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''Tiktaalik'' is a genus of extinct [[Sarcopterygii|sarcopterygian]] (lobe-finned fish) from the Late [[Devonian]] period, with many features akin to those of tetrapods (four-legged animals).<ref name="Nature">{{cite journal |last1=Daeschler |first1=Edward B. |author-link1=Ted Daeschler |last2=Shubin |first2=Neil H. |author-link2=Neil Shubin |last3=Jenkins | first3=Farish A. Jr. |author-link3=Farish Jenkins |date=6 April 2006 |title=A Devonian tetrapod-like fish and the evolution of the tetrapod body plan |journal=Nature |volume=440 |issue=7085 |pages=757–763 |bibcode=2006Natur.440..757D |doi=10.1038/nature04639 |issn=0028-0836 |pmid=16598249 |doi-access=free }}</ref> It is one of several lines of ancient sarcopterygians to develop adaptations to the oxygen-poor shallow water habitats of its time—adaptations that led to the evolution of tetrapods.<ref name="scientificamerican">{{cite journal |last=Clack |first=Jennifer A. |author-link=Jenny Clack |date=December 2005 |title=Getting a Leg Up on Land |journal=Scientific American |volume=293 |pages=100–107 |issue=6 |bibcode=2005SciAm.293f.100C |doi=10.1038/scientificamerican1205-100 |issn=0036-8733 |pmid=16323697 }}</ref> Well-preserved fossils were found in 2004 on [[Ellesmere Island]] in [[Nunavut]], Canada.<ref>{{cite journal |last=Easton |first=John |date=23 October 2008 |title=''Tiktaalik's'' internal anatomy explains evolutionary shift from water to land |url=http://chronicle.uchicago.edu/081023/tiktaalik.shtml |journal=University of Chicago Chronicle |issue=3 |volume=28 |issn=1095-1237 |access-date=2012-04-19 }}</ref>
''Tiktaalik'' lived approximately 375 million years ago. [[Paleontology|Paleontologist]]s suggest that it is representative of the transition between non-tetrapod vertebrates such as ''[[Panderichthys]]'', known from fossils 380 million years old, and early tetrapods such as ''[[Acanthostega]]'' and ''[[Ichthyostega]]'', known from fossils about 365 million years old. Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, [[Neil Shubin]], to characterize ''Tiktaalik'' as a "[[Tetrapodomorpha|fishapod]]."<ref>{{cite news |last=Wilford |first=John Noble |author-link=John Noble Wilford |date=5 April 2006 |title=Scientists Call Fish Fossil the 'Missing Link' |url=https://www.nytimes.com/2006/04/05/science/05cnd-fossil.html |newspaper=[[The New York Times]] |access-date=2015-05-17}}</ref><ref>{{harvnb|Shubin|2008}}</ref> Unlike many previous, more fish-like transitional fossils, the "fins" of ''Tiktaalik'' have basic wrist bones and simple rays reminiscent of fingers. They may have been [[weight-bearing]]. Like all modern tetrapods, it had rib bones, a mobile neck with a separate pectoral girdle, and lungs, though it had the gills, scales, and fins of a fish.<ref name="Nature" /> However in a 2008 paper by Boisvert at al. it is noted that ''Panderichthys'', due to its more derived distal portion, might be closer to tetrapods than ''Tiktaalik'', which might have independently developed similarities to tetrapods by convergent evolution.<ref>{{cite web|url=http://uu.diva-portal.org/smash/get/diva2:221262/FULLTEXT01 |title=Pectoral fin info |publisher=uu.diva-portal.org
Tetrapod footprints found in Poland and reported in ''[[Nature (journal)|Nature]]'' in January 2010 were "securely dated" at 10 million years older than the oldest known [[Elpistostegalia|elpistostegids]]<ref>{{cite journal |last1=Niedźwiedzki |first1=Grzegorz |last2=Szrek |first2=Piotr |last3=Narkiewicz |first3=Katarzyna |last4=Narkiewicz |first4=Marek |last5=Ahlberg |first5=Per E. |author-link5=Per E. Ahlberg |date=7 January 2010 |title=Tetrapod trackways from the early Middle Devonian period of Poland |journal=Nature |volume=463 |issue=7227 |pages=43–48 |doi=10.1038/nature08623 |issn=0028-0836 |pmid=20054388 |display-authors=3 |bibcode=2010Natur.463...43N |s2cid=4428903 }}</ref> (of which ''Tiktaalik'' is an example), implying that animals like ''Tiktaalik'', possessing features that evolved around 400 million years ago, were "late-surviving relics rather than direct transitional forms, and they highlight just how little we know of the earliest history of land vertebrates."<ref>{{cite journal |author=<!--Staff writer(s); no by-line.--> |date=7 January 2010 |title=Four feet in the past: trackways pre-date earliest body fossils |url=http://www.nature.com/nature/journal/v463/n7277/edsumm/e100107-01.html |type=Editor's summary |journal=Nature |volume=463 |issue=7227 |issn=0028-0836}}</ref>
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The idea that animal and plant species were not constant, but changed over time, was suggested as far back as the 18th century.<ref>{{cite journal |last1=Archibald |first1=J. David |date=August 2009 |title=Edward Hitchcock's Pre-Darwinian (1840) 'Tree of Life' |url=http://www.bio.sdsu.edu/faculty/archibald.html/Archibald09JHB42p561.pdf |journal=[[Journal of the History of Biology]] |volume=42 |issue=3 |pages=561–592 |doi=10.1007/s10739-008-9163-y |issn=0022-5010 |pmid=20027787 |citeseerx=10.1.1.688.7842 |s2cid=16634677 }}</ref> Darwin's ''On the Origin of Species'', published in 1859, gave it a firm scientific basis. A weakness of Darwin's work, however, was the lack of palaeontological evidence, as pointed out by Darwin himself. While it is easy to imagine [[natural selection]] producing the variation seen within [[genus|genera]] and families, the transmutation between the higher categories was harder to imagine. The dramatic find of the [[Archaeopteryx#History of discovery|London specimen]] of ''[[Archaeopteryx]]'' in 1861, only two years after the publication of Darwin's work, offered for the first time a link between the [[class (biology)|class]] of the highly derived birds, and that of the more basal [[reptile]]s.{{sfn|Darwin|1859|loc=[http://www.readprint.com/chapter-2217/Charles-Darwin Chapter 10]}} In a letter to Darwin, the palaeontologist [[Hugh Falconer]] wrote:
