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Transitional fossils are the fossilized remains of transitional forms of life that illustrate an 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. Numerous examples exist, including those of primates and early humans. Image File history File links Question_book-3. ... For other uses, see Fossil (disambiguation). ... This article is about biological evolution. ... This cladogram shows the relationship among various insect groups. ... Wikipedia does not have an article with this exact name. ... It has been suggested that Clade be merged into this article or section. ... This is a very tentative list of vertebrate transitional fossils (fossil remains of a creature that exhibits primitive traits in comparison with more derived life-forms to which it is related). ... The following charts give a brief overview of several notable primate fossil finds relating to human evolution. ...

According to 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 fossils", but on a cladogram representing the historical divergences of life-forms, a "transitional fossil" will represent an organism at the point where indivual lineages (clades) diverge. The modern evolutionary synthesis refers to a set of ideas from several biological specialities that were brought together to form a unified theory of evolution accepted by the great majority of working biologists. ... This article or section does not adequately cite its references or sources. ... Greek clados = branch) or phylogenetic systematics is a branch of biology that determines the evolutionary relationships of living things based on derived similarities. ... A clade is a term belonging to the discipline of cladistics. ...

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These diagrams plot the set of 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.

Image File history File links Download high resolution version (976x580, 12 KB) Hominin species known to science in 1850, plotted as boxes indicating their range of cranial capacities and range of appearance in the fossil record (as of 2005). ... Image File history File links Download high resolution version (976x580, 16 KB) Hominin species known to science in 1900, plotted as boxes indicating their range of cranial capacities and range of appearance in the fossil record (as of 2005). ... Image File history File links Download high resolution version (976x580, 21 KB) Hominin species known to science in 1950, plotted as boxes indicating their range of cranial capacities and range of appearance in the fossil record (as of 2005). ... Image File history File links Download high resolution version (976x580, 33 KB) Hominin species known to science in 2002 for which cranial capacity can be estimated, plotted as boxes indicating their range of cranial capacities and range of appearance in the fossil record (as of 2005). ... Tribes Gorillini Hominini and see text Homininae is a subfamily of Hominidae, including Homo sapiens and some extinct relatives, as well as the gorillas and the chimpanzees. ...

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.^[1] He noted the limited collections available at that time, but described the available information as showing patterns which followed from his theory of descent with modification through natural selection.^[2][3] Indeed, Archaeopteryx was discovered just two years later, in 1861, and represents a classic transitional form between dinosaurs and birds.^[3] For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ... The 1859 edition of On the Origin of Species First published in 1859, The Origin of Species (full title On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life) by British naturalist Charles Darwin is one of the pivotal... This article is about evolution in biology. ... For other uses, see Natural selection (disambiguation). ... Species A. lithographica Meyer, 1861 (type) Synonyms See below Archaeopteryx (from Ancient Greek archaios meaning ancient and pteryx meaning feather or wing; pronounced Ar-kay-op-ter-iks ) is the earliest and most primitive known bird to date. ...

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.^[3]

Examples of transitional fossils

See also: Evolution of the horse, Evolution of cetaceans, and 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 horses 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^[4] and various hominids considered to be proto-humans. Reconstruction, left forefoot skeleton (third digit emphasized yellow) and longitudinal section of molars of selected prehistoric horses The evolution of the horse involves the gradual development of the modern horse from the fox-sized, forest-dwelling Hyracotherium. ... The cetaceans (whales, dolphins and porpoises) are descendants of land-living mammals, and remnants of their terrestrial origins can be found in the fact that they must breathe air from the surface; in the bones of their fins, which look like huge, jointed hands; and in the vertical movement of... Othniel Charles Marsh (1831-1899) Othniel Charles Marsh (October 29, 1831 - March 18, 1899) was one of the pre-eminent paleontologists of the 19th century, who discovered and named many fossils found in the American West. ... Binomial name Equus caballus Linnaeus, 1758 The horse (Equus caballus, sometimes seen as a subspecies of the Wild Horse, Equus ferus caballus) is a large odd-toed ungulate mammal, one of ten modern species of the genus Equus. ... It has been suggested that Clade be merged into this article or section. ... Binomial name Ambulocetus natans Thewissen et al. ... Binomial name Daeschler, Shubin & Jenkins, 2006 Tiktaalik (IPA pronunciation: ) is a genus of extinct sarcopterygian (lobe-finned) fishes from the late Devonian period, with many features akin to those of tetrapods (four-legged animals) [1]. It is an example from several lines of ancient sarcopterygian fish developing adaptations to oxygen... Genera The hominids are the members of the biological family Hominidae (the great apes), which includes humans, chimpanzees, gorillas, and orangutans. ... Species Homo sapiens See text for extinct species. ...

