Transitional fossil: Difference between revisions

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{{Paleontology|cTopic=Fossil record}}

A '''transitional fossil''' is any [[fossil]]ized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.<ref name="Freeman">{{harvnb|Freeman|Herron|2004|p=816}}</ref> This is especially important where the descendant group is sharply differentiated by [[gross anatomy]] and mode of living from the ancestral group. These fossils serve as a reminder that taxonomic divisions are [[human]] constructs that have been imposed in hindsight on a continuum of variation. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. Therefore, it cannot be assumed that transitional fossils are direct ancestors of more recent groups, though they are frequently used as models for such ancestors.<ref name=Prothero/>

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In 1859, when [[Charles Darwin]]'s ''[[On the Origin of Species]]'' was first published, the fossil record was poorly known. Darwin described the perceived lack of transitional fossils as, "... the most obvious and gravest objection which can be urged against my theory," but explained it by relating it to the extreme imperfection of the geological record.<ref>{{harvnb|Darwin|1859|pp=[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 that followed from his theory of [[evolution|descent with modification]] through [[natural selection]].<ref>{{harvnb|Darwin|1859|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=359 341–343]}}</ref> Indeed, ''[[Archaeopteryx]]'' was discovered just two years later, in 1861, and represents a classic transitional form between earlier, non-avian [[dinosaur]]s and [[bird]]s. [[List of transitional fossils|Many more transitional fossils]] have been discovered since then, and there is now abundant evidence of how all [[class (biology)|classes]] of [[vertebrate]]s are related, including many transitional fossils.<ref name="NS2645">{{cite journal |last=Prothero |first=Donald R. |authorlink=Donald Prothero |date=1 March 2008 |title=Evolution: What missing link? |url=https://www.newscientist.com/article/mg19726451.700-evolution-what-missing-link.html?full=true |journal=[[New Scientist]] |issue=2645 |pages=35–41 |issn=0262-4079 |doi=10.1016/s0262-4079(08)60548-5 |volume=197}}</ref> Specific examples of class-level transitions are: [[Evolution of tetrapods|tetrapod]]s and [[Evolution of fish|fish]], [[Origin of birds|birds and dinosaurs]], and [[Evolution of mammals|mammals and "mammal-like reptiles"]].

The term "missing link" has been used extensively in popular writings on [[human evolution]] to refer to a perceived gap in the [[Hominidae|hominid]] evolutionary record. It is most commonly used to refer to any new transitional fossil finds. Scientists, however, do not use the term, as it refers to a pre-evolutionary view of nature.

==Evolutionary and phylogenetic taxonomy==

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[[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.]]

"Missing link" is still a popular term, well recognized by the public and often used in the popular media.<ref name="zimmer">{{cite journal |last=Zimmer |first=Carl |authorlink=Carl Zimmer |date=19 May 2009 |title=Darwinius: It delivers a pizza, and it lengthens, and it strengthens, and it finds that slipper that's been at large under the chaise lounge [sic] for several weeks... |url=http://blogs.discovermagazine.com/loom/2009/05/19/darwinius-it-delivers-a-pizza-and-it-lengthens-and-it-strengthens-and-it-finds-that-slipper-thats-been-at-large-under-the-chaise-lounge-for-several-weeks/ |journal=[[Discover (magazine)|The Loom]] |type=Blog |___location=Waukesha, WI |publisher=[[Kalmbach Publishing]] |accessdate=2011-09-10}}</ref> It is, however, avoided in the scientific press, as it relates to the concept of the great chain of being and to the notion of simple organisms being primitive versions of complex ones, both of which have been discarded in biology.<ref name="NS2645">{{cite journal|last=Prothero|first=Donald R.|authorlink=Donald Prothero|date=1 March 2008|title=Evolution: What missing link?|url=https://www.newscientist.com/article/mg19726451.700-evolution-what-missing-link.html?full=true|journal=[[New Scientist]]|volume=197|issue=2645|pages=35–41|doi=10.1016/s0262-4079(08)60548-5|issn=0262-4079}}</ref> In any case, the term itself is misleading, as any known transitional fossil, like Java Man, is no longer missing. While each find will give rise to new gaps in the evolutionary story on each side, the discovery of more and more transitional fossils continues to add to our knowledge of evolutionary transitions.<ref name="NS2645" /><ref name="nomorelink">{{cite news |author=<!--Staff writer(s); no by-line.--> |title=Newly found fossils could link to human ancestor |url=http://www.cbc.ca/news/technology/newly-found-fossils-could-link-to-human-ancestor-1.930070 |work=[[CBC News]] |___location=Ottawa, Ontario, Canada |publisher=[[Canadian Broadcasting Corporation]] |date=8 April 2010 |accessdate=2015-05-19 |quote=It's tempting to call the new species a 'missing link' between earlier species and modern humans, but scientists say the concept no longer applies, given new knowledge of human evolution. [...] Researchers now say the evolution of humans consisted of a number of diverse species in many branches, not a single smooth line from ape-like species to humans.}}</ref>

===Punctuated equilibrium===