<blockquote>Had the Solnhofen quarries been commissioned—by august command—to turn out a strange being à la Darwin—it could not have executed the behest more handsomely—than in the ''Archaeopteryx''.<ref>{{cite journal |last=Williams |first=David B. |date=September 2011 |title=Benchmarks: September 30, 1861: Archaeopteryx is discovered and described |url=http://www.earthmagazine.org/article/benchmarks-september-30-1861-archaeopteryx-discovered-and-described |journal=EARTH |issn=1943-345X |access-date=2012-02-24 |archive-url=https://web.archive.org/web/20120507072544/http://www.earthmagazine.org/article/benchmarks-september-30-1861-archaeopteryx-discovered-and-described |archive-date=7 May 2012
Thus, transitional fossils like ''Archaeopteryx'' came to be seen as not only corroborating Darwin's theory, but as icons of evolution in their own right.<ref>{{harvnb|Wellnhofer|2009}}</ref> For example, the Swedish [[encyclopedic dictionary]] ''Nordisk familjebok'' of 1904 showed an inaccurate ''Archaeopteryx'' reconstruction (see illustration) of the fossil, "ett af de betydelsefullaste paleontologiska fynd, som någonsin gjorts" ("one of the most significant paleontological discoveries ever made").<ref>{{harvnb|Leche|1904|pp=[http://runeberg.org/nfba/0758.html 1379–1380]}}</ref>
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The term was first used in a scientific context by [[Charles Lyell]] in the third edition (1851) of his book ''Elements of Geology'' in relation to missing parts of the [[Geologic time scale|geological column]], but it was popularized in its present meaning by its appearance on page xi of his book ''[[Geological Evidences of the Antiquity of Man]]'' of 1863. By that time, it was generally thought that the end of the [[last glacial period]] marked the first appearance of humanity; Lyell drew on new findings in his ''Antiquity of Man'' to put the origin of human beings much further back. Lyell wrote that it remained a profound mystery how the huge gulf between man and beast could be bridged.<ref>{{cite journal |last=Bynum |first=William F. |date=Summer 1984 |title=Charles Lyell's ''Antiquity of Man'' and its critics |journal=Journal of the History of Biology |volume=17 |issue=2 |pages=153–187 |doi=10.1007/BF00143731 |jstor=4330890 |s2cid=84588890 |issn=0022-5010 }}</ref> Lyell's vivid writing fired the public imagination, inspiring [[Jules Verne]]'s ''[[Journey to the Center of the Earth]]'' (1864) and [[Louis Figuier]]'s 1867 second edition of ''La Terre avant le déluge'' ("Earth before the Flood"), which included dramatic illustrations of savage men and women wearing animal skins and wielding stone axes, in place of the [[Garden of Eden]] shown in the 1863 edition.{{sfn|Browne|2003|pp=130, 218, 515}}
The search for a fossil showing transitional traits between apes and humans, however, was fruitless until the young Dutch geologist [[Eugène Dubois]] found a skullcap, a molar and a [[femur]] on the banks of [[Solo River]], [[Java]] in 1891. The find combined a low, ape-like [[skull roof]] with a brain estimated at around 1000 cc, midway between that of a chimpanzee and an adult human. The single molar was larger than any modern human tooth, but the femur was long and straight, with a knee angle showing that "[[Java Man]]" had walked upright.<ref>{{harvnb|Swisher|Curtis|Lewin|2001}}</ref> Given the name ''[[Homo erectus|Pithecanthropus erectus]]'' ("erect ape-man"), it became the first in what is now a long [[list of human evolution fossils]]. At the time it was hailed by many as the "missing link," helping set the term as primarily used for human fossils, though it is sometimes used for other intermediates, like the dinosaur-bird intermediary ''[[Archaeopteryx]]''.<ref>{{harvnb|Reader|2011}}</ref><ref>{{cite web |last=Benton |first=Michael J. |author-link=Michael Benton |date=March 2001 |title=Evidence of Evolutionary Transitions |url=http://www.actionbioscience.org/evolution/benton2.html |website=[[actionbioscience]] |publisher=[[American Institute of Biological Sciences]] |___location=Washington, D.C. |access-date=2012-03-29 |archive-date=26 April 2012 |archive-url=https://web.archive.org/web/20120426203938/http://www.actionbioscience.org/evolution/benton2.html
[[File:Punctuated Equilibrium.svg|thumb|upright=1.7|Sudden jumps with apparent gaps in the fossil record have been used as evidence for [[punctuated equilibrium]]. Such jumps can be explained either by [[macromutation]] or simply by relatively rapid episodes of gradual evolution by natural selection, since a period of say 10,000 years barely registers in the fossil record.]]