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. It has been suggested that Clade be merged into this article or section. ... Paleontology, palaeontology or palæontology (from Greek: paleo, ancient; ontos, being; and logos, knowledge) is the study of prehistoric life forms on Earth through the examination of plant and animal fossils. ... This cladogram shows the relationship among various insect groups. ...

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. In phylogenetics, a group is monophyletic (Greek: of one stem) if all organisms in that group are known to have developed from a common ancestral form, and all descendants of that form are included in the group. ... Greek clados = branch) or phylogenetic systematics is a branch of biology that determines the evolutionary relationships of living things based on derived similarities. ... A crown group is a living monophyletic group or clade, consisting of the last common ancestor of all living examples, plus all of its descendants. ...

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:

• "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. Species A. lithographica Meyer, 1861 (type) Synonyms See below Archaeopteryx (from Ancient Greek archaios meaning ancient and pteryx meaning feather or wing; pronounced Ar-kay-op-ter-iks ) is the earliest and most primitive known bird to date. ... For other uses, see Platypus (disambiguation). ...

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.

Misconceptions

It is commonly claimed by critics of evolution that there are no transitional fossils.^[5][3][6] Such claims may be based on a misunderstanding of the nature of what represents a transitional feature^[5] or may be an active tactic by creationists seeking to distort or ignore the evidence that exists.^[3] The claim has been called a "favourite lie" of creationists by Donald Prothero which is "manifestly untrue".^[3] Creationism is generally the belief that the universe was created by a deity, or alternatively by one or more powerful and intelligent beings. ...

A common creationist argument is that no fossils are found with partially functional features, although this is more to do with irreducible complexity than transitional fossils. It is plausible, however, that a complex feature with one function can adapt a wholly different function through evolution. 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. Creationism is generally the belief that the universe was created by a deity, or alternatively by one or more powerful and intelligent beings. ... Irreducible complexity (IC) is an argument made by proponents of intelligent design that certain biological systems are too complex to have evolved from simpler, or less complete predecessors, through natural selection acting upon a series of advantageous naturally occurring chance mutations. ...

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 living beings, 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 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 periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution.^{[citation needed]} Punctuated equilibrium (or punctuated equilibria) is a theory in evolutionary biology which states that most sexually reproducing species will show little to no evolutionary change throughout their history. ... Stephen Jay Gould (September 10, 1941 – May 20, 2002) was an American paleontologist, evolutionary biologist, and historian of science. ... Dr. Niles Eldredge (born August 25, 1943) is an American paleontologist, who, along with Stephen Jay Gould, proposed the theory of punctuated equilibrium in 1972. ...

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. Binomial name Dart, 1925 [1] Australopithecus africanus was an early hominid, an australopithecine, who lived between 2-3 million years ago in the Pliocene. ... Taung Child refers to the fossil of a skull specimen of Australopithecus africanus. ... Pithecanthropus erectus redirects here. ... Binomial name (Dubois, 1892) Synonyms † Pithecanthropus erectus † Sinanthropus pekinensis † Javanthropus soloensis † Meganthropus paleojavanicus Homo erectus (Latin: upright man) is an extinct species of the genus Homo. ... Trinomial name Homo erectus pekinensis (Black, 1927) Peking Man (sometimes now called Beijing Man), also called Sinanthropus pekinensis (currently Homo erectus pekinensis), is an example of Homo erectus. ...

Footnotes

New Scientist is a weekly international science magazine covering recent developments in science and technology for a general English-speaking audience. ... Year 2007 (MMVII) was a common year starting on Monday of the Gregorian calendar in the 21st century. ... is the 170th day of the year (171st in leap years) in the Gregorian calendar. ...

References

• Darwin, Charles (1859), On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1st ed.), London: John Murray, <http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=1> 

For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ...

See also

• List of transitional fossils
• Evidence of common descent
• Gawis cranium

This is a very tentative list of vertebrate transitional fossils (fossil remains of a creature that exhibits primitive traits in comparison with more derived life-forms to which it is related). ... While on board HMS Beagle, Charles Darwin collected numerous specimens, many new to science, which supported his later theory of evolution by natural selection. ... Asahmed Humet holding the Gawis cranium, shortly after he discovered it. ...

External links

• Transitional vertebrate fossils FAQ, at talk.origins
• Hominid species FAQ, at talk.origins
• Transitional species in insect evolution (discusses the evolution of termites from cockroaches, with particular focus on extant intermediate species)
• Transitional fossils and Old Earth Creationism, at Answers In Creation
• No Missing Link? Evolutionary Changes Occur Suddenly, Professor Says - ScienceDaily.com, Feb.12, 2007. Professor Jeffrey H. Schwartz's article online - MIT Press Journals