvnIS4zMs&sa=X&oi=book_result&resnum=1&ct=result 
| accessdate=September 21, 2008 
| publisher=University of Chicago Press 
| location=Chicago, Ill. ;London 
| unused_data=|publisher University of Chicago Press 
}}</ref> The last few decades of the 20th century saw a renewed interest in [[mass extinction]]s and their role in the evolution of life on Earth.<ref>{{cite book
|last=Bowler|first=Peter J.|authorlink=Peter J. Bowler 
|title=''Evolution:The History of an Idea'' | pages=351–352
|publisher=University of California Press|year=2003|isbn=0-52023693-9
}}</ref> There was also a renewed interest in the [[Cambrian explosion]] that apparently saw the development of the body plans of most animal [[phylum|phyla]]. The discovery of fossils of the [[Ediacaran biota]] and developments in [[paleobiology]] extended knowledge about the history of life back far before the Cambrian.<ref name="Marshall2006Explaining" />

Increasing awareness of [[Gregor Mendel]]'s pioneering work in [[genetics]] led first to the development of [[population genetics]] and then in the mid-20th century to the [[modern evolutionary synthesis]], which explains [[evolution]] as the outcome of events such as [[mutation]]s and [[horizontal gene transfer]] providing [[genetic variation]], with [[genetic drift]] and [[natural selection]] driving changes in this variation over time.<ref>{{cite book
|last=Bowler|first=Peter J.|authorlink=Peter J. Bowler 
|title=''Evolution:The History of an Idea'' | pages=325–339
|publisher=University of California Press|year=2003|isbn=0-52023693-9
}}</ref> Within the next few years the role and operation of [[DNA]] in genetic inheritance were discovered, leading to what is now known as the [[central dogma of molecular biology|"Central Dogma" of molecular biology]].<ref>{{ cite web 
| author=Crick, F.H.C. | title=On degenerate templates and the adaptor hypothesis | year=1955 
| url=http://genome.wellcome.ac.uk/assets/wtx030893.pdf |format=PDF| accessdate=October 4, 2008 
}}</ref> In the 1960s [[molecular phylogenetics]], the investigation of evolutionary "family trees" by techniques derived from [[biochemistry]], began to make an impact, particularly when it was proposed that the human lineage had diverged from [[ape]]s much more recently than was generally thought at the time.<ref>{{cite journal 
| author=Sarich V.M., and Wilson A.C. | title=Immunological time scale for hominid evolution 
| journal=Science | volume=158 | issue=3805| month=December | year=1967 | pages=1200–1203 | url=http://www.sciencemag.org/cgi/content/abstract/158/3805/1200 | accessdate=September 21, 2008
| doi=10.1126/science.158.3805.1200 
| pmid=4964406 
}}</ref> Although this early study compared [[protein]]s from apes and humans, most molecular phylogenetics research is now based on comparisons of [[RNA]] and [[DNA]].<ref>{{ cite book 
| author=Page, R.D.M, and Holmes, E.C. | title=Molecular Evolution: A Phylogenetic Approach| page=2
| year = 1998 | isbn=0865428891 
| publisher=Blackwell Science 
| location=Oxford 
}}</ref>

==See also==
* [[Archaeobiology]]
* [[Timeline of paleontology]]
* [[List of publications in biology#Paleontology|Important publications in paleontology]]
* [[Fossil]]s
* [[History of paleontology]]
* [[Evolutionary history of life]]
* [[Geology]]
* [[Dinosaur]]s
* [[Radiometric dating]]
* [[Fossil collecting]]
* [[List of transitional fossils]]
* [[List of notable fossils]]
* [[List of fossil sites]] ''(with link directory)''

{{clear}}

==Notes==
{{reflist|group=derivation}}

==References==
{{Reflist | 2}}

==External links==
{{wikibooks}}
{{wiktionary}}
{{wikiversity2|Topic:Paleontology}} 
{{portalpar|Paleontology}}
*[http://paleobiology.si.edu/ Smithsonian's Paleobiology website: a good introduction]
*[http://www.ucmp.berkeley.edu/FAQ/faq.html University of California Museum of Paleontology FAQ About Paleontology]
*[http://www.paleosoc.org The Paleontological Society]
*[http://www.palass.org The Palaeontological Association]
*[http://www.paleoportal.org The Paleontology Portal]
*[http://fossilinsects.net/index.html International Palaeoentomological Society]

{{commonscat}}
{{Biology-footer}}

[[Category:Earth sciences]]
[[Category:Geology]]
[[Category:Paleontology| ]]
[[Category:Zoology|Paleontology]]
[[Category:Fossils|*]]
[[Category:Historical geology]]

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