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==Sources==
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* {{cite book |last1=Gingerich |first1=Philip D. |author-link1=Philip D. Gingerich |last2=Russell |first2=Donald E. |year=1981 |title=Pakicetus inachus, a New Archaeocete (Mammalia, Cetacea) From the Early-Middle Eocene Kuldana Formation of Kohat (Pakistan) |url=http://deepblue.lib.umich.edu/bitstream/handle/2027.42/48501/ID352.pdf?sequence=2&isAllowed=y|type=Research report |series=Contributions from the Museum of Paleontology |volume=25 |issue=11 |___location=Ann Arbor, MI |publisher=[[University of Michigan Museum of Natural History|Museum of Paleontology, University of Michigan]] |pages=235–246 |issn=0097-3556 |lccn=82621252 |oclc=8263404 }}
* {{cite book |last=Gould |first=Stephen Jay |author-link=Stephen Jay Gould |year=1980 |title=The Panda's Thumb: More Reflections in Natural History |edition=1st |___location=New York |publisher=[[W. W. Norton & Company]] |isbn=978-0-393-01380-1 |lccn=80015952 |oclc=6331415 |title-link=The Panda's Thumb (book) }}
* {{cite book |last=Haeckel |first=Ernst |author-link=Ernst Haeckel |title=The Evolution of Man |year=2011 |orig-
* {{cite book |last=Lamarck |first=Jean-Baptiste |author-link=Jean-Baptiste Lamarck |year=1815–1822 |title=Histoire naturelle des animaux sans vertèbres |language=fr |___location=Paris |publisher=Verdière |lccn=07018340 |oclc=5269931 }}
* {{cite book |last=Lovejoy |first=Arthur O. |author-link=Arthur Oncken Lovejoy |year=1936 |title=The Great Chain of Being: A Study of the History of an Idea |url=https://archive.org/details/greatchainofbein0000love |url-access=registration |series=William James Lectures, 1933 |___location=Cambridge, MA |publisher=[[Harvard University Press]] |lccn=36014264 |oclc=192226 }}
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* {{cite book |last=Reader |first=John |year=2011 |title=Missing Links: In Search of Human Origins |others=Foreword by Andrew Hill |edition=Enlarged and updated |___location=Oxford; New York |publisher=[[Oxford University Press]] |isbn=978-0-19-927685-1 |lccn=2011934689 |oclc=707267298 |url-access=registration |url=https://archive.org/details/missinglinksinse0000read }}
* {{cite book |last=Shubin |first=Neil |author-link=Neil Shubin |year=2008 |title=Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body |___location=New York |publisher=[[Pantheon Books]] |isbn=978-0-375-42447-2 |lccn=2007024699 |oclc=144598195 |url=https://archive.org/details/yourinnerfishjou00shub_0 }}
* {{cite book |last1=Swisher |first1=Carl C.
* {{cite book |last=Wellnhofer |first=Peter |author-link=Peter Wellnhofer |year=2004 |chapter=The Plumage of ''Archaeopteryx'': Feathers of a Dinosaur? |editor1-last=Currie |editor1-first=Philip J. |editor1-link=Philip J. Currie |editor2-last=Koppelhus |editor2-first=Eva B. |editor3-last=Shugar |editor3-first=Martin A. |display-editors = 3 |editor4-last=Wright |editor4-first=Joanna L. |title=Feathered Dragons: Studies on the Transition from Dinosaurs to Birds |series=Life of the Past |___location=Bloomington, IN |publisher=[[Indiana University Press]] |isbn=978-0-253-34373-4 |lccn=2003019035 |oclc=52942941 }}
* {{cite book |last=Wellnhofer |first=Peter |title=Archaeopteryx: The Icon of Evolution |year=2009 |others=Translated by Frank Haase; foreword by [[Luis M. Chiappe]] |edition=Revised English edition of the 1st German |___location=München |publisher=Verlag Dr. Friedrich Pfeil |isbn=978-3-89937-108-6 |oclc=501736379 }}
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