Transitional fossil 331755 226012983 2008-07-16T13:06:49Z 87.15.239.130 {{Refimprove|date=December 2007}} '''Transitional fossils''' are the [[fossil]]ized remains of '''transitional forms''' of life that illustrate an [[Evolution theory|evolutionary]] transition. They can be identified by their retention of certain primitive ([[plesiomorphic]]) traits in comparison with their more [[derived]] relatives, as they are defined in the study of [[cladistics]]. '''"Missing link"''' is a popular term for transitional forms. [[List of transitional fossils|Numerous examples]] exist, including those of [[List of human evolution fossils|primates and early humans]]. According to [[Modern evolutionary synthesis|modern evolutionary theory]], all populations of organisms are in transition. Therefore, a "transitional form" is a human construct of a selected form that vividly represents a particular evolutionary stage, as recognized in hindsight. Contemporary "transitional" forms may be called "[[living fossil]]s", but on a [[cladogram]] representing the historical divergences of life-forms, a "transitional fossil" will represent an organism at the point where individual lineages ([[clade]]s) diverge. {| align="right" cellpadding="4" width="180px" style="margin:0 0 1em 1em; background: #f9f9f9; border: 1px #aaa solid; border-collapse: collapse;" |- |1850 |[[Image:Hominins 1850.png|200px|Described Hominin species, 1850]] |- |1900 |[[Image:Hominins 1900.png|200px|Described Hominin species, 1900]] |- |1950 |[[Image:Hominins 1950.png|200px|Described Hominin species, 1950]] |- |2002 |[[Image:Hominins 2002.png|200px|Selection of described Hominin species, 2002]] |- |colspan="2"|<small>These diagrams plot the set of [[Homininae|Hominin]] species known to science as of a given year. Each species is plotted as a box showing the range of cranial capacities for specimens of that species, and the range of dates at which specimens appear in the fossil record. The sequence of diagrams shows how an apparent "missing link" or gap between species in the fossil record may become filled as more fossil discoveries are made.</small> |} ==Transitional fossils and the theory of evolution== In 1859, when [[Charles Darwin]]'s ''[[On the Origin of Species]]'' was first published, the fossil record was poorly known, and Darwin described the lack of transitional fossils as "the most obvious and gravest objection which can be urged against my theory", but explained it by the extreme imperfection of the geological record.<ref>{{harvnb|Darwin|1859|p=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=297 279-280]}}</ref> He noted the limited collections available at that time, but described the available information as showing patterns which followed from his theory of [[evolution|descent with modification]] through [[natural selection]].<ref>{{harvnb|Darwin|1859|p=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=359 341-343]}}</ref><ref name="NS2645">[[New Scientist]], [[27 February]] [[2008]], Issue 2645, pages 35-40, "Evolution: What missing link?" by [[Donald Prothero]]</ref> Indeed, ''[[Archaeopteryx]]'' was discovered just two years later, in 1861, and represents a classic transitional form between dinosaurs and birds.<ref name="NS2645" /> Many more transitional fossils have been discovered since then and it is now considered that there is abundant evidence of how all the major groups of animals are related, much of it in the form of transitional fossils.<ref name="NS2645" /> ===Examples of transitional fossils=== {{seealso|Evolution of the horse|Evolution of cetaceans|Evolution of mammalian auditory ossicles}} Though the evolution of the horse and its relatives, as [[Othniel Charles Marsh]] assembled surviving fossils in his reconstruction of the evolution of [[horse]]s in the form of a single, consistently developing lineage with many "transitional" types, is often cited as a family tree with a number of clear transitional fossils, modern [[cladistics]] gives a different, multi-stemmed shrublike picture, with multiple innovations and many dead ends. Other specimens cited as transitional forms include the "walking whale" ''[[Ambulocetus]]'', the recently-discovered lobe-finned fish ''[[Tiktaalik]]''<ref>{{cite book|last=Shubin|first=Neil|title=Your Inner Fish|publisher=Pantheon|date=2008|isbn=9780375424472}}</ref> and various [[Hominidae|hominid]]s considered to be [[Homo (genus)|proto-human]]s. ==Transitional forms and cladistics== Before the general acceptance of [[cladistics]] in [[paleontology]], evolutionary trees were often drawn as the emerging of one group from another. The transitional forms were placed at the borders of these. With the establishment of cladistic methods, relationships are now strictly expressed in so-called [[cladograms]], illustrating the branching of the evolutionary lineages. The different so-called 'natural' or '[[monophyletic]]' groups form nested units that do not overlap. Within cladistics there is thus no longer a transition between established groups, but a differentiation that occurs within groups, represented as a branching in the [[cladogram]]. In this context, transitional organisms can be conceptualized as representing early examples on the different branches of a cladogram, lying between a particular branching point and the "[[crown-group]]", i.e. the most-derived group, which is placed at the end of a lineage. ==Transitional forms vs. 'intermediate' forms== The terms 'transitional' and 'intermediate' are for the most part used as synonyms; however, a distinction between the two can be made:<!--is this distinction made in the professional literature?--> * "Transitional" can be used for those forms that do ''not'' have a significant number of unique derived traits that the derived relative does not possess as well. In other words, a transitional organism is morphologically close to the actual common ancestor it shares with its more derived relative. * "Intermediate" can be used for those forms that ''do'' have a large number of uniquely derived traits not connected to its derived relative. According to this definition, ''[[Archaeopteryx]]'', which does not show any derived traits that more derived birds do not possess as well, is transitional. In contrast, the [[platypus]] is intermediate because it retains certain reptilian traits no longer found in modern mammals and also possesses derived traits of a highly specialized aquatic animal. Following this definition, all living organisms are in fact to be regarded as intermediate forms when they are compared to some other related life-form. Indeed there are many species alive today that can be considered to be transitional between two or more groups.<!--giving an example here will clarify this apparent misunderstanding of cladistics--> ==Misconceptions== It is commonly claimed by critics of evolution that there are no transitional fossils.<ref name="CC200">{{Cite web | url = http://www.talkorigins.org/indexcc/CC/CC200.html | title = Claim CC200 at www.talkorigins.org | accessdate = 2007-06-19}}</ref><ref name="NS2645" /><ref>[The Scientific Case for Creation http://www.creationscience.com/onlinebook/LifeSciences27.html], Life Sciences, 23. Fossil Gaps</ref> Such claims may be based on a misunderstanding of the nature of what represents a transitional feature<ref name="CC200" />, or may be an active tactic employed by [[creationist]]s, seeking to distort or to discredit evolution theory.<ref name="NS2645" /> The claim has been called a "favourite lie" of creationists by Donald Prothero which is "manifestly untrue".<ref name="NS2645" /> A common [[creationist]] argument is that no fossils are found with partially functional features. However, there is evidence that a complex feature with one function can adapt to a wholly different function through evolution in a process known as [[exaptation]]. The precursor to, for example, a wing, might originally have only been used for gliding, trapping flying prey, and/or mating display. Nowadays, wings may still have all of these functions, while also being used for active flight. Although transitional fossils elucidate the evolutionary transition of one life-form to another, they only exemplify snapshots of this process. Due to the special circumstances required for preservation of remains, only a very small percentage of all life-forms that ever have existed can be expected to be represented in discoveries. Thus, the transition itself can only be illustrated and corroborated by transitional fossils, but it will never be "caught in the act" as it were. Critics of evolution often cite this argument as being a convenient way to explain the lack of 'snapshot' fossils that show crucial steps between species. However, progressing research and discovery are managing to fill in gaps. The theory of [[punctuated equilibrium]] developed by [[Stephen Jay Gould]] and [[Niles Eldredge]] and first presented in 1972<ref>Niles Eldredge and Stephen Jay Gould, 1972. "Punctuated equilibria: an alternative to phyletic gradualism" In T.J.M. Schopf, ed., ''Models in Paleobiology.'' San Francisco: Freeman Cooper. pp. 82-115. Reprinted in N. Eldredge ''Time Frames.'' Princeton: Princeton Univ. Press. 1985.</ref> is often mistakenly drawn into the discussion of transitional fossils. This theory, however, pertains only to well-documented transitions within taxa or between closely related taxa over a geologically short period of time. These transitions, usually traceable in the same geological outcrop, often show small jumps in morphology between extended periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution. ==Missing links== A popular term used to designate transitional forms is "missing links". The term is especially used in the regular media, but is inaccurate and confusing. In reality, the continuing discovery of more and more transitional fossils is further adding to the knowledge of evolutionary transitions. The term probably arose in the nineteenth century where the awaited discovery of a "missing link" between humans and so-called "lower" animals was considered to be the final proof of evolution. The discovery of ''[[Australopithecus africanus]]'' ([[Taung Child]]), [[Java Man]], ''[[Homo erectus]]'', ''[[Sinanthropus pekinensis]]'' (Peking Man), etc. are also vital to the study of links. ==Footnotes== {{reflist|2}} ==References== *{{Citation | last = Darwin | first = Charles | author-link =Charles Darwin | year = 1859 | title =[[On the Origin of Species]] by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life | edition =1st | publication-place = London | publisher =John Murray | url =http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=1 | accessdate = }} *{{Citation | last = Sunderland | first = Luther | author-link =luther sunderland | year = 1988 | title =[[Darwin's Enigma]] | edition =1st | publication-place = El Cajon, CA | publisher =Master Books | url =http://www.answersingenesis.org/creation/v14/i4/fossils.asp | accessdate = }} ==See also== *[[List of transitional fossils]] *[[Evidence of common descent]] *[[Gawis cranium]] ==External links== * [http://talkorigins.org/faqs/faq-transitional.html Transitional vertebrate fossils FAQ], at [[talk.origins]] * [http://www.talkorigins.org/faqs/homs/species.html Hominid species FAQ], at talk.origins * [http://darwinblog.blogspot.com/2005/12/transitional-species-in-insect_24.html Transitional species in insect evolution] (discusses the evolution of [[termite]]s from [[cockroach]]es, with particular focus on [[Extant taxon|extant]] intermediate species) * [http://www.answersincreation.org/transitional_fossils.htm Transitional fossils and Old Earth Creationism], at [[Answers In Creation]] * [http://www.sciencedaily.com/releases/2007/02/070210170623.htm No Missing Link? Evolutionary Changes Occur Suddenly, Professor Says] - ScienceDaily.com, Feb.12, 2007. Professor [[Jeffrey H. Schwartz]]'s [http://www.mitpressjournals.org/doi/abs/10.1162/biot.2006.1.4.357 article online] - MIT Press Journals * [http://www.bbc.co.uk/dna/h2g2/alabaster/A4351628 H2G2 Article on Transitional fossils] - By H2G2 Article member "Kiteman"; goes over Transitional fossils. [[Category:Fossils]] [[Category:Evolutionary biology]] [[da:Overgangsform (fossil)]] [[de:Missing Link]] [[es:Fósil transicional]] [[fr:Forme transitionnelle (évolution)]] [[he:החוליה החסרה]] [[it:Anello mancante]] [[nl:Fossiele overgangsvormen]] [[ja:ミッシングリンク]] [[pl:Brakujące ogniwo ewolucji]] [[pt:Fóssil de transição]] [[fi:Puuttuva rengas]] [[sv:Felande länk